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I have written this article for my Land Rover Club and thought it might be a reasonable Tech article for this site. However, I cannot seem to post it in the Tech section, so I will post it here.
It took me a long time to put this together, so I hope it is of benefit to others.
Batteries and your vehicle.
Battery Basics
As a battery charges or discharges the current it stores, there are chemical reactions happening in the battery. Basically as it discharges the surface of the plates get a coating of lead sulphate and the charging process forces that lead sulphate back off and returns the plates to lead.
The main thing from this is that it takes more effort to reverse the process than what the reaction produced in the first place. It takes about 10% more amps than it produced. The reaction to reverse the process (charge the battery) is controlled by the voltage. It takes at least 0.7 volts over what the battery is sitting at to start any reaction at all and the recommended voltage is around 14.3 volts for a flooded battery and 14.7 volts for an AGM at normal room temps.
These voltage readings are based on the battery not being charged for at least 6 hours. A battery is classed at 100% charge when it sits at 12.7 volts, 75% at 12.4 Volts, 50% at 12.2 Volts, 25% at 12.0 volts, and flat at 11.9 volts.
Power stored in the Battery.
People tend to use the Amp Hour rating of a battery as an indicator of how many amps they can use in the battery. This is not really the case. The Amp Hour rating is only how much power you can get out of the battery at a constant amp draw over 10 hours over a 10 hour period before it get down to 10 volts. So for a 90 amp hour battery you can draw 9 amps for 10 hours before the battery gets to 10 volts.
However, the quicker you draw the current out of the battery, the less amps you will get out of it. For example, CCA or Cold Cranking Amps is how many amps you can get out of the battery for 30 seconds at -18C before the battery drops below 7.2 volts. CCA means little to people in Australia as we are unlikely to be trying to start a car in -18C temperatures. Hot Cranking Amps is what amps you can get for 30 seconds at 27C. So for my example AGM battery, it has 770 CCA or 1080 HCA. 30 seconds is only 120th of an hour. So the ability to only provide a total of 180 over 120th of a hour means it only supplied 9 amp hours of power and not the 91 amp hours it was rated at. If you were doing other heavy amp drawing work with the battery, like winching, it would only last 2 minutes at 530 amps (18 amp hours), 5 minutes at 350 amps (29 amp hours), or 10 minutes at 240 amps (39 amp hours). So you cannot simply divide the amp hour rating of a battery by the amps you are drawing to work out how long the battery will last. The more you draw the less you will get out of the battery.
Battery Charging
You need to reverse the chemical reaction to charge a battery. This requires both voltage and amps. It is the voltage that opens the door and it is then the amps that go through it. The more the door is opened, the more amps that can go through it. The door gets harder to open the colder it is and therefore requires more voltage. Put too much voltage in and the reaction becomes too great and the battery will start to heat up and gas. The maximum voltage is usually listed at 15 volts during the main bulk charging process. How far the door is opened is also about the voltage being charged at and the voltage or state of charge of the battery. As the battery becomes charged the opening created will reduce and therefore the amps that can be put into the battery will reduce.
The actual charge times will be slightly different from battery to battery depending on its internal resistance, etc. The following is an example of one battery manufacturer on a 100 amp hour AGM battery discharged by 55%. On a charger that can put out 20 amps and at room temp of 25C, it would take 47 hours to fully recharge the battery at 13.5 volts, 18 hours and 13.8 volts, 10 hours at 14.4 volts, and only 8 hours at the recommended voltage of 14.7 volts.
The amount of voltage required to recharge a battery changes by 0.25 volts for every 10C change in the battery temperature. The higher the temperature the lower the volts required. Therefore in the above case you would move all the voltages up 0.25 volts at 15C or down 0.25 volts at 35C.
There are basically 3 stages a battery goes through when charging. These are the Bulk, Absorption, and Float Stages.
The Bulk stage is when the battery is very low. The Voltage being applied to the battery allows large amps to be put back into the battery and it charges up quickly. This stage ends when the charging manages to get the voltage in the cells up to the voltage being used to charge it. When using the correct voltage, this process generally ends when the battery is back up to 70% to 80% of capacity. The lower the voltage being used the lower the percentage of recovery that is obtained during this stage.
