LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[NahanniV]

Originally Posted by downunder

What did you do the chart with? Thanks

I used a program called Logview: LogView Startseite

Connected to a PowerLog 6S (similar to CellLog 8S).

[diugo]

Originally Posted by deckofficer

I'm just letting voltage differential determine charging current when the cells are in parallel for balancing. I set the PS at 3.7 volts on a 80~90% DOD cells and it starts out charging at about 4.5 amps. 3 days later as cell voltage is coming up, it now is only putting 1.8 amps into the cells.

Now I see your situation---0.03C @ 400Ah is 12 amps. So technically you should have stopped charging before you began!

What is the parallel cell voltage right now at 1.8A?

[deckofficer]

Originally Posted by diugo

Now I see your situation---0.03C @ 400Ah is 12 amps. So technically you should have stopped charging before you began!

What is the parallel cell voltage right now at 1.8A?

It has dropped to 1.6 amps and cells are 3.46, 3.46, 3.46, 3.47

[diugo]

Originally Posted by deckofficer

It has dropped to 1.6 amps and cells are 3.46, 3.46, 3.46, 3.47

Are you conducting a test with them right now? If not, could you take one cell out of the parallel pack, put just it on your 3.7V PS, and report the cell's voltage and current?

[goboatingnow]

Isn't C/30 regarded as the cutoff point ?


Dave

[diugo]

Originally Posted by goboatingnow

Isn't C/30 regarded as the cutoff point ?

In standard CCCV charging, yes. But Bob is using only CV. That's why I am intrigued. Bob, if you turn off the charger, what does the parallel voltage fall back to?

[deckofficer]

The charge current is determined by voltage differential, so one cell or 4 in parallel, the PS at this voltage will only push 1.6 amps. This is why my 50 amp charger programmed for LiFePO4 charges at 14.8 volts, as this is the fastest way to charge with a delayed taper, than that of a lower voltage. If you have errant cells, then having the charger cut out at a lower voltage would insure no cell jumps above 4.0 volts, but the voltage taper would start sooner and the time to fully charge would be longer. That is why I balanced so many times because in the beginning one cell would jump above 4.0 volts before the others even reached 3.4 volts and on discharge a different cell dropped to 2.5 volts before the others had dropped to 3.3 volts. Now this bank discharges and charges with very tight voltages among the cells, taking out the worry of using the 50 amp charger.

[deckofficer]

Originally Posted by diugo

In standard CCCV charging, yes. But Bob is using only CV. That's why I am intrigued. Bob, if you turn off the charger, what does the parallel voltage fall back to?

It doesn't fall back, stayed at 3.46 volts.

[diugo]

Bob, I think the mistake is the 3.7V. You're charging along the asymptote, so I would set the PS no higher than 3.45V.

I think you are done charging. Especially if you consider that the PS has already returned 220Ah to the cells---(4.5+1.6)/2 * 24 * 3.

[goboatingnow]

Fundementally this isn't a good way to charge Li. There must be active cutoff , then you can charge CV followed by CV. I would have thought that using CV unless you set the voltage quite high , ( and Li can handle that once you have fast cutoff. ) nothing would really flow into the cells., ie the charging currents would turn put to be very low

Dave

[deckofficer]

This is the last balancing cycle for this pack. The last cycle I did the same, and after it the voltages were tight to each other. They are even tighter this time.

[diugo]

Originally Posted by goboatingnow

Fundementally this isn't a good way to charge Li. There must be active cutoff , then you can charge CV followed by CV. I would have thought that using CV unless you set the voltage quite high , ( and Li can handle that once you have fast cutoff. ) nothing would really flow into the cells., ie the charging currents would turn put to be very low

Obviously there is flow, 1.6 amps' worth. I think it comes down to a question of energy. Energy removed and returned. If you know you took 100Ah out of a battery, CV can be used to return it---but only if you keep good track of the amp-hours in. Adjusted for Peukert for good measure. Probably why CV isn't generally used!

[deckofficer]

The reason I picked 3.7 volts is that is what each cell would be (balanced) at the end of the fast 50 amp charge.

[diugo]

3.7V is a good educated guess for 50 amps. The correct value is the sum of the cell's fully charged voltage (3.45V) and the product of the cell's internal resistance (3 millohms) and the charge current: 3.45 + (0.003 * 50) = 3.60V. Things are quite different with your PS charging four cells in parallel. First, the internal resistance is divided by four. Second, the charge current is much less. The correct voltage here is 3.45 + (0.0075 * 1.6) = 3.462V.

[NahanniV]

Originally Posted by diugo

...the cell's fully charged voltage (3.45V)

Where did you get 3.45V from ?