AC Power Pack for Cordless Power Tools

[Bob Scott]

A couple of weekends ago, we found out that our roof leaks BADLY. This is not the time of year for roof shingle replacement. This is the rainy season in Vancouver, and will be until Spring sometime. (The early failure of the shingles is subject to a class action suit against the now defunct asphalt shingle maker, but that's another story.) So we screwed a giant cheap tarp with camo coloring onto the roof using 3 inch screws and 2X4's to hold it down. All we needed was my son's power drill for tools. Here's where the problem begins:

The cordless drill/screwdriver had two batteries, both dead from lack of use. They are not used much because we are not professional power tool users and seldom use them. I bet a lot of home owners have this problem with the modern battery operated power tools: batteries dead when you finally need the thing. So, I found my corded 1/2" drill vintage circa 1950 in the garage. (I rescued it free from a trash pile a while back. It's as old as I am, but all it needed was gear lube.) That did the job.

Does anyone else think that making an AC power supply for cordless tools would make a good project article? Gut a dead battery and install a switching power supply inside the battery case. Of course you then have a corded power tool, but at least you don't have to hunt around for a 1950 Zephr drill and an extension cord.

[CeaSaR]

Happy post holidays Bob!

Sorry to hear about the roof and the ensuing battery problem. Both really bite.

I think that would be a good idea for an article. How many people could actually
use something like that? My guess is, quite a few. Look at all the battery "replacers"
sold. Think "As Seen On TV". Come up with a generic board that can change voltage
enough to suit the range of tools out there and you'll be the next "ASOT" millionaire.
(Me, I'll invest in the extension cord market place, should this take off.)

Hmmm, you've pointed out a real problem with these tools for the general masses.
They lose charge and eventually capacity over non-use time. Maybe you could
come up with a battery tender that would top it off every couple of days and run a
full discharge/charge cycle once every 2 weeks or so. Yeah, it may sound wastefull,
but it can actually lengthen the life of a battery, with the added bonus of keeping
the battery charged for use. (Sounds like a business proposition to me... )

CeaSaR

(Remember, you heard my idea here first... )

[MrAl]

Hi Guys,


Actually i happen to have a ton of experience with this issue, not that i am happy about that though

As you probably have guessed, i had problems with my portable drills in the past too. Since they came up with every single drill i had and very bad with the last one i bought, i decided to do something about it. I ended up with three or four fairly good solutions, each with their own advantages and disadvantages, but nonetheless pretty good. See if you agree.

1) The problem of the battery pack always going dead because you didnt charge it recently.
Solution: Built a special charger with a timer that only charges the battery pack once per day, just enough to replace the cells internal self discharge. The self discharge is what makes the pack go dead while you are not using it, and thus when you go to use it it doesnt work and you have to wait for a charge.
Now i built a special charger with voltage sensing and all the buttons and LEDs you can think of, but there is a more simple way to get this solution. That is, to use a plug in AC timer and set it for one hour a day or maybe even less. This will keep the pack up to charge but will not burn up the cells as what happens if you leave the charger plugged in directly all the time.
This solution lasted 5 years, the life of the new NiCd's i replaced. Replacing the cells again would take me another 5 years, but instead i turned to the next solution.

2) Use a lead acid battery instead.
If you dont need to use the drill every day then this is a good solution. This comes in two forms: (A) you buy a 7.5Ahr battery that will power the drill, or (B) you buy a jump pack made for automobiles (usually have around 15AHr battery or better). Either way, you get 12v any time you need it and you can make up your own cord for the drill. I happened to have another battery pack that i gutted and converted to a cord with 12v cigar lighter plug on one end and so that plugs into the drill and into the jump pack. I used a 12v cigar lighter jack for the smaller 12v lead acid battery and use that when i use that smaller battery. The issue of how to carry this around with you comes up too, so what i did was use a small camera case and stick the 12v 7.5Ahr battery into that when i want to carry it with me. The big jump pack is much bigger and so is harder to carry, but i did carry that a few times.
The nice thing about using a 12v 7.5Ahr battery is that you have roughly 4 times the run time that ordinary NiCd cells will bring because that's the higher ampere hour rating at work which of course means much longer run time for your drill.
Charging is about once every 6 months if you are not using it.