The Absorption stage commences when the bulk stage completes. During the absorption stage the amount of amps that the battery will take is steadily reduced as the battery becomes more charged. It will generally finish when the amount of amps being absorbed by the battery drops to between 1 and 5 amps.
The Float stage is mainly to maintain the battery at its state of charge and maybe put the last couple of percent of capacity back into the battery. At this stage you want the reactions in the battery to calm down to stop it overcharging and causing gassing, etc. Therefore the voltage should be dropped to a float voltage around the mid 13 volts.
If is often listed that it can take 4 times longer to get the battery from the beginning of the absorption stage to the battery being fully charged, than it did to get the bulk of the power back into the battery through the bulk stage. This means that you can get the battery back up to 70% to 80% capacity quite quickly if you want to use that power again.
There are suggestions that you can charge a flat battery with another battery. This is highly unlikely. Even if you had a dead flat battery at 11.9 volts and a fully charged battery at 12.7 volts, there is going to be a minimal transfer of current before the fully charged battery comes within the 0.7 volts of the dead one
Float charge
The float voltage for you main battery does not have a lot of relevance as it is always being used and will not stay at a fully charged state for long. However, for an auxiliary battery, that is used rarely on camping trips, etc, it will become fully charged and stay that way for extended periods of time. Therefore the float charge becomes more important. Putting in a voltage higher than the float charge will cause the battery to overcharge, overheat, and give off gasses (even for an AGM). The float charge for an AGM in the back of your car at 20C is 13.8 volts. Under the bonnet at 35C it is 13.4 volts. This is below the voltage that your charging system is putting out. Therefore it is wise to disconnect your auxiliary battery when you do not plan to use it for a while and connect it back up a couple of days before you plan to use it again. You should connect it up every 3 to 6 months for a short time to top it up.
Your Cars electrical system and its affect on Batteries
As listed previously, the voltage that a battery charges at will have a significant affect on the charging of a battery. That is an AGM battery at 13.5 volts will take 47 hours to recharge from 45% compared to 10 hours at 14.4 volts. Therefore Voltage Drop has a significant effect on charging a battery.
The voltage drop of a wire is determined by the surface area of the copper core (the area across the end of the cable) and the amps you are putting through it. The number of strands in that core is not important for DC voltage, only AC voltage where there is a “skin” effect.
For reasons only known to Land Rover, they use only relatively small cables for the main power supplies from the alternator to batteries and batteries to starter motor, as well as, associated earths.
The voltage drop is calculated by the length of the wire in metres (including the return earth wire) multiplied by current draw in amps multiplied by the resistance of copper at 0.017, all divided by the cross section area of the wire in square millimetres.
The main power cables for my Range Rover have 37 copper strands, each with a diameter of 0..75mm. This gives a cross sectional area of 16.3mm2. So at a middle of the range output from the alternator at say 60 amps the voltage drop over 3 metres (1.5 metre there and 1.5 back) will be (3x60x.017)/16.3 or 0.19 volts. Up this to 85 amps and the voltage drop increases to .27 volts and at 120 amps .38 volts. So this is the voltage drop just from the alternator to the battery without worrying about the rest of the wiring.
To determine the actual voltage drop on your car you do not need to know the formula, you only need a multimeter. Start the car and put on as many electrical items as possible to simulate the maximum draw you will place on the alternator. Switch on headlights, aircon, rear window demister, driving lights, thermo fans, etc. While using either the body of the alternator or the negative terminal on the battery for the negative cable of the multimeter, put the positive lead of the multimeter on the charge terminal of the alternator and then to the positive terminal of the main battery. The difference in voltage between the two readings is that caused by the charge wire from the alternator to the battery. Then while keeping the positive lead of the multimeter on either the positive terminal of the battery or the charge terminal of the alternator, move the negative lead of the multimeter between the body of the alternator and the negative terminal of the battery. The difference between the two readings is the voltage drop from the earth connections/wires between the alternator/motor and the negative terminal of the battery. The voltage drop of the charge wire and the earthing added together is the total voltage drop you are suffering due to Land Rovers wiring.
