Thread: Power consumption of cordless drills - tech question

Two things -
1. Need to know the voltage to calculate the amperage
2. RPM is not relevant to current draw, except in free air. If you think about it, a drill going flat out may slow down to half speed when heavily loaded, and a stalled drill (0 rpm) draws maximum power... so how heavily you load it is the critical question.
Best way to answer the question is empirical - get some heavy duty alligator leads and use them to connect the removed battery via an ammeter so you can actually measure the current draw.

Ahhh yeah the voltage! 18v luggerite. Re the second bit - that's what I mean about the draw changing during thr drilling, but no way to measure it because I don't have that drill or the ammeter! That's why I was saying just pick a wattage of say 500, to cover a pretty decent draw (in fact I would have thought that to be very high for wood drilling with a really good brad point, which I do have).

Answer: 67 holes.
It's pretty meaningless though, as battery performance varies enormously with the state of charge, and the recovery time between holes.
The other thing is that 500 Watts is actually very high for a cordless drill. Simple application of Watt's Law says that an 18V drill is drawing 27 amps to consume 500 Watts - this is why the batteries get hot! (I = W/V so 500W / 18V = 27.7 Amps)
Each hole takes 5 seconds (5/3600 or 0.001389 of an hour) at 27.7 Amps so 0.001389 x 27.7 = 0.3845 Ah per hole.
2.6 Ah battery life / 0.03845 Ah is 67.6 holes.
In fact, I'd expect only 2/3rds of that before the drill became too slow to go on...

Well I'll be damned! That is precisely the answer I got on 12th Feb last year, and tonight I was seeking independent verification or correction, mainly because I recall someone saying that Amp hours actually meant something different to what you and I obviously agree on. Good on you luggerite, made me day you did!

Edit: and yes I understand you have to have all those caveats, and is reasonably meaningless unless you nail down all the other vague factors. Actually I would think that you'd get many,many more holes of the type I'm thinking of - you know, a typical woodworking shop type of hole in timber.

Yeah, I did simplify things cos this was only very ballpark theory. Actual Amp hours is subject to all sorts of weird things like duty cycle and Peukert's equation and a million miles of bumpf. Essentially you need to know what the manufacturer meant when they said "2.6 Amp hours" - what their testing regime for that specification was. It's cos of these variations that you might notice these days that car batteries are no longer quoted in Amp hours, but in CCA - Cold Cranking Amps - because this has a strictly defined testing regime.
That's why I summarised all of that by adding in the bit about "2/3rds" of that predicted life.
This link adds a bit more info about the issues (although it is written with lead acid rather than Li-Ion batteries in mind)
What is an Amp Hour and How to Calculate Battery Capacity | OverlandResource.com – Overland expedition travel info.

Originally Posted by FenceFurniture

Edit: and yes I understand you have to have all those caveats, and is reasonably meaningless unless you nail down all the other vague factors. Actually I would think that you'd get many,many more holes of the type I'm thinking of - you know, a typical woodworking shop type of hole in timber.

Depends whether you're drilling jarrah or radiata pine!

Remember that while the older style of batteries go slower and slower, the Li-ion tend to maintain their full performance (not quite the case in reality) until they reach a point of discharge and stop dead, which when I think about it, may have been what Luggerite was refering to when he said they would stop at 2/3rds

Regards.
Paul

Hi Paul, these would be new batteries. Typically, my Makita 18v drill goes and goes and goes, and then doesn't, as you say, but I would say that 95% or more of power is used first at full (or desired) output.

This is the same reason that I favour Bic cigarette lighters over the cheap(er) and nastier ones.

Originally Posted by FenceFurniture

Hi Paul, these would be new batteries. Typically, my Makita 18v drill goes and goes and goes, and then doesn't, as you say, but I would say that 95% or more of power is used first at full (or desired) output.

Hello Brett

I don't know at what point they cut out. I might try putting a multi meter on the batteries and see if I can get a handle on that one. Perhaps others know more of the design parameters.

Regards
Paul

Brett

I've done some quick checks on my Panasonic 14.4V impact driver. It has a 3.3ah battery, but the principle is still the same.

Just be aware that my multi meter is a cheapie and at best a guide. It is about $40 worth and not a "Fluke", which is the brand that many electricians favour. So what I am saying is look at the percentage more than absolute values. Because of that I have rounded the figures to one decimal place althouh my meter does display two decimal places.

I had a battery ready to go fully charged and it read 16.5V. Although I expected more than 14.4V, this was a little higher than I was expecting, hence my coments above.

The battery already in the impact driver indicated 13.5V and I had been using it for some time. I started to run it down, but realised that unloaded it might take a while. So I got a hex head self drilling timber screw about 40mm long and proceeded to drill it into radiata pine. At the end of the first screw it hesitated. During the second screw it stopped several times. That was it. Closer to dead than I had thought. I measured the voltage at 12.5V.

This represents about 75% of the full voltage and I suspect is the point at which it is designed to cut out. I don't know if this is for protection of the motor or the battery. It is beyond my knowledge.

Out of interest I fitted the fully charged battery and drove a screw. It was about twice as quick as the earlier attempts I made.

Voltage is probably the limit for the drill working. AHrs are the time it takes to get to that point, and this will vary according to the workload asked of the drill.

I saw a test conducted by "AWR" a number of years ago. They drove chipboard screws in to (funnily enough) chipboard. The performance of each dill was measured by how many screws they could put into the material.

Regards
Paul

Good on you Paul, that's useful info. When I said 95% I was referring to available useful power if you know what I mean, rather than voltage.

Isn't this a bit like "how long is a piece of string"

Only if you want to find out how long the string is.