Testing AGM batteries

[dbsoundman]

OK all, if you saw my last thread you can see it appears that I have a faulty battery or batteries. I have two Renogy 200ah AGM batteries currently. To be honest, I won't be mad if I have to replace these as I'll go straight to Lithium batteries. However, I want to definitively determine if my batteries are indeed faulty. I'm not finding much in the way of AGM battery tests online, at least tests that apply to non-starter batteries. Anyone have any particular procedures? Should I just disconnect one battery at a time and repeat a load test overnight?

[Booyah45828]

Contact renogy and see how you want them to be tested. We have a carbon pile load tester and a resistance tester at the shop. I prefer load testing because it seems to find faulty batteries better, whereas the resistance tester would say the battery is okay. But some companies don't like their batteries load tested, and require resistance testing instead.

There's also ways to perform a 20 hour load test using a headlight and checking the voltage at the end, but I'm not familiar enough with this way in order to tell you how to do it.

[dbsoundman]

Originally Posted by Booyah45828

Contact renogy and see how you want them to be tested. We have a carbon pile load tester and a resistance tester at the shop. I prefer load testing because it seems to find faulty batteries better, whereas the resistance tester would say the battery is okay. But some companies don't like their batteries load tested, and require resistance testing instead.

There's also ways to perform a 20 hour load test using a headlight and checking the voltage at the end, but I'm not familiar enough with this way in order to tell you how to do it.

Renogy chat basically recommended checking each battery individually with a multi meter, and then making sure they're both individually fully charged. Basic stuff I planned to try anyway. They didn't seem to have any further testing procedures, which is fine.

[Rucker]

Here's what I would do for each AGM or lead-acid battery. Batteries should be tested individually.
-Wire up a shunt, if you don't already have one connected. This will measure the current going into out coming out from the battery.
-Charge the battery up to full state of charge. Note voltage.
-Let sit overnight. Note voltage drop if any. Apply the charger again until the battery is fully charged.
-Apply a continuous load of no more than 10-20% of the battery capacity. If it is a 100 amp battery, no more than 20 amps. This can be lights, inverter loads, anything really except for inductor motors (so not a fridge). Something that will draw current steadily out of the battery.
-Note both amps drawn and voltage at regular intervals-like hourly. Draw the battery down to about 20% remaining.

Non-lithium batteries will have a fairly linear voltage curve. That is, the voltage will drop consistently with the current usage until the battery is nearly depleted.

Your shunt will tell you how many amps the battery was able to provide.

[dbsoundman]

Originally Posted by Rucker

Here's what I would do for each AGM or lead-acid battery. Batteries should be tested individually.
-Wire up a shunt, if you don't already have one connected. This will measure the current going into out coming out from the battery.
-Charge the battery up to full state of charge. Note voltage.
-Let sit overnight. Note voltage drop if any. Apply the charger again until the battery is fully charged.
-Apply a continuous load of no more than 10-20% of the battery capacity. If it is a 100 amp battery, no more than 20 amps. This can be lights, inverter loads, anything really except for inductor motors (so not a fridge). Something that will draw current steadily out of the battery.
-Note both amps drawn and voltage at regular intervals-like hourly. Draw the battery down to about 20% remaining.

Non-lithium batteries will have a fairly linear voltage curve. That is, the voltage will drop consistently with the current usage until the battery is nearly depleted.

Your shunt will tell you how many amps the battery was able to provide.

I think the easiest way to do this is to disconnect one of the two batteries, reconfigure my solar charge controller (which only charges from shore power currently) with the new battery bank capacity (200 instead of 400 ah), let it charge, then do the discharge test using the charge controller to monitor as it has its own shunt.

[Ronnie]

Originally Posted by dbsoundman

I think the easiest way to do this is to disconnect one of the two batteries, reconfigure my solar charge controller (which only charges from shore power currently) with the new battery bank capacity (200 instead of 400 ah), let it charge, then do the discharge test using the charge controller to monitor as it has its own shunt.

Yes as long as you can verify voltage readings. Full charge, then disconnect and let sit for a short while then measure voltage with a volt meter. Should 12.9-13 volts. Then reconnect and load test.


as AGM batteries get old they will not charge up to full voltage, might see only 12.6. So lower then 12.9 is a good sign of a battery loosing capacity.

[Rucker]

Originally Posted by dbsoundman

I think the easiest way to do this is to disconnect one of the two batteries, reconfigure my solar charge controller (which only charges from shore power currently) with the new battery bank capacity (200 instead of 400 ah), let it charge, then do the discharge test using the charge controller to monitor as it has its own shunt.

That will probably work. How is the charge controller interconnected with the battery-is there a bus bar everything connects to?
And can you shut off the charging function while you're testing?

And completely unrelated I think you said your new inverter is a whopping 5K something or other-that can draw over forty amps continuously. If the wiring from the battery isn't sized for that kind of load (~420 amps at 12VDC, 210 amps at 24VDC, 105 amps at 48VDC depending on how the input is configured) you'll need to upsize the conductor or, if you want a quick and dirty solution for testing, fuse the wire for its ampacity and let 'er pop if you exceed its rating while you get the right wiring in place.

If you have questions about this in particular speak up, we will happily consult.

[Rucker]

Originally Posted by Ronnie

Yes as long as you can verify voltage readings. Full charge, then disconnect and let sit for a short while then measure voltage with a volt meter. Should 12.9-13 volts. Then reconnect and load test.


as AGM batteries get old they will not charge up to full voltage, might see only 12.6. So lower then 12.9 is a good sign of a battery loosing capacity.