Say you then put an auxiliary battery in the rear of your car. You then run the recommended cable that is rated at 50 amps for charging. Such a cable is called a 6B&S and has a surface area of 13.56mm2. But if you tried charging that battery at 50 amps the voltage drop would be 8 metres (4 there and 4 back) x 50 x 0.017, with it all being divided by 13.56 , giving .50 volts. So the voltage has dropped from 14.3 volts at the alternator, to 14.0 volts at the main battery to 13.5 volts at the auxiliary battery. So you are now looking at a 47 hour charge time to recover the battery from 45% charge at a temperature of 25C. If it is colder in your car, the charge time increases even further to a point that if it gets below 10C it will not fully charge.
Now make a couple of changes and put the auxiliary battery under the bonnet on the opposite side to that of the main battery and in the same location. Firstly the temperature around the battery will increase. In this example we will use 35C as the temp. We now use 0B&S cable to and from the alternator. This cable has a surface area of 49.2mm2. So at putting 50 amps into the battery the voltage drop will now only be 1 metre of cable (500mm there are 500mm back) x 50x 0.017, divided by 49.2. Giving a voltage drop of only .02 volts. Due to the higher temps the 8 hour charge voltage decreases to 14.4 volts and the 10 hour figure to 14.1 volts. Since you are now charging at 14.3 volts, you will effectively obtain the best charge rate.
But batteries do not like too much heat. They really start getting affected at over 40C and should not be charged at over 50C. So if locating a battery under the bonnet it is best to keep it towards the front of the engine bay where it does not get as hot and keep it away from exhausts and turbo chargers that generate a lot of heat.
The voltage that a battery is charged at will significantly affect the life of the battery and the charge it will hold. Odyssey batteries compared charging one of their AGM batteries at the recommended voltage to a voltage of only 0.5 volts below that voltage. The overall life of the battery reduced from over 400 cycles to just 150 and the amount of current stored in the battery started to drop off considerably after just 75 cycles.
Even though the heat under the bonnet will also reduce the life of the battery, you will still get many years use out of it at the under bonnet temperatures.
Therefore it is considerably better to locate an auxiliary battery under the bonnet than in the back of the car.
Use of a single auxiliary battery or multiple batteries
There are dual battery systems available that allow you to use the main battery for auxiliary power at the same time as drawing power from the auxiliary battery.
First thing to consider when doing this is not to draw your main battery below 60% (12.3 volts) or you may not be able to start your car. A starter motor for a petrol motor will draw around 450 amps and you can double this figure for a diesel. When drawing 450 amps through Land Rover wiring, you will suffer a voltage drop of 1.7 volts. The large current draw drops the voltage at the battery to around 7.5 volts, so you are getting very little voltage to the starter motor. Taking also into account that your battery, when fully charged, can only supply the amps for a petrol motor for less than 1 minute, at 60% charge it is down to 35 seconds. So you will be in trouble if it does not start first or second go with less than 60% charge.
The second consideration is the charge characteristics of a battery. As mentioned earlier, it can take 4 times as long to recover the last 30% of the battery than the first 70%. So if I have one 100 amp hour battery and drain it. If the full recharge time is 10 hours, I am going to get 70 amp hours of power back in 2 hours and the other 30% will take 8 hours. If I then split this usage across two 100 amp hour batteries they are both going to be drain to 50%. So I can now only recover 40% (50% to 70% across 2 batteries), or 40 amp hours of the power used in a relatively quick time of around 1 hour 10 minutes. But it will take another 10 hours to recover the rest. This is because you are now splitting the charge across two batteries in the part of the charge cycle that does not allow a lot of amps to be added to the battery.
So you will be better off if you can get away with using one battery instead of two. This is because you can recover a lot more power more quickly moving from campsite to campsite.
The Alternator
The alternator works by attempting to maintain a given voltage. The voltage drops as items draw electrical power. The alternator will then increase the amperage that it is putting out to maintain the predetermined voltage. It will keep on doing this up to the maximum amperage that it can put out. After that point the voltage across the system will start to drop.
On some alternators there is a “battery” connection on the back. This is the point that it measures the voltage that it is trying to maintain. By rights this wire should be ran to the positive terminal of the main battery and the alternator will then maintain the voltage to the battery point. However, many of this type of alternator are wired up with this wire just going to the output terminal of the alternator. This means that the voltage at the battery is lower due to the voltage drop in the wiring to the battery. It is best to put the wire to the battery so that the voltage over the whole car system is maintained at a more appropriate voltage.
