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For my battery-powered tool sets (drill, saber saw, jig saw, flashlight, vacuum, etc.) I have Craftsman and Black & Decker. Both are 18 V systems, and both have approximately 20 V "wall-wart"-type chargers. Neither system would indicate when the battery pack was fully charged. Both chargers have an LED that lights up to show "charging" but that does not indicate (by the light going off, or by any other means) when the charge is complete. Instead, the owner's manuals say to "charge for at least 7 hours", or something like that.

The Craftsman charger died 4 months ago. It suddenly went from ~20 VDC TO 3 VDC. The B&D charger died 2 weeks ago; 0 VDC. This isn't the first, or even the fifth, time I've had a wal-wart-type charger die on me.

I bought an 18 V WORX tool battery-pack charger (with a battery pack) for a buck. It is a "desktop"-style charger and seems much more robust. It charges both my Craftsman and B&D battery packs just fine. The WORX charger has a (third-lead) thermistor, and since the other 2 systems don't, I charge the Craftsman and B&D battery packs through the WORX battery pack so that the thermistor is happy. That involves a spider-web of wiring:

charging battery pack thru another battery pack that has thermistor

It all works fine, but (finally, my question!) I want to know: Is there a way to determine when the battery packs are fully charged, so I can plug in another battery pack to be charged?

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    \$\begingroup\$ Are you sure it is a NiCd battery pack and not a NiMH battery pack? The charge termination detection is slightly different for the two types, and since you are asking about charge termination, it is better to be sure of that information. The reason I ask is that NiCd battery packs are increasingly rare. \$\endgroup\$
    – user57037
    Commented Oct 29, 2017 at 22:46
  • \$\begingroup\$ batteryuniversity.com/learn/article/… \$\endgroup\$ Commented Oct 30, 2017 at 2:56

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NiCADs and NiMH both have the property that, at constant current charge, when they reach full charge they will show rapidly rising voltage. Rapid temperature rise, too, although standard NiCAD chargers will operate at a pretty relaxed (nominal 16 hours) charge regime, so temperature isn't a problem. This is not true of fast-charge NiMH, and temperature sensing can be important in terminating such chargers.

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    \$\begingroup\$ This answer is misleading. When voltage is used to terminate charge, the charger looks for a DECLINING voltage (-dV/dt). As far as I know, no NiMH or NiCd charger uses the positive slope of the voltage to terminate charge. There is a rapid rise in voltage, but it precedes full charge. \$\endgroup\$
    – user57037
    Commented Oct 30, 2017 at 1:59
  • \$\begingroup\$ Yes, both battery-pack systems are Ni-Cd. The rapid temperature rise when fully charged may be the reason for the WORX using a thermistor in the charging system. I didn't even know it when I first posted my question, but the WORX lights up green when (I assume) the battery is fully charged. It looks like charging through the WORX, then, is the easiest way to do what I want. Is there any potential problem with charging a second battery pack through the first one (that has a thermistor)? \$\endgroup\$
    – mike232
    Commented Nov 9, 2017 at 22:13
  • \$\begingroup\$ Terminating at voltage rise is incorrect. Peak voltage could be used for termination, but more reliable way is voltage decrease, as already explained in one comment. \$\endgroup\$
    – juhist
    Commented Oct 23, 2022 at 17:18
  • \$\begingroup\$ @WhatRoughBeast Your first claim "when they reach full charge they will show rapidly rising voltage" is just wrong - the voltage is dropping slightly when full, and the drop comes as follow-up of a temperature increase - at least in case of fast-charging. So either a -dV/dt detection or a temperature rise detection is used. \$\endgroup\$
    – datenheim
    Commented Jan 30, 2023 at 18:39

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