Do I have a dud Lifepo4 battery?

[salmirabile]

I bought this battery the other day:

https://www.ebay.co.uk/itm/12V-20Ah-...72.m2749.l2649

My problem is this....I understand lifepos aren't meant to change voltage very much until almost depleted but under some load of 4a which equates to around 0.2C, the voltage drops like a stone to 11.2v!! Is that normal or do i have a fake/dud battery?

Also, after a full charge at 13.8v, it goes down to 13.4v after 10 minutes without any load and then after having the 4a load on for around 30seconds, the voltage reads 13.3v which is 90% and then after 4hrs i put on another 4a load for another 30 seconds and it now reads 13.2v...how can this battery lose so much capacity so quickly?

 

[snoobler]

Welcome to the forum.

13.8V isn't fully charged unless you're charging it for many hours. Full charge is typically achieved at 14.4-14.6V.

Battery voltage drops when a load is applied. This loaded battery voltage is not indicative of its state of charge. Connection quality and wire size also impact the voltage drop. Readings taken at the battery may vary from readings taken at the load.

I recommend you fully charge the battery to 14.4-14.6V and reassess.

 

[salmirabile]

thanks for your reply.

I'm gonna do another test today and see what happens.
my balance charger wouldn't charge it beyond 13.8 but I'll try and force charge it and see.
I know battery voltage drops under load but it certainly shouldn't fall 2v in a lifepo4 which is practically at it discharge value, especially when 0.25c load is applied. This is what's confusing me.

All my readings are at the battery terminals, cabling is over gauged and the total circuit wiring is around 20cm long.

 

[Park Kitchings]

I seem to have the same problem with a new Battleborn battery. When a lithium is at 13 volts, what percentage of charge is this?

 

[upnorthandpersonal]

When a lithium is at 13 volts, what percentage of charge is this?

Almost impossible to tell - it depends at what C rate it's been charging, how many hours it's been at rest, and how long it's been on the charger. See this graph:

So your 13V can be anywhere between 80% (or even higher) and 50% (or even lower) depending on the circumstances. The voltage delta between 90% and 20% is less than 0.8V!

 

[salmirabile]

So, it turned out that my battery was either faulty or fake! I bought another lifepo4 from Ultra max and it was perfect. with the same 4a load on my new 24ah battery, the voltage drop was 0.2v!

My conclusion is the cells in the faulty battery were, in fact li-ion 18650 cells. I haven't looked at the cells as they're completed sealed and couldn't be bothered to strip it apart. I got a refund anyway.