Most modern alternators do not have the “battery” connection and simply maintain the voltage based on the voltage at the output terminal.
Safety requirements of batteries inside the car or boot
Basically these requirements are that the battery must be sealed and properly restrained. If you use a AGM battery like an Optima, it is classed as being sealed and you can place it by itself . If you use a conventional flooded battery, it must be in a sealed container, with that container vented to the outside of the vehicle. If you run the cabling to the second battery under the car, it must be fixed every 600mm and not be in a position where it can be damaged by road debris.
Some auxiliary battery kits sold only have a fuse at the charging end of the cable to the second battery. Remember that the second battery is also a source of power if the cable shorts out and a fuse should also be placed at the auxiliary battery end of the cable.
Batteries and Winching
This example is based on my example battery, being a Odyssey PC1350 AGM battery that is rated at 770CCA and 91AH.
With most Warn winches pulling 4,000b on the 3 winding of cable on the drum, it will be pulling around 320 amps. If your alternator can only supply around 40 amps due to its size, other current draw, or the voltage drop in the it would mean a draw of 280 amps on the battery. Based on the example battery, you would be able to winch for around 7 minutes until you flatten the battery.
Assuming that you get a 120 amp alternator wired correctly and that 100 of these amps can go to the winch. It now means that your batteries now only have to supply 220 amps instead of 280 amps, This now means that with the example battery you can now winch 4,000b for 12 minutes instead of 7. That is a 70% increase in winch time for dropping your draw from the battery by 20% as you can now use over 40 amp hours of power instead of 30 due to the slower current draw.
If you then add a second battery, you may think that you will only double the amount of winching time you get and you would be wrong. Reducing the amp draw from 220 amps from one battery to 110 amps each from 2 batteries will increase your winch time from 12 minutes to around 31 minutes. So the advantage of having a two batteries for winching will increase the amount of time you can winch significantly.
The above figures were for a 4,000b pull, The maximum for most Warn 9,000lb winches is around 7,000b when you are on the 3rd layer of cable. At this pull it is drawing 480 amps. So with the example battery, and with the alternator contributing 100 amps to the pull, the battery will only last 4 minutes. Even with dual batteries you will only get about 14 minutes.
It took me a long time to put this together, so I hope it is of benefit to others.
Batteries and your vehicle.
Battery Basics
As a battery charges or discharges the current it stores, there are chemical reactions happening in the battery. Basically as it discharges the surface of the plates get a coating of lead sulphate and the charging process forces that lead sulphate back off and returns the plates to lead.
The main thing from this is that it takes more effort to reverse the process than what the reaction produced in the first place. It takes about 10% more amps than it produced. The reaction to reverse the process (charge the battery) is controlled by the voltage. It takes at least 0.7 volts over what the battery is sitting at to start any reaction at all and the recommended voltage is around 14.3 volts for a flooded battery and 14.7 volts for an AGM at normal room temps.
These voltage readings are based on the battery not being charged for at least 6 hours. A battery is classed at 100% charge when it sits at 12.7 volts, 75% at 12.4 Volts, 50% at 12.2 Volts, 25% at 12.0 volts, and flat at 11.9 volts.
Power stored in the Battery.
People tend to use the Amp Hour rating of a battery as an indicator of how many amps they can use in the battery. This is not really the case. The Amp Hour rating is only how much power you can get out of the battery at a constant amp draw over 10 hours over a 10 hour period before it get down to 10 volts. So for a 90 amp hour battery you can draw 9 amps for 10 hours before the battery gets to 10 volts.
However, the quicker you draw the current out of the battery, the less amps you will get out of it. For example, CCA or Cold Cranking Amps is how many amps you can get out of the battery for 30 seconds at -18C before the battery drops below 7.2 volts. CCA means little to people in Australia as we are unlikely to be trying to start a car in -18C temperatures. Hot Cranking Amps is what amps you can get for 30 seconds at 27C. So for my example AGM battery, it has 770 CCA or 1080 HCA. 30 seconds is only 120th of an hour. So the ability to only provide a total of 180 over 120th of a hour means it only supplied 9 amp hours of power and not the 91 amp hours it was rated at. If you were doing other heavy amp drawing work with the battery, like winching, it would only last 2 minutes at 530 amps (18 amp hours), 5 minutes at 350 amps (29 amp hours), or 10 minutes at 240 amps (39 amp hours). So you cannot simply divide the amp hour rating of a battery by the amps you are drawing to work out how long the battery will last. The more you draw the less you will get out of the battery.
