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LiFePo4 Out of Balance

Hello, My CALB CA400 LiFePo4 cells, (4 cells in series, 12v) (8 1/2 years old) are getting out of balance towards the end of charge @ 14.2 volts. Readings are as follows-

 

Victron Multiplus  @ 14.21v Absorb 1 Hour (minimum setting)

Cell 1 3.498v

Cell 2 3.394v (lowest)

Cell 3 3.625v

Cell 4 3.670v (highest)

▲v = 276m/V

 

At rest –

Cell 1 3.350v

Cell 2 3.333v (lowest)

Cell 3 3.369v

Cell 4 3.372v (highest)

▲v = 39m/V

Do you think the active balance on the TAO might improve this over time? I don't usually charge this high but needed to take them close to 100% for a camping stint in the Motorhome.

I want to buy the TOA BMS when finances allow.

Thanks - Leigh

Hi Leigh,

Based on the end of charge voltages the unbalance seems important.  If we consider cell 4 as full (3.67v), cell 3 at 3.39v might be only 95% full.  5% of 400 Ah = 20 Ah.

Yes the active balancer in the TAO  BMS would get those cells balanced within 5-7 hours (on the above assumption).  For the first full charge you want to reduce the Multiplus charge current after one cell has reached 3.40 volt, like that it will take a longer time before the highest cell reaches 3.65v... giving more time to the balancer to do its work (if you have the Victron Cerbo, the BMS can send CANbus commands to the Victron equipment to reduce charge current to do that automatically)

The challenge if you do not balance the cells rapidly is that the highest cells are getting more stressed and will age (loose capacity) faster the other ones... increasing the unbalance... (spiral of death).

Meanwhile you should set the bulk/absorption voltage so that no cell goes above 3.55v.

Phil

Thanks Phil,

That might explain why my whole pack is down to about 380A/H from 400 on my BMV 712 smart shunt.

I don't have a Cerbo yet, would it be better to get the GX monitor with it or get your monitor with the TAO? I plan on drilling my Victron BMV 712 shunt to add your current sensing.

The LiFe batteries have sure made it all very involved. but would never go back to LA.

In the meantime I have a charger that could bump that low single cell up a bit would that be advisable?

I greatly appreciate your help - Cheers - Leigh

 

 

 

I do not have the GX monitor as all the information from the Cerbo is available on the phone / tablet... and I never need to look at it.

In any case if you want the BMS to send CAN messages, you need the TAO Monitor as it is the Monitor that handles all the communication (WiFi, CANbus).  The only tasks I perform in relation to the lithium battery is to look at the Monitor twice a day to be sure all is OK.

Yes, you can bump the low cell with a DC source - but under close supervision.  Better (if your DC source allows it) is charge the battery to about 13.8V, then connect all the cells in parallel and gradually charge them to 3.65V (manual top balance)

Phil

Hi Phil,

Does the charge current have to be reduced for your active cell balancing to work correctly?

Scenario is 750 watts of solar @ 40v nominal  to an Outback MX60 charging 30 amps into a 12 v 400a/h LiFe bank.

Current wont reduce till the absorption 14.2v  phase.

Thanks - Leigh

Hello Leigh,

No you do not have to reduce the charge current in normal situation where you start with balanced cells and the BMS only has to do minor corrections.

But if the cells unbalance is important, the BMS will not have enough time to balance the cells before the chargers are turned off (or go to float).  In that case you have two options:

  1. have the  BMS send a CAN message to the Cerbo to reduce the  charge current when a cell voltage reaches 3.45 or 3.50 V (the current must be as low as possible while allowing the BMS to balance - you need to try different values for the current - maybe around 10A), then when a cell voltage reaches 3.55V, reduce the current to 2 or 3A.  Start with a charge voltage of 13.8V, then progressively increase to 14.6V like in option 2 (always keep a trigger to stop the charge if a cell voltage reaches 3.65V)
  2. check what I suggest in this topic: https://www.taoperf.com/forum/topic/95/

Let me know how you go.... Phil