Resurrect dead NiMH tool pack?

[Techjunkie]

I have an aftermarket 3.0AH 18v NiMH battery made to fit/work with DeWalt 18v cordless tools. I bought it several years ago, but feel like it hasn't been used enough to be completely dead (the OEM NiCd packs served me much longer). Also, it didn't die a slow obvious death, but just wouldn't take a charge one day. Several minutes after placing the pack on the DeWalt charger, the charge indicator changes to fast blink, replace pack. Repeated attempts to charge produce the same results every time. Using the "recondition" button on the charger makes no difference.

I opened the cell pack and found that one of the 15 Panasonic 0606 MH sub-C cells had almost no charge. The rest measured between 1.1-1.2V. I charged the suspect cell individually using magnets, wire and a single cell NiMH charger. That brought the dead cell back to 1.25v open. Pack voltage measured 18+V open after that but the pack still wouldn't run the cordless drill even with no load attached. I put it back on the charger, but the charger indicates "replace pack" every time.

I tested the built-in thermistor circuit, which I also repaired (the contact had broken) and it measures the same resistance when cool as an OEM pack, so I'm pretty sure that's not it.

I am purely guessing that the DeWalt charger measures the internal resistance of the pack by measuring the current that the pack draws at a given charge voltage and when it's way out of range, kills the charge and changes the indicator to "replace pack".

Should I try to charge every single cell individually, or is there an easier way to try to recondition the entire pack, maybe by trickle charging it with another charger? (I have a laptop power supply that puts out 20V max 3.25A that I can adapt.)

 

[Techjunkie]

I trickle charged it with the 20V laptop charger at 100mA for a while and after a few hours, and 20v open on the pack, it was able to turn the drill very slowly. After that, I put it on the DeWalt charger and wudduya know, it completed the charge cycle for the first time. It doesn't have nearly the torque of the pack that I rebuilt myself, but the fact that it's working at all gives me hope. I was able to stall the drill with ease compared to the good pack. Now it's on the charger again.

Should I try the reconditioning mode, or should I open the pack and charge each cell individually to try to recondition it?

 

[Lynx_Arc]

if you have a nimh dewalt charger (smart) you could try just taking the bad cell out (or bypass it) it will reduce the voltage of the drill but if it is a very weak battery it may be worse having it in the pack than not.

 

[paulr]

If a cell is crapping out that would explain the low torque. You could try replacing the bad cell but it may be better to replace all the cells (hmm, 15 of them, ouch).

The aftermarket pack used panasonic cells? If it's a no-name pack, I wonder if the cells are counterfeit.

 

[yellow]

as You seem very avid with modding --> a perfect chance to use the "Wildflyer method"
http://www.wildflyer.de/
[edit] a test shows that the link gives no hit :thinking: [/edit]
... anyway it would have been in german.

Short explanation: every cell gets a "Bypass" from positive to negative of a resistor in series with a normal diode.

The Idea is, that the resistor constantly has a small current flowing and the diode ensures that the cell is not discharged too much
(Diode has a drop of about 0.6-0.8 V, below nothing happens. So the cell stays at that voltage - which is not chritical for Ni-Cad/Ni-MH)
that small, contant current "removes" any bad chemistry inside the cell and thus should revive the pack.
The more cells, the better it is said to work.
... You only need enough space to put the things in. And must have access to each individual cell contacts to solder them on)


PS: I would choose a low current of about 100 mA/day, the one at wildflyer had his packs with 100 mA per hour
(he is a scale aircraft flyer, these guys have other ideas about "storing" or "usual time between charging and using" of their packs)

 

[was.lost.but.now.found]

I must have missed something so please correct my error, but I don't believe DeWalt makes a NiMH battery, do they? They only make NiCad or Li-ion, right? That means your charger is meant only for NiCad batteries. NiMH chargers can charge NiCad, but NiCad chargers cannot charge NiMH. I don't know what might be going on besides that, and I can't explain why is charges sometimes, but unless I missed something this is likely the reason for your problems and the charger no longer likes your cells.

 

[paulr]

That is a good point, you shouldn't charge a nimh pack with a nicd charger. I have a rather nice Bosch drill with a nicd pack and sometimes think of rebuilding the pack with nimh cells, using a hobby charger to charge the nimh pack. But it seems like a hassle to not be able to use the drop-in charger that came with the drill.

 

[SilverFox]

Hello Was.lost.but.now.found,

The question of the day...

What happens when you charge a NiMh battery pack with a NiCd charger?

Tom

 

[was.lost.but.now.found]

Not sure. I'd guess it has something to do with the charger looking for incorrect signal of charge termination. Upon inspection, Battery University seems to indicate that the end of charge voltage drop is much smaller on NIMH than on Nicad, meaning the Nicad charger will not accurately identify the miniscule drop produced by NIMH cells. This will result in consistent overcharge and damage of the cells.

How did I do?

 

[SilverFox]

Hello Was.lost.but.now.found,

Pretty good...

NiCd chemistry chargers are set to charge at a higher rate, and are looking for a larger value of the -dV for termination.

Since NiMh cells are usually higher capacity than the NiCd cells they are replacing, the higher charge rate is usually not an issue.

However, to get the most cycle life from a NiMh battery pack, the -dV is usually set to around 5 mV/cell, or a little less. A NiCd chemistry charger is looking for 10 - 15 mV/cell. NiMh cells will show this, but they will heat up in the process and the extra heat will reduce cycle life.

So, you can charge a NiMh battery pack on a NiCd charger, but it will end up hotter and will not give you good cycle life. When you charge a NiCd pack on a NiMh charger, it just doesn't get fully charged. If a normal NiMh pack charged on a NiMh charger gives you 300 - 400 charge/discharge cycles, charging on a NiCd charger will reduce that to around 200 - 300 cycles.

Not the end of the world, but a reduction in performance.

Tom