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Convert a 7.2V cordless drill to Lithium power

Picture of Convert a 7.2V cordless drill to Lithium power
So...lets say you happen to get you hands on a beautiful old-school BOSCH PBM V-1 cordless drill.

But it came with a couple of drawbacks:
A) The Ni-Cd battery pack is useless (makes sense after all those years).
B)  It didn't come with a charger.

So you come to the logical conclusion. The cost of a new battery pack is more that you ar willing to pay,
and its impractical to rebuild the pack with new No-Cd cells as you don't have a charger to begin with!

So what do you do???

Convert to Lithium Power of course!  :-)

All you'll need is the following:

1)  2 x 18650 li-ion UNPROTECTED batteries (branded is better than generic Chinese) 
2)  One holder for 2 18650 batteries (cheep on eBay or locally)
3)  A small rotary tool (I used a Dremel with metal cutting discs)
4)  Soldering iron (25W should be enough)
5)  Hot glue gun (super glue, optional)
6)  Tool to open up the old battery pack (I ended up using the Dremel)

Its really simple, so...lets begin!
 
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Step 1:

First thing first...

You need to determine the voltage of the pack you have on your drill.
This should be  printed on the pack itself.
My pack was a 7.2Volt one (using six Ni-Cd sub-C cells).
So it makes sense to use two 18650 lithium batteries (at 3.7V nominal voltage each)

So we need to replace the old dead Ni-Cd cells with the new lithium batteries.

First of all we take the old battery pack and pry it open.
Some are easier than others. Some even have screws but others (like mine will require a Dremel)
Try not to do much damage as we will need to put the bottom half back on.

The only thing we will need from in there is the battery pack contacts and a small plastic piece that holds them.
Discard the old cells properly (and be careful of any toxic acid residue on them)


onemoroni19 months ago
I just started seeing this idea and it is great! I see many great rechargeable tools in the thrift stores and thought what a pity the cost to get new batteries, but now they can be easily re purposed. It would be nice to have more technical information and sources.
makya onemoroni19 months ago
I hate to say it like this, but if you are not used to using LiPo batteries, you should not use this idea. Lipo fires are bad! These are predominantly used in R/C race cars, and are required to be charged in a fire-proof container, as they become unstable if they drop below 3.2-3.4 volts per cell, and can spontaneously combust.
if you do use Lipo, make sure you use a dedicated LiPo charger and a low voltage alarm like I listed above.

IMO, you are bettter served with a Lithium Ion pack, which are more stable and can also be bought at R/C plane suppliers, and are more stable at low voltage.
onemoroni1 makya9 months ago
makya, I think you ment to post rather than reply to my post. Good information.
ironsmiter1 year ago
have to say, going with unprotected cells because of under-current issues is DANGEROUS!!!

There's a REASON they put current limits into those protection circuits.
Draw the power too fast, and you can kill your cells dead.
You'll need replacement cells every couple months of regular use.
Just not worth it.
Remember the big to-do a few years ago with the exploding and burning laptop batteries? The reason today's batteries are so much safer is because they changed the protection circuitry so the cells aren't pushed as hard.

A MUCH better solution, if you have room, is to use a 2s2p battery setup, with protected cells. That will MORE than double the power, AND make the individual cells last a lot longer. At the least, use a protection circuit with a higher rating. Consider looking at "pack protection" circuits.
If you DON'T have room for all that, consider BETTER protected cells.
They are out there. Some Redilast and Tennergy can source over 4 amps!
Sorry, bad form on my part... SHOULD have started with
GREAT IDEA! and a lovely write up.
I love the fact that you're saving that old workhorse from an untimely demise.
The saying "they don't make them like they used to" is VERY true. Especially for these old, nearly indestructible power tools.

This is Reduce, Reuse, Recycle at it's best.
billgeo (author)  ironsmiter1 year ago
It's true you started off a little "strong"... :-)

However my experience with using this drill is that you don't end up
over-discharging the batteries simple because of the "human factor".
In other words the drill will have no power and you will pull out the
batteries to recharge them before you can actually do damage to them.

But I am planing to do some more measurements and calculations
in the near future, and then I will be able to get back with more info on that.
But I still think there are no protection circuits that can provide the current
the motor needs to run (and also be cost effective).
makya billgeo9 months ago
there are Lipo alarms available that monitor the voltage of the cells and start beeping when they hit low voltage without damage to the cells. the alarms don't limit current, just monitor voltage. they plug into the balance lead on the battery
http://www.hobbyking.com/hobbyking/store/__41185__On_Board_Lipoly_Low_Voltage_Alarm_2s_3s_USA_warehouse_.html?strSearch=lipo%20alarm
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