But, as time goes by, the amp-hr capacity will drop, and the even when new, does the battery actually have the amp-hr capacity listed, or does the label even show an amp-hr capacity?
This page shows how we measured the actual capacity of our house battery. You can adapt the procedure to measure the capacity of your house battery.
You can run this battery capacity test when the batteries are new to confirm the manufacturers amp-hr rating, and then run it once a year or so to see how the batteries are holding up. Or, if you do something to abuse your batteries, you can run the capacity test after the abuse and see how much damage you did 🙂
Our house battery is two 6 volt flooded lead acid golf cart batteries hooked in series to make a 12 volt battery. The nominal capacity of these batteries is about 200 amp-hrs, but they vary and in some cases are not even listed. The actual measured capacity of our newish golf cart batteries proved to be far less than the nominal 200 amp-hrs.
The procedure used here could be used to test AGM or flooded lead acid batteries, and with some changes even Lithium batteries.
One reason for us wanting to know the actual capacity our house batteries is that to set up our Victron Battery Monitor system, you have to input the capacity of the house battery. It uses this capacity to report the State Of Charge (SOC) of your battery. Its helpful to know the SOC of your battery to let you know how much juice you have left and to avoid running your battery down to too low an SOC and damaging it. But, if you think (for example) that your battery capacity is 200 amp-hrs but the actual capacity is 150 amp-hrs, then the SOC reported on your battery monitor will be optimistic by about 50% too high and may cause you to discharge the battery too far. The SOC meter really does you no good unless you have a good number to enter for you battery capacity.
Procedure to Measure Battery Capacity
The idea is to hook up a fixed load to the battery and to use the measure current through the load to estimate total capacity. The test starts with a fully charged battery and ends when the battery voltage drops down a fully discharged level.
So, if you hook up a load that draws 5 amps, and the battery goes for 30 hours before it is discharged, then the amp-hr capacity is (5 amps)*(30 hrs) = 150 amp-hrs.
Since battery capacity depends fairly strongly on how heavily the battery is loaded, it seems to make sense to pick a test load that is about the same as the average load that your house battery.
For us, the average load on our house battery about 4 amps, so we tried to pick a load that would load the battery at about 4 amps. We had a 250 ft coil of #18 lamp wire lying around, and hooking the two 250 ft halves of this wire in series produced a current of about 3.8 amps, which seems close enough (the resistance is about 3.2 ohms).
We used the Victron battery monitor to measure the voltage, current, and amp-hrs used to do the test. But, if you don’t have a battery monitor, a meter that can measure voltage and current is sufficient.
- Fully charge the battery – make sure you start with a fully charged battery.
- Turn off all the regular loads on the battery (lights, fridge, … everything)
- Turn off solar charging
- Turn off the connection to the van battery.
- Set up to measure voltage and current as the test goes forward
- Connect the test load to the house battery
- Keep measuring and recording the voltage and current as the test goes on.
- When the battery voltage reaches the fully discharged level, remove the load and charge the battery. See below for setting cutoff voltage.
This will take a while — if you have a battery that is sized to last a couple days out in the field, then its going to take longer than a couple days to fully discharge the battery.
You have to choose a voltage that corresponds to fully discharged. We used 11 volts because 1) there is very little left in the battery at 11 volts, 2) some 12 volt devices will cutout automatically at 11 volts, so effectively you have run out of useful capacity at 11 volts, and 3) as far as I can determine a deep cycle lead acid battery won’t be harmed by this very infrequent deep discharge as long as you fully charge the battery after the test. You should make sure that you don’t let the battery voltage go below 11 volts as this may harm the battery and/or make it hard to recharge. If you want to be more conservative, stopping the test at 11.5 volts would probably also do the job as voltage is falling pretty rapidly at this voltage.