18v drill battery rebuild questions
Hey guys, I've got a DeWalt 18v drill with tired battery packs. They will run the drill but run time is way down. I charged one up and then took it apart and here is what I found:
-Most cells at 1.3 v.
-Two at 1.28
-Two a 1.29
-One at 1.4
-Total voltage charged: 19.5
-Voltage when drill is running but not under load: 18.4
-After running drill for 60 seconds voltage while running drops to 17
-Battery voltage no load after above run: 18.8
Most of the websites I've checked say to find the one or two bad cells and replace them but I don't see any individual bad cells. So is the whole set over the hill? Any rebuild options or should I just go for new packs? These are about 4 years old and only get part-time use.
I hate to drop a C-note on a pair of packs if these are salvageable.
Try recovering them instead.. Try this.
Find a REALLY small load. Lets say 100mA.
R=V/I, 180ohms for 18v.
60W 120v bulb = About that. (240ohms on resistance. .less when cold). Less is OK, More is not. (less wattage that is)
Connect it across the 18v drill pack once its discharged enough to no longer spin your drill, And measure the voltage now and then. I believe those 18v packs are 15 cells, So you want to wait untill it hits 0.4v per cell, or 0.4v * 15, or about 6v
Once thats done, Recharge it fully, And repeat. Your charger may initial refuse to charge it due to low voltage, but most trickle charge it in that state and it should quickly perk up and start charging, Failing that find a 24v+ supply and hook the battery to it in series with the light bulb, untill the pack voltage exceeds 15v. (Should only take a minute or two)
Discharge to 0.4v per cell has been shown to actualy recover Nicad capacity. The only problem is discharge below 1v per cell can easily reverse some cells as the internal resistance is VERY high below 1v per cell, and any high current reverse is very damageing, Hence the use of very low current.
Test each cell individually under load, a 6V lantern bulb makes a good test light. I bet you find a few of them dropping down in voltage while the test light is hooked up.
While I am not an authority on "dry cell" nicads, I have been maintaining aircraft nicads for many years, and I assume the chemistry is similar. I have seen "reversed" cells in a 24V battery go as high as 6v. This is when the nominal voltage is about 1.3V.
What this means, is that assuming a 20 cell battery, you might have 19 cells at + 1.3vdc, and one cell at - 6.0vdc, for all intents & purposes, you now have a 18.7 vdc battery!
As for as nicads not developing a "memory", I wish someone would notify Saft America and Marathon, (the manufacturers of Aircraft Nicad batteries) so they can amend their maintenance & inspection schedules. Currently, the batteries require a full discharge, cell shorting, and recharge to 120% of capacity every 100 hrs of aircraft operation.
Last edited by Thruthefence; 12-12-2011 at 02:11 PM.
When my packs die I just buy all new cells. I don't mess around trying to find the weak ones. If you are capable and shop around, they can be rebuilt for about half of what a new pack costs.
THANKS for the fast response, guys. Back to the shop to try your suggestions. Boostinjdm, you may be right in the end but my inherent cheapness won't let me do that until I've spent more than $100 worth of my time trying to save these!
I have bought rebuilt cores from here:
The price per 18v core is $42. I'm not associated with them, just a satisfied customer.
I've wondered if buying one of these and stripping the batteries out would be a practical source for rebuild.
You didn't say what type of cells they are.
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I've done that, or something like it, in the past, buying leftover/overstocked etc. battery packs and rebuilding others with the contents. It can work, but difficulty varies considerably with layout. I rebuilt a few batteries for an old Milwaukee, and extended its life for years. But when I opened up a DeWalt 18 volt battery, I found the way they're stacked rather intimidating, and I think it might be difficult to roll your own. You may find it less daunting, but I suggest anyone planning this open up a pack first and see what you're getting into.
Originally Posted by armedandsafe
Bruto's right. It's not a quickie job, as you have to do some planning on how you're going to lay out the cells and the connections. I suggest some close color prints of the existing one, so you have a guide as you go along.
One of the main incentives for rebuilding your own is that you can usually get Sub-C's with a considerably higher capacity than the original cells. I remember my first rebuild came with 1800 ma cells while 15 3300 ma cells could be had for an amount that was still far less than a whole replacement battery.....and if you opt for the replacement battery, you're likely to get the same low capacity cells as the original.
Like most things we venture into, the first time can be a bit of a challenge, but after that first one, it's a breeze.....plus you're going to wind up with a battery pack that is far superior than the original.