Potential top balance issue

[hammick]

I am top balancing my 16s pack in parallel using the power supply Will recommended. I am on day three and the pack is taking over 10 amps and all batteries are under 3.4 volts at their terminals. With the charger turned off I get continuity across the farthest distanced positive terminals at 0 ohms and the same for the negative terminals. However, when I turn on the charger I don't get continuity and the farthest distanced positive terminals have 261 ohms of resistance and the farthest distanced negative terminals have 284 ohms of resistance and no continuity. The negative connections are all pipe hanger. The positives are buss bars which are tied together with pipe hanger.

Is it normal to have such resistance and no continuity when charging? I'm wondering if the pipe hanger is not a great idea for charging at over 10 amps. It is not getting hot. Thanks for any advice. I hate to spend a week charging these batteries only to find I have a problem or even worse damage them.

BTW pack voltage was at 3.358 when I started the top balance. By my calculation that was over 90% charged.

 

[hankcurt]

You can't get a valid resistance measurement on an energized circuit.

Your ohm meter works by supplying a small amount of current and then measuring the voltage drop that it creates. If you try to measure an energized circuit with an ohm meter, it is going to see the voltage drop from the external current that is running through the circuit and give a false reading. At worst, if the voltage is too high, it can destroy the meter.

The only way to measure resistance on an energized circuit is to use your meter to measure voltage drop and current separately and then calculate the resistance. ( Ohms = volts / amps )

 

[hammick]

You can't get a valid resistance measurement on an energized circuit.

Your ohm meter works by supplying a small amount of current and then measuring the voltage drop that it creates. If you try to measure an energized circuit with an ohm meter, it is going to see the voltage drop from the external current that is running through the circuit and give a false reading. At worst, if the voltage is too high, it can destroy the meter.

The only way to measure resistance on an energized circuit is to use your meter to measure voltage drop and current separately and then calculate the resistance. ( Ohms = volts / amps )

Thanks. That makes a lot of sense and explains why my resistance readings were all over the place. I did remove the charger and observed that the resistance is higher on the negatives which is all pipe hangar. Does having more resistance on the negative parallel path cause issues or just slow the balancing down a little? I will be using nice buss bars capable of handling 280 amps when they are installed.

UPDATE: Actually I give up on the resistance testing. With the charger disconnected I am getting different readings depending on which leads I use. Thankfully both sets of leads show the same voltage.

 

[hankcurt]

There are other people here that have more experience with balancing cells that might have better advice than I do.

However, the one thing that I see is that you should invert your negative buss bar assembly. If you trace out the length of the current path for the cells on the top of the stack, it is shorter than the length of the current path for the cells at the bottom of the stack. Thus there will be more bus bar voltage drop for the bottom cells, and they will get less current from your charger than the top cells. If you flip the negative assembly over so that it is fed from the bottom, the current path is the same length for all of the cells. This will help them arrive at the same voltage at the same time.

 

[BarkingSpider]

If you have some 1/2" or 3/4" copper pipe in your garage, flatten it in a vice or even beat it flat with a lump hammer, drill holes for the battery terminals and brighten it up with some wire wool (or coca cola!). Not the prettiest, but very effective bus bars.

 

[hammick]

There are other people here that have more experience with balancing cells that might have better advice than I do.

However, the one thing that I see is that you should invert your negative buss bar assembly. If you trace out the length of the current path for the cells on the top of the stack, it is shorter than the length of the current path for the cells at the bottom of the stack. Thus there will be more bus bar voltage drop for the bottom cells, and they will get less current from your charger than the top cells. If you flip the negative assembly over so that it is fed from the bottom, the current path is the same length for all of the cells. This will help them arrive at the same voltage at the same time.

Thanks. I believe you are correct. The positive and negative connections should be opposite. I had just enough pipe tape left to do another run on the bottom. I can cut off the top negative run but I figure the negatives are using no buss bars so I might as well leave it. Hopefully my voltages start coming up.

As far as buss bars I don't have time to make any right now. I need to have this top balance done no later than Wed so I can pack it all up and take it to our off grid place.

 

[hammick]

I am over four days into this top balance and I am not making much progress. Individual cells are between 3.372 and 3.385 volts. Pack voltage is at 3.40 when charging. Has it taken this long for any of you guys when using the 10 amp power supply?

 

[Bud Martin]

It took me almost two days to top balance my four 90Ah batteries to 3.6V, the batteries Voltage were about 3.28V on the batteries when I first receive them. I top balance at 5A. So for that many 280Ah batterires it will take a long time, I will find out how long when I get my four 272Ah batteries this month.
BTW, you should use Ohm mode instead of Continuity mode when trying to measure resistance, and your probe leads also have resistance so when you try to measure very very low resistance you need to subtract the probe leads resistance from the reading unless your meter has ZERO function to zero out the probe resistance.

