Change cordless drill cells to Li-Ion

I have a couple of drills that have completely dead batteries in them.

They are all 18v batteries. I took them apart and they are composed of 15 NiCD cells in a series arrangement.

What I am thinking about doing is changing these for 5 protected 18650s for a total of 18v.

Questions:

Would this work fine as long as proper polarity is observed?

Would the motor respond differently?

I understand that I couldn't use the original charger. I have a hobby charger that's designed for Li-Ion cells.

Am I missing something that would prevent me from doing this?

*edit* I am assuming that protected cells should definitely be used to protect from both under and over voltage, as well as current draw, correct?

I'd also use the cells with the charging tabs pre-installed to make wiring easier.

Is this the right place for this, or is there a more appropriate forum I can ask this at?

Ordinary protected cells are LiCo and can't take more than 2C discharge. This makes them completely unsuitable for most power tool applications.

You really want LiFePO4 or LiMnNi cells, which can handle much higher currents and don't blow up. As a result of their relative safety, they're (AFAIK) always sold unprotected. Of course, abusing them with overdischarge, etc. can still ruin the battery before its time -- but at least it'll go softly into that good night. One good source of these is disassembling Li-ion power tool batteries, but that could get counterproductive in your case.

Even if you knew your application would work with LiCo, you still wouldn't want to stack a bunch of individually protected cells. (BTW, I've never seen individually protected cells with solder tabs -- does someone actually sell them?) When you're building a permanently assembled pack, you want a single circuit for the entire pack, and you'll probably still want one with safe chemistry cells.

I understand you may be looking for an interesting and useful project, rather than simply to get replacement battery packs, but if you're just after functionality, you might consider getting e.g. Milwaukee V18 Li-ion battery packs, and simply stuffing the guts in your existing batteries whole? They have 5 IMR18650 or IMR26700 (depending on the model) cells and a PCB with fuel gauge preassembled, and if you look around for good deals, are just about as cheap as buying stuff separately to build your own.

Is there something wrong with using regular 18650s?

If I understand correctly they typically rated at a discharge of 3c. With the average being 2400mAh i'm looking at up to 7.2 amps safe draw.

I'm not sure what the draw is on most drills. Any idea on this?

There is enough room in the battery case to probably squeeze 10 in there to do 5s5p to double the capacity and the maximum current draw.

Can you provide any good links about building a protection circuit?