Too Busy For Words - The PaulWay Blog

01 11 2013

Fri, 01 Nov 2013

Converting cordless drill batteries

We have an old and faithful Ryobi 12V cordless drill which is still going strong. Unfortunately, the two batteries it came with have been basically killed over time by the fairly basic charger it comes with. I bought a new battery some time ago at Battery World, but they now don't stock them and they cost $70 or so anyway. And even with a small box from Jaycar connected to the charger to make sure it doesn't cook the battery too much, I still don't want to buy another Nickel Metal Hydride battery when all the modern drills are using Lithium Ion batteries.

Well, as luck would have it I recently bought several LiIon batteries at a good price, and thought I might as well have the working drill with a nice, working battery pack too. And I'd bought a nice Lithium Ion battery balancer/charger, so I can make sure the battery lasts a lot longer than the old one. So I made the new battery fit in the old pack:

First, I opened up the battery pack by undoing the screws in the base of the pack:

There were ten cells inside - NiMH and NiCd are 1.2V per cell, so that makes 12V. The pack contacts were attached to the top cell, which was sitting on its own plinth above the others. The cells were all connected by spot-welded tabs. I really don't care about the cells so I cut the tabs, but I kept the pack contacts as undamaged as possible. The white wires connect to a small temperature sensor, which is presumably used by the battery charger to work out when the battery is charged; the drill doesn't have a central contact there. You could remove it, since we're not going to use it, but there's no need to.

The new battery is going to sit 'forward' out of the case, I cut a hole for my replacement battery by marking the outline of the new pack against the side of the old case. I then used a small fretsaw to cut out the sides of the square, cutting through one of the old screw channels in the process.

I use "Tamiya" connectors, which are designed for relatively high DC current and provide good separation between both pins on both connectors. Jaycar sells them as 2-pin miniature Molex connectors; I support buying local. I started with the Tamiya charge cable for my battery charger and plugged the other connector shell into it. Then I could align the positive (red) and negative (black) cables and check the polarity against the charger. I then crimped and soldered the wires for the battery into the connector, so I had the battery connected to the charger. (My battery came with a Deanes connector, and the charger didn't have a Deanes connector cable, which is why I was putting a new connector on.)

Aside: if you have to change a battery's connector over, cut only one side first. Once that is safely sealed in its connector you can then do the other. Having two bare wires on a 14V 3AH battery capable of 25C (i.e. 75A) is a recipe for either welding something, killing the battery, or both. Be absolutely careful around these things - there is no off switch on them and accidents are expensive.

Then I repeated the same process for the pack contacts, starting by attaching a red wire to the positive contact, since the negative contact already had a black wire attached. The aim here is to make sure that the drill gets the right polarity from the battery, which itself has the right polarity and gender for the charger cable. I then cut two small slots in the top of the pack case to let the connector sit outside the case, with the retaining catch at the top. My first attempt put this underneath, and it was very difficult to undo the battery for recharging once it was plugged in.

The battery then plugs into the pack case, and the wires are just the right length to hold the battery in place.

Then the pack plugs into the drill as normal.

The one thing that had me worried with this conversion was the difference in voltages. Lithium ion cells can range from 3.2V to 4.2V and normally sit around 3.7V. The drill is designed for 12V; with four Lithium Ion cells in the battery, it ranges from 14.8V to 16.8V when fully charged. Would it damage the drill?

I tested it by connecting the battery to a separate set of thin wires, which I could then touch to the connector on the pack. I touched the battery to the pack, and no smoke escaped. I gingerly started the drill - it has a variable trigger for speed control - and it ran slowly with no smoke or other signs of obvious electric distress. I plugged the battery in and ran the drill - again, no problem. Finally, I put my largest bit in the drill, put a piece of hardwood in the vice, and went for it - the new battery handled it with ease. A cautious approach, perhaps, but it's always better to be safe than sorry.

So the result is that I now have a slightly ugly but much more powerful battery pack for the drill. It's also 3AH versus the 2AH of the original pack, so I get more life out of the pack. And I can swap the batteries over quite easily, and my charger can charge up to four batteries simultaneously, so I have something that will last a long time now.

I'm also writing this article for the ACT Woodcraft Guild, and I know that many of them will not want to buy a sophisticated remote control battery charger. Fortunately, there are many cheap four-cell all-in-one chargers at HobbyKing, such as their own 4S balance charger, or an iMAX 35W balance charger for under $10 that do the job well without lots of complicated options. These also run off the same 12V wall wart that runs the old pack charger.

Bringing new life to old devices is quite satisfying.

posted at: 08:41 | path: /tech | permanent link to this entry


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