diy solar

diy solar

How to determine if Lifepo batteries are bad

S

Sonnyboy

New Member
Joined
Jul 17, 2022
Messages
92
I have a solar setup that includes two lifepo4 batteries in parallel that power a very small freezer and a couple 6w led lights on motion sensors. After several cloudy days they need to be brought in and recharged with a battery charger. When I do that, I hook up a two year old agm that has been used a lot. It always seems to last longer and it’s significantly smaller. Is there a way to tell if your batteries are defective? I also get off charging info from the MT50.”(video) when running the LifePO4 batteries…volts bounce up and down with no draw on the system.

 
Last edited:
sunshine_eggo said:
Are there loads present in the system during that video?

Have your batteries ever been charged below freezing?
Click to expand...
No load and they are located in an insulated box in a shed. But yeah, it has been below freezing. I have another exact same battery (weize 100ah) in a van and that has been working fine and is not in an insulated box.
 
What do they do when charged with the other charger? Does it read voltage, amps, track ah, or anything?

I think you either have a faulty charge controller in the shed, a bad connection between the charge controller and the batteries (probably not directly on the batteries since you seem to be redoing that frequently and symptoms don't change), or faulty batteries. If the batteries had bms's with communication, or removable lids, you could look at what the cell voltages are doing.

When the agm is hooked up in the shed, does it charge properly?
 
Vigo said:
What do they do when charged with the other charger? Does it read voltage, amps, track ah, or anything?

I think you either have a faulty charge controller in the shed, a bad connection between the charge controller and the batteries (probably not directly on the batteries since you seem to be redoing that frequently and symptoms don't change), or faulty batteries. If the batteries had bms's with communication, or removable lids, you could look at what the cell voltages are doing.

When the agm is hooked up in the shed, does it charge properly?
Click to expand...
The agm seems to do far better and is only 70ah. The lifepo: I don’t think the connection or controller are bad since the same controller and connections work so well for the agm. I bought three weize 100ah at the same time and the one in my van still works great, albeit with much less of a load put on it. The only thing I can think of is that when I initially hooked up the system, I wasn’t paying attention and briefly connected positive to negative on the batteries and there was a spark. Maybe I fried something in one or both. That would be a bummer. I don’t get why the voltage/amperage input from the panels would fluctuate like it does.
 
Normally, shorting of the battery terminals together would activate short circuit protection. I don't know what is normal if you short two batteries together. I would expect the same with no negative outcome, but I can't be sure.

I'm not seeing voltage/amperage input from the panels changing much at all.

43.6V/.3A in
Battery voltage varies wildly rom 12.9V/.9A with an immediate jump to 14.4V/.8A
Then battery voltage cuts to 11.2V.

The immediate cut off of the input suggest BMS over voltage protection; however, the voltage shouldn't drop like that.

That really looks like a BMS issue. Over voltage protection should just stop charging and hold the peak voltage for a bit.
 
Well input from the panels would be the 43v on the left in your video and it didn't fluctuate wildly, although if the controller has nowhere to put it because the battery 'disconnects' or 'disappears' it will definitely throttle down the PV simultaneously in response to 'losing' the battery.

When you say the agm does better, do you mean during discharge or during charge? If it lasts longer in discharge that means your lithiums are only letting you use a tiny fraction of original Ah, which is bad. But if the agm ALSO doesn't charge worth a damn that would be good news as it would likely point to an issue with wiring or charge controller which are cheaper than 200ah of lifepo4.

As far as shorting the batteries together, the only way the bms would be 'expected' to fail from that would be if it said 'not suitable for series' which would mean the bms was not rated for exposure to 24v, 36, 48v etc and shorting two fully charged batteries together in opposite polarity would be ~29v at ludicrous amps, however briefly.
 
Do you think a Load Tester would determine if the batteries are shot?

I can’t watch the video with the low bandwidth connection i have. I also wonder with BMS low voltage cutoff if something happened to the internal electronics with the panels hooked up wanting to push out full amps, but the BMS cutting off for long periods.
 
Vigo said:
Well input from the panels would be the 43v on the left in your video and it didn't fluctuate wildly, although if the controller has nowhere to put it because the battery 'disconnects' or 'disappears' it will definitely throttle down the PV simultaneously in response to 'losing' the battery.

When you say the agm does better, do you mean during discharge or during charge? If it lasts longer in discharge that means your lithiums are only letting you use a tiny fraction of original Ah, which is bad. But if the agm ALSO doesn't charge worth a damn that would be good news as it would likely point to an issue with wiring or charge controller which are cheaper than 200ah of lifepo4.

As far as shorting the batteries together, the only way the bms would be 'expected' to fail from that would be if it said 'not suitable for series' which would mean the bms was not rated for exposure to 24v, 36, 48v etc and shorting two fully charged batteries together in opposite polarity would be ~29v at ludicrous amps, however briefly.
Click to expand...
I think the agm lasts longer in discharge. I don't know why the lithium seems to charge to 14v and then drops down to 12 with no load. I am going to keep an accurate log and check this for sure. I just had to bring in the agm and recharge it after a few days of snow. In response to chrisski's post, do you think a load tester would be worth buying to know the lithium battery state?
 
Do these batteries have built-in low temperature charge protection (they won't charge below freezing)?

If not, do they have secondary protection?

Is there any chance they were charged below freezing?

If no low-temp charging protection and they were charged below freezing repeatedly, they're cooked.
 
Last edited:
MisterSandals said:
Can you post a pic that shows your 3 battery wiring?
Click to expand...
It's not three. Its just one agm that I use when the lithium drain too low. Then I charge the lithium inside and swap out when the agm gets too low. we don't get much such here. The two lithium are just in parallel.
 
sunshine_eggo said:
Do these batteries have built-in low temperature charge protection (they won't charge below freezing)?

If not, do they have secondary protection?

Is there any chance they were charged below freezing?

If no low-temp charging protection and they were charged below freezing repeatedly, they're cooked.
Click to expand...
They are Weize 100ah batteries. According to their ad and Will Prowse's review, they do have low temp protection. Unless mine are faulty somehow, I don't think this is the problem.
 
Sonnyboy said:
I think the agm lasts longer in discharge. I don't know why the lithium seems to charge to 14v and then drops down to 12 with no load. I am going to keep an accurate log and check this for sure. I just had to bring in the agm and recharge it after a few days of snow. In response to chrisski's post, do you think a load tester would be worth buying to know the lithium battery state?
Click to expand...
If the battery drops to 12V quickly and with no load after charging to 14V, that does indicate an issue with the battery. This is, if I understand, in the house charging with a Lithium battery charger after it had run down after a few days outside? What make and model charger are you using, and does it have a LiFePO4 setting?
 
wholybee said:
If the battery drops to 12V quickly and with no load after charging to 14V, that does indicate an issue with the battery. This is, if I understand, in the house charging with a Lithium battery charger after it had run down after a few days outside? What make and model charger are you using, and does it have a LiFePO4 setting?
Click to expand...
I am using the Epever 5415an 50 amp scc
 
You must log in or register to reply here.

Similar threads

B
What Happens to Solar Power When Batteries Are Full
2
Replies
21
Views
771
Mattb4
Mattb4
S
LifePO4 heater pads
Replies
2
Views
241
mactech
M
O
How to Wire a 24 volts Lifepo4 Bank
Replies
3
Views
168
timselectric
T
C
Battery discharge question
Replies
6
Views
272
Conductor570
C
C
Changing over to lifepo
Replies
3
Views
179
Vigo
V
Back
Top