36 Volt SLA charger for 36v Tool Batteries

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theyerb
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36 Volt SLA charger for 36v Tool Batteries

Hello all- there have been a few posts regarding using an SLA charger with different battery types, but haven't exactly answered my question. I just switched to lithium-ion for my ebike from SLA. I am now running two Bosch 36 volt tool batteries (BAT836's) in parallel. To charge them, I tried using the "intelligent" (SLA) charger that came with my hub motor kit, but it charged the two batteries up to 43.3 volts. Do you think the overcharge may have damaged the cells? And why do you think it is overcharging them? Is it ok to go a bit over 42 volts, or is it a strict limit? btw, the cells in these batteries are Konions.

Thx :)

Dave-s
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Re: 36 Volt SLA charger for 36v Tool Batteries

A fully charged 36v SLA will read 43.2v.
Each Lead Acid cell's nominal voltage is 2v, but fully charged it is 2.4v. Each 12v battery is comprised of 6 cells, so a fully charged 12v SLA will read 14.4v (6*2.4v).
3*14.4v = 43.2v.
The problem with the Lithium batteries that come with power tools is that they are not truly 36v. The packs are comprised of 10 Lithium cells.
Hot off the charger each cell's voltage is 3.65v, so this is why they are called 36v (10*3.65v = 36.5v).
But the nominal voltage of the Lithium batteries is 3.2v so the nominal voltage of a Lithium power tool pack is only 33v.
If you are charging a 36v pack with an SLA charger you are charging the cells way above their maximum voltage.
This may not do any damage in the short term, but you are shortening the life of the pack.

theyerb
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Re: 36 Volt SLA charger for 36v Tool Batteries

Dave-s,

Thanks for the insight. I believe most lithium-ions, LifePo4 aside (A123s for instance) have a peak voltage of 4.2 volts per cell and a nominal voltage of around 3.7, non? In that case, I figured 43.3 wasn't too much higher than 42 volts...

I guess I have 3 options: to find a new charger, shrug it off and it might hurt the batteries, or add another Konion in the series to raise the max voltage of each pack.

Cheers!

Dave-s
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Re: 36 Volt SLA charger for 36v Tool Batteries

A123 cells are LiFePo chemistry, so they too are 3.7v peak, 3.2v nominal.
But I did a search, and found that the Konion cells are 4.2v peak, so I guess they are Lithium-Cobalt or Lithium-Manganese.
You can string two diodes serially to your SLA charger and they will drop the charging voltage by about 1.4v.

reikiman
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Re: 36 Volt SLA charger for 36v Tool Batteries

As I understand it, the LiFePO4 charging algorithm is the same as SLA charging algorithm. But as noted above the cutoff voltages are different. Further that LiFePO4 are more easily harmed by over charging as well as easily harmed by discharging too deeply. It's possible on some SLA chargers to adjust the charging voltage and they'll work fine for this purpose.

As I understood it the nominal voltage for any LiFePO4 is 3.2v per cell, not just A123. The nominal voltage for Lithium-Cobalt is 3.7v per cell. Li-Co != LiFePO4.

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