Low battery ah with normal voltage

[Jonathan123]

I have a 12v 200ah LifePo4 battery that gets seasonal use in the warm weather. I’ve been monitoring the voltage all winter while it’s been sitting unused in my basement and finally plugged it in to a charger today to get it ready for the season. The voltage was about 13.04 so I understood there was about a 50% state of charge based on charts I’ve been looking at. From what I understand that’s a decent state of charge for storage over several months.

When I plugged in the charger the display indicated only about 3.x amp hours in the battery. It’s been charging all day and is up to well over 100 now, hopefully on the way back to 200. But I don’t get the very low amp hour reading in light of the 13.04 voltage reading.

This is all still new to me and I’m totally in the dark. What’s going on here?

 

[sunshine_eggo]

What display?

Anything that tracks SoC or capacity remaining will be completely inaccurate after sitting a significant period of time. BMS and battery monitors need to be synced at least a couple times a month to maintain any kind of accuracy.

 

[Jonathan123]

Thanks @sunshine_eggo. The display is on the battery charger. So I should ignore the battery charger AH reading then?

 

[sunshine_eggo]

Thanks @sunshine_eggo. The display is on the battery charger. So I should ignore the battery charger AH reading then?

You should likely understand what it means and then apply it properly, or ignore it if its meaning is not understood. Also, if it's accuracy is not verified, it should only be viewed as an approximation.

From what I understand that’s a decent state of charge for storage over several months.

When I plugged in the charger the display indicated only about 3.x amp hours in the battery

Is this upon connecting the charger, or is it because it had been charging long enough to input 3.x Ah?

It’s been charging all day and is up to well over 100 now, hopefully on the way back to 200. But I don’t get the very low amp hour reading in light of the 13.04 voltage reading.

Your confusion confuses me. I don't see how the two values are in any way related, so you relating the two to each other is what I don't understand. The charger can't possibly know the SoC or how much Ah is in the battery upon connecting.

The charger is likely reporting how much Ah it has put into the battery, not how much is in it, i.e., "you have allowed me to put 3.X Ah into this battery since starting the charge. I don't know if this battery is 5Ah or 5,000Ah, but I'll keep reporting what I've input until I'm done."

If that's the case, I would estimate the charger will report 100-140Ah input when charge is complete.

13.04 is certainly less than 50%. Most raw cells ship at 3.29V or 13.16V total, and they are typically 30-50%. there are variations in these "look-up" tables equating voltage to SoC. Here's another:



That chart suggests you're between 30-35% SoC.

The only thing that can reliably know the state of charge (or Ah remaining) of a LFP battery is a properly configured and regularly synced battery monitor that continuously COUNTS the Ah in and out of the battery and calculates SoC based on the net in/out and the KNOWN rated capacity.

 

[Jonathan123]

Ah ok that makes more sense. This was upon connecting the charger to the battery. The idea that the charger was reporting how much it had input makes much more sense. Thank you.

Re the voltage chart you posted, I’ve seen those numbers for gel/agm but different values for Lifepo4 than for other 12v battery types. However that’s secondary in this case since I have the charger and the battery is now on solar for the season.

 

[sunshine_eggo]

Ah ok that makes more sense. This was upon connecting the charger to the battery. The idea that the charger was reporting how much it had input makes much more sense. Thank you.

Re the voltage chart you posted, I’ve seen those numbers for gel/agm but different values for Lifepo4 than for other 12v battery types. However that’s secondary in this case since I have the charger and the battery is now on solar for the season.

Nope. That's for LFP.

here's AGM.



Worth noting that the RESTING voltage of a 100% SoC AGM is lower than LFP @ 30%.

What did the charger end at?

 

[Jonathan123]

After it read full (which literally displays “Full”) I let it sit for about 6 hours and then it measured 13.4v on my multimeter. I never saw a final AH reading but assume you were right about that part. Now it’s up in Maine powering some 12v lights and a dc fridge and charging up via pv.