Extending a drill battery: cable size

I had a great plan to make an extension lead for my 18V Makita drill. I bought a broken drill and a dead battery off eBay; my plan is to cut the handle off the drill and strip out the battery and connect the two with a cable so that you can keep the drill battery in your bag or stuffed down your top to keep it warm (in cold Alpine caves).

Allegedly my 18V drill has a max power of 400W, so should pull around 20A max. You can then through various online voltage drop calculators rapidly reach the conclusion that you need chunky cable if you don't want to drop more than 0.2V or so (over 3m or so). So off I went to buy some 3 core YY cable with 16mm^2 wire... (I only really need 2 fat wires but you need one to carry the battery thermistor signal).

As I'm sure anyone more familiar with cabling will know, 16mm^2 is a BIG cable. In fact the 8m of cable (to do a pair of battery extenders) weighs about 4 kg...

Now I reckon I can drop to 4mm^2 while only losing about half a volt, but according to this page:
https://www.elandcables.com/cables/yy-cable
even though there is only a quarter as much copper in 4mm^2 instead of 16mm^2 the cable still weighs over 200g a metre.

I know other people have done this - what did they use? (have they just failed to consider voltage drop properly?  :-\ )

I could always run the thermistor cable in a thinner wire on the outside to get it down to 2 core but that seems messy.

Imperial have done something similar [1, 2, 3] for previous generation 14.4 V Makita drills, although we use LiPo hobby batteries rather than proprietary Makita ones. I don?t know off hand  exactly what cable diameter we use (and can?t check as the drills are currently in Slovenia for our expedition [4]), but would guess <3 mm diameter copper roughly, but length probably not much more than a meter.

I have to say 4 m cable sounds very long to me. Apart from anything else I imagine it would be quite easy to get in a tangle whilst bolting.

We based our design off some tests done by YUPC [5] in which it was found that the drills have a bit of tolerance in terms voltage they?ll take without complaining (indeed the LiPo?s we use are 14.8 V).

It would be interesting to know how much of this transfers to newer Makita products, does anyone have any experience?

[1] - https://www.union.ic.ac.uk/rcc/caving/newzealand/articles/2015-03-12-drills.html
[2] - https://www.union.ic.ac.uk/rcc/caving/newzealand/articles/2015-03-15-newdrills.html
[3] - https://ukcaving.com/board/index.php?topic=24719.0
[4] - https://ukcaving.com/board/index.php?topic=25199.0
[5] - http://durmitor.yucpc.org.uk/misc/drill_batteries.php

andrewmc: your figures surprised me, but they are right.
If you want to keep the voltage drop down to 0.2V at 20A in your 3m of cable you need a conductor area of just over 10mm^2.
I have only seen one remote battery setup, and I think it used 2.5mm^2 twin & earth, so there would have been a fair voltage drop when it was really working hard.

But the internal resistance of a Li Ion cell (is that what Makita use?) seems to be about 150 milliohms at cave temperatures, so 3V will be lost across that at 20A.
That 60W input will warm the battery up quite quickly, and the internal resistance seems to drop to less than 0.1 ohm at about 30C, so things will get better.

But if you are still going to lose perhaps 4 times as much warming the battery up as warming the thinner cable, is it worth going for the thin cable?

Your reasons for thinking of doing this sound great and people seem to agree with your numbers.

The creativity of making your own drill battery sounds exciting, just about as exciting as waking up on the sofa with a homemade affair spitting molten plastic and carpet behind you and having to throw it out of the back door before it explodes...

I have no first hand experience of this, but I have helped to remove a carpet after it happened, thankfully I was out caving when it occurred.

If you value a low home insurance premium (and your life) then it may be a good idea to stay away from this kind of tinkering.

The connector contact resistance is far more significant than the cable resistance. Use 2.5mm sq silicone rubber insulated. Drop me a PM if you'd like me to send you some.

We use 16mm ho7 rubber cable at work for 63amp cables so think you have the wrong size.
4mm is for 32amp so check the specs on HO7 rubber cable, depending on temperature you may need artic flex?

I think mud is typically a bigger factor than anything else when it comes to resistance.

We (the YUCPC link above) used some fat-ish mains cable and it was fine. If you do a DIY battery then some resistance is beneficial for LiPos (which have a much higher power density than LiIons).

FWIW, having used both standard drill batteries as well as the sort of set-up you describe, I have a preference for standard drill batteries. Extra cables are a just a faff in my opinion.

The aim is not to modify my drill or my drill batteries in any way, just to provide an extender so you can - optionally - connect the drill and the battery via a cable (with the appropriate terminals at either end).

littlelegs said:

We use 16mm ho7 rubber cable at work for 63amp cables so think you have the wrong size.
4mm is for 32amp so check the specs on HO7 rubber cable, depending on temperature you may need artic flex?

Click to expand...

Those rated amperages will probably be based off 230V, max wire temperature, assumed wire packing density and allowable voltage drop based on regulations which are probably not relevant to this project. You can put far more than 32A through that cable before it actually melts (or more likely the insulation melts) but the voltage drop will exceed regulations.

andrewmc said:

The aim is not to modify my drill or my drill batteries in any way, just to provide an extender so you can - optionally - connect the drill and the battery via a cable (with the appropriate terminals at either end).

Click to expand...

Ah, sorry. I misunderstood. We didn't notice any real difference between having the battery directly attached to the drill and connecting the same battery via the mains cable when we ran the tests linked above. I think we may have done some similar calculations before then, but decided to just give it a shot with what we had available, which worked fine.

Do you have a feel for what difference it makes having the drill battery warmer in a ~0 degree environment?

MarkS said:

Do you have a feel for what difference it makes having the drill battery warmer in a ~0 degree environment?

Click to expand...

yes - absolutly - my main drill = 28v dewalt with lion batteries - and on a conservation job [ above ground ] - in sub zero conditions - the batteries had to be " incubated " by CNCC // NE volunteers - to deliver any usefull work -

straight out box - they died in seconds - despite full charge before job . but incubated for 15 min - inside people clothes - they all gave full preformance despite the cold

i think they had to be re warmed after lunch breaks - but in " constant use " - they kept preformance

Rubber cable (at least H07) is heavy but I'm not sure the YY cable will stand up to use...
The Arctic cable is lighter, though.

so a 18V system is usually 5 cells in series. 3.6v * 5 however thats their "resting" voltage. Most chemistries will be charged at 4.2V so thats 21V and so is "only" 19A. I would expect the power to drop as the battery voltage drops. It doesn't help much but it is 1A lower.
(https://hackaday.io/project/21052-18v-li-ion-drill-battery-pack-rebuild).
at 400W a 2Ah battery would last ~ 5 minutes too so I suspect that the 400W might be a lie

Under load our 14.4V Makita battery voltage dropped to 12.6 V from a fully charged voltage of 16.1 V. We were pulling 23 A drilling 8 mm holes with that.