Lead Acid Battery Capacity Discharge Test.

How to really test your battery.

This was originally written for owners of motorhomes and caravans with second batteries, so the text has that scenario in mind. It applies to all normal battery types, AGM, Sealed Lead Acid, VSLA, flooded (the old fashioned vented batteries), car batteries etc.

And, this test concept can be applied to almost all battery types from watch cells to the largest cells used in telephone exchanges and for standby power.

Caution with standby battery banks, as there are safety issues with the sheer size of the system and associated high voltages, and possible extremely high fault currents).
Caution with solar banks with associated inverters, as there many safety issues.
You need to remove and isolate any battery before testing, and take extreme care.

End Point Voltage
Almost inevitably when I talk to others about discharging cells down to their “end point” of 1.8 volts per cell or so, they shudder and think that this will harm the cell. Well, this type of test is that which has been carried out for many years in real industrial situations.
It will not harm the battery if done occasionally; it might slightly harm your battery if done once a day for a month.

Process
What we are doing is a high confidence discharge test of the actual capacity of a battery.
This is to be measured in “hours” at a known discharge current, and then compared to manufacturer’s specification to make a judgment of capacity, or end of battery life.

You will need an accurate voltmeter, and preferably an accurate ammeter, and some test loads (light bulbs if nothing better is available). If you don’t have an ammeter, you could still do the test, relying somewhat on the indicated wattage of the light bulbs, as this in practice is reasonably reliable.

1 – Determine the normal load in your specific case, as this discharge test should be done approximating the normal service usage. A small to medium motorhome with say a situation of a single 120 Amp Hr battery and a BD35 compressor type frig and LED lights, and some LCD TV use should be done at C/20 (capacity “C” in Amp Hrs divided by the “twenty hour” rate). This test is done at 6 amps discharge current.

Larger situations with a heavier usage current as normal, should be done at the C/10 rate.

The tests are done at these standard industry rates, and these are also the rates normally specified by the manufacturer. You may find that your battery is specified at some other rate. You may have trouble obtaining suitable loads to do the higher current C/10 test.

If in doubt, use a C/20 test, as it is easier to set up and control, and somewhat safer for the user, and for the battery.

Do not test if the ambient temperature is abnormally high or low, or if the manufacturer does not specify capacity rating offsets for temperature. (This gets rather complicated).
For 120 AH battery, C/20 is 6 amps, C/10 is 12 amps.
For 100 AH battery, C/20 is 5 amps, C/10 is 10 amps.

Using an example of a single Fullriver 12 volt HGL120-B (120 Amp Hr) AGM battery
Using an example of a single Fullriver 12 volt HGL120-B AGM battery, rated at 120 Amp Hrs at the C/20 rate of 6 Amps with an endpoint voltage of 1.75 volts per cell or 1.75x6 = 10.5 volts at the terminals.
The manufacturer rates the endpoint voltage as “Cutoff Volts per Cell” of 1.75 V/Cell”.
Manufacturers spec sheet is here.
http://www.fullriver.com/products/ad...HGL120-12B.pdf

A new battery with a 6 amp load will last 20 hours or a bit more, before the battery voltage falls to the test discharge endpoint of 10.5 volts.

A battery which is not new will give a shorter period of test time, and you have to then judge arithmetically, and by instinct, the actual capacity and remaining life. (If it doesn’t last 10 hours, the battery is in a poor state of capacity).

Although the manufacturer is using a constant rate of discharge, you can approximate this by manually adjusting the rate over the time of the discharge test, (by switching in extra load) and still achieve reasonable accuracy. (In an industrial test, the battery discharge tester automatically keeps the discharge current constant).

Charge before Test
Charge the battery fully first, preferably with its own normal charger, so that you eventually test the battery capacity in its intended usage situation. For AGM types, be aware that you need to use a charger with a higher voltage than vehicle alternator voltages, preferably with 14.5 volts charge and 13.8 volts floating after charge.

Prepare for the test after the battery is fully charged, and when you have the time to watch and adjust the current manually with bulbs to keep it at 6 Amps approx, without interrupting the current at any time, except for a brief time to set currents at the test start.
You may wish to consider starting it at 4-5 am and check at hourly, as you will have 20 hrs to wait, and will have to consider practicalities and safety aspects.
Do not do an “unwatched” overnight test for safety reasons.

(You do not need to wait for the battery to “settle” after a full charge, but if you have abnormally discharged the battery recently, you do need to cycle the battery with its normal load for say five charge/discharge cycles, or for two days before this test).

2 – Use 12 volt auto filament type light bulbs in parallel to act as a load. Wire it up but don’t start the discharge test yet.

Use good test cables with adequate copper size and solid connections, soldered, or by using lamp sockets etc.

Use 1 watt bulbs (“reading lamp” size) to load to 0.1 Amp, 5 watt bulbs (“parking” size) for 0.4 Amp, 21 watt bulbs (“stop” size) for 1.75 Amps, 50 watt headlamp bulbs for 4.2 Amps, and 65 watt headlamp bulbs for 5.4 Amps. All these are approximate, depending on manufacturer, and the fact that filament bulbs are non-linear resistors, changing resistance slightly with filament temperature as they “light up”.

In our specific test here, to start with, use a 65 watt bulb in parallel with a 5 watt bulb and maybe a 1 watt as well, to get a measured current of as close to 6 amps as you can get. Allow for the heat radiated by the lamps.

3 – Start the test after the battery is fully charged, and when you have the time to watch and adjust the current manually with bulbs to keep it at 6 Amps approx, without interrupting the current. You may wish to start it early morning and check at hourly. If you are using an ammeter in circuit all the time, and it has ranges, use it on the highest range available eg 10 Amps. Economy digital multi range meters, with ammeter functionality, may fail after a time if used at more than 50% of the rated range on relatively high currents for long periods. Turning the DMM to “OFF” does not lessen this risk; it has to be removed from circuit, unless continuously rated.

You will need to add more bulbs over the discharge time, as the battery terminal voltage falls, especially at the end of the test. Have an extra 1 watt and 5 watt bulb ready to parallel up if required. Alligator clips are useful when adding these smaller lamps.

Time the discharge from start to where the battery terminal voltage falls to 10.5 volts overall, measured only at the battery terminals, making very sure that you have a good connection (electrically) with your voltmeter test leads.
A new HGL120-B battery has a rated hour life of 20 hours at a discharge of 6 Amps.

4 – Assess your battery against manufacturer specification for your actual battery only, not against anything else.

5 – Make a judgment allowing for any compromises that affected the test, temperature, load current interruptions, accuracy of voltmeter ammeter etc.
These concepts are applicable to most types of larger batteries.


Auto starting batteries like the N70ZZ series are sometimes used in motorhomes, but manufacturers don’t normally give test specifications in AmpHrs, as these batteries are not designed for this type of service. They normally test at approximately 75 to 90 AH.

Century Batteries rates its N70M marine battery at 100 Amp Hr, and this gives us a “guide”(It is a different battery), but Century doesn’t give any further spec such as endpoint voltage.
In the case of no spec available, use a C/20 discharge rate, and a presumed endpoint voltage of 1.8 volts per cell for lead acid batteries.
In other cases like this, contact the manufacturers Technical Advice, perhaps on the phone. Sometimes they will give an estimate of “equivalent AmpHr rating”, so you can test the battery.

This is not the time or thread, but the old fashioned “Tong Tester” high current tester is actually not a bad test for a starter battery, as it simulates the designed service.

This post might be over kill for some viewers, but I thought it best to be reasonably thorough, without being overly pedantic.
Hopefully of assistance to someone.

Cheerio, mike