Switching power supply for power tools? - Page 1
Forum section: electronics / projects

author: maverick
author_rank: Newbie
timestamp: June 04, 2012, 01:39:48 pm
content: Hi guys, I have a number of Makita 18V lithium-ion cordless power tools. Since replacement batteries are rather expensive and the fact that I'm normally not too far away from a power outlet, I was thinking about possibly powering them with an 18V switching power supply. I could simply drill a hole into the case of a dead battery, pass a power cord through it and use the existing contact system, however, I'm not too sure about how powerful a power supply I'll need. I have measured the running current draw by the drill from the battery and it is about 5 amps. However, the starting or inrush current is about 30 amps! What is the correct way to cater for this inrush current? And is there a way to cater for it without having to buy a massive 30 or 40 amp power supply?Just for info, I found that the battery packs actually uses 10 Li-Mn cells in 5S2P config. The specification for the individual cells is 1.5 amp-hours and a max discharge rate of 10 amps which equals roughly 7C. So given the 5S2P config, the pack should be have a capacity of 3 amp-hours and be capable of a max discharge rate of 7x3amps=21 amps. This initially (before my current measurement test) led me to think that a 25A power supply would be adequate.Thanks very much!

author: digsys
author_rank: Unknown
timestamp: June 04, 2012, 02:03:40 pm
content: ... oooorrrr you could use a trick I've often used. Make up an 18V battery pack with enough capacity to ~run your longest session.Set up a trickle charger (or smarter) to keep it "topped up", The battery pack then supplies the heavy load current AND takes careof the back EMF spikes !! That way you only need a simple / cheap charger to look after storage. Everyone wins :-)

author: SeanB
author_rank: Super Contributor
timestamp: June 04, 2012, 04:07:55 pm
content: I did the same, just used a 12V 7Ah SLA battery on a short lead to replace the 14.4V really expensive to replace battery pack. Tossed the original 1h charger and just use a simple trickle charger to keep a battery charged for it.

author: FenderBender
author_rank: Super Contributor
timestamp: June 04, 2012, 10:01:18 pm
content: I don't really see the problem with a corded drill/saw etc. I actually really like corded tools and electronics because you never have to worry about batteries dying or any of the associated problems.However, a 18V 25A supply is not only hard to find but REAAAALLY expensive,. I would estimate between $200-300 for a reasonable quality one if you could even find one. Whereas batteries, like a SLA will be able to accommodate your high current needs without needing a ridiculous power supply to do the whole thing for you. You can simply charge it when you are not using it and run your tools off the battery when you do need it.

author: johnwa
author_rank: Frequent Contributor
timestamp: June 05, 2012, 09:41:02 am
content: Quote from: maverick on June 04, 2012, 01:39:48 pmYou will probably find that the current is significantly higher when the tool is under load.Quote from: maverick on June 04, 2012, 01:39:48 pmThere is nothing in the tool that will limit the current to within the specifications of the batteries. The stall current could easily be 50A or more. While there will probably be a fuse of some sort, I would expect this to be sized not to blow under momentary motor stall conditions. Having said that, I think your estimate of 25A is probably reasonable for normal operating conditions.There is probably nothing too critical about the power supply voltage - you could maybe take it up to 24V for intermittent use, or use it at 12V with some loss of performance. This might give you more options for power supplies. The only possible concern, other than being mindful of the duty cycle, would be the voltage ratings of any electronic speed control. I think some manufacturers sell tools in a range of different voltages, and use the same motor in all of them. They just put more cells in the battery packs for the more expensive ones...Quote from: SeanB on June 04, 2012, 04:07:55 pmYes, I have done this too, made up a long lead to clip the drill onto a car battery. You really have to watch the voltage drop in the wiring at these power levels - I found that a few metres of 0.75mm2 figure 8 cable really killed the performance.

author: maverick
author_rank: Newbie
timestamp: June 06, 2012, 07:10:09 am
content: Thanks to all who've responded so far. Given the lower cost and high current capability of lead-acid batteries, I think that is probably a better option. It would also allow me to go at least "semi-cordless" after I've charged it. The only problem is that after a bit of googling, I can't seem to find any lead acid batteries in the 18V-20V range. They seem to go 6, 12, 24. Since three 6V batteries in series would be a pain in the butt, would it be possible to combine a single 24V battery with some kind of voltage regulator? Keeping in mind that if I go with a voltage regulator, it would need to be capable of handling high output currents. Thanks again!

author: digsys
author_rank: Unknown
timestamp: June 06, 2012, 09:11:47 am
content: QuoteNONO, You're back to using silicon and no better off than originally. Your choices are 12V or bite the bullet andmake up 3x6V pack or use 2nd hand laptop LiION packs , often avialable cheap, many at app14.4V.If you have to go with a 24V SLA etc, just use a lamp etc in series to dump the 6V ... it is wasting energy though.

author: maverick
author_rank: Newbie
timestamp: June 07, 2012, 04:00:10 am
content: @digsys: It would seem to me that the 24V SLA is the only practical choice with enough voltage. My question now is, if I go with this option, does the resistor I choose have to have a wattage rating high enough to cater for that inrush of 30A? Thanks.

author: digsys
author_rank: Unknown
timestamp: June 07, 2012, 04:31:43 am
content: QuoteThere are MANY ways to put together a nice high power resistor. First, you need to calculate ~ a value. The way I'dcalculate it is - Using the drill at ~ 80% load, I'd want to wash off say 8V (a 24V battery is ~26V in fact). ie SO at Full lockI'd want just under the 18V, a reasonable safety margin. At light loads the supply will be over 18V, but that doesn'tmatter much. There's not enough energy to overheat the motor.As far as making a resistor - you can rat an old bar heater, and just rewind the wire around a non-metal rod.There's heaps of places you can find dern nice heater wire, cars etc If you want it looking nice, there are plenty of100W-200W wirewound resistors around in industry. I use a bank of 9x 100W 20R to discharge EV battery packs.On one occasion, when a motor controller hiccuped, they got hit with 750A+ at 200VDC for 1-2 secs. It was spectacular,and the resistors are fine (OK a little stretched :-) )