new to all of this, but trying to balance some new eve 280 ah cells (8s), while in series...(i just don't want to disassemble everything to put them in parallel since they are built into a compression jig in an awkward box...). the JBD bms is trying to balance things above 3.4v but one cell is higher than the rest. i tried charging the lowest cell individually (while still wired in series) up to the median with a small benchtop charger which worked well. but how to bring down the highest? bridge that cell with a resister individually? how big a resistor? 3.2v, 280 ah cell, it's at 3.45v, everything else is at 3.375. or run a small device off it??? what runs on 3.5 v? flashlight bulb? i'll try letting the bms finish the job once i get everything more or less close together. or will i just be chasing my tail once everything gets a bit higher? thanks for help/suggestions
alternative series top balancing methods???
- Thread starter chemosabe
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BentleyJ
Solar Wizard
I can understand why you don't want to take everything apart just to do a top balance, lot of work that may need to be repeated a few times over the years. Letting the BMS balance the cells while the battery is in Float charge mode is really the easiest way when the cells are in series. It does take a long time especially with 280Ah cells. JK BMS sells a 5A and even a 10A active balancer, Bluetooth. I have one of each but they are a bit expensive, $300 & $500. They come with a set of cables with alligator clips for attaching to the battery posts temporarily. The 10A version would sure speed things up if your budget can handle it. I'm finding that my 4 year old 48V, LFP DIY battery needs top balancing about every 5-6 months and its different cells each time. Just today I was float charging at 54.8V (3.425V per cell) and 2 of the cells were reading 3.7+ volts. The 2 amp active balancer will eventually get the job done but tomorrow I'm going to connect the big balancer.
Samsonite801
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When I assembled my six 4s batteries together and started using it, I didn't have time to do a final top balance (or I might've missed one of the packs when I did top balance), so I had one pack that had a cell off voltage (high from the rest).. It stayed like that for several days and didn't balance itself.
I did a search on Google and found that people had luck enabling 'balance on discharge and charge' as well (instead of only 'balance on charge'), so I tried it, and within 2 days it balanced out on its own. Then if you want, you can turn it back to only 'balance on charge'. I just left mine on both ways, and it seems fine. I may turn it off at some point but don't really see any side effects from having it on in my case since I have plenty of capacity.
Another way to get faster balancing if needed, is to add in a JK Active Balancer, like they make 1a, 2a, 4a, 5a, 10a, etc in those. I have a couple of 2a JK Active Balancers I tried with two 16s packs and they balanced the pack out pretty quickly.
Some people may say that's not the right way to do it, but I don't care, it worked...
I did a search on Google and found that people had luck enabling 'balance on discharge and charge' as well (instead of only 'balance on charge'), so I tried it, and within 2 days it balanced out on its own. Then if you want, you can turn it back to only 'balance on charge'. I just left mine on both ways, and it seems fine. I may turn it off at some point but don't really see any side effects from having it on in my case since I have plenty of capacity.
Another way to get faster balancing if needed, is to add in a JK Active Balancer, like they make 1a, 2a, 4a, 5a, 10a, etc in those. I have a couple of 2a JK Active Balancers I tried with two 16s packs and they balanced the pack out pretty quickly.
Some people may say that's not the right way to do it, but I don't care, it worked...
Last edited:
RoySalisbury
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I have the same issue. I am currently top balaicing each cell in the pack one at a time with a desktop charger (to 3.60v down to about 0.2amp). It's taking awhile, but I'm hoping it finally gets them in sync.
I've ordered an active balancer, and had I known these issues ahead of time (I know, there here in the forum), I would have got the JK BMS and not the JBD BMS. Biggest thing for me was that I could only find the JBD ones (Overkill) in stock in the US. Everything else was weeks on a boat from China.
I've ordered an active balancer, and had I known these issues ahead of time (I know, there here in the forum), I would have got the JK BMS and not the JBD BMS. Biggest thing for me was that I could only find the JBD ones (Overkill) in stock in the US. Everything else was weeks on a boat from China.
Ampster
Renewable Energy Hobbyist
You don't need big Ohms but if you goes below 5 Ohms you need big Wattage resistor. The lower the Ohms the faster one cell will drain.
mikefitz
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A 12v car headlamp bulb will work well as a load. Make up some suitable leads and manually connect across the high cell (whilst the battery is under charge) for several seconds whilst observing the cell voltage.
With a little practice the discharge duration estimate becomes easier. If things are happening too fast lower the charge volts.
The 'high cell' will vary in the pack so you will need to move the bulb load from cell to cell as needed.
Mike
With a little practice the discharge duration estimate becomes easier. If things are happening too fast lower the charge volts.
The 'high cell' will vary in the pack so you will need to move the bulb load from cell to cell as needed.
Mike
last (stupid?) idea- can i connect my small benchtop charger in reverse polarity to drive the voltage down on the highest cell? say overdrive the cell voltage by a very small amount in reverse (while it's still wired in series in the pack)???? trying to avoid more purchases... thanks thanks thanks
BentleyJ
Solar Wizard
Well if a zero resistance connection between + and - results in a dead short, what will happen when there is reverse (negative) polarity applied to the battery? Sounds a lot like pouring gasoline on a fire.
Ampster
Renewable Energy Hobbyist
Yes you can until it lets out the smoke like mine did. Some have protection so nothing will happen. After that experiment, I used a big resistor. It has to be done at near the top to get best results.
mikefitz
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The bench top charger in reverse is plain stupid.
Use the car headlight bulb as I suggested, it's safer than a fixed resistor and more effective due to the non linear resistance.
The worst case is, it lights up, the fact that it illuminates gives a good indication of making a good contact with the buss bars.
The obsessive aim to get cell balance within a few mV at high charge voltages is not needed for a practical battery in many applications.
Mike
Use the car headlight bulb as I suggested, it's safer than a fixed resistor and more effective due to the non linear resistance.
The worst case is, it lights up, the fact that it illuminates gives a good indication of making a good contact with the buss bars.
The obsessive aim to get cell balance within a few mV at high charge voltages is not needed for a practical battery in many applications.
Mike
Hedges
I See Electromagnetic Fields!
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A four-quadrant power supply set to target voltage could do it.
What if you set up a one-quadrant CV/CC supply, put a resistor across it so it operates in two quadrants, connect through a resistor to high cell?
Cell discharges through resistor, tapering to target voltage.
More reasonable approach is just put a volt meter across cell, connect a resistor, and bleed off charge, checking occasionally.
Unlike charging near-full to excessive voltage and having it run away, this discharge is starting from up the curve to down in the flat area. So plenty of time to take a break and not miss the action.
In general, though, series charging with BMS until high voltage disconnect followed by individual cell charge CV/CC seems good, never having to break apart the pack.
What if you set up a one-quadrant CV/CC supply, put a resistor across it so it operates in two quadrants, connect through a resistor to high cell?
Cell discharges through resistor, tapering to target voltage.
More reasonable approach is just put a volt meter across cell, connect a resistor, and bleed off charge, checking occasionally.
Unlike charging near-full to excessive voltage and having it run away, this discharge is starting from up the curve to down in the flat area. So plenty of time to take a break and not miss the action.
In general, though, series charging with BMS until high voltage disconnect followed by individual cell charge CV/CC seems good, never having to break apart the pack.
thanks for saving me from stupid ideas. and thanks for limiting my purchases (like a new benchtop charger ; ) ) i will definitely try the headlight bulb - just have to figure out how to get it out of my car. and thanks for the suggestion about balance on discharge. will try that too.
