View Full Version : Saving your batteries

03-19-2006, 01:42 AM
I recently commented on another thread about modifying the el-cheapo battery chargers that come with the e-cheapo electric drills we all know and love. The nicads don't last not so much because of poor quality batteries but because of the crummy charger. I have about 4 of these drills and the chargers for the 18 volt models are supposed to charge the pack in about 3 hours.

The problem is that the charger has no control circuits at all. It's just a wall transformer, with a diode and a current limiting resistor in the charger base.

In order to charge the battery pack in just 3 hours a very potentially unsafe charge rate must be used. If the battery is left on charge longer than it takes to reach full charge then all of the charging power after that is converted to heat in the battery. In short order the batteries in the pack overheat and electrolyte is vented. This damages the batteries and it only takes a few such episodes to kill the pack. Leaving your batteries on one of these cheap chargers overnight is a death sentence for the batteries.

There is a simple and cheap solution. All you need to do is add another current limiting resistor to the charger base. This will lengthen the time it takes to recharge the battery pack but it will save it from being damaged so easily.

First pic:

You can tell if you have one of these chargers by the extremely simple circuit board as seen in this pic. It has two LEDs plus their resistors, a diode and a current limiting resistor to the battery pack. Other such chargers will be similar. Also, another way to tell is simply that with the cheap chargers they never stop charging the battery and the charge light on the charger base stays on all the time.

A wirewound 10 to 20 ohm, 5 to 10 watt resistor (indicated by arrow) is added in series with either lead that runs to either of the connectors to the pack. Extra cooling holes should be drilled in the base as the resistor will become somewhat warm.


Tuck the resistor in whatever space is available:


Holes should also be drilled in the top of the case for cooling taking care not to damage the wiring or circuit board. Put it back together and you are done.


On this unit the original charge current was around 0.8 amps. For ordinary nicad batteries the rule of thumb for safe charging current with no danger of overheating and venting is 1/10 of the amp hour capacity. That means that if a cell is rated at 2500 milliamp hours capacity it may be charged at a maximum rate of around 250 milliamps without problems. This rate gives time for the gasses produce by charging to recombine in the electrolyte withou causing enough pressure to open the vent.

After I added the 10 ohm resistor the charging current fell to about 0.28 amps. This is close enough to the safe rate that the batteries may be left on charge for a long period without damage. Of course, it will take longer to charge the batteries but that beats killing them with overheating because you forgot to take them off charge at just the right time.

If your charger was supposed to charge in around three hours like mine then with this modification it will take perhaps four times longer. You can just leave it overnight and it will fully charge the battery pack without damaging it. Note that the charge indicator LED may be much dimmer. This is normal and reflects the fact that the charging current is lower.

03-19-2006, 02:46 AM
All three of the battery packs I've taken apart had a thermal breaker/sensor on the side of the trailing battery. It has a separate lead and connection to the battery charger. I haven't purchased any new drills for a while but my battery packs and chargers all have three connections.

I had the understanding that this is how the charger throttles the charge rate.

03-19-2006, 02:53 AM
It will vary. This one has a thermistor in series with the charging circuit to the battery in a feeble attempt to regulate the current. Seeing that the battery is well above the themistor outside the case it isn't going to pick up much of the heat. The ones with the third wire probably have the thermistor in the battery pack. Even then it is a very poor excuse for charge control.

03-19-2006, 02:53 AM
This is an excellent tip,Evan.Overcharging is exactly what killed my last 2 packs.(Left them on overnight http://bbs.homeshopmachinist.net//frown.gif)Since it's cheaper to buy a new drill than to buy a new pack,I now have a number of "spare parts" for my current drills.I took the old packs apart,and found only 1 of the cells was actually dead.
Thus I now have a good supply of cells to make 6 or 7 cell radio control packs http://bbs.homeshopmachinist.net//smile.gif.
I didn't see your previous post relating to this,but I will look at my chargers tomorrow to see if they can be easily modified as per your instructions.
Thanks a lot for the info.Hans.


Your Old Dog
03-19-2006, 07:26 AM
The Nicad camera batteries we used to use at work also had a thermistor circuit and a simple wall charger for charging at home. Our rule of thumb was to charge the batteries laying flat on a floor or table top and if the floor or table top was ever so slightly warm the batteries were done.

We are grateful for the Nimh technology. I would never spend another nickel for nicads for anything. Nimh are damn near worry free and work in cold weather and after long storage.

03-19-2006, 08:35 AM
I have seen peak chargers. I think they shut off when the voltage peaks and starts falling? I have an RC truck the batt. charger I purchased has a timer on it. I usually connect my V meter to it and shut it off when the voltage peaks. The batts do get warm when charging. How warm is too warm?

03-19-2006, 09:31 AM
There is nothing wrong with nicads. They are better than NiMH for applications that require very high amounts of current for short periods, like power tools. Nothing can push out the amps like a nicad. The main problem is the chargers.

Timers are better than nothing but that doesn't account for the state of charge of the battery when put on charge. Voltage sensing is much better but still not the best as it doesn't account for the overall condition of the pack. The best is a combination of voltage sensing and accurate temperature sensing with ramping of the charge current. The best nicad chargers also have a "maintainer" cycle that finishes that charging with a low repetition rate short duration pulse of high current after charging is finished. Properly charged and maintained nicads can last as much as 20,000 charge cycles. Nicads on satellites have lasted as long as 20 years.

How warm is too warm? If the battery feels slightly warm that is OK. If it feels definitely hot or "really warm" that is too warm. That's the best you can do to judge it without a proper sensing system.

J Tiers
03-19-2006, 10:25 AM
That is a really basic charger.

The DeWalt etc are not like that, and don't need the resistor, nor will it actually work in them.

I wouldn't recommend trying that with any decent charger.... just the dollar store ones.

03-19-2006, 10:51 AM
Agree, but it is suprising what tools include such crap chargers. I recently bought another 18 volt drill that is really nice. It has a two speed transmission, adjustable clutch and electronic braking. It's perfect for power tapping as it stops instantly. It's a really nice drill but it came with that POS charger.

On the other hand, my set of 18 volt garden tools came with a smart charger that turns off when finished.