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I've converted an existing 18v DC battery pack (for a power tool) to use replaceable 18650 cells. My plan is to charge the cells in a "smart charger" and discard the OEM charger. I'm only getting about 1/2 of use from the tool before power cuts out. This is an original battery pack which failed in service and I suspect the board may be toast.

If I wire up the 5 cells (in series) directly to the load pins that connect to the tool, do I need to consider some sort of discharge or thermal protection inline as well? Specifically, I'm looking at This one which is good for 40V. Thoughts?

(update: Considering the 5S 21v 18v BMS board here since it's a little smaller https://www.ebay.com/itm/272724717408) Thanks

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  • \$\begingroup\$ Is one of the cells bad? How do you balance charge it? \$\endgroup\$ – winny Jun 4 '18 at 19:02
  • \$\begingroup\$ The conversion uses removable cells. I charge them with a Nitecore intellicharger. \$\endgroup\$ – Will T. Jun 4 '18 at 21:48
  • \$\begingroup\$ That solves charging. Still, if one has less capacitance than the other, you will reverse the polarity of that cel eventually. Can you make sure they have roughly the same capacitance? \$\endgroup\$ – winny Jun 4 '18 at 22:13
  • \$\begingroup\$ I don't know how I would measure capacitance in circuit so maybe I can consider accepatble lifetime for each cell. Does it vary widely by battery vendor? I have a bike light which uses four of these 18650s (Ultrafire) and they seem to last at least through the fall and winter balance charging them every other day. I've noticed several times that some of the cells will start out charging with one LED lit, while others start with two. Is this what you are referring to? If I could get 50 charges before replacing cells I'd be thrilled. OEM pack only lasted about 10. \$\endgroup\$ – Will T. Jun 4 '18 at 22:35
  • \$\begingroup\$ Do you have access to a known load, say light bulb, resistor or similar, a multimeter and a stopwatch? If yes, then you can determine the capacitance. It sounds to me like cell imbalance unless you have old laptop batteries (high capacitance but low peak discharge rate) and a very high powered tool, at which point all cells will sag and not just one. What does the datasheet for the cells say? \$\endgroup\$ – winny Jun 5 '18 at 6:09
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should be ok, if max. current of your power tool is not too high (10A Relais)

Maybe you want to check cell voltages of your old pack too, sometimes there is just one cell with low voltage. Charge (only) this one (at its terminals) to the average voltage (or al litle higher) of the others and try again. If problem still exists the cell is of untypical low capacity.

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  • \$\begingroup\$ I've already removed all the old cells from the OEM pack but that strategy will be much easier with the "revised" pack. I've checked voltage on the old cells in the past, some will be 4.2v while a couple others will be much lower (something like 3.x volts). I've never checked them after pulling them out of the charger but that would have been a good idea. I found a different BMS board for my circuit - added it above. What's the 2S/4S/5S on these? Looks like the number of cells it supports but why 'S'? \$\endgroup\$ – Will T. Jun 8 '18 at 10:00
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Number of cell in series, 4S means 4 cells in series

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