Gamer World

How to rebuild cordless tool battery packs

Cordless tool batteries are a topic that could make for a decent sized book! To keep this reply shorter(but it’s still long!)I would like to make it clear that I am speaking ONLY of Ni-Cads; trying some of these things with other batteries and cells can result in an explosion right in your face. We need to begin with the basics. A battery is simply a group of individual cells; please keep this in mind as we go along.

A Ni-Cad cell likes to be discharged fairly deeply before charging, but a Ni-Cad battery does not. With any battery, one or more cells will be weaker than the rest and overuse can reverse-charge one or more cells which cannot be corrected. This is caused by the stronger current running through a weak cell while it can no longer add to the current; therefore the current flow in that cell will be going backwards.

With a Ni-Cad Dewalt battery, charge it when performance just begins to fall off and it will last a lot longer. Heat also kills Ni-Cads so don’t run the battery hot and let it cool before charging it. I’ve had some of my batteries last over 6 years in almost daily use because I follow those rules no matter what!

Ni-Cads of any given size differ. Most cordless packs consist of a number of 1.2 volt cells in a size known as “Sub C”, because they are slightly smaller than an ordinary “C” cell. Their capacity varies from 600ma to over 1400ma and the price follows. In the name-brand packs you’ll find cells(usually unmarked)that run to the higher end of the scale. The “Harbor Freight” etal packs use lower capacity cells. Quality of the cells varies too, with some designed to be rapid-charged while the cheaper ones unable to withstand that high rate of charge for long(if at all).

Again, the name brands(DeWalt, Milwaukee, Craftsman, Ryobi, etc)are rapid-chargers, the cheaper ones aren’t. Ni-Cads are all better off with a slow charging cycle but there’s not much loss with the rapid-charge type and the time difference(1 hour vs 5+ hours)makes them worthwhile. Chargers also differ. While some will adapt to charging a lesser than intended capacity, others will just overcharge the cheap cells and ruin thewm.

While the cheap cells can be used to rebuild a battery, you won’t be getting the same performance that a new pack will give. It may be good enough if your application is medium duty, but heavy usage will show this to be true. I know because I have rebuilt dozens of batteries and I run them hard at work every day. My tools and Makita batteries have a hard life keeping up with me!

One of the characteristics of a Ni-Cad cell is a tendency to create “dendrites”, which are tiny build-ups of metallic particles inside the cell which effectively create a short circuit that gets worse with time. Excessive charging or discharging(especially the cheap cells)aggravates the situation. Dendrites can be burned off by ‘flashing’ a higher voltsge and current across the cell. This is the ‘repair’ method you hear so much of and it works for this condition. Observe correct polarity and use a car battery but don’t apply the current for long, just a few brushes like you’re striking a match.

Too long and the cell can explode. Remember we’re talking cells, not the entire battery, and each cell must be flashed indivudially. And keep your distance and sparks far from the hydrogen at the car battery; remember the Hindenburg? Nuff said!

Which brings us to the actual repairing and rebuilding of Ni-Cad packs. If you have one or two bad cells you can replace them with like cells from another old pack, thus restoring it. If several are bad replace them all(saving the good ones for the above purpose later on if you wish). From the factory most batteries are connected through stainless steel ribbons which are spot-welded on. This is done as overheating a cell will ruin it or cause it to explode. The spotwelding is done so rapidly no heat buildup occurs. Soldering is possible here but it takes a lot of heat applied quickly and removed quickly, as well as a good flux.

Roughening the surfaces with sandpaper seems to help. As a Ham radio nut I’ve soldered extensively and this is the toughest soldering job I know of. The internet has plans for making your own spot welder but I haven’t built one so you’re on your own there. The specialized spot welders for bosch batteries cost more than you’re going to want to pay!

Now the good part: Replacement cells can be had with tabs already spot welded on and these tabs are very solderable so use them only. You’ll be glad you did! Buy the best cells you can find, again the extra quality is well worth the cost. The downside is that you’re probably going to spend nearly as much(or even more!)than a new or factory rebuilt pack. Sorry for that bad news and if you know of a good source for quality cells cheap please let me know!

I cheat-I buy new Craftsman packs at K-Mart(much cheaper than at Sears and nearly top-of-the-line cells)and use those for my rebuilds. I’ve bought a new 19.2 volter there for $19.99 on sale. My last 18V batch of Sub-C NiCads cost me $62 after shipping and were a lesser capacity to boot.

By studying the way the battery goes together you can usually cut the Craftsman apart in a way that minimizes the soldering you’ll need to do. If you have space you can even take off the positive or negative connection(according to the battery you’re working on)from a different place. All that matters is that all cells are in series, that the new take-off point goes to the right connector in the pack case, and that you’re added connection can handle the current. If I’m out of copper ribbon(which I make from the gutter man’s scraps)I use 12ga stranded wire if I can fit it.

However you do it, be certain that a short circuit is impossible or you’ll find yourself with a fire at best or an explosion at worst, not to mention the melted plastic all over everything(hopefully not your charger where a short will usually occur). If you use plastic to insulate be sure it can handle a lot of heat; I like to use the heavy cardboard from a notepad back. So long as it’s kept dry it makes a great insulator but it will ignite easily if a short heats things up.

Sorry for the long reply but like I said, there’s a lot more to cordless batteries that meets the eye and we haven’t even touched on the Nimh’s or the Lithiums! If the subject intrigues you check out the technical forums at www.battery-company.com.au

You might even luck out and find a good deal on the replacement pack you need there. I’ve used them several times and I can reccomend them but they don’t carry as extensive a line of replacement packs as they used to.

About the Author

Battery company is an operated online retailer of high-quality electronics accessories, specialising in laptop batteries and laptop adapter, battery chargers, camera batteries, camcorder batteries, power tool batteries and mobile phone batteries. Battery company has quickly grown to one of the top online Australian retailers of electronics accessories, offering generic product brands that provide the same quality as the big brand names but at half the price. Now at www.battery-company.com.au, we offer customers a wide selection of products at unbeatable prices, together with personalised customer service and fast shipping for our customers.

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