Care and Feeding of Deep Cycle Batteries

By Peter Hers
September 2003

My first 4x4 trips were to Botswana to provide radio support for the Toyota Desert Race.  I took along two large 'ordinary' lead-acid batteries to run the radio equipment, but really had no idea of the right way to manage them.  On the first trip I ran into problems after a few hours, with the battery voltage dropping too low to run the transmitter, and we had to run the vehicle engine for a couple of hours to recharge the batteries.  Not really satisfactory, but we managed.

When we bought our own 4x4, our first trips were relatively short, and we used a simple cooler box to keep food and drink cool.  However, as our trips got longer and more challenging, I worked on developing a second battery system.  We learned a little more on each trip, and gradually improved the system over time.

There is a large amount of useful information on batteries and battery systems available on the web, and I have borrowed freely from many web sites. This article arose from my desire to summarise the most important facts, and make it accessible to a wide range of people. This article has also been published in Four Wheeling, the monthly journal of the Four Wheel Drive Club of South Africa.




Just about every motor vehicle contains a standard 12 volt lead-acid battery.  This is used primarily to power the starter motor.  Once the engine has started, all the power required to run the engine, lights, indicators, radio, etc. is supplied by the alternator. You can use the motor battery to run small loads, such as radio, inside lights, etc for a short time without the engine running, but these can easily run the battery flat if used for extended periods without recharging the battery.

If you are using your 4x4 or other SUV to go off camping in the wild blue yonder, you will soon have call to use a 12 volt supply for many different purposes.  The following are a few examples: 

·                    Fridge or freezer: These will generally use a current of between 4 and 10 amps.  The current is drawn intermittently, as the fridge mechanism turns itself on and off to regulate temperature. The longer the fridge is connected, the more the battery is discharged.

·                    Winch:  This device is used very rarely (only when someone is stuck!), but when it is used the current drain can be large.  The total effect on the battery is related to the time that the winch motor is running.

·                    Two-way radio: In normal communication, the transmitter is only used for short bursts, so the load on the battery is low.  However, if you are transmitting for long periods then this may start to have an effect on the battery.

·                    Lighting:  Fluorescent lights use very little current, but again the drain from the battery is proportional to the time that the light is switched on. Spotlights can use a large amount of current and will quickly drain your battery.

It is not a good idea to run the above devices off the fitted battery in your vehicle, as you may run the battery flat and then will be unable to start the engine. This is bad!  Even if you can start the engine by using jumper cables from another vehicle, you will have done permanent damage to the battery, as it is not designed to be deeply discharged – Each time you do this, the usable capacity of the battery is reduced, and hence the life of the battery is cut short. A normal car battery can provide a very large current for a short time, as when starting, but should not be regularly discharged more than a few percent of its total capacity.

So the answer is to provide a second battery specifically for your extra devices. Generally the second battery will be a Deep Cycle battery, so-called because it is designed to cope with being deeply discharged over and over again, something which would ruin a car battery very quickly.  Depending on how deeply it is discharged each time, a deep cycle battery can withstand several hundred total discharge/charge cycles.  For example, if the depth of discharge is limited to 50% of capacity, you can expect at least 500 discharge/charge cycles.

Battery Capacity

Battery capacity is expressed in Ampere-Hours, ie: The number of amperes (amps) which can be provided, multiplied by the number of hours for which this current could be maintained. The size of the battery in your vehicle is generally determined by the amount of current required for cranking, ie: running the starter motor for sufficient time to start the motor. Some common capacities used in passenger cars are 36 AH, 45 AH (small sedans), 66 AH (mid range), 88 AH, up to 102/105AH.  The capacity is usually marked on the battery, or included in the model number (eg:  “2766” = 66 AH battery).  Otherwise you can easily determine the capacity by measuring the physical size of the battery and checking with the supplier.

Size vs. capacity is generally the same for normal and deep-cycle batteries.

Rules for maintaining your deep cycle battery

More batteries are ruined by incorrect charging practices than by any other cause. The main culprit is the formation of insoluble sulphate, which occurs whenever the battery remains discharged for a period of time. Here are a few basic rules which will help you to keep your valuable second battery in excellent condition for a long time.

1.                  Always recharge the battery back to 100% as soon as possible after discharging. Whenever you return from a trip, ensure that it is properly charged before storing.

2.                  Always keep the battery fully charged except when actually on a trip. It is good practice to top up the battery charge every 3 months or so.