Battery Charging
You need to reverse the chemical reaction to charge a battery. This requires both voltage and amps. It is the voltage that opens the door and it is then the amps that go through it. The more the door is opened, the more amps that can go through it. The door gets harder to open the colder it is and therefore requires more voltage. Put too much voltage in and the reaction becomes too great and the battery will start to heat up and gas. The maximum voltage is usually listed at 15 volts during the main bulk charging process. How far the door is opened is also about the voltage being charged at and the voltage or state of charge of the battery. As the battery becomes charged the opening created will reduce and therefore the amps that can be put into the battery will reduce.
The actual charge times will be slightly different from battery to battery depending on its internal resistance, etc. The following is an example of one battery manufacturer on a 100 amp hour AGM battery discharged by 55%. On a charger that can put out 20 amps and at room temp of 25C, it would take 47 hours to fully recharge the battery at 13.5 volts, 18 hours and 13.8 volts, 10 hours at 14.4 volts, and only 8 hours at the recommended voltage of 14.7 volts.
The amount of voltage required to recharge a battery changes by 0.25 volts for every 10C change in the battery temperature. The higher the temperature the lower the volts required. Therefore in the above case you would move all the voltages up 0.25 volts at 15C or down 0.25 volts at 35C.
There are basically 3 stages a battery goes through when charging. These are the Bulk, Absorption, and Float Stages.
The Bulk stage is when the battery is very low. The Voltage being applied to the battery allows large amps to be put back into the battery and it charges up quickly. This stage ends when the charging manages to get the voltage in the cells up to the voltage being used to charge it. When using the correct voltage, this process generally ends when the battery is back up to 70% to 80% of capacity. The lower the voltage being used the lower the percentage of recovery that is obtained during this stage.
The Absorption stage commences when the bulk stage completes. During the absorption stage the amount of amps that the battery will take is steadily reduced as the battery becomes more charged. It will generally finish when the amount of amps being absorbed by the battery drops to between 1 and 5 amps.
The Float stage is mainly to maintain the battery at its state of charge and maybe put the last couple of percent of capacity back into the battery. At this stage you want the reactions in the battery to calm down to stop it overcharging and causing gassing, etc. Therefore the voltage should be dropped to a float voltage around the mid 13 volts.
If is often listed that it can take 4 times longer to get the battery from the beginning of the absorption stage to the battery being fully charged, than it did to get the bulk of the power back into the battery through the bulk stage. This means that you can get the battery back up to 70% to 80% capacity quite quickly if you want to use that power again.
There are suggestions that you can charge a flat battery with another battery. This is highly unlikely. Even if you had a dead flat battery at 11.9 volts and a fully charged battery at 12.7 volts, there is going to be a minimal transfer of current before the fully charged battery comes within the 0.7 volts of the dead one
Float charge
The float voltage for you main battery does not have a lot of relevance as it is always being used and will not stay at a fully charged state for long. However, for an auxiliary battery, that is used rarely on camping trips, etc, it will become fully charged and stay that way for extended periods of time. Therefore the float charge becomes more important. Putting in a voltage higher than the float charge will cause the battery to overcharge, overheat, and give off gasses (even for an AGM). The float charge for an AGM in the back of your car at 20C is 13.8 volts. Under the bonnet at 35C it is 13.4 volts. This is below the voltage that your charging system is putting out. Therefore it is wise to disconnect your auxiliary battery when you do not plan to use it for a while and connect it back up a couple of days before you plan to use it again. You should connect it up every 3 to 6 months for a short time to top it up.
Your Cars electrical system and its affect on Batteries
As listed previously, the voltage that a battery charges at will have a significant affect on the charging of a battery. That is an AGM battery at 13.5 volts will take 47 hours to recharge from 45% compared to 10 hours at 14.4 volts. Therefore Voltage Drop has a significant effect on charging a battery.
The voltage drop of a wire is determined by the surface area of the copper core (the area across the end of the cable) and the amps you are putting through it. The number of strands in that core is not important for DC voltage, only AC voltage where there is a “skin” effect.
