I have a home-made battery pack, and am wondering if it is safe to charge.

Background - I won a cheap chinese bike light and 4x18650 battery pack. The battery pack was particularly nasty - I suspect it was 90% air. I have since lost the battery pack when it fell off my bike.

It came with a US wall charger labelled as

Li-ion Charger
Input: 100-240V~ 50/60Hz 0.35A
Output: 8.4V =-=-= 1000 mAh

(that symbol is supposed to be a horizontal bar with three dashes below it.)

I have used two CR123A 3.6V batteries in series, and this runs the light part nicely. They are "protected" ones, confirmed by the slightly longer length and the small vent holes around the positive terminal. These were clipped into a battery holder, wires were soldered to tabs, and the whole thing was covered in heatshrink for water protection.

http://img.dxcdn.com/productimages/sku_8683_1.jpg from dx.com

Questions The battery pack is literally batteries and wire. Should there be anything else inside the battery pack, or does the "protected" part of the battery cover that?

Is this charger suitable to charge this battery pack?

I guess it will take 90-120 minutes to charge - is that reasonable based on 1000 mAh and 880 mAh capacity of each battery?

Edit - since asking, I connected the new pack to the charger and left it for 3 hours. The charger's LED started as red, and then went green after about 150 mins, suggesting it sensed the chargedness of the pack and dropped to a lower power state.

  • 2
    \$\begingroup\$ Related but not quite dupes electronics.stackexchange.com/questions/24228 and electronics.stackexchange.com/questions/207396 \$\endgroup\$
    – Criggie
    Commented May 10, 2017 at 8:43
  • 7
    \$\begingroup\$ I mean, they are called TrustFire ... who wouldn't trust them? \$\endgroup\$
    – PlasmaHH
    Commented May 10, 2017 at 8:44
  • 1
    \$\begingroup\$ typical for cheap battery pack; no worries. the voltages don't sound like a good match. \$\endgroup\$
    – dandavis
    Commented May 10, 2017 at 8:47
  • 3
    \$\begingroup\$ @PlasmaHH Or they put as warning: Trust. They'll fire. \$\endgroup\$ Commented May 10, 2017 at 8:56
  • 1
    \$\begingroup\$ MAX charge rate for 880 mAh LiIon is usually either 880 mA or 440 mA and varies with model and brand. The 1000 mAh chrger MAY charge at 1000 mA or 500 mA or ... . 1000 is "a bit high" or "much too high" depending on what current the batteries are specd for. Alas, you are unlikely to find a guaranteed spec for those batteries. People cell outer shells on Alibaba with names like that for battery makers to use on their batteries. They may be quite good and may be junk and you can't tell. Using a proper LiIon charger at say 500 mA is safer if available. More MAY be OK. \$\endgroup\$
    – Russell McMahon
    Commented May 10, 2017 at 11:24

1 Answer 1


Will the protection circuitry keep it protected while charging? Most likely, although I would be very cautious with it. It would be a good idea to be 100% sure it has protection circuitry. If the cells have protection circuitry then nothing else is needed, if not, then other protection circuitry is definitely needed.

Will the charger work? Yes, it should. But if it charges at 1A then it will most likely put unnecessary strain on the batteries and shorten their life. Typically you want to charge batteries at less than 1C (which for those 880mAh cells would equate to 880mA). I know there are cells out there that can support much higher rates without major impact to cycle life, however most cells are not made for that.

Is that charge time reasonable? Yes, 2-2.5hrs is a perfectly reasonable charge time. This is due to the fact that it charges slower once it hits the voltage limit.


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