battery cell voltage 3.5 vs 3.3

[jvbutter01]

according to my BMS after sitting on charger for almost 2 weeks now, i continue to have #4 cell (on pos output of battery) at 3.5v. The #1,2,3 cells are 3.33 , 3.33, 3.35. The Victron Energy BMV-712 Smart Battery Monitor, shows it NOT charging. I assume this is due to bms shut off from cell 4 at 3.56v. BMS setting is
Overvoltage 3.65v
Over voltage release 3.55
Pack over voltage 14.6v
Pack over voltage release 14.2v

I do have small scale power tester. Same unit WillP uses..

8 In 1 150w 180w Digital Battery Capacity Tester Voltmeter Adjustable Constant Current Electronic Load Charger Meter Indicator - Buy Digital Battery Capacity Tester,Usb Meter Indicator,External Battery Charger Product on Alibaba.com

8 In 1 150w 180w Digital Battery Capacity Tester Voltmeter Adjustable Constant Current Electronic Load Charger Meter Indicator - Buy Digital Battery Capacity Tester,Usb Meter Indicator,External Battery Charger Product on Alibaba.com

www.alibaba.com

I used this 1x to drain down the battery voltage in that cell to 3.3 as the others, hooked back up is series config battery, and put back on charger. I know the battery charger PD9145AL will give a boost charge to 14.4v every 15min / 21hr cycle.
I feel there is some capacity potential there left on the table. I will use this in my RV and would like to use all the capacity i can for this type charger.
What are my options for getting this single cell down enough to get all 4 batteries to charge up to 3.5 max? Do I need to tear down the battery into parallel cells again, set them back on the smaller 12v/10a charger?

 

[Steve_S]

Well, I would start with obvious things first (a case of BTDT).

~ WARNING ~
DISCONNECT the BMS from the BMS HARNESS before furtling about with the harness, connectors & disconnecting/reconnecting busbars ! THAT is the easiest way to fry your BMS and make once in a lifetime Magic Smoke appear.

Check the busbars attached to C4, ensure they are Clean, have no burrs or ridges factory stamped BBars usually have a slight ridge on the "exit side" of the hole when it was stamped out. That can & will mess with reading cell voltages & IR.

Ensure the contact faces on the cell terminals are also clean, free of wax, oils etc, even a light scuff with emergy cloth (super fine only) and then wipe down with rubbing alcohol, wipe down the busbars too. The side that makes contact with the cells may also benefit from alight emery cloth sanding. (you can also use Wet/Dry (black) sandpaper 400 grit or finer up to 1000 grit.

Also verify the crimped connector contact on the cell for the BMS Lead, even a little loose can throw off readings. (That's bitten me a couple of times, I now crimp & solder the darn things,( Rule fo 3 applied, I hate getting to 4 )

When reassembling the busbars onto the cells, you can use a "wee dab" of NoAlox / OxGuard onto the "contact surface patch" of the busbar which sits on the cell. A very very thin light wipe is all that is needed, and NOT on the screw threads or in the screw hole. Then tighten down the screws (be careful the aluminium is soft and you can strip the threads). IF using Grub Screws, spin them down to the bottom of the hole, then back out 1-1/2 turns, hold tight with the Allen Key as you use a small wrench to tighten down the nut, while not allowing the screw to go deeper (*1). Recommended Option as this is in an RV and mobile and subject to the issues with that, using Lock Washers is good BUT, I have found that Serrated Washers actually work better because they grab all around. Alternately (not the best) you can use a very small drop of Loctite on the BUT THREAD only, you cannot let it get into the cell threads... NOTE that Loctite is NOT dielectric, meaning it will not pass electricity. Another alternate (this one works good actually) is once everything is tightened down and all tested as good a drop of "Nail Polish" at the top of the threads @ the nut will prevent the but from backing off. (Shadetree mechanic fix which works for light stuff).

(*1) If you break through the top of the cell, it's garbage ! Easy to do and one Must avoid that.