For reasons only known to Land Rover, they use only relatively small cables for the main power supplies from the alternator to batteries and batteries to starter motor, as well as, associated earths.
The voltage drop is calculated by the length of the wire in metres (including the return earth wire) multiplied by current draw in amps multiplied by the resistance of copper at 0.017, all divided by the cross section area of the wire in square millimetres.
The main power cables for my Range Rover have 37 copper strands, each with a diameter of 0..75mm. This gives a cross sectional area of 16.3mm2. So at a middle of the range output from the alternator at say 60 amps the voltage drop over 3 metres (1.5 metre there and 1.5 back) will be (3x60x.017)/16.3 or 0.19 volts. Up this to 85 amps and the voltage drop increases to .27 volts and at 120 amps .38 volts. So this is the voltage drop just from the alternator to the battery without worrying about the rest of the wiring.
To determine the actual voltage drop on your car you do not need to know the formula, you only need a multimeter. Start the car and put on as many electrical items as possible to simulate the maximum draw you will place on the alternator. Switch on headlights, aircon, rear window demister, driving lights, thermo fans, etc. While using either the body of the alternator or the negative terminal on the battery for the negative cable of the multimeter, put the positive lead of the multimeter on the charge terminal of the alternator and then to the positive terminal of the main battery. The difference in voltage between the two readings is that caused by the charge wire from the alternator to the battery. Then while keeping the positive lead of the multimeter on either the positive terminal of the battery or the charge terminal of the alternator, move the negative lead of the multimeter between the body of the alternator and the negative terminal of the battery. The difference between the two readings is the voltage drop from the earth connections/wires between the alternator/motor and the negative terminal of the battery. The voltage drop of the charge wire and the earthing added together is the total voltage drop you are suffering due to Land Rovers wiring.
Say you then put an auxiliary battery in the rear of your car. You then run the recommended cable that is rated at 50 amps for charging. Such a cable is called a 6B&S and has a surface area of 13.56mm2. But if you tried charging that battery at 50 amps the voltage drop would be 8 metres (4 there and 4 back) x 50 x 0.017, with it all being divided by 13.56 , giving .50 volts. So the voltage has dropped from 14.3 volts at the alternator, to 14.0 volts at the main battery to 13.5 volts at the auxiliary battery. So you are now looking at a 47 hour charge time to recover the battery from 45% charge at a temperature of 25C. If it is colder in your car, the charge time increases even further to a point that if it gets below 10C it will not fully charge.
Now make a couple of changes and put the auxiliary battery under the bonnet on the opposite side to that of the main battery and in the same location. Firstly the temperature around the battery will increase. In this example we will use 35C as the temp. We now use 0B&S cable to and from the alternator. This cable has a surface area of 49.2mm2. So at putting 50 amps into the battery the voltage drop will now only be 1 metre of cable (500mm there are 500mm back) x 50x 0.017, divided by 49.2. Giving a voltage drop of only .02 volts. Due to the higher temps the 8 hour charge voltage decreases to 14.4 volts and the 10 hour figure to 14.1 volts. Since you are now charging at 14.3 volts, you will effectively obtain the best charge rate.
But batteries do not like too much heat. They really start getting affected at over 40C and should not be charged at over 50C. So if locating a battery under the bonnet it is best to keep it towards the front of the engine bay where it does not get as hot and keep it away from exhausts and turbo chargers that generate a lot of heat.
The voltage that a battery is charged at will significantly affect the life of the battery and the charge it will hold. Odyssey batteries compared charging one of their AGM batteries at the recommended voltage to a voltage of only 0.5 volts below that voltage. The overall life of the battery reduced from over 400 cycles to just 150 and the amount of current stored in the battery started to drop off considerably after just 75 cycles.
Even though the heat under the bonnet will also reduce the life of the battery, you will still get many years use out of it at the under bonnet temperatures.
Therefore it is considerably better to locate an auxiliary battery under the bonnet than in the back of the car.
Use of a single auxiliary battery or multiple batteries
There are dual battery systems available that allow you to use the main battery for auxiliary power at the same time as drawing power from the auxiliary battery.
