Balancing question - how long?

[stevelex]

I've got 8 200ah lifepo4 cells. They are in parallel for top balancing.

I've got a power supply that is pushing amps. It says it is pushing 10 amps @ 3.44 volts (I set a limit of 3.65 volts). That's 34 watts approx.

It's been running for almost 3 days with very small increases in the voltage (It started at 3.37 volts).

Even if the cells were 50% depleted, that's 2400 wh. At 34 watts, that's 70 hours.

Am I to assume the batteries were more than 50% depleted?

How long will this take?

This is the power supply I have: https://smile.amazon.com/gp/product/B087TK6ZM2/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

 

[snoobler]

This is very common.

8 * 200Ah = 1600Ah

Cells tend to ship at 50%, so 800Ah

800/10 = 80 hours is the likely minimum to achieve balance.

The vast majority of the charge occurs in a very very very narrow voltage range.

The spike at the end can occur over the course of just a few hours.

Important:

1. You've read the top balancing guide in Resources and are following its recommendations and are not following best practice by choice or due to a lack of BMS on hand.
2. You set the PS voltage BEFORE the pack was connected.
3. You're looking at CELL voltage, not the voltage reported by the power supply.

There will be a notable difference between cell voltage and supply voltage due to voltage drop in the leads and in the power supply itself. You will likely see the PS voltage hit 3.65V and start tapering current well before the cells hit 3.65V.

 

[stevelex]

This is very common.

8 * 200Ah = 1600Ah

Cells tend to ship at 50%, so 800Ah

800/10 = 80 hours is the likely minimum to achieve balance.

The vast majority of the charge occurs in a very very very narrow voltage range.

The spike at the end can occur over the course of just a few hours.

Important:

1. You've read the top balancing guide in Resources and are following its recommendations and are not following best practice by choice or due to a lack of BMS on hand.
2. You set the PS voltage BEFORE the pack was connected.
3. You're looking at CELL voltage, not the voltage reported by the power supply.

There will be a notable difference between cell voltage and supply voltage due to voltage drop in the leads and in the power supply itself. You will likely see the PS voltage hit 3.65V and start tapering current well before the cells hit 3.65V.

Ok, thanks. I'm following will's balancing video exactly. I have a slightly different power supply, but that's it. I even swapped the leads of the power supply for 4 awg to reduce any losses. I have a BMS, but it is not hooked up - it will be hooked up when I assemble the 24v battery from the 8 cells. So it should balance in the next day or so.

 

[RCinFLA]

Also appears your meter is reading high. Based on time you have spent and likelyhood cells started around 50% SOC your initial reading should have been about 3.300v on cells, not 3.370. This assumes the 3.370v was before you applied charging current.

If 70 mV doesn't seem like much, difference between 50% AND 85% SOC is only about 35 mV. Cell voltage begins to rise faster at about 3.38v which is about 90% SOC.

Another common mistake is using cheap leads that ship with power supply having too much voltage drop from power supply to battery. At the least the last 10% takes a long time. At the worst, folks crank up power supply voltage limit to get cell voltage up to 3.65v and end up overvoltage at end of charge.

 

[snoobler]

Another common mistake is using cheap leads that ship with power supply having too much voltage drop from power supply to battery. At the least the last 10% takes a long time. At the worst, folks crank up power supply voltage limit to get cell voltage up to 3.65v and end up overvoltage at end of charge.

personally, I think he has a winner if the unit still reads 3.44V and 10A after nearly 3 days of charging.

 

[snoobler]

Ok, thanks. I'm following will's balancing video exactly. I have a slightly different power supply, but that's it. I even swapped the leads of the power supply for 4 awg to reduce any losses. I have a BMS, but it is not hooked up - it will be hooked up when I assemble the 24v battery from the 8 cells. So it should balance in the next day or so.

The guide is more detailed and up-to-date, vs. the video.

 

[Factory400]

10A at 3.xx volts is rather tiny in relation to a pack this size - 33 watts or so..

My strategy was to series my pack and charge at 10A at 27v or so which is is 270 watts. Looking for any cell jumping ahead of the others, it got me to a higher SOC reasonably quick and then I put them all in parallel for the final push to 3.65v.

My pack is 16x 200Ah and would have taken ages to complete.

 

[snoobler]

10A at 3.xx volts is rather tiny in relation to a pack this size - 33 watts or so..

My strategy was to series my pack and charge at 10A at 27v or so which is is 270 watts. Looking for any cell jumping ahead of the others, it got me to a higher SOC reasonably quick and then I put them all in parallel for the final push to 3.65v.

My pack is 16x 200Ah and would have taken ages to complete.

Which is the "best practices" I mention in the guide above. Series w/BMS first, then parallel to top 'em off.

 

[stevelex]

Also appears your meter is reading high. Based on time you have spent and likelyhood cells started around 50% SOC your initial reading should have been about 3.300v on cells, not 3.370. This assumes the 3.370v was before you applied charging current.

If 70 mV doesn't seem like much, difference between 50% AND 85% SOC is only about 35 mV. Cell voltage begins to rise faster at about 3.38v which is about 90% SOC.

Another common mistake is using cheap leads that ship with power supply having too much voltage drop from power supply to battery. At the least the last 10% takes a long time. At the worst, folks crank up power supply voltage limit to get cell voltage up to 3.65v and end up overvoltage at end of charge.

