Max draw out of protected 18650

[Fallingwater]

I want to buy four of these, with the intention of using them in a model drill (Dremel lookalike) to make it cordless. I haven't yet decided if I'll use three or four cells (the drill is rated at 7 to 18 volts).
The drill draws about an amp when spinning but not being used for anything, but the draw increases considerably when it has some resistance (such as when using a cutting disk). I need the cells to endure draws up to two and possibly even three amps.
I know 1C or slightly higher discharge is fine for the cells themselves, but I'm not sure if the protection circuitry will allow it.

 

[TigerhawkT3]

The circuits probably aren't set at less than 4A. Many are set at 5A. I would go with more popular (expensive) cells like AW's or Pilas, as the quality on the smaller brands isn't always good. Turbo DV8 got a couple Ultrafire/Trustfire/something 10440s that just couldn't handle his L0D-CE, while my AWs were just fine.

 

[mdocod]

Power tools can draw high current when loaded, electric motors are like a dead short when stalled out. I would not use normal li-ion cells in that application at all.

either build a NIMH pack, OR, if you really want to go with something li-ion, use 4 of these cells:
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=3737
they are LiFeP04 and very safe, they can deliver loads up to around 15-20 amps safely, and if you stalled the motor and managed to draw more temporarily you wouldn't have any real safety issue as I'm quite confident they could probably deliver temporary "burst" rates of 50+A with minimal damage.. And even when damaged that chemistry does not react violently. You take a hit on capacity but I think it's well worth it IMO... install your own low-voltage limiter to help preserve the cells from being over-discharged..

FYI: I'm pretty sure this is the same *type* of cell being used in the new "black and decker VXP" system (is that the name? VXP? not 100% sure if that's right now that I think about it)...

 

[Fallingwater]

Well, if the current draw exceeds the safe point the protection circuitry should open the circuit. That's what it's there for, and that's why I'm not getting unprotected cells...
And it's not like I'd keep the drill stalled for an hour, anyway. Actually I don't think I ever stalled it while operating it...

 

[mdocod]

the other question is, how high is the current going to ramp just to spin the thing up... Bulbs often exceed the current limit of the cell for a fraction of a second, if they do if for a few too many fractions of a second before they get warmed up thy trip the circuit, a motor takes a lot longer to spin up than a bulb does to get hot. hook up that current meter again and take a look at what happens when you turn it on. (So I know you don't intend on stalling out a motor all the time, but keep in mind that it always starts from a state of "stall" when "off.")

 

[Yenster]

I don't believe there is any protection circuit in these batteries that would shut off current. These 'protected' batteries don't have the short-circuit protection, so these will put out a LOT of current without tripping....let's say enough to melt a tailcap switch spring (yeah I've done it). According to the specs on that switch, that's about 7 amps.

 

[Fallingwater]

I have some supercheap protected RCR123s that are very definitely short-circuit safe. I know that for a fact, as I accidentally shorted one and it went open-circuit before any damage was done. Putting it in the charger for a second or two restored it to normal operation.
I'd be surprised if big fat 18650s weren't protected against this sort of thing.

The motor in the model drill takes longer to spin up than that in my AC powered Dremel clone. The Dremel is almost instant, while the model drill takes maybe a fifth of a second, and so doesn't draw very much current. I don't think I've ever gotten it to draw more than 2.5 amps, and that was when trying purposefully to stop it to see how much I could get it to draw.
It's not a large tool.

I'll take measurements again tomorrow.

 

[Yenster]

I have some supercheap protected RCR123s that are very definitely short-circuit safe. I know that for a fact, as I accidentally shorted one and it went open-circuit before any damage was done. Putting it in the charger for a second or two restored it to normal operation.
I'd be surprised if big fat 18650s weren't protected against this sort of thing.

Fallingwater, I assure you that these Trustfire 18650s from DX don't have the short-circuit protection, so they will not 'go open'. However, these Trustfire 18650s do, and can be restored to normal operation by putting in the charger just like you mentioned.

EDIT: Sorry, I took closer look at my batteries and realized that it was these other batteries that fried by tailcap. These other batteries don't have any PCB on the bottom so they must be unprotected.

 

[LEDite]

Fallingwater;

There is no way to know the actual design specs.

Manufacturers of quality cells have data on maximum discharge rates on their web sites. That's the only way to know the correct answer.

My guess on these cells would be ~2.7 amps (1.8 amps x1.5C).

Larry Cobb