Once assembled & before the nuts are sealed with loctite or such if you decide you want to use it. Do a High Amp Charge and watch temps, if anything warms up, that is the result of resistance & likely a poor connection. Bus bar may warm, a single cell itself can also do so. Retighten and verify (no power going in when you do) then try again once everything is all cool. Last test, a heavy discharge for 15 minutes, again to verify nothing get's warm

You did not mention the cells you are working with. IF they are commodity cells (not premium priced Matched/Batched & Binned) their tolerances at the bottom & top vary. It is quite common for them to top out at 3.55V and to settle between 3.50-3.4V when they rest post charging. The cells will diverge starting from 3.35V but will diverge significantly at higher voltages. 1mv per Amp Hour of storage capacity is not unusual, which means for 100AH cells you can expect to see 100mv divergence... Such incidences & observations have led to the interest in Active Balancing systems for some folks (not to be confused with Passive Balancing which only burns off voltage from hi cells, where Active transfers voltage from Hi cells to Lo Cells).


This is likely helpful for you:
General LiFePO4 (LFP) Voltage to SOC charts/tables 12/24/48V (diysolarforum.com)

Hope it Helps, Good Luck
Steve

 

[jvbutter01]

@Steve_S Thanks for the info... tearing down now.. and sure @#$% there are some rough edges to the factory buss bars... got some 1/8x1 copper bar to make my own, but I figured I could use the provided bars for the beginning steps. Well maybe not, yes the provided bars do have burs... I will keep working through your steps and update later. i leave for a week at the parents so it will be a few days until i have update. thanks for your detailed information.

 

[GSXR1000]

Did you too balance the cell with all in parasol before u assembled the pack?

@Steve_S Thanks for the info... tearing down now.. and sure @#$% there are some rough edges to the factory buss bars... got some 1/8x1 copper bar to make my own, but I figured I could use the provided bars for the beginning steps. Well maybe not, yes the provided bars do have burs... I will keep working through your steps and update later. i leave for a week at the parents so it will be a few days until i have update. thanks for your detailed information.

 

[jvbutter01]

I did do a top balance, but could only get to 3.4v with the charger.

 

[GSXR1000]

I did do a top balance, but could only get to 3.4v with the charger.

Did current go-to 0 amps?
Was it a charger or constant voltage power supply?

 

[jvbutter01]

That is a great question...The amazon only says...

• ✔ MULTI-PROTECTION: Over Voltage Protection; Over Current Protection; Over Temperature Protection; Over Power Protection; Auto Constant Current & Voltage; Professional Shipping Protection; CE Certified.✔ EFFECTIVE COOLING: Whole Side Cooling Design; Large Temperature Control Cooling Fan.

 

[GSXR1000]

That is a great question...The amazon only says...

• ✔ MULTI-PROTECTION: Over Voltage Protection; Over Current Protection; Over Temperature Protection; Over Power Protection; Auto Constant Current & Voltage; Professional Shipping Protection; CE Certified.✔ EFFECTIVE COOLING: Whole Side Cooling Design; Large Temperature Control Cooling Fan.

Did you set it to 3.65 volts while all cells are in parallel ? Let it run until current went to 0. It could take days even a week

 

[jvbutter01]

yes originally it had run for 2 weeks, 10 a max charge on a 280ah battery will teak time. I took off all only had 1 on the charger, and in less than 6hr it ballooned up. so i have stopped. From what i have read, i need to series them, connect to standard charger at 45a and charge them up. Then pull them apart, and parallel them, then put on small bench charger to top off. I just don see why the single sell jumps up to 3.5v and BMS shuts off it off. I know the bms should, but why the single cell goes to 3.5 after being at 3.3 before hooking back in series. I will put another cell in its place and see what happens. Also considering putting in in different position

 

[Steve_S]

yes originally it had run for 2 weeks, 10 a max charge on a 280ah battery will teak time. I took off all only had 1 on the charger, and in less than 6hr it ballooned up. so i have stopped. From what i have read, i need to series them, connect to standard charger at 45a and charge them up. Then pull them apart, and parallel them, then put on small bench charger to top off. I just don see why the single sell jumps up to 3.5v and BMS shuts off it off. I know the bms should, but why the single cell goes to 3.5 after being at 3.3 before hooking back in series. I will put another cell in its place and see what happens. Also considering putting in in different position

Quick & Dirty answer, these unmatched cells have different Internal Resistances which impacts how they will take a charge or how they'll discharge (voltage dip under load is the indicator for that). These cells seem to really have a preference to settle out at 3.5V (from observing my own cells as well as the feedback from others here). Fortunately between 3.50-3.65 it's a steep rise & therefore not that many AH in the grand scheme of things to give up.... And besides, it's within that 10% off the top safety margin anyways.