 

[hankcurt]

I believe you ordered the 272 ah cells and have 16 of them in parallel. They should be shipped with no more than 50% charge, and preferably no more than 30% charge.

The pack total amp hours in parallel is 16 x 272 = 4352 ah. If you have to add 50% of that, you must add 2176 ah. At 10 amps, that will take 217.6 hours, which is a little over 9 days.

 

[hammick]

I believe you ordered the 272 ah cells and have 16 of them in parallel. They should be shipped with no more than 50% charge, and preferably no more than 30% charge.

The pack total amp hours in parallel is 16 x 272 = 4352 ah. If you have to add 50% of that, you must add 2176 ah. At 10 amps, that will take 217.6 hours, which is a little over 9 days.

That is correct. However I charged these batteries up in 4s series packs using a 40 amp automotive charger. The resting voltage of the 16s pack in parallel before I started the top balance was 3.358 volts. I can't remember exactly how long the batteries rested but it was over a couple hours.

 

[hankcurt]

Without knowing the precise starting charge, it is impossible to know when it will finish.
Reading posts like this one reinforces the importance of patience and also just how quickly the voltage will rise at the end.

 

[hammick]

Without knowing the precise starting charge, it is impossible to know when it will finish.
Reading posts like this one reinforces the importance of patience and also just how quickly the voltage will rise at the end.

Thank you. I hadn't read that one. I going now to put my power supply into CV mode at 3.65 I think I would cry if I bloated my cells after all this time invested.

Also I forgot to mention that I have been setting the power supply to CV mode at 3.5 volts before I go to bed so the amps have been lower overnight. I am hoping tomorrow is the magic day.

 

[Bud Martin]

That is what did too, I set it to 3.5v first then once they reach that point then I turn it up to 3.6V and keep an eye on it every hour.

 

[hammick]

Let's try this again a little less Redneck this time. I think I was getting a ton of voltage drop with the plumbers tape. Used copper plumbers tape this time and voltages are coming up nicely. Night and day actually from my last attempt. All batteries were at 3.34v when I started this afternoon and I should be balanced at 3.5v soon. I'll then take it up to 3.6v and call it good. You can get the copper plumbers tape at Lowes.

 

[hammick]

So I completed my top balance at 3.6 volts. They were all reading the same voltage when the amps went to zero. I let them rest for about a half hour and then connected them in series (16s). I let them rest overnight for about 10 hours. I then put a 3 amp load on them for about thirty seconds and let them rest for six hours. My resting voltages (listed below) are all over the place. Difference between highest and lowest cell is .068 volts. Is this normal for fully charged batteries?

1. 3.524
2. 3.522
3. 3.519
4. 3.459
5. 3.476
6. 3.527
7. 3.488
8. 3.514
9. 3.534
10. 3.496
11. 3.516
12. 3.471
13. 3.490
14. 3.512
15. 3.519
16. 3.466

 

[hammick]

I pulled 9 amps out of them and let them rest for 1.5 hours. All cells reading between 3.347 - 3.349 volts. I'm going to check again at 85% SOC but things are looking good. I'll never charge these above 85% unless doing a maintenance top charge.

 

[Bud Martin]

Hello, I am still new and learning about the LiFePO4. I will be getting my 280aH soon, So what is the acceptable Voltage variation between cells after balancing, during dis charge, etc? <50mV? 40, 30, etc. Thanks.

 

[hammick]

Hello, I am still new and learning about the LiFePO4. I will be getting my 280aH soon, So what is the acceptable Voltage variation between cells after balancing, during dis charge, etc? <50mV? 40, 30, etc. Thanks.

I'm not expert but I started discharging my 16s pack this afternoon. Discharging at 7ah and all cells are between 3.327 - 3.329 volts with 23ah removed as I type this.

Mine are the Lischen 272ah cells but the one I tested gave over 285ah.

 

[Bud Martin]

Thanks for the replay. 272aH Lishen are also the cells I order from Basen. So far I did not find what the accepatable Voltage difference should be acceptable, hope some expert will chime in.
BTW, what capacity tester did you use? I just receive one with big fan like Will has on his video today.

 

[hammick]

Thanks for the replay. 272aH Lishen are also the cells I order from Basen. So far I did not find what the accepatable Voltage difference should be acceptable, hope some expert will chime in.
BTW, what capacity tester did you use? I just receive one with big fan like Will has on his video today.

An expensive one but I'm returning it. It is supposed to discharge up to 40 amps. On a single cell it would only do 34 amps. On my 16s pack it will only do 7 amps.

Battery capacity tester