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Too late to top balance with BMS?

kolek

kolek

Inventor of the Electron
Joined
Sep 29, 2021
Messages
416
I've been trying to top balance a 16 cell battery in sets of 4 (in parallel, using a charger), but it's taken 1 week and I've made little progress on the first set of 4.
Here are the voltages of the 4 cells I have tried to charge:
3.343V
3.340V
3.333V
3.332V
(They didn't charge evenly because for the first 5 days of charging, I had the both the positive and negative charging wires on the terminals of the first cell, rather than splitting it between the 1st and 4th cell).

All the other cells are about 3.292V.

The charger is a 60A charger but it's charging at about 4.5A, I'm using 18AWG wire. LF280 batteries.

Is there too big a difference between these cells to just use my JK-BMS to charge all 16 cells together?

At this stage what's the best course of action? Charging in sets of 4 is going to take a month or more at this rate.
 
Jk bms can probably manage a 12S battery, depends on model. Depends if you have a charger that will do 36V. Victron smartsolar will for example.
 
squowse said:
Jk bms can probably manage a 12S battery, depends on model. Depends if you have a charger that will do 36V. Victron smartsolar will for example.
Click to expand...
My charger is the Kikusui PAK6-60A, it does 0-6V at up to 60 amps. I thought I was a freakin' genius buying a low voltage high amperage charger because I thought that would enable me to charge real fast at low voltage. Apparently I was an idiot.
 
kolek said:
My charger is the Kikusui PAK6-60A, it does 0-6V at up to 60 amps. I thought I was a freakin' genius buying a low voltage high amperage charger because I thought that would enable me to charge real fast at low voltage. Apparently I was an idiot.
Click to expand...
But its only done about 7A average? Sounds like fraudulent specs. It might benefit from better cables.
 
Way less than 7A, it's doing like 4.5A, I guess because the differential between 3.65V and 3.3V is so small?
Either that or the power supply is bad, or the wires are too thin.
 
kolek said:
Way less than 7A, it's doing like 4.5A, I guess because the differential between 3.65V and 3.3V is so small?
Either that or the power supply is bad, or the wires are too thin.
Click to expand...
I just converted 18awg to mm2. Yes thats way too small. 60A would want 10mm2 cables. The voltage drop on the cables is throttling the current.
I would still use the bms to charge the cells first though,
 
squowse said:
I just converted 18awg to mm2. Yes thats way too small. 60A would want 10mm2 cables. The voltage drop on the cables is throttling the current.
I would still use the bms to charge the cells first though,
Click to expand...
Wouldn't the wires be getting warm if they were throttling the current?
 
The current is low - so no. It finds it’s equilibrium. Even at 4.5A you will be getting 0.2V drop on 1metre cables. 60A requires 10mm2 even at a very short length. Thicker is obviously better.
 
kolek said:
Wouldn't the wires be getting warm if they were throttling the current?
Click to expand...
Not really, as the voltage drop is so small.

Your power supply seem to have possibility to use external voltage sensing that connects another set of (thinner) wires to the load and senses the voltage drop in the cables.

With external voltage sense you can use even long wires and voltage drop doesn't matter as long as the wires are enough to handle the current without overheating. For 60A I'd use minimum of 6AWG or 10mm2. Sense wires can be any thin wire (ie 14 to 22AWG) you happen to have.
 
Guys, this was really helpful.
squowse said:
I just converted 18awg to mm2. Yes thats way too small. 60A would want 10mm2 cables. The voltage drop on the cables is throttling the current.
I would still use the bms to charge the cells first though,
Click to expand...
@squowse Thanks a lot for your help!
So I can learn from this blunder, what online calculator are you using to get this info?
 
MattiFin said:
Not really, as the voltage drop is so small.

Your power supply seem to have possibility to use external voltage sensing that connects another set of (thinner) wires to the load and senses the voltage drop in the cables.

With external voltage sense you can use even long wires and voltage drop doesn't matter as long as the wires are enough to handle the current without overheating. For 60A I'd use minimum of 6AWG or 10mm2. Sense wires can be any thin wire (ie 14 to 22AWG) you happen to have.
Click to expand...
@MattiFin Amazing you actually found an English user manual for my power supply. Looks like it was typed using an old IBM Selectric typewriter and looks like the date on the manual is 1989. When I searched I couldn't find one. I think this model was mainly sold in Japan.
 
I just connect the JK BMS and charge the battery. Monitor the cell voltages when you get up near fully charged and see how the balancer gets on. If there are any cells lagging badly behind, connect your charger to that cell. Let the balancer cut off the high cell voltages.
 
If you build all 16 then the 4 will be way more out of balance than they would have been from scratch, but it will be possible.

I would parallel all them for a couple days first at least.
 
kolek said:
Way less than 7A, it's doing like 4.5A, I guess because the differential between 3.65V and 3.3V is so small?
Either that or the power supply is bad, or the wires are too thin.
Click to expand...
Do not expect a LiFePo4 cell to sit on 3.65V. You will never seen that. They are fully charge sitting on 3.35-3.38V after few hours of been charge. In fact. Do not charge to 3.65V. There is no benefit anything over 3.45V.
 
kolek said:
Wouldn't the wires be getting warm if they were throttling the current?
Click to expand...
Are you using little alligator clips to connect your charger to your 4 cells in parallel? This is a known problem that its solved with ring terminals.

Happen to have a pic of your charging setup?
 
kolek said:
I've been trying to top balance a 16 cell battery in sets of 4 (in parallel, using a charger), but it's taken 1 week and I've made little progress on the first set of 4.
Here are the voltages of the 4 cells I have tried to charge:
3.343V
3.340V
3.333V
3.332V
(They didn't charge evenly because for the first 5 days of charging, I had the both the positive and negative charging wires on the terminals of the first cell, rather than splitting it between the 1st and 4th cell).

All the other cells are about 3.292V.

The charger is a 60A charger but it's charging at about 4.5A, I'm using 18AWG wire. LF280 batteries.

Is there too big a difference between these cells to just use my JK-BMS to charge all 16 cells together?

At this stage what's the best course of action? Charging in sets of 4 is going to take a month or more at this rate.
Click to expand...

Why don't you just have all 16 connected in parallel?
 
MisterSandals said:
Are you using little alligator clips to connect your charger to your 4 cells in parallel? This is a known problem that its solved with ring terminals.

Happen to have a pic of your charging setup?
Click to expand...
Not using alligator clips was the only thing I did right. I am using ring terminals, too bad the wire I chose was too thin.
 
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