Lifepo4 Battery health?

[Jackytwoshoes]

I have a chunky 48v 18Ah Lifepo4 Battery. It was laid up for a couple of months, and maybe charged once during that period.

The other day it seemed to be completely flat.. no LEDS, no drive, no nothing. I think that the capacity seems to be a bit reduced and I've a suspicion that it's losing charge of it's own accord overnight.. Does that point to a dodgy cell or several? Can I get it checked/ fixed/ reconditioned or should I just use it until it completely dies...

 

[Nealh]

Lifepo4 (LFP) are regarded as one of the safest lithium chems, the voltage range is low with 3.2v nom voltage 2.8 - 3.6v is the typical usage range. Even charged to 3.6v it is known that some may discharge/balance out at 3.4v or there about.
Severe self discharge is either down to bad cells, loose wire contact or faulty BMS.
Does the battery charge at all ?

 

[Jackytwoshoes]

It does, I'm still using it daily. Can I simply measure the voltage with a multimeter just now when it's fully charged and measure again before I go home?

 

[Jackytwoshoes]

Okay, charger says fully charged and I just tried a multimeter. It said 54v. Is that high?

 

[Nealh]

54v isn't typically high but not bad, bad.
48v lifepo4 is 16 cells in series or 16 x 3.2v for a nominal value of 51.2v, 58.4v is a max charge rate but is the top end at 3.65v per cell.
3.6v (95%) or lower is a better charge cut off rate, though it isn't unknown for lifpo4 to settle at 3.5v per cell after charging. 56v for a 48v one.
54v = 3.375v per cell if they are balanced, the only way to know if the pack is balanced is to manually check the 16 cell groups which means opening up the battery to do so via the BMS leads.

How old is the battery ?

 

[Jackytwoshoes]

I'm not sure how old it is, I bought it second hand from a member of this forum. I'm a little shy of opening up the battery, but maybe there's nothing to fear..

I've just measured the battery again after it's been at rest, fully charged for an hour and it's at 53.8v now.

 

[Nealh]

Not much to fear in opening a battery if you are a practical person and can use a volt meter with some guidance. The battery may likely be fine though loosing some capacity thru ageing.
Post a pic or two of the battery.

If the battery has been idle charge/discharge it a few times to see if it balance charges a bit higher first. With a good balanced battery you can disconnect the charger once it is charged but if cells are out of balance let it sit for an extra hour or three to let the the BMS carry out it's job. The BMS bleed resistors release voltage very slowly for balancing and often 100mv is the release/charge voltage used so it can take many hours even if the charge light is green. It may take 3 or 4 charge cycles for balance if out to rectify it self it at all.

 

[Jackytwoshoes]

That's very interesting, thanks. I always tend to unplug it as soon as the light goes green for fear of over charging as the charger doesn't inspire me with confidence. But with the knowledge that it's doing some good, I'll leave it charging a bit longer. Is there a danger in leaving it too long?

 

[Nealh]

Lookslike the pack could do with a rewrap in heat shrink.
Access to the BMS is impossible without cutting the shrink so don't go there unless you have shrink wrap to replace it.

If it is unbalanced there is chance it might rebalance but you will need to let the charge/balancing to do its job so you will have to leave on to do it's job the BMS will bleed down any high voltage. The charger will only provide current that the BMS ask's for, most chargers are cc/cv and only supply that what is asked of them.

Measure voltage every hour for three hours to see if it changes and then check final voltage and resting voltage the next day.