3) Use a computer power supply 12v at whatever amperes you need.
For this i have to use one of my higher power 12v computer power supplies complete with little jumper to turn it on and little load resistor on the +5v supply. It works, but i dont have the full current output yet as i need to get a slightly bigger power supply, or at least one that can put out more amps on the 12v line.
Because i design all kinds of electric circuits i of course carefully considered building my own 40 amp power supply just to power the drill. The homework required is minimal, but the problem is that the inductor cost gets a little high. There's also the issue of where to get the 12vac to power the supply (or whatever voltage is needed). I considered using a 35 amp transformer. I also considered running off line, which would not require a transformer, but then the issue of isolation comes up. Using an inductor only means the drill would not be isolated from the line. Using a specially wound transformer like those little 'regulated' dc wall warts use might work, but i didnt want to get into winding a 40 amp transformer.
I figured i might as well use the PC power supply as i didnt have to buy anything.

The issue of how to measure your drill current comes up too, and the solution to that seems to be to use a clamp on ammeter capable of going up to 50 amps or 100 amps DC. Note here that some clamp on ammeters only work with AC and not DC, so you have to make sure the spec's say DC also or else it wont work at all. My drill draws 35 amps stalled so i want at least a 40 amp power supply.

Oh yeah, all of the solutions above require either a 12v drill or a 9.6v drill or something like that. I suspect that an 18v drill will also run off of a 12v battery just a little slower than usual, but i have not actually tried this so you'd have to experiment a little if you want to use a 18v drill. A 14v drill should work just fine though.

As a side note, in Europe i have read that they are now starting to use NiZn cells in portable tools. I dont think they are doing that in the USA yet however, or if they ever will. The run time may or may not improve a little, but the lead acid will still beat out any small cell like these.

[frhrwa]

I really like the idea of a charger that charges on a programmed time frame, then discharges the battery on a timed program also.. that way you get the discharged battery recharged like it had been used, hopefully extending the life and not giving it that "memory" function (short life span charge).. that would be a marketable item for sure!!.. someone can make some serious money with all the cordless tools available..

[MrAl]

Hello again,


Mine lasted 5 years and i didnt have to discharge it, just charge it every day a little bit that's all.

[Einar M]

For 2400 mAH batteries typical self discharge rates are 1 mA for NiCd and 100 uA for NiMH. Rather than a timer setup it would be easier to supply this current thru a resistor, posssibly putting a LED in series so you can see whats going on. If the LED is too dim then add a PNP trans this way: connect E to V+ , B to the current setting R to B+ , and the C to the LED thru another R and gnd. I don't have any long term experience to say how well this will work ; the 5 yrs Al lists isn't all that great ; 10+ would be what I would shoot for.

[dacflyer]

i hate that the drills are getting bigger and heavier.
i still use a 7.2 volt drill , i have rebuilt the battery pack about 6 times so far over the last 15yrs.
ya thats right, 15+ yrs. a older model B&D drill.
only thing i hate is that the ni-cads i put in it, tend to only last a year or so, before they start loosing capicity.. usually i'd goto the hobby shop, but a 7.2V R/C battery pack, and slightly modify it so that it will work in the origional battery pack case.
the charger it has is a brute force charger, the battery pack has a thermal switch in it.
when the thermal switch trips, then the charger shuts off. you press a button on the charger to start the charging process.
might try nickle-hydrades next time..

[MrAl]

For 2400 mAH batteries typical self discharge rates are 1 mA for NiCd and 100 uA for NiMH. Rather than a timer setup it would be easier to supply this current thru a resistor, posssibly putting a LED in series so you can see whats going on. If the LED is too dim then add a PNP trans this way: connect E to V+ , B to the current setting R to B+ , and the C to the LED thru another R and gnd. I don't have any long term experience to say how well this will work ; the 5 yrs Al lists isn't all that great ; 10+ would be what I would shoot for.

Hi there,


A few mA into the battery might not work, because the charge acceptance goes down quite a bit for lower currents. For higher currents it is much higher thus if you use a higher current for a short period of time it tends to work pretty well to replenish the self discharge.
I agree however that 5 years isnt as great as i had hoped, but it isnt too bad for those cells.

If you try any experiments let us know...