Yes, actually that was not the meter, that was the power supply. So this is good info. The batteries were in the 3.27 range if I recall, so yea, they were pretty low. They are at 3.33 as of this afternoon. It'll take a while, but I'm not going to touch it. Camper isn't ready anyway. Working on sink and drains. Thx for the info.

 

[snoobler]

Please reassure us that you set the PS to 3.65 prior to connecting to the battery, and 4awg... I think that the thickest I've heard of for this exercise. Good choice.

 

[stevelex]

Please reassure us that you set the PS to 3.65 prior to connecting to the battery, and 4awg... I think that the thickest I've heard of for this exercise. Good choice.

Yes, the max voltage on the DC power supply is set to 3.65v. I used 4awg because I got a bunch from the junkyard so I have a lot sitting around and I have a lot of lugs as well, so...that's what I used. Here's a photo of the charger and wires I took Tuesday morning at 9:30am.

 

[snoobler]

Nice!

The only thing I would suggest is to place the (+) and (-) leads at opposite ends of the battery. That's just a best practices thing that conforms to Victron's "Wiring" guide, link #2 in my sig. It insures all cells see the same resistance back to the source. In a balancing exercise, it doesn't matter much. I try to force best practices to establish good habits as I am a high entropy prone individual.

 

[Factory400]

4AWG is hardcore commitment to WINNING!

I have considered making a simple circuit board for protecting this balancing setup. Essentially a pre-set 3.65 volt relay trigger that disconnects the power supply at a precise voltage. So many of the low cost PSU's are not particularly reliable and the knobs are easy to bump up to a damaging voltage. I had a few similar ones that had scratchy pots that would make the settings a bit dubious.

 

[stevelex]

Nice!

The only thing I would suggest is to place the (+) and (-) leads at opposite ends of the battery. That's just a best practices thing that conforms to Victron's "Wiring" guide, link #2 in my sig. It insures all cells see the same resistance back to the source. In a balancing exercise, it doesn't matter much. I try to force best practices to establish good habits as I am a high entropy prone individual.

That's a good point. Will do.

 

[snoobler]

4AWG is hardcore commitment to WINNING!

I have considered making a simple circuit board for protecting this balancing setup. Essentially a pre-set 3.65 volt relay trigger that disconnects the power supply at a precise voltage. So many of the low cost PSU's are not particularly reliable and the knobs are easy to bump up to a damaging voltage. I had a few similar ones that had scratchy pots that would make the settings a bit dubious.

No doubt. To maintain 10A on many units, it takes setting them to 3.9V or higher.

You thinking a little Arduino with a relay?

 

[Factory400]

No doubt. To maintain 10A on many units, it takes setting them to 3.9V or higher.

You thinking a little Arduino with a relay?

All analog for a protection circuit - no ADC or firmware to mess things up.

It would be a basic comparator that latches a mechanical relay until it is reset by a pushbutton or power cycle. Only a few components, precise and reliable. Perhaps some jumpers to select voltages for a few diffierent chemistries.

I would be nervous hooking up an expensive bank of LFP's and walking away for hours or days only to find that the PSU glitched to 4.1 volts or something.

 

[JoeHam]

I would be nervous hooking up an expensive bank of LFP's and walking away for hours or days only to find that the PSU glitched to 4.1 volts or something.

Just search the forum for “puffed” or “bloated” cells. Only takes minutes of inattention at an inopportune time.

I think there is a real need for a failsafe item like that.

 

[stevelex]

Just an update: On Saturday sometime the charger showed no current and reached the desired voltage. So that was 5 days. Of course, now it's 23 deg F outside and I'm not going to be working on the camper until it warms up, so the cells are sitting here, charged and balanced, in my kitchen, until then.

 

[Nol]

May I ask, did it take 5 days to charge it from 3,44V to full at 3,65V ?
What is the best way to balance : parallel or serie ?
If parallel I have to buy a similair charger but if in serie goes faster or better I can charge then with the 20A Renogy AC-DC charger
For your information my 4 x 280Ah cells are on my way and I already receive a 120A BMS

 

[snoobler]

Nol Resources page. Top balancing guide. Read it and follow it exactly.

 

[Nol]

I have seen and read that but I was curious if it take 5 days to charge it from 3,44v to 3,65v
I can not find an advice if parallel of serie is better of faster

 

[snoobler]

I have seen and read that but I was curious if it take 5 days to charge it from 3,44v to 3,65v
I can not find an advice if parallel of serie is better of faster

Your statement above indicates you have either NOT read the guide, or you do not understand it. Please re-read it carefully.

If you read this thread, your question is answered. Cells started at 3.27 and took 5 days to charge to 3.65 @ 10A with a period of taper at the end.

 

[Nol]

I did read it but maybe not carefully enough so I download it and read it again, thanks and I follow it.
I also going to buy this 10A charger, I don´t know the size of the leads but I will replace them. Is 8 agw good or should I use 6 or 4 ?

 

[snoobler]

Bigger is better. The OP of this thread was successful using 4. The means of connecting to the power supply and the cells is just as important. Needs to be a good, solid connection.

 

[SV_Stray_Cats]

Just an update: On Saturday sometime the charger showed no current and reached the desired voltage. So that was 5 days. Of course, now it's 23 deg F outside and I'm not going to be working on the camper until it warms up, so the cells are sitting here, charged and balanced, in my kitchen, until then.

Drain them back to 40% if you are going to leave them for a while! They will be much more comfortable and won't degrade - that's why they ship them that way.