58 replies to this topic

[buckland]

This is probably a one up question.
Recharging my LiFePo4 battery to then balance the cells. It is on charger. BMS is balancing. Read out says 3.22 on each cell(4).
The question is the BMS % readout is 15%. This is real low though the Volts are 12.88v. What does it signify? (Time of photo it was 7%)
 Adjustments.JPG   361.44K   47 downloads

Edited by buckland, 17 June 2020 - 11:33 PM.

[Vic Harder]

Huh.  Puzzler.  The 82 mv number is the delta between highest and lowest cell.  That works out to 2.54%, not 7%.   A question for rando, methinks.

[rando]

The '%' value on the balancer display is just a guess at the SOC based on the battery voltage.   For LiFePO4 batter 12.71 V is pretty much flat, the normal resting voltage is 13.2-13.3V, so an estimate of 7% SOC is pretty reasonable.    Remember that for lithium batteries, using voltage for SOC is pretty much useless as the difference between fully charged (13.3V) and about 20% SOC (13.1V) is tiny. 

[buckland]

Much appreciate that info. I was at a loss on it. The trickle charger has brought the cells up to 12.9 V Today I will put it on a regular 12 v charger for a few hours and see if it will go up to 13.5 V Thanks again.

Rob

[buckland]

I have a portable flex 100w panel and a spare controller. I wired it up to charge the packs and it is now finally increasing in % SOC. I think this is good however this morning all 4 packs were balanced... as the charging increased they went out with one very strong one. I intend to leave on the balancing after I get up at 100%. (Hopefully)

 

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[buckland]

After two days on a 4amp charger, the BMS indicated a 60% charge and voltage of 13.33. Then hooked up a small inverter 150 watt and plugged in a 110 v fan to see what happens. After an hour on that load I took the photo below. Down To 33% and 13.19.
What this tells me I am not sure. I was wondering why the battery would not charge over 60%... and why it fell so rapidly. Individual bad cells in a pack (3)? Thoughts?  IMG_0723.jpg   165.92K   26 downloads

Edited by buckland, 20 June 2020 - 01:22 PM.

[CougarCouple]

Hi buckland
Was the charger still putting out to the battery? What was the voltage off the charger, amp output? If the charger was still charging the battery sound like it just needed more time.

I don’t know how long the charger you have would take to bring the battery up to 100%.

What you had plugged in sounds like you lost around 1/3 rd of a fully charged battery.

Russ

[PaulT]

I would be inclined to withhold judgement until going through a few charge/discharge cycles.  It may be that the BMS will equalize the cells over several cycles And improve overall performance.

 

Paul

[buckland]

Russ, No that was with the charger off (when it was on it read 80%) soon as I took it off it dropped to 60%. Seemed no matter how long the charger was on 80% was max. That said ... it is sitting at 13.20 V at 35 % after using it to run a fan/inverter.

I could run it down a bit more and then put it back on the charger for a day. I was not aware that a few charge cycles could affect the performance.

Paul, Will run through a few charges and load this weekend and see if it affects the percentage.

Constantly learning 

Thanks

Rob

Edited by buckland, 20 June 2020 - 09:03 PM.

[rando]

That looks really good to me, the cells are staying in balance both charging and discharging. 

 

Remember, the display on the balancer is only guessing at what the battery SOC is based on the voltage it sees, which doesn't work very well on lithium to start with, and really doesn't work when the battery is not at rest.   When you are loading it down with the inverter, the voltage drops, so the balancer display drops the SOC because it is only a linear function of the measured voltage.   When you are charging the voltage rises, and all of a sudden the balancer thinks the SOC is high.

 

For now I would ignore the '%' number on the balancer, it is causing more confusion than it is worth.  Either use a real current counting battery monitor, or the 'resting' voltage if you don't have a battery monitor.