Saving / Upgrading an Old Cordless Screwdriver to NiMH

I've got a handy little cordless screwdriver which didn't last long with the stock NiCd (Nickel-Cadmium) batteries. Seems like the factory batteries we're really sorry. Worked good for the first six months - then, as usual, wouldn't hold a charge. Such a shame to toss such a handy little device just because the factory went cheap on the batteries.

Well, I have a bunch of NiMH (Nickel Metal Hydride) batteries laying around from another project (don't ask). These AA batteries I have came from Amazon.com at $2 bucks, each. I think I bought three dozen to get that price.

Anyway, the four junk NiCd batteries are C size, but only 2/3 height, rated at 1400 mA-h. So, my 2500 mA-h AA batteries are going to be much more capable. Plus, the NiMH have a much better discharge curve that Ni-Cd.

Step 1: Opening Space

There were a bunch of ribs molded into the plastic to hold the odd shaped Ni-Cd batteries. Using a pneumatic die grinder with a cut-off wheel, plus a grinding stone wheel, it was just a matter of determination to get rid of all the plastic that was in the way. The hard part was preserving the mounting posts for the screws which hold the cover together.

Step 2: Solding Together a New Battery Pack & Repacking

My batteries don't have mounting tabs. No worries, I snapped off the tabs off the original batteries and soldered them onto the new batteries. To make a connection between batteries, I used some copper solder wick - some nice 3/16" wick worked great.

Step 3: Short Circuit!

Wow, do those batteries ever get hot when short circuited!

I had just packed the case together - was just driving down the last screw in the case when I noticed it. The handle was warm. In fact, it was starting to get hot. Maybe it was just the left over heat from the glue gun. I had put a little lump of glue inside to hold one of the batteries.

But no, it was getting hotter!

Time to reopen the case again and see what the problem was. Boy, everything looked OK. But, the batteries were smokin hot! Gosh, I hope I didn't hurt them. Eight bucks in batteries down the tubes?

My guess is the solder wick in the middle section got touching. It was just the bottom two batteries that were really hot.

A little anti-static bag cut up made for a good plastic shield. I guess I was lucky the short happened right away. Better to find the short now!

With the plastic shield in-place, I reassembled the case once again.

Step 4: Finished

All done, and back on charge. The stock charger is rated at 6V / 160mA. I think I can just leave the screwdriver on charge continuously without any worries.

Thanks,
Jim

Comments

[simplyconnected]

I am bothered by 'discharge rates'. I have replaced some ni-cads in my 12-volt drill with brand new sub-C cells (Tenergy 3300 Ni-MH) only to find that most of the torque simply wasn't there.

I know if you short a fully-charged ni-cad, it will explode because they have a great discharge rate. The Ni-MH's I used had a very low discharge rate and I found that different cells of the same capacity are simply not suited for power tools.

Then I saw an ad for Ni-MH sub-C cells on eBay that boasted "30-amp discharge rate". I haven't bought them yet but I wanted to run this by you first.

By all rights, when your cells shorted, they should have exploded.

[jimk3038]

I had not considered the maximum discharge rate when I made the switch to NiMH batteries. I have no idea if my NiMH batteries are better or worst than NiCd batteries.

If you really want to build a super battery pack you might want to consider a lithium polymer (Li-Po) battery pack. The Li-Po battery makes a claim to fame of super high discharge rates. Also, Li-Po batteries are much lighter than most other types. Lots of folks use Li-Po batteries for remote control airplanes. Only downside to Li-Po is the re-charge hassle. You gotta use a "smart" charger that has a Li-Po charge mode. Otherwise, Li-Po batteries WILL explode if overcharged. However, I've seen Li-Po packs that are advertised with a max discharge rate of 50C. So, a 1Ah battery pack could dump 50 amps into a load for about 10 minutes!

Sorry I couldn't be more helpful.

Good Luck,
Jim

[lemonie]

You're using the same Ni-Cd charger for the Ni-MH replacements?

L

[jimk3038]

Yep,

First, I tested charging using my bench power supply. I set it to match the factory supply at 6V / 160mA. Note, the factory supply actual starts out around 8V when unloaded. Then, when loaded, the output drops to 6V / 160mA. So, I set my bench supply at 8V / current limited to 160mA. 

While charging, the power supply is in current limit mode at 160mA. After a day of charging the supply is still in current limit mode. The batteries never got even warm with this charging rate.

I read somewhere else, it's not a good idea to trickle charge Ni-MH for days, and days. Unlike Ni-Cd, the Ni-MH will decay with the trickle charge. So, I guess we'll charge them overnight and them use them until they need another charge.

Thanks for the question,
Jim

[lemonie]

I was thinking that (since your Ni-Cd pack failed) you might need to be careful charging these or have to replace them a bit too soon.
I found this technical sheet which might be useful.

L

[jimk3038]

I read your "technical sheet" link. Good stuff. However, the bulk of the info was "flash" charging the Ni-MH batteries.

I simply want to use an easy recharge method, like a trickle charger. The info you provided suggests (at lest for Panasonic batteries) the trickle charge should be 0.033 to 0.05CmA. So, for my 2500mA-h batteries, that would be 80 to 125mA. Right in-line with what the factory charger is doing.

The other tip from your link suggests not to let the trickle charger carry on forever. Here is the quote from the link:

The overcharging of nickel-metal hydride batteries, even by trickle charging, causes a deterioration in the characteristics of the batteries. To prevent overcharging by trickle charging or any other charging method, the provision of a timer to regulate the total charging time is recommended.

So, once the batteries need charging, use the trickle charger for 10 to 20 hours and then unplug them from the charger. Don't leave the batteries charging continuously.

Thanks for the tips,
Jim