Using an multi-cell Efest charger to balance charge

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Sharkey   1 µW

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Using an multi-cell Efest charger to balance charge

Post by Sharkey » Aug 03 2018 6:35pm

I'm contemplating using a multi-cell, in this case a 6 cell, charger to balance a 10s10p 18650 pack. The idea would be to use the balance leads to bring all parallel connected cells up to the 4.2 volt cut off point for the charger. I realize that probably the only way this would work is to connect every other 10p grouping to the charger using the balance wires. And then when those cells have been brought up to the 4.2 volts (where the charger shuts off), reconnect the remaining cells that were not charged in the first set of cells. My idea is based on the fact that each of the 10 series connected parallel groups has a balance wire connected to the positive terminal and another one to the negative terminal. So... starting with cell 1, balance wire 1 is connected to the positive terminal and balance wire 2 is connect to the positive terminal of cell 2. And... this is also the negative terminal of cell 1 since they are connected together in series. And that's why I'm assuming I couldn't charge adjacent cells at the same time. In other words, for cells 2 through 9 share the remaining balance wires. And of course, the 10th cell would use a balance wire connected to its negative terminal which, of course, is the negative terminal of the entire pack. As a result, in theory, all 10s groups would be at 4.2 volts. I.E. top balanced. Which, to me, seems better than using a BMS which, as I understand it, brings the pack up to where at least one of the parallel cells is at 4.2 volts and then bleeds off voltage to the same voltage level of the lowest cell group in the pack. This seems a bit inefficient and backward to me and you end up with a pack at the voltage of the lowest cell.

I'm looking for reasons why this would either not work or not be a good idea. I do realize that is less automatic than using a BMS, but I don't need the BMS function to stop discharge of the pack below a safe point. This is an auxiliary pack in parallel with the primary pack which is protected with a BMS and thus will be shut down before reach a damaging low voltage.

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Alan B   100 GW

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Re: Using an multi-cell Efest charger to balance charge

Post by Alan B » Aug 03 2018 8:22pm

Balance chargers like the BC168 or CB86 balance charge 6 adjacent cell groups using 6 independent isolated programmable DC-DC converters internally. They periodically stop charging for a moment to measure all the cells accurately, and then resume.

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amberwolf   100 GW

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Re: Using an multi-cell Efest charger to balance charge

Post by amberwolf » Aug 04 2018 2:21am

The idea will probably work, but I would recommend something other than any product from Efest or similar places. :/

Some mentions just here on ES:
search.php?keywords=efest&terms=all&aut ... mit=Search


Keep in mind that if the paralleled unprotected pack does not actually connect thru the BMS of the protected pack, that BMS cannot protect the paralleled pack at all--it will not shut off current from it, it will only shut off current from the pack the BMS is already connected to protect.

If it is possible to parallel the new pack to the existing one at the cell level, then you dont' need any extra charger/etc; the BMS and charger of the existing one will do all the work for you.


As far as the balance charger goes...as long as you never "cross the streams", by connecting any wires across series pack connections, you can balance any paralleled cells in whatever way the chargers work.

If you have a 6s charger, you'd connect the most negative 6 parallel groups to the charger, then disconnect those when full. Then connect the most positive 4 parallel groups to the charger, then disconnect those when full.


There are 10s balance chargers, too, though you may need to split the pack in sections, breaking series connections, with some balance chargers. IIRC, some of the 10s chargers do not charge 10s packs; they charge two 5s packs that are not in series with each other. Make sure the charger you intend to use does not require this.

Sharkey   1 µW

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Re: Using an multi-cell Efest charger to balance charge

Post by Sharkey » Aug 05 2018 2:15am

Thanks so much to all who responded.

Good point regarding relying on the stock BMS controlling the primary battery. I had made the assumption that motor controller was integrated into the Giant Quick-E removable battery module and that when the BMS sensed the low voltage shut off point, it would not allow further output to the controller, thus also protecting my auxiliary pack as well. I still don't know if my assumption is correct or not. And I do have an accurate voltmeter that gives me the current voltage mounted in plain view that gives me aggregate voltage all the time. So if I'm wrong and I also don't heed what my voltmeter is indicating, it's my own fault for any damage to the auxiliary pack.

Regarding splitting the 10s10p into two 5s10p units, that's precisely what I did so that I could mount them low and on either side of the bike. Then simply connecting those two units in series and then directing that in parallel to the main battery. I'm sure I'm not the first to use the build method I used, but I've never seen a DIY video featuring this method. I first built the 10p groupings by arranging the cells, first, 4 cells, side by side, in a row with the remaining 6 cells arranged with 3 on each side of the 4 cell row. After spot welding the connecting nickel strips, I then stacked 5 of these 10p groups on top of each other in series with balance wires connected to each of those interconnections. I had previously determined that this arrangement would just slide into a piece of 3" X 4" PVC gutter downspout which creates an inexpensive, easily made, and nice looking enclosure. This gives me two 18 volt packs in tubes about 18 inches long. Those are then joined electrically in a junction box to give the desired 36 volt pack.

Again, thanks for your very useful comments. I really appreciate it.

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Re: Using an multi-cell Efest charger to balance charge

Post by amberwolf » Aug 05 2018 7:04pm

Sharkey wrote:
Aug 05 2018 2:15am
After spot welding the connecting nickel strips, I then stacked 5 of these 10p groups on top of each other in series
FWIW, if spot welding packs together, youll get better connections and lower resistance if you do the series connections first, and then the parallel ones. Only works that way if you do series between every cell, which is more work and more nickel, but gets better performance, less heat, and greater reliability (since the series connections handle the entire pack current, and parallel ones don't).

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