NiCd battery problem.

[Gardener]

I've got a 24v NiCd battery pack (backpack version) which powers a Felcotronic electronic pruner. It has provided me good service over the years, and I still get a full day out of the battery pack.

Today I took it out of the truck and realized the toggle switch was on, and the unit had completely discharged over the course of several days. Now when I hook it up to either of my chargers I get a fault, and charging won't initiate.

Suggestions? I consider hooking it up to some sort of trickle charger for a day or two, perhaps a modified low voltage/low output wall wart. I'll pull the battery brick out of the enclosure, as there is a control board in there and I intend to avoid damaging that.

 

[Greenman]

I know that on the lithium battery packs used in some tools the control circuits won't allow a charge once the battery's drop below a certain point. If that's the issue, a trickle charger might be able to get it above the threshold. You would have to bypass the control circuit to do it. I've actually seen this done once in a youtube video. I believe it was AVE.

 

[Steltek]

Since it was working well before and it is a NiCad battery, I suspect that the trickle charger idea would work. I've revived several 18v Dewalt NiCad batteries for my drills in the past using a similar method.

With Dewalt NiCads, the safety shutoff circuitry is in the charger and not the batteries so you can just directly trickle charge the battery pack itself without bypassing the circuit board in the battery. I don't know if that would be the case with yours, though, so it isn't worth taking the chance if you can easily separate the pack.

I doubt you'd need a day or two, though, maybe just 15-20 minutes to re-establish a seed charge detectable to the charger. In a pinch on my old Dewalts, I've even recovered one by directly connecting it to a second fully charged battery via 14 gauge wires for 5 minutes or so.

 

[Gardener]

Thanks guys. I got it to take a charge, The charger is wired to a 1/4 stereo phone plug which inserts into the battery pack. The instructions say to attach the charger to the pack prior to plugging it in to an AC outlet. I reversed that order, allowed the charger to initiate up to a flashing green (indicates a bad connection) and then plugged it into the battery, hot-swap style. It immediately went to red (charging in process) instead of flashing red (aborted charge/fault). Used the low output charger, nothing fried.

 

[shortylickens]

You got your money worth.
Upgrade to nickel metal hydride.

 

[Steltek]

You got your money worth.
Upgrade to nickel metal hydride.

If those pruners are what I think they are (a new kit like his is probably over 2 grand), just replacing the battery pack with a NMIH pack would be probably be over $400 with sales tax. And, if OP's current charger won't charge NMIH battery packs, that would probably put the cost of the upgrade up north of $500.

Don't blame him for wanting to salvage his current battery.

 

[Gardener]

Yup, 2 grand kit. When it goes I'll replace/rebuild the 20 cell nicad sub-c battery brick, ~$100. The original pack has lasted 20 years, may have another 10 years in it. The shears themselves are in good shape, and replacement components are still available...and I have stockpiled wear items.

Seems to be no worse for the wear after the accidental discharge, tests 25 volts at full charge (20 cells at 1.2).

 

[mindless1]

^ I am not familiar with that battery pack but something is off.

"IF" It has 20 NiCd cells in it, then the full charge voltage should be closer to 28V, not 25V. While it is called a 24V pack, and probably does have 20 cells to cause that voltage rating, the unloaded full charge voltage of a NiCd (or NiMH for that matter) is much closer to 1.4V than 1.2V per cell. The 1.2V is just the nominal, average voltage during the discharge cycle.

I'm not suggesting that you need to do anything, if it still provides acceptable power to the tool and runtime then I would wait till that is no longer true, but the odds are that you have at least two cells shorted out or several that are weak, or even more shorted cells if the pack uses paralleled series of 20 cells to achieve higher capacity.

 

[Gardener]

I think that NiCds are know for holding a consistent voltage throughout their discharge cycle...consistent and always lower than comparable NiMH and Alkalines. However I have another pack I'll check it against for comparison.

 

[mindless1]

Not knowing how your pack is designed, if it is more elaborate than a typical power tool pack, it might have had a control board for charge and/or discharge cutoff purposes, using high forward drop mosfets (at the time) or even transistors in series with the cells to lose another fraction of a volt, but at full charge each cell should be around 1.4V. If you can open the pack and check individual cells with a multimeter, you should see which are not taking a full, if any charge.