[HH] universal power tool battery

[Tom Metro]

Greg London wrote:
> ended up getting makita 18v lithium ion tools.
> Their charger will fully charge a 18v, 3amphour battery in 30 minutes.

When I was looking at various power tool battery packs that might be
repurposed, I considered those. It's a "flat" pack that slides into the
socket, right? And if I remember correctly, it comes in both a full
capacity and smaller "light" version.

The problem with the premium power tool brands is that there is often a
sizable markup on the batteries, so if you are picking a battery to be
adapted to other tools, it isn't the best choice.

I did, however, run across many threads on model airplane forums where
people were buying up Dewalt LiOn battery packs[1] because it was
actually an economical way to acquire the A123 made cells. (A123[2][3]
is a local battery manufacturer; An MIT spinoff that makes a unique iron
phosphate (LiFePO4)[4] version of LiOn batteries that is supposed to be
safer (eliminates thermal runaway) and have other superior electrical
characteristics.)

1. http://www.flyelectric.ukgateway.net/lithium-a123.htm
2. http://www.a123systems.com/
3. http://en.wikipedia.org/wiki/A123Systems
4. http://en.wikipedia.org/wiki/Lithium_iron_phosphate_battery

It could be argued that Dewalt is not as much of a premium tool brand as
it was prior to being acquired by whoever it is that owns Black &
Decker. The latter's mid to low-end tools come out of the same factories
and use some of the same parts as the Dewalt products now. Both product
lines use A123 batteries in their LiOn packs.

Also, unless you need the protection circuit supplied with the Dewalt
pack, the A123 cells are now readily available through distributors and
are probably cheaper bought that way.


> Not sure how li-ion compares to nimh...

I believe by volume the capacity is no better or even worse, but LiOn is
lighter, and thus a higher energy density, but most importantly provides
far more charge-discharge cycles. Around 1000 vs. a few hundred for
NiCd/NiMH. They also offer low self-discharge, so they have good shelf life.

They have a flatter discharge curve, with a pronounced 'knee' at the
end. That combined with the over discharge protection circuits will
result in a tool that appears to just stop working without much prior
warning, compared to the way a NiCd tool will just wind down. (I've
heard contractors complain about this until they get used to it.)

A big difference I noticed when switching fro NiCd to LiOn in my
Craftsman cordless drills is that is is very easy to trip the over
current protection. Try drilling a 3/4" hole in a beam and if the bit
snags at all, the battery shuts down. After sitting in the charger for a
few minutes, it's usable again. I haven't yet figured out whether its
just a matter of time (say for a PTC to cool off) or if the charger
actively does something to reset the battery.


> I have beat the crap out of their tools and they keep kicking ass.

The wind blew my Hitachi 12V LiOn impact driver off a 2-story roof onto
pavement, and aside from the battery popping off, it was unharmed. So
yeah, pro-quality tools can be impressively built.


> What little I've read says the tool somehow communicates with
> the battery, or the battery communicates with the tool.
> So, I wasn't sure about putting them in series.

Makes you wonder what sort of information they are exchanging. The need
for such communications is far greater when you have "dumb" packs, but
all of the LiOn packs I've heard of include protection circuitry. For
example, "dumb" NiCd packs often have a temperature sensor embedded
inside wired to the pack's connector so the charger can halt charging a
pack that is overheated from use or charging.

The LiOn protection circuit does need some high power silicon to act as
the cut-off switch when the safety circuit trips. Conceivably a
manufacturer could save money by using that transistor in a PWM circuit
controlled by electronics in the tool as a cheaper way to implement
variable speed.


> like an oscillating multitool for doing plunge cuts.

This is a good example of a specialty tool that you don't get a lot of
gain out of it being cordless. I don't use my oscillating multitool
often, but when I do it tends to be for a prolonged period, for things
like sanding, or cutting through metal lath backed plaster. Dragging out
a cord for that isn't much of an inconvenience, and you gain a more
powerful tool that runs indefinitely.

So although I've seen a bunch of cordless versions, I've been satisfied
with the inexpensive corded version from Harbor Freight:
http://www.harborfreight.com/variable-speed-multifunction-power-tool-67537.html

(I also own their older, single-speed model.)
I've never used the Fein original that pioneered the market segment, but
the HF version seems to do the job fine.

I feel the same way about "Dremel" type tools. I owned an early cordless
 model and found the battery weight and run time to be far more of a
hindrance than the cord it eliminated.

 -Tom