As mentioned before, I have been Thrash Testing this past week... Boy that really get's the cells to "show their colours" ohh boy ! Seeing some pretty wild deviations and differentials pending on what I am doing Loads or charging. One enlightening test was to have only One Pack live and pushing it with 200A draws and then 150A Charging, Holy Macaroni ! That really tests the internal IR and it is quite noticeable. Even after my "abusive testing" the packs all like to charge up and they settle & sit at 3.45V till the cattle wander by. Luckily the Active Balancers level them up quite nicely.

The consequence which I don't like... I have two cells (one per 280AH pack) that are slightly weaker than the rest, and I have 2, 175AH in my other packs which have also shown issues... So I am also running through a "process" and am waiting on my TekPower TP1540E DC Adjustable Switching Power Supply 15V/40A Digital Display, to arrive which I will use to "medic" the weaklings individually.... I'm not tearing down the assembled packs now, only if I absolutely have to as the packs are installed onto the rack in the powerhouse, which would make that a major adventure for me. I figure that 3.60V at 30-35A will top the cranky cells nicely and then I'll trickle them up, only doing 1 cell at a time. Darn shame it's not here yet, as I am snowed in for at least 2 more days, the couriers can't even get here ATM. LMAO... I wanted Rural/Remote, I got it ! dang inconveniences...

For anyone interested, link to the Powersupply:
Amazon.com: TekPower TP1540E DC Adjustable Switching Power Supply 15V 40A Digital Display

 

[jvbutter01]

Thanks @Steve_S for your info. Yes I figured like anything, you can have 10 of something, and 1-2 will be superior, and 1-2 will be weaker... manufacturing isn't 100% perfect all the time. I don't know what is worse, 1 above the wrest, or 1 below the rest. If its above, you will cut off potential to the others when charging up. if its below the rest, then you are getting cut off sooner. BMS will stop the risk of over or under charge. I'm away from home now. When I get back, i will see about getting the other 3 charge up closer to 3.5 individually charging. Maybe get it up closer to the single 3.5 and then series them back up and try discharge / charge cycle and see what happens. Maybe I will get your new PS on order, this 40a will get the cells charged up quicker then my little 10a charger?

Current bench charger: TUFFIOM DC Power Supply Variable 0-10A/0-30V​

Would it be worth the price? yours putting out 4x the amps to charge them up quicker? and get the 3 other cells up to 3.5 to match. I can't use my RV 40 charger,

Progressive Dynamics PD9145ALV 12V​

it only works in 12v setup. Not individual 3.2v cells. thoughts?

 

[Steve_S]

Well, in my particular case, I chose the 15V / 40A capable to give myself some flexibility. My installation (no grid here) forces that as a requirement. So if I have to use my small Inverter Genny, or my big guy, or run off my FLA Bank->Inverter->Variable Power supply->LFP Cell, I've gotta be ready to do so. Also, TBH, I haven't the patience to babysit cells charging for long hours with a darn genny running, so if I can push an individual cell at 40A till it saturates @ 3.50-3.55 and holds it above 3.45 I'd be tickled pink. Ideally I want to have all my packs living between 3.1V to 3.4 which is the main power curve anyways.

There always has to be a screwball in the works... You know, Murphy's Laws at work... 97% of the time it can be worked around. Fortunately, 100% is unnecessary unless your completely OCD and can't help it. LOL

 

[jvbutter01]

nope, not OCD, but I am tired of waiting for this 280 to fill up 5ah at a time... its a little annoying. I'm on the fence anyway about getting 8 more. Not like I need them now, but I think will expand my 2 x 4cell 12v, parallel setup to a 2 x 8cell 24v pack for the TT. I have a 20ft run to make front of trailer to back for inverter, so it would be better. good deals to be had right now. So I could use the better charger on those also. its just that $189 price tag.

 

[Solarfun4jim]

Quick & Dirty answer, these unmatched cells have different Internal Resistances which impacts how they will take a charge or how they'll discharge (voltage dip under load is the indicator for that). These cells seem to really have a preference to settle out at 3.5V (from observing my own cells as well as the feedback from others here). Fortunately between 3.50-3.65 it's a steep rise & therefore not that many AH in the grand scheme of things to give up.... And besides, it's within that 10% off the top safety margin anyways.