I think there's also some good discussion on the Internets regarding the impact of high discharge versus what I recommended-namely, that higher discharge rates make the battery appear that it doesn't have the expected capacity. I think.

I do know that if your batteries are bad you'll see interesting behavior-things like very quick voltage sag and your inverter will start beeping way sooner than you expect, for instance.

The testing we're talking about is sort of 'is my battery bad?' testing. Once you've ruled out bad batteries you can do successively heavier discharges to see how the batteries operate under various loads.

[dbsoundman]

I reconfigured the batteries tonight, using judicious electrical tape just to be sure. 2/0 wire really doesn't like to bend or move so I doubt things will shift overnight but you never know. Set up the charge controller to expect a 200 ah system instead of 400, and turned on the charger. I have a lamp timer that runs the charger for several hours then shuts it off "automatically", so I'm letting that run its course. Battery voltage was I think 12.6 without the charger running, so that seems about normal. The actual load on the battery is less than 5W, just the power supply for my cellular modem and two LED lights. In the morning I'll put a heavier load on it and see what happens.

[Rucker]

Excellent, very interested in your results.

[Ronnie]

Originally Posted by Rucker

I think there's also some good discussion on the Internets regarding the impact of high discharge versus what I recommended-namely, that higher discharge rates make the battery appear that it doesn't have the expected capacity. I think.

I do know that if your batteries are bad you'll see interesting behavior-things like very quick voltage sag and your inverter will start beeping way sooner than you expect, for instance.

The testing we're talking about is sort of 'is my battery bad?' testing. Once you've ruled out bad batteries you can do successively heavier discharges to see how the batteries operate under various loads.

I am not suggesting a high load discharge test like the load test you would do on a car battery, but in fact meaning to refer to the load test you suggested, like the 20 hour load test. Perhaps load test is not really the right word, capacity test?


My preference for testing batteries is to take them out and put them on the bench, and charge them fully. Let them sit for 24 hours check the voltage, then run a load based on what the battery is designed for.

[cadillackid]

Originally Posted by Ronnie

Yes as long as you can verify voltage readings. Full charge, then disconnect and let sit for a short while then measure voltage with a volt meter. Should 12.9-13 volts. Then reconnect and load test.


as AGM batteries get old they will not charge up to full voltage, might see only 12.6. So lower then 12.9 is a good sign of a battery loosing capacity.

doesnt it depend on the AGM battery? some AGM batteries are designed to be "car batteries" so it seems even brand new they settle down at closer to 12.6-12.8 when let to sit for a few hours.. the ones I use from batteries plus are designed like that.. they are considered dual purpose deep cycle batteries..



my house batteries in the DEV bus are designed as deep cycle UPS vatteries and tend to live higher at 12.8 - 13.0 or so after a few hours sitting...

[Ronnie]

Originally Posted by cadillackid

doesnt it depend on the AGM battery? some AGM batteries are designed to be "car batteries" so it seems even brand new they settle down at closer to 12.6-12.8 when let to sit for a few hours.. the ones I use from batteries plus are designed like that.. they are considered dual purpose deep cycle batteries..



my house batteries in the DEV bus are designed as deep cycle UPS vatteries and tend to live higher at 12.8 - 13.0 or so after a few hours sitting...

Yes, true. However The renogy batteries( which he has) I have used before and are a deep cycle only, and should be close to 13 . My optima dual purpose also should be 13 volts . This is the spec Optima gives.

[dbsoundman]

The charger turned off around 4am, here’s how things are going so far. Not a super official bench test but I still think this will provide some information. By my calculation I should get around 80 hours of usage at 15w before I hit 50%. Obviously I won’t wait that long, but hopefully some trends will emerge today, or maybe I’ll increase the load soon.

[Rucker]

This is great. Is the app your solar charge controller app?

[dbsoundman]

Originally Posted by Rucker

This is great. Is the app your solar charge controller app?

Nope, just Home Assistant with a custom app (really a script) I wrote in Python that grabs the data from the Bogart WF2030 web interface for my TM2030 charge controller. It pushes the data via MQTT protocol to my Home Assistant system at my actual house.

[Rucker]

Originally Posted by dbsoundman

Nope, just Home Assistant with a custom app (really a script) I wrote in Python that grabs the data from the Bogart WF2030 web interface for my TM2030 charge controller. It pushes the data via MQTT protocol to my Home Assistant system at my actual house.

I was gonna say, I'll revise my opinion of that solar charge controller if their app is looking that good.

Kudos-wouldn't surprise me if someone wanted to tap your knowledge in this space.

[dbsoundman]

Originally Posted by Rucker

I was gonna say, I'll revise my opinion of that solar charge controller if their app is looking that good.

Kudos-wouldn't surprise me if someone wanted to tap your knowledge in this space.

I've had plans to share it once I was sure it worked. It took some trial and error but it seems stable now. I'll try to do a writeup on the Home Assistant stuff I've done sometime soon.

In other news, I think I have a bad battery...look at that voltage drop around 10:40am! This is with a constant ~15 watt load.

[Rucker]

I'd say. AGM is right at 100% SOC at 12.85 volts, and at about 0% at 11.65 volts. From my calculations you got two and a half amps out of that battery from full charge to discharge...

This is just the one battery, correct?

[Booyah45828]

If it's tanking like that with a 1 amp load I concur. How does that app determine SOC%? Amps in/amps out over capacity? Can't be based off volts because 11.6 is what, 20%?