First thing to consider when doing this is not to draw your main battery below 60% (12.3 volts) or you may not be able to start your car. A starter motor for a petrol motor will draw around 450 amps and you can double this figure for a diesel. When drawing 450 amps through Land Rover wiring, you will suffer a voltage drop of 1.7 volts. The large current draw drops the voltage at the battery to around 7.5 volts, so you are getting very little voltage to the starter motor. Taking also into account that your battery, when fully charged, can only supply the amps for a petrol motor for less than 1 minute, at 60% charge it is down to 35 seconds. So you will be in trouble if it does not start first or second go with less than 60% charge.
The second consideration is the charge characteristics of a battery. As mentioned earlier, it can take 4 times as long to recover the last 30% of the battery than the first 70%. So if I have one 100 amp hour battery and drain it. If the full recharge time is 10 hours, I am going to get 70 amp hours of power back in 2 hours and the other 30% will take 8 hours. If I then split this usage across two 100 amp hour batteries they are both going to be drain to 50%. So I can now only recover 40% (50% to 70% across 2 batteries), or 40 amp hours of the power used in a relatively quick time of around 1 hour 10 minutes. But it will take another 10 hours to recover the rest. This is because you are now splitting the charge across two batteries in the part of the charge cycle that does not allow a lot of amps to be added to the battery.
So you will be better off if you can get away with using one battery instead of two. This is because you can recover a lot more power more quickly moving from campsite to campsite.
The Alternator
The alternator works by attempting to maintain a given voltage. The voltage drops as items draw electrical power. The alternator will then increase the amperage that it is putting out to maintain the predetermined voltage. It will keep on doing this up to the maximum amperage that it can put out. After that point the voltage across the system will start to drop.
On some alternators there is a “battery” connection on the back. This is the point that it measures the voltage that it is trying to maintain. By rights this wire should be ran to the positive terminal of the main battery and the alternator will then maintain the voltage to the battery point. However, many of this type of alternator are wired up with this wire just going to the output terminal of the alternator. This means that the voltage at the battery is lower due to the voltage drop in the wiring to the battery. It is best to put the wire to the battery so that the voltage over the whole car system is maintained at a more appropriate voltage.
Most modern alternators do not have the “battery” connection and simply maintain the voltage based on the voltage at the output terminal.
Safety requirements of batteries inside the car or boot
Basically these requirements are that the battery must be sealed and properly restrained. If you use a AGM battery like an Optima, it is classed as being sealed and you can place it by itself . If you use a conventional flooded battery, it must be in a sealed container, with that container vented to the outside of the vehicle. If you run the cabling to the second battery under the car, it must be fixed every 600mm and not be in a position where it can be damaged by road debris.
Some auxiliary battery kits sold only have a fuse at the charging end of the cable to the second battery. Remember that the second battery is also a source of power if the cable shorts out and a fuse should also be placed at the auxiliary battery end of the cable.
Batteries and Winching
This example is based on my example battery, being a Odyssey PC1350 AGM battery that is rated at 770CCA and 91AH.
With most Warn winches pulling 4,000b on the 3 winding of cable on the drum, it will be pulling around 320 amps. If your alternator can only supply around 40 amps due to its size, other current draw, or the voltage drop in the it would mean a draw of 280 amps on the battery. Based on the example battery, you would be able to winch for around 7 minutes until you flatten the battery.
Assuming that you get a 120 amp alternator wired correctly and that 100 of these amps can go to the winch. It now means that your batteries now only have to supply 220 amps instead of 280 amps, This now means that with the example battery you can now winch 4,000b for 12 minutes instead of 7. That is a 70% increase in winch time for dropping your draw from the battery by 20% as you can now use over 40 amp hours of power instead of 30 due to the slower current draw.
If you then add a second battery, you may think that you will only double the amount of winching time you get and you would be wrong. Reducing the amp draw from 220 amps from one battery to 110 amps each from 2 batteries will increase your winch time from 12 minutes to around 31 minutes. So the advantage of having a two batteries for winching will increase the amount of time you can winch significantly.
The above figures were for a 4,000b pull, The maximum for most Warn 9,000lb winches is around 7,000b when you are on the 3rd layer of cable. At this pull it is drawing 480 amps. So with the example battery, and with the alternator contributing 100 amps to the pull, the battery will only last 4 minutes. Even with dual batteries you will only get about 14 minutes.