As mentioned before, I have been Thrash Testing this past week... Boy that really get's the cells to "show their colours" ohh boy ! Seeing some pretty wild deviations and differentials pending on what I am doing Loads or charging. One enlightening test was to have only One Pack live and pushing it with 200A draws and then 150A Charging, Holy Macaroni ! That really tests the internal IR and it is quite noticeable. Even after my "abusive testing" the packs all like to charge up and they settle & sit at 3.45V till the cattle wander by. Luckily the Active Balancers level them up quite nicely.

The consequence which I don't like... I have two cells (one per 280AH pack) that are slightly weaker than the rest, and I have 2, 175AH in my other packs which have also shown issues... So I am also running through a "process" and am waiting on my TekPower TP1540E DC Adjustable Switching Power Supply 15V/40A Digital Display, to arrive which I will use to "medic" the weaklings individually.... I'm not tearing down the assembled packs now, only if I absolutely have to as the packs are installed onto the rack in the powerhouse, which would make that a major adventure for me. I figure that 3.60V at 30-35A will top the cranky cells nicely and then I'll trickle them up, only doing 1 cell at a time. Darn shame it's not here yet, as I am snowed in for at least 2 more days, the couriers can't even get here ATM. LMAO... I wanted Rural/Remote, I got it ! dang inconveniences...

For anyone interested, link to the Powersupply:
Amazon.com: TekPower TP1540E DC Adjustable Switching Power Supply 15V 40A Digital Display

All this info your imparting steve, whilst putting these cells through their paces is highly beneficial. My 8 xuba 280's on a 2nd top balance to 3.65v settled thereafter at 3.58v. On series connection again, after a capacity test and recharge, i noticed the cells start to diverge right at 3.43v/cell. Up to that point they had remained right within 10mv. Charge rate was only 0.1C though. I'm thinking that, in general, my solar charge rate will be around the 0.1C and my max discharge rate just shy of 0.5C (more likely 0.2C most of the time), so if only charging at 3.4V and discharging to 3.1V , i might as well leave the balancing off altogether.
Do you have any 'feel' for the degree of cell drift when they aren't pushed hard....might they stay balanced for years?
Is all your experimentation on these cells with regards to pushing them hard?
It is great to get this 'real life experience' on these cells....so thanks for that.

 

[Steve_S]

Well... insider info, if you want more 280's at current price, better order soon, Amy is down to <700 pcs and they are flying out the door fast... the next batch will be at a higher price due to EVE pricing change (she was just at EVE corporate negotiating the next batch). I don't know by how much per cell price will increase but they will be going up. She's als just made a deal with a new ship[ping broker with better options & prices...
~ Unofficial Amy is @ Luyuan (she's no longer got anything to do with Xuba.) She & her husband took over Luyuan.
Shenzhen Luyuan Technology Co., Ltd. - Lifepo4 battery, LTO battery (alibaba.com)

Yes, I am seeing similar that they really do prefer to settle & sit around 3.45 +/- a bit. But I expect that may resolve a bit better once I update the BMS Firmware which Jason just issued a few hours ago.

My configuration is to be able to handle 200A draw off 24V. I used everything rated to 300A just so I'm covered for surges as my Inverter can draw up to 500A surge for 12,000W. I've ever hit 300A even with my Mig Welder plugged in or even my compressor (but those two are too rude to run off solar, really... they are far from polite).

The standing "drift" is minimal from what I see BUT I am live 24/7/365 so there is always something going on with the batteries, they rarely sit in "storage mode" as such.

One point that is rarely mentioned by anyone, I haven't any idea why it's just not mentioned...
The battery voltage drop under load is often unconsidered.
Example: I just made my 1st pot of coffee (yeah, 03:00 hrs I keep very odd hours)
Battery Bank (LFP) Voltage was sitting at 26.2V with 10A load (computer & communications systems routers, hub, sat modem, and my fridge)
Turned on Coffee maker, add 66A load and with 76A pull the battery voltage dropped to 25.2 while perking for 6 minutes (it's all it takes).
Within 2 minutes post load they settled back to 26.2V.

