This thread comes from an Indiegogo discussion: User: anton.fetzer (2w)
I received my battery, but when setting it up with the app, it asks me what brand my cells are. My cells are from recycled laptop batteries and power banks that I found in the trash. That’s the reason why I bought this battery case. Which “brand” should i select if I have a wild mix of old cells from many different manufacturers?
Also, I couldn’t find any information about how much current the BMS can rebalance. It would be great to see the rebalancing current in the app in real time.
Can you also provide a table with the voltage difference limits that the BMS uses? It already complains when the difference is more than 50mV, which is extremely low. When I plug in my charger, the cells instantly get out of balance, and the charging stops. Would it be possible to manually change the voltage difference limits to accommodate older cells?
Replies:
GOUACH Batteries (2w): hello anton, using a mix of cells seems risky and you might encounter a lot of unbalancing issues and finally not great performance. I would really recommend finding the same model of cells for the entire pack. You must check that all cells have the same voltage, capacity and impedance before you assemble them. You should select a model that is most conservative like the EVE.
the bms will start rebalancing with about 50mA if the unbalance is bigger than 20mV. 50mV is not bad so it should not complain. The fact that the pack gets out of balance when you plug the charger shows that the pack is really not in good shape.
no you cannot change these parameter but really it is for your safety.
anton.fetzer (2w): Can you add an option for “mixed” cells in the app? Or for cells with high internal resistance? Why does the app need to know which cells I am using?
GOUACH Batteries (1w): hello Anton, no this is not possible as the BMS can not treat each cell differently (and there is no bms on the market that could do that as the cost of the electronics would be huge).
the app doesn’t need to know which cell you are using but it asks it so that it can setup the BMS properly. Some parameters (like operating temperature, maximum current etc) depend on the cell and these are set during this phase.
if you use a mix of cells, the setup should be the most conservative so you will have bad perfomances.
anton.fetzer (1w): Of course, the BMS should not treat each cell differently, but it would be great to have the option for more aggressive rebalancing. Ive been doing some testing with the battery and the rebalancing is very slow. Especially because the BMS shuts off after only a few minutes of inactivity and stops the balancing. I have left the battery plugged in for 24 hours and it still says its only 78% full with 0.2V difference between the highest and lowest cell group.
Last year, I built a different battery using a mix of old recycled cells that I spot welded together with a 20€ BMS and that BMS keeps rebalancing until all cell groups are the same. Ive been using it for almost a year now and it works perfectly. I expected the same from the Infinite battery kit.
I was under the assumption that the whole point of the infinite battery is to allow the re-use of old cells and mixed cells. Even if you start with a batch of identical cells, once some cells are worn out, you have to swap them out, and everyone will end up with a mix of different cells in a few years.
Hello Anton,
When the BMS shuts down, it’s the main micro controler that shuts off but the BQ chip still continues to monitor the battery and still balances. At the moment, it is configure to balance the cells only when they are over 3,9V as it seems to waste energy when you are not charged enough.
The balancing current is not huge but it will balance. What you are asking is not really the intended usage of the BMS. I think, if you want to use refurbished cells, the minimum is to fully charge the cells so you they should already be balance before assembly. If after this the voltage is not the same it measn that some of your cell have high discharge rate which is a bad sign.
The example you mention with another BMS may be that you just used better cells.
Would it be possible to add some indicator to the app to show that it is actually balancing or showing which cell group is currently being balanced?
Ive left the battery for several days plugged in and three cell groups are at 4.15 while several are at 3.95. So to me it looks like the BMS is not balancing.
Could you also provide an explanation for how the balancing is done?
Does the BSM discharge the highest cell group through a resistor?
Before I added the cells to the battery I kept them at 3.5V so that they contain less energy in case something goes wrong.
Self discharge might be an issue and I will keep checking the voltages over the next few days.
ballancing from 4.15 to 3.95 is going to take forever. I would really recommend to disassemble the cells and charge them all to 4,2 with cell charger. Would require much less time.
The balancing is done by discharging each group of cell that is considered higher than the other in some resistance. Since the energy is disipated in heat, we cannot go too fast. The BQ chip that handles this will discharge all even groups for a while and then all odd groups because it would create too much heat to balance all groups at once.
This is in the worst case when you have one group which is lower than the other and that all other groups should be discharged to reach the lower level.
A Bit more information in the app would be great!
Also some kind of history data of the battery would be great, but I’m sure such features will be coming.
I’ve built my 2 batteries with cells from defunct packs. In one I combined the working cells of two packs from the same brand. The other has all the cells from one pack.
So far it has been working great.
@Anton one limitation to almost any BMS is, it can’t see what’s going on inside one group of cells. Each group has 4 cells in series and if you put an empty cell in a group with 3 full cells it will be charged with a very high current.
No, each group has 4 cells in parallel. That means the BMS sees them as one large cell.
I do not understand what you are trying to say with that.
Of course, but did you read my message? That is not my problem. Ive made sure that all my cells were 3.5V ±0.1 V before I assembled the pack. But due to small differences in capacity the first group of cells reached 4.2V while some other goups were 3.95V. I let the pack sit plugged in for several days, but I do not see any progress in balancing. My other battery that I built from the same cells with a 20€ BMS from Aliexpress does not have this problem.
Excuse me, yes in paralell you are right. But the point is, if you use cells with different capacities and/or with different chemistries in one group of 4 that might be a problem since the bms is not aware of whats going on inside these groups.
Why would the BMS need to know what is going in within one cell group?
Cells in parallel behave like one large cell.
As long as the voltage range of the cells is roughly the same, how could there be a problem running them in parallel?
I would even go as far as saying that combining cells with different internal resistances is a good idea. That way the low resistance cells dampen the voltage drop while the “slow” cells provide additional capacity. The BMS does not need to be aware of that.