3.                  Never discharge the battery below 10,5 volts

The design of how you intend to use your battery is important.  If your fridge and other devices consistently discharge the battery too far, the life of the battery will be reduced, deep cycle or not.  A good rule of thumb is to purchase a battery with double the capacity you require for all your devices  – See below for an example of this calculation.

The life of your battery will also be reduced if it is consistently kept at a high temperature.  Therefore, placing a second battery within the engine compartment of your vehicle is not the best choice, although it may be your only choice. A better location would be under a seat, as in the Defender and some other vehicles, or in another cavity under the floor, or even in a removable box, as is available from various sources.

State of charge

Although there are many different types of batteries, you will generally be using a sealed maintenance-free deep cycle battery.  These all contain a built-in hydrometer which acts as a charge indicator, visible through a little window in the top of one of the cells.  The indicator provides a rough indication of the state of charge, as follows:


State of Charge (SOC)


SOC above 70%

Dark / Black

50% - 70% SOC – Recharge when possible


Below 50% SOC – Recharge before using

Clear / Yellow

Danger : Electrolyte level low. Do not test or charge; return to supplier.

As you can see, this does not give you a very precise measurement of the state of charge.  With totally sealed batteries you cannot check the specific gravity of the electrolyte, so the only way is to use an accurate voltmeter.  Measurements should be taken with no load on the battery. The following table should be usable with most standard fitted batteries as well as deep cycle batteries.

Open circuit voltage at 26.7ºC

State of charge











The voltage is dependent on the temperature, and decreases slightly with increasing temperature, and vice versa.

Testing the capacity of your battery

Checking the voltage gives you an idea of the state of charge of your battery.  Most battery service centres will perform a simple load test to check if the battery can deliver the kind of current needed, usually to drive the starter motor.  Although these measures are useful, they do not tell you the actual capacity of the battery, ie:  How long it will be able to continue delivering current before the voltage drops too far.

The only way to really test battery capacity is with a full load test.  You may need to do this if you possess or have acquired a 2nd hand battery and need to know its true capacity.

1.                  This test should not be carried out often, as this will reduce the life of the battery. 

2.                  Charge battery to 100% capacity (See next section below).

3.                  From the nominal battery capacity, calculate the current to discharge it over 20 hours. You may need to check with the supplier to determine the capacity. (Eg: For a 66 AH battery, Current = 66/20= 3.3 amps.)

4.                  Find one or more old motor lamps which when placed in parallel or series draws exactly 3.3 amps from the battery.

5.                  Monitor the battery voltage and time from when you start the load test.

6.                  When the battery voltage drops to 10,5 volts disconnect the load.  (Don’t let this run overnight, as you will discharge below 10,5 volts, miss the timing, and damage the battery!)

7.                  Immediately recharge the battery back to 100%.

 From the above you can calculate the capacity of the battery as follows:

Capacity = (Hours to reach 10,5 volts) / 20  expressed as a percentage.

A battery is considered unusable when the capacity drops below 80%.    

Charging your battery

Consider first the standard battery fitted to your vehicle.  During starting a high current is used, but only for a short time.  Once the engine has started the alternator takes over and provides all the electrical power required for the vehicle, and recharges the battery at the same time.  So after a short amount of daytime driving your battery should be up to 80%-90% charge again. Generally your standard fitted battery will not attain 100% charge, but will be close enough.

However, during night-time the lights on your vehicle can easily consume most of the available output of the alternator, leaving little left over to recharge the battery. If your battery is in good condition, and you have no other loads on the battery then this situation is quite satisfactory, as the loss of charge consumed by starting will be only a few percent of the total capacity, and can be recovered during the next few hours of daylight driving.

However, the same does not apply to your second battery. For example, if you are running a fridge, then this could easily discharge your deep cycle battery below 50% capacity.  The lowest point in the cycle will be in the morning, as the fridge has been running all night, with no way of recharging the battery.

External battery chargers

With an “intelligent” charger, the best possible time to fully recharge a battery is within 5-8 hours. You cannot do it in a shorter time without damaging the battery, and this applies to both normal and deep cycle batteries. 

