What's everyone using for battery capacity testing?

[pollenface]

I want to test some old tired AGM's down to "inverter alarm voltage" to see if it's worth keeping them.

I have an inline power meter and 500w inverter, I usually start with a large load and reduce it towards the end of the cycle to keep things happening quickly but to also minimize volatge sag as the battery nears alarm voltage. Towards the end I use the standby power of the inverter alone.

I'm sure this is not optimal, what are your thoughts?

 

[Bob B]

If you discharge AGMs that low .... make sure you charge them back up right away. They are not designed to be discharged that low.

 

[pollenface]

If you discharge AGMs that low .... make sure you charge them back up right away. They are not designed to be discharged that low.

That wasn't my question.

 

[Brucey]

I want to test some old tired AGM's down to "inverter alarm voltage" to see if it's worth keeping them.

I have an inline power meter and 500w inverter, I usually start with a large load and reduce it towards the end of the cycle to keep things happening quickly but to also minimize volatge sag as the battery nears alarm voltage. Towards the end I use the standby power of the inverter alone.

I'm sure this is not optimal, what are your thoughts?

If you have a Victron SmartSolar SCC you can use the history tab in Victron Connect to track how many Ah you've fed in.

 

[pollenface]

If you have a Victron SmartSolar SCC you can use the history tab in Victron Connect to track how many Ah you've fed in.

That's a fair point, although I didn't think it was an accurate representation with lead acid with their higher internal resistance, or is it?

 

[Lt.Dan]

You cant base capacity of a lead acid from charging because of columbic efficiency and peukert effects. This does not give you an accurate representation of what will come OUT of the battery.

Best way is with a DC Shunt between the battery and the inverter.

 

[Brucey]

That's a fair point, although I didn't think it was an accurate representation with lead acid with their higher internal resistance, or is it?

Ah yes sorry I'm lfp.

 

[sunshine_eggo]

When I test capacity of lead acid batteries, I use reserve capacity, i.e., the # of minutes it will deliver 25A until 10.5V is reached.

In batteries smaller than 500Ah, you will not technically take them to 0% because there will be some charge left in the battery, so you're minimizing the risk of damage (which is often overstated). 5X 60W halogen/incandescent bulbs pulls about 25A @ 12V, and it's a good approximation when you consider the entire voltage range.

I built a quick test rig that included a 750W MSW inverter (that actually cuts out at 10.5V), 5 cheap bulb sockets in parallel (for variable loads if I needed to pull current in increments of about 5A) and an AC powered analog clock that would just stop running when the inverter shut off.

That allowed me to test them unattended and get a time value at the end. I compared the minutes measured to the minutes rated and computed a state of health.

After I did the proof of concept and tested a few batteries, I simply went back to my icharger 3010B that can discharge a 12V @ 25A to 10.5V and measure the Ah, Wh and record the time to discharge. Results were pretty consistent with my "analog" tester.