[MrAl]

i hate that the drills are getting bigger and heavier.
i still use a 7.2 volt drill , i have rebuilt the battery pack about 6 times so far over the last 15yrs.
ya thats right, 15+ yrs. a older model B&D drill.
only thing i hate is that the ni-cads i put in it, tend to only last a year or so, before they start loosing capicity.. usually i'd goto the hobby shop, but a 7.2V R/C battery pack, and slightly modify it so that it will work in the origional battery pack case.
the charger it has is a brute force charger, the battery pack has a thermal switch in it.
when the thermal switch trips, then the charger shuts off. you press a button on the charger to start the charging process.
might try nickle-hydrades next time..

Hi there dac,

Yeah i still use my 9.6v drill but i considered using a 12v one too.

When you charge the pack do you keep it charging all the time? One year sounds like constant charging which will kill the pack in a year or less many times. Try the timer and see if that helps.
NiMH cells dont charge the same way NiCd cells do, so i would guess they would die even sooner.

[dacflyer]

i do not charge them, til they start to get weak. and then once on the charger, it only charges for about 20-30 min, then cuts off..no trickle charging. i only charge as needed. i use the drill bout once a week or so..

[MrAl]

Hi dac,


Oh ok, well it could be low quality cells then. I had two packs made with very low quality cells and they only lasted for 6 months even with careful charging only when needed.
Maybe try a different supplier.

[haklesup]

Some good suggestions to preserve the battery. Excatly what you do for NiCd, NiMH and LiIon do differ somewhat though.

In any case I want to address the question about the feesability of the AC adapter.

I see this is a similar problem to the question, how do I make an AC-DC power supply to operate an automotive amplifier in the home?" The answer to that is; it will cost you more to make a high current DC supply than it would to buy a purpose built AC powered home amplifier in the first place.

The DC motor in the cordless drill is a high current load especially when really torquing a screw or bolt. You can look at the battery pack peak current or tools spec sheet to get an idea of the size supply you need. IN any case, a 12V (or 9.6V or whatever) 5 to 10A supply isn't dirt cheap. At least you don't need a regulated supply but you need one beefy enough so that voltage does not drop out under load.

Maybe its not that bad on cost for a 2A to 10A supply but it's probably bigger than the battery. A project would be squeezing a supply with those specs into a battery pack case. You have to worry about form factor, heat dissipation and AC line safety. Its a great idea but why don't the tool makers offer them, there must be a reason. Even if you don't really need them, people would buy them if the manufacturers showed them it could be useful through advertizing so I think the market does exist. I suspect the engineered result would not meet expectations either from a cost or performance standpoint.

A similar thing goes on with cell phone chargers. Ever notice the car chargers top off a battery in less than an hour and the wall charger takes several hours. You could have a faster home charger but it would be big, hot and expensive.

[MrAl]

Hi again,


Hackle:
I have found that i need a PC computer power supply to power my drill. Mine takes much more than 10 amps to power. In fact, i have a nice 10 amp 0-30v power supply that will not power the drill correctly because with only 10 amps the torque isnt nearly enough.
I have measured the stall current at 35 amps. It runs on a PC computer power supply that's made for 16 amps but i suspect it is putting out more current than that. I hope to get a higher current PC computer power supply at some point, but i dont use the drill that much anyway and when i do i usually use one of the 12v lead acid batteries instead. The 7.5Ahr battery and the 15Ahr battery run it just fine with plenty of torque.

So i guess what i am saying is that if someone really wants to run off the mains supply then a PC computer power supply is probably the cheapest route, unless they want to build a non isolated off line DC supply or they can get a surplus heavy current transformer that puts out 12v (or whatever they need) and build a bridge rectifier out of some nice heavy current diodes and a few caps.

The lead acid batteries work out pretty well though, and i just charge them after use.

[Dean Huster]

Problem is that NiCd cells have such low internal resistance that it's difficult to make an AC power supply that can deliver the current required by a nearly-stalled drill that's ramming in 4-inch deck screws. And the series resistance of the power wiring is of major consequence. As cheaply as you can get 120vac drills, it'd be much better to go that route rather than try to jerry-rig something that will work poorly best and still keep you teathered not only to the AC mains, but to a bulky power supply to boot.

[dacflyer]

my suggestion is if you want a battery that will last all day, build you a battery belt, like the tv camera guys have, a 12v 7ah battery would last a lot longer than the drill battery itself.
just make sure to use heavy enough wire from battery belt to the drill..
cheaper than to try to make a PS..
a fanny pack works good to hold the battery, oh don't forget to fuse it