I was given a "Power Grinder" brand(made in China) rotary tool, and its cordless. I've plugged it into the charger, and the red light came on, and is still on 6 hrs. later. The tool works, but I don't know if its fully charged. It didn't come with any sort of manual, or instructions. I searched for a manual online, but couldn't even get the tool to come up in the results. I don't know if I'm damaging the battery, or if it actually takes this long to charge. Any help is much appreciated! Thanks, Mark![enter image description The pic is the tool with the battery attached, at the bottom.here]1

  • A free tool may have a battery that is not capable of fully charging rechargeable battery’s have a limited life no matter what type. Several will gain a deeper charge by fully discharging and recharging several times, if the battery gets warm in recharge unplug let cool and try to top off the charge, heat kills rechargeables.
    – Ed Beal
    Commented Feb 8, 2020 at 22:03

1 Answer 1

  1. It's normal for a battery to self-discharge over several months.
  2. It's normal for a cheap tool (Black & Decker tier) to take 12-18 hours to fully charge.
  3. What damages such tools is being left on the charger for days and days.

The reason is because their charger is not using "everything we know about batteries". By contrast, a Tesla supercharger can calibrate charge rates to take a Tesla Model 3 from 25% to 75% in 25 minutes at a rate that would take another Tesla from 90% to "fiery explosion". Most batteries want a bulk charge rate in the middle of their range to about 85% and a low current taper charge near the end of cycle.

In the case of cheap chargers, rather than shift gears from "bulk slam-charge" to "low taper" to "maintain", the cheap charger simply does the taper charge continuously!

Which makes the charge comically slow. But results in de minimus risk to the battery.

Note that it doesn't transition to a maintenance charge either; that's why you can't leave it on charger indefinitely.

  • Leaving the battery discharged for long periods is also damaging the battery.
    – Solar Mike
    Commented Feb 8, 2020 at 5:57
  • @SolarMike Not for NiCd or NiFe, which is what most cheapie tools are. Commented Feb 8, 2020 at 6:09
  • Someone gave a tool with nicads in it - not been used in several years and battery is stuffed... so experience seems to say batteries should be left discharged for long periods.
    – Solar Mike
    Commented Feb 8, 2020 at 6:11
  • 1
    @SolarMike Is there a "not" missing from your last comment? Commented Feb 8, 2020 at 15:41

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