Capacity Testing - Discharge testing batteries

You can not test deep cycle batteries with handheld test equipment in a matter of minutes. It takes 8 hours to ensure your batteries are fully charged before spending 8 hours or more discharging them before another 8 hours to recharge. (assuming the batteries are OK), if they cannot hold capacity these tests can be done in hours once you have ensured the battery is first fully charged because the discharge and recharge time will take a much shorter time due to no capacity.

How do you perform a test, there are many variations, some include:

1. Reserve capacity test. Reserve capacity is a discharge at 20deg C and 25A constant for X number of minutes which will be stated on your battery as RC. If RC is 200 then that's 200 minutes at 25A discharge until the battery reaches an end voltage of 10.75v. If your test lasts 100 minutes then your capacity is 50% of its rated new Ah.

2. AH rating test. If a battery is rated at 200Ah @ 20 hours that's 5 amps per hour for 20 hours till an end voltage specified by the battery manufacturer. They often quote in VPC or volts per cell. So for a 12v battery you need to multiply the number by 6. 1.8vpc = 10.8v when testing a 12v battery. You need to discharge a fully charged battery at 5 amps for 20 hours and if the battery lasts 10 hours before the voltage reaches 10.8v then that's 50% of its rated capacity.

3. Not recommended but some distributors supply 75A discharge testers because that's a test used on golf cart batteries that they distribute. Golf cart batteries are also normally 6v 250Ah or larger (can be 415Ah) so a 75A discharge test is only 1/2 or less the batteries rated capacity. If you perform a 75A constant current discharge test on a 100Ah battery you certainly aren't going to get a real-world test result on a 100Ah battery when the test would only last 30 mins on a brand new healthy battery but your tested battery only lasts say 17 mins. It's easy to quote a large percentage number differential or failure when you've only tested for a few minutes and each minute represents 2-3%. So this test needs to be put in perspective of reliability to your situation. It is, however, a great way to ensure you buy a new battery from the battery test company.

Last point, a discharge test is using a deep cycle, your battery only has X number of those depending on how you've used your battery so they really only need to be done when there's an issue or if performing a system upgrade and you want to assess the suitability of the batteries to perform their new duties.