Instructables

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)


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!

I have to agree with ironsmiter here. His comment rather refers to max current draw under operation that each li-ion cell can handle (which is way lower than NiCd or NiMH batteries); not to be confused with the low voltage cutoff aspect.

Distributing the current-burden on each cell is achieved; as suggested by ironsmiter; by having two or more in parallell for each planned cell. This distributes the current draw from each cell by I/nC, where I is current and nC refers to the number of cells in parallell. Thus for 7.4v rionsmiter suggests 2p2s (2 in parallell 2 times in series) amounting to a total of 4 cells.

A look at the spec sheets from major battery manufacturers on 18650-cells gives some valualbe insights on the limitations of these cells . Li-ion cells need much more "babying", than previous batteries. Risk of fire if badly treated highly present. A note worth making for people doing this @ home for the first time.

Great project! Kudos.

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 billgeo1 year 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
KarolJ29 days ago

You have to implement a low-voltage cutoff circuit between your battery and the drill. Otherwise, you will run the risk of discharging the cells beyond a sacred point for Li-ion and your battery will die (or heavily degrade).

billgeo (author)  KarolJ28 days ago

Yeah, you are right! Thank you for the comment!

There should be a low voltage cut off there,

and I am making one using an ATtiny micro, because when there is a lot of current

draw on the battery the voltage dips a lot momenteraly and thats problematic when using comparators.

However, I reality I found that the drill becomes useless for lack of power before the LiPo cells reach a point of destructive discharge, so you charge them or replace them anyway...

KarolJ billgeo27 days ago
Agreed. ATtiny solution is pro! Thumbs up.
onemoroni11 year 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.
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.
makya, I think you ment to post rather than reply to my post. Good information.