As charge get's lower within the battery bank, that dip can actually be larger while under loads, if low enough at rest, the dip can actually result in a Low Volt Disconnect on the inverter or EVEN THE BMS can trip on low volt if "one" cells tickles the trigger threshold. (Man that is a PITA ! ) but with more than one pack in a bank that absorbs most of these sorts of things as the packs share out the load when properly configured. To that end my Type-A Virgo Personality kicked into over drive and all my wires are length matched to within 2mm from battery pack to common DC Bus. Crazy EH ! but it works... LOL

A NOTE / REMINDER... I am also running all my packs with QNBBM-8S Active Balancers, with Chargery BMS8T's with their passive balancing OFF. When it comes to the top side of balance etc that has an effect. So Far I can say it works a TREAT and keeps things very nicely topped off and level. So far I am pleased with these units... once the new PS arrives and I can re-top my cells individually as I have really thrashed them and they can benefit from some love (and give me piece of mind too), I'm going to also try a few other little tricks I'm pondering.

The QNBBM-8S is $144 USD + S&H etc. if curious.
China 8s 24V LiFePO4 Lithium Ion Lead Acid Lto AGM Gel Flooded 18650 Battery Equalizers - China Lithium Ion Battery Equalizer, 8s Battery Equalizer (made-in-china.com)

 

[Solarfun4jim]

Well... insider info, if you want more 280's at current price, better order soon, Amy is down to <700 pcs and they are flying out the door fast... the next batch will be at a higher price due to EVE pricing change (she was just at EVE corporate negotiating the next batch). I don't know by how much per cell price will increase but they will be going up. She's als just made a deal with a new ship[ping broker with better options & prices...
~ Unofficial Amy is @ Luyuan (she's no longer got anything to do with Xuba.) She & her husband took over Luyuan.
Shenzhen Luyuan Technology Co., Ltd. - Lifepo4 battery, LTO battery (alibaba.com)

Yes, I am seeing similar that they really do prefer to settle & sit around 3.45 +/- a bit. But I expect that may resolve a bit better once I update the BMS Firmware which Jason just issued a few hours ago.

My configuration is to be able to handle 200A draw off 24V. I used everything rated to 300A just so I'm covered for surges as my Inverter can draw up to 500A surge for 12,000W. I've ever hit 300A even with my Mig Welder plugged in or even my compressor (but those two are too rude to run off solar, really... they are far from polite).

The standing "drift" is minimal from what I see BUT I am live 24/7/365 so there is always something going on with the batteries, they rarely sit in "storage mode" as such.

One point that is rarely mentioned by anyone, I haven't any idea why it's just not mentioned...
The battery voltage drop under load is often unconsidered.
Example: I just made my 1st pot of coffee (yeah, 03:00 hrs I keep very odd hours)
Battery Bank (LFP) Voltage was sitting at 26.2V with 10A load (computer & communications systems routers, hub, sat modem, and my fridge)
Turned on Coffee maker, add 66A load and with 76A pull the battery voltage dropped to 25.2 while perking for 6 minutes (it's all it takes).
Within 2 minutes post load they settled back to 26.2V.

As charge get's lower within the battery bank, that dip can actually be larger while under loads, if low enough at rest, the dip can actually result in a Low Volt Disconnect on the inverter or EVEN THE BMS can trip on low volt if "one" cells tickles the trigger threshold. (Man that is a PITA ! ) but with more than one pack in a bank that absorbs most of these sorts of things as the packs share out the load when properly configured. To that end my Type-A Virgo Personality kicked into over drive and all my wires are length matched to within 2mm from battery pack to common DC Bus. Crazy EH ! but it works... LOL

A NOTE / REMINDER... I am also running all my packs with QNBBM-8S Active Balancers, with Chargery BMS8T's with their passive balancing OFF. When it comes to the top side of balance etc that has an effect. So Far I can say it works a TREAT and keeps things very nicely topped off and level. So far I am pleased with these units... once the new PS arrives and I can re-top my cells individually as I have really thrashed them and they can benefit from some love (and give me piece of mind too), I'm going to also try a few other little tricks I'm pondering.

The QNBBM-8S is $144 USD + S&H etc. if curious.
China 8s 24V LiFePO4 Lithium Ion Lead Acid Lto AGM Gel Flooded 18650 Battery Equalizers - China Lithium Ion Battery Equalizer, 8s Battery Equalizer (made-in-china.com)

Steve,
Great points made as regards the LVD under loads when at a low SOC....i hadn't considered that. Has me thinking i might set my victron multi at 25.6v(3.2v/cell) and the chargery at 3.0v (will just need to see how much capacity i can get from a 3.4 - 3.2v range).