There are various different methods you can use to recharge your battery. One of the most common is to use an external constant current charger which is set to deliver not more than 12% of the reserve capacity rating of the battery.  It is important to monitor the state of charge, and it is also important not to overcharge the battery.  A timer that will cut off the charger after the correct time is valuable, and should not be disabled.  The following table shows recommended battery charging rates and times for fully discharged batteries:

Reserve capacity

Charging current


32 AH or less

3 A

15 hours

32-50 AH

4 A

21 hours

50-68 AH

5 A

22 hours

68-100 AH

6 A

23 hours

Over 100 AH

10 A

24 hours

Another method is to use a constant voltage, or “automatic”, charger.  This supplies a regulated voltage of 14,4 volts, and should stop charging when the battery has reached full charge.  This will generally be over a period of 10 hours.  You will need a charger with a capacity of at least 10% of your battery, eg: for a 66 AH battery, you will need a charger with a capacity of  about 7 amps.

Charging your 2nd battery within your car

The above discussion relates to ideal conditions, at home or in the workshop.  However, when you are on the road your battery will encounter different conditions from day to day.  If you are driving for a few hours every day (and your battery is mounted or kept in the car, not left back at camp!) then you should be able to recharge it to a reasonable level each day.  The degree to which your alternator is able to recharge the battery depends on how much current it can supply. If you are really serious you may want to upgrade the standard alternator in your car, or even mount a second one specifically for your second battery.

As mentioned above, you will not be able to get much current for recharging while driving at night, as all the available power is used for the lights. Another good reason not to drive at night!

Your 2nd battery cannot simply be connected in parallel with the ordinary car battery; All this will give you is a larger capacity battery, but they will both run down together:  If this happens you may be unable to start your car when you need to, and will also damage the ordinary battery as discussed above.  So you need a switching system of some sort which will only connect the 2nd battery when the engine is running and there is available charging current from the alternator.  One way of doing this is with a solenoid-operated switch; Another is to use a heavy-duty diode, which only allows current to flow in one direction.  Whichever solution is chosen, it is important to ensure that you do not have excessive current flowing when the battery is first connected, as this could cause melting of insulation in your wiring, giving rise to all sorts of electrical problems and at worst a vehicle fire.  Professional systems are available which combine all the controls necessary. There are quite a few competing systems available. Perhaps a comparison could form the subject of a later article.

If you are planning to stay at one location for a few days, without much driving, then you cannot expect to recharge the battery each day – Remember that it will take 8-10 hours to completely charge a battery, even if sufficient current is available.  The only solution here is to use a solar panel.  These are fairly expensive but work extremely well, particularly in Africa with our excellent sunshine.

Worked example

Let’s assume that we are on a trip where we travel for about 6 hours every day.  We have a fridge, and some lights which we use in the evening and in our tent. First we calculate how much power (capacity) we will require between charging opportunities.

The fridge draws 5,5 amps when in cooling mode, and 0,5 amp when in standby. For simplicity let’s assume that it is in cooling mode for 50% of the time. This gives us an average current drain of  (5,5+0,5)/2=3 amps.  Therefore between charging opportunities, we will discharge roughly 18 hours x 3 amps = 54 AH.

The lights consume 40 watts, and will be run for about 4 hours each night.  The current drain is roughly 40/12=3,3 Amps, and the total discharge 4x3,3 = 12 AH.

The total discharge each night will therefore be 54+12 = 66 AH.  If we wish to get a reasonably long life out of the deep cycle battery, we should plan to discharge it only 50% or so.  A suitable size battery would be 105AH – Above this size it gets expensive, and is also large and heavy.

The driving time of 6 hours each day is a little low to achieve a full top up of the battery each day, but it should be able to bring it up to 90% of capacity each day, which is quite satisfactory.

While on the trip it is important that the battery is not discharged below 10,5 volts.  This can be achieved by an automatic cut-out, although this only lets you know when it finally cuts out. An alternative is to continually monitor the battery voltage.  This can be done by installing a voltmeter either on the battery box or in the car, so that it can be watched. If the voltage drops too low, then you will have to do without your fridge, or go for a long drive to recharge!

Finally, when you return home, make sure that the battery is properly recharged as soon as possible.

Important warning

The danger of overcharging your battery cannot be overemphasized!  When charged at a voltage higher than about 14,8 volts, a lead-acid battery will generate both hydrogen and oxygen.  This mixture is highly explosive.  All it needs is a spark in the wrong place and the battery will explode.  I have had contact with three separate people where this has happened; Luckily they all still have their sight.  Two cases occurred in the battery compartment under the seat of a Defender, where the gas had collected.

