Renogy LIFEPO4 discharging while isolated.

[Jamie Marshall]

Hi all
I have a 50ah lifepo4 from renogy that is discharging while isolated.
The issue with Renogy is resolved as they are sending me a new battery and their communications where very good.

I'm hopping that now warrenty isn't an issue I might fix the thing, as it would be a real shame to waste it.

I used it a few times, but it is relatively new.
Having noticed something wrong, I fully charged it via the renogy dcdcmppt 30 driving back from a trip. It was at full at 14.2-14.4v and the dcdc had stopped charging. I could confirm the capacity was full via the smart shunt.

I then isolated the positive at the battery.
The next day the voltage was 13.4v
4 days later the voltage was 12.8v (flat)

Given the battery was isolated I can only assume that the BMS must be faulty or the cells must be faulty.
I don't know much about lifepo4 raw cells, but I've never heard of such an issue.
If its the BMS I can imagine stripping it out for a new one, but this might be a waste of time and money.

Has anyone seen this before? Is it likely the BMS? or the Cells?

Thanks so much for reading. Hope I'm making sense.

 

[meetyg]

The BMS should certainly not draw that much power in 4 days.
Hard to tell what's faulty, but I suspect the BMS.
If they let you keep the existing battery and you know how to carefully disassemble it, I would try to fully charge and the disconnect the BMS. That will tell you if the BMS is faulty or the cells.
You may be able to reuse the cells with a new BMS.

 

[Rainman65]

I have two of the same 50Ah batteries which I use to power trolling motors in my wife’s and my kayaks.
If I leave them disconnected for a day, they may or may not drop back to 12.8 volts. They will not take additional charge.

The motors have a voltmeter on them which also shows 12.8V when plugged in. As soon as we start motoring they stay on 12.8V (under load) but when we stop they go up to 13.2V.

The batteries seem to work perfectly fine.

I don’t know if this affects capacity or not as after 2 hours of fishing yesterday they still had 13.2V using a voltmeter and they took around 30mins each to charge with a 20a charger. They work perfectly for our needs.

I determined the BMS goes to sleep (Experts - is this a thing?) after a period of inactivity.

 

[mikefitz]

It could well, be that the BMS goes to sleep. An alternative is that the charge path is opened by the BMS due to cell overvolts under charge. This would introduce a 0.6 volt drop across the BMS, thus indicating 13.4 - 0.6 = 12.8 at the terminals for a charged battery. Note after charging the resting voltage will be near 13.4 volts at the battery cells.
Suggest that the problem battery is loaded or charged to establish that a 'wake up' of the BMS occurs.

Mike

 

[Jamie Marshall]

There is somthing very odd going on. I'm glad to hear it's not just me.
I've opened my battery to test the bms and now find it now sits at 13.3v for the last 2 days.
It should not be charged by all other accounts, having been at 12.8v and dropping dramatically under load.

I get 13.3v at either side of the bms, does that mean it can't be asleep?

I have a victron charger in the post, as up until now I could only charge while towing the trailer. This should help verify things.

 

[mikefitz]

No drop across the BMS indicates its operating with the charge and discharge paths active.

Note the BMS will open the charge path at low temperatures.

 

[Rainman65]

27 degrees Celsius here at the moment, so we can exclude low temperature. Both batteries are at 13.4v when fully charged. The voltmeter on the motors tends to under read. I will leave a more accurate voltmeter plugged in on our next fishing trip on or around the 27th December.

 

[Jamie Marshall]

It could well, be that the BMS goes to sleep. An alternative is that the charge path is opened by the BMS due to cell overvolts under charge. This would introduce a 0.6 volt drop across the BMS, thus indicating 13.4 - 0.6 = 12.8 at the terminals for a charged battery. Note after charging the resting voltage will be near 13.4 volts at the battery cells.
Suggest that the problem battery is loaded or charged to establish that a 'wake up' of the BMS occurs.

Mike

Thanks for the help.
Could you explain the voltage drop during bms sleep time please. I don't understand it properly. Thanks

 

[Jamie Marshall]

Update. Battery is/was fully charged, victron blue smart could only give it 0.5ah.

After charging, the batt measures 13.06v at terminals and 13.54v before bms. Charger was set to 14.2v smart lithium lifePO4 mode.
Is the voltage difference the over voltage protection kicking in?

Looking back to before, how was the battery giving the appearance of being completely flat?
When it was measuring 12.8v I loaded it with 3.4amps running the fridge and the voltage dropped to 12.4v.
I was convinced that this was a flat battery.
1. I thought I would damage the battery using it.
2. My independant low voltage cut out was going to be triggered.
3. I didn't think I would have any power to use.

@Rainman65 do you find that the voltage only bounces back once you put a charge on the terminals?
What do you think the pattern is?
You seem to use your batteries around this issue, all be it disconcerting.
Thanks

 

[Rainman65]

The voltage bounces back after I put a load on the battery. Charging makes no difference.

Strangely, it needs a load, and it reads normal after I turn off the load, not during the initial load. After this cycle the battery works perfectly.

Have a merry Christmas!