Generally a “maintenance free” battery is sealed.  However, if there is a build-up of gas, it can escape via the pressure relief valve and fill the box containing the battery.  Always make sure that any gas can escape before connecting or disconnecting.  If at all possible, ensure that no current is being drawn when connecting or disconnecting battery cables, to minimize the possibility of a spark.  If you suspect that your battery has been overcharged then take it back to your dealer for servicing.

Note on electric fridge connections

When operating on a12 volt DC supply, the efficiency and effectiveness of your fridge or cooler box is very dependent on the actual voltage reaching the electronics in the box.  Some cooler boxes make use of a fairly small diameter wire for the 12 volt connection.  The result of this is that, when maximum current is being drawn, there is a measurable voltage drop in the supply cable.

I have a cooler box which worked tolerably well, but its performance last year in the middle of summer during our trip to view the eclipse was disappointing.  Back at home I checked out the supply cable, and found that I was losing about 1 volt at maximum current.  This equates to a loss in available power at the fridge end of 15%, which has a major effect on the cooling mechanism.  And the consequence of this is that the fridge does not cool properly.

Upgrading the size of the cable and the connectors at both ends solved the problem, and gave me a very satisfactory fridge. ( I still wonder why the manufacturer didn’t pay attention to this?  Surely it means that large numbers of purchasers will be dissatisfied with their purchase, and all for 2 metres of copper wire!)

Here you can see how I replaced the DC connector on the cooler box with a more substantial connector, and the new cable, which connects from a standard vehicle outlet to the cooler.

Note on power connectors built into vehicles

All vehicles have at least a cigarette lighter somewhere in the vehicle, which you may think is a convenient point to use for your fridge and other devices.  However, carefully read your instruction manual to make sure that it can cope with whatever current you plan to extract from it.  The cigarette lighter socket is usually not designed to provide a large current for an extended time, and you could find that the wiring up to the socket heats up, or even gives rise to a vehicle fire.

My Pajero has a separate connector which is designed to provide a higher current, and the wiring up to the socket, as well as the fuse behind it, are designed to provide up to 8 amps.

Notes on miniature sealed lead-acid batteries

Over the last few years we have seen increasing use of miniature totally sealed lead-acid batteries in work lanterns, spot lights, etc.  You may also use one of these as a more substantial power source for a portable radio, fluorescent light or video camera, etc.  Generally these batteries have three cells, and provide a nominal 6 volts. Capacities are usually within the 5-10 Ampere-Hours range.

Many of these devices are supplied with two charging circuits, allowing them to be recharged either from the mains supply, or from your car 12 volt system.  As the battery is built into the unit it is not easily accessible, and you may not even be able to find a point at which to measure the battery voltage. Some of these devices include a charge indicator which gives a rough estimate of the state of charge.

These batteries behave in exactly the same way as deep cycle batteries, so the same rules apply if you want to maintain the battery capacity and hence useful life:

1.                  Do not allow the battery to become completely discharged.

2.                  Always recharge as soon as possible after discharge. 

3.                  Follow the provided charging instructions, and use only the supplied charging device.

4.                  Do not exceed the maximum charging times, usually about 12-24 hours.

5.                  Fully recharge the battery before storing.

6.                  If left unused for long periods, apply a top-up charge on a regular basis to maintain charge.  A good period would be every 3 months.

Features desirable in 2nd Battery Systems

The following is a short list of the features which should be available in any second battery system.

1.                  The ability to charge the 2nd battery from the vehicle charging system.

2.                  The ability to isolate the 2nd battery from the primary battery.

3.                  The ability to safely connect the 2nd battery to the primary battery for emergency starting purposes.

4.                  Monitoring of the 2nd battery voltage and State of Charge.

5.                  Automatic low voltage cut-out or alarm.

Before installing any 2nd battery system, check what voltage is available from your alternator and control system.  Some vehicles provide a voltage which does not have sufficient “head room” to allow the use of electronic or diode isolation systems.  If you install an electronic system in one of these vehicles you will find that you cannot ever properly charge your second battery. 


Deep Cycle Battery Frequently Asked Questions, by Bill Darden

Battery Council International

Sure Power Industries brochures on battery isolators, equalizers, low voltage disconnects, etc.  Look at the excellent “Introduction to Batteries and Charging Systems” (1.72MB)

Exide Batteries: BCI Group Numbers, Dimensional Specifications, Polarity and Terminals

Battery Charging Basics –

Battery References and information links

Deep Cycle Batteries FAQ – Northern Arizona Wind & Sun


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