diy power adapter and battery packs for cordless power tools?

[jason41987]

i was wondering if anyones ever created their own battery pack using AA NiMh cells for cordless power tools?.... an NIMH AA battery is i believe 2.9Ah, 1.2v, meaning 15 of these would give 18v neccessary to power the tool and an Ah rating higher than the standard battery..

i figure if i create a project box capable of holding 15 AA batteries, three rows of 5 stacked on top of eachother wired in series, id get the voltage and the capacity i need... then i could plastic-weld the top half of an old battery pack on top of this so itll fit in the tool... then when the batteries all die from being too old, i can open up the compartment, empty the batteries, and buy a new pack of rechargables

question is... how would this work with the orginal charger...if i use the top off the old battery pack, it should fit the standard charger too

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another thing ive thought of doing is building a power supply.. like a power strip i can plug into my wall or a heavy duty extension cord thatll convert 110v AC to 18v DC...perhaps at a 1500w maximum output.. heck a 1000w maximum should be plenty to run multiple tools.. then i can just wire a socket to the tool itself and run a removable cord between the tool and the power supply

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so... what do you guys think of these ideas of lowering the total cost over the life of these tools, extend their usage long after they change the form factor for the batteries, and also give me the option to go corded when im at a work-bench and dont need portability?

 

[Nigel Goodwin]

so... what do you guys think of these ideas of lowering the total cost over the life of these tools, extend their usage long after they change the form factor for the batteries, and also give me the option to go corded when im at a work-bench and dont need portability?

I suggest you look more closely - you will find that most cordless battery packs already use standard sized cells (usually AA), simply cut the packaging open and replace them (as most people have been doing for decades).

 

[jason41987]

yeah, i know they typically use round cells... my idea was to have a battery door on the bottom i can pop open and remove the internals without much hassle.. if im not mistaken they are connected by strips of metal soldered together... but, it should be relatively inexpensive to buy bulk power cells and replace them inside the battery, shouldnt it?

and what about my idea of building a 1000w 18v DC power supply?

 

[Nigel Goodwin]

yeah, i know they typically use round cells... my idea was to have a battery door on the bottom i can pop open and remove the internals without much hassle.. if im not mistaken they are connected by strips of metal soldered together... but, it should be relatively inexpensive to buy bulk power cells and replace them inside the battery, shouldnt it?

Yes, and as I said, it's commonly done.

and what about my idea of building a 1000w 18v DC power supply?

Not much point having cordless tools and then add a cord to them?.

If you want a corded tool then buy one, they are much cheaper than cordless tools, and much more powerful as well.

 

[3v0]

Most of my tools sit unused most of the time so the only cordless one I own is a 12V drill.

All of my other drills, saws, routers and sanders are corded and generally last a long long time. About the only corded units I have had fail are craftsman.

 

[audioguru]

The old Ni-Cad cells in cordless tools can provide much more current than newer Ni-MH cells. For high current they need to be welded together with metal strips not with simple spring contacts used in toys.
Ni-Cad has cadmium that is extremely toxic and poisonous. Therefore they are not available as consumer products anymore.

 

[dr pepper]

I have re-stuffed drills for people a couple of times, the batteries were not quite standard, sub c seems to be popular in these kind of packs, but you can buy them from component suppliers.
Making your own is possible so long as the batteries you use are the same type as the charger, ie if the charger is meant to charge nimh's then use nimh's in the pack.
I've done this in reverse to, I was given a pack for a dewalt which fell down some scaffolding as was totalled, its now a battery pack for my pushbike lights.

 

[jason41987]

i believe milwaukee battery packs use 18650 cells in them

 

[jason41987]

yeah.. i just looked it up... milwaukee uses 18650 size cells, they use 5 of them in the compact battery, 10 in the extra capacity battery... the compact battery is 1.5ah and about 2.8ah for the XC... umm, however, i can get 2.8Ah 18650 cells, five for about $35, giving the compact battery pack the capacity of the bigger battery pack, but the weight and balance of the smaller one.... and if i put these batteries in the XC battery pack id have nearly a 5ah pack... wow, cant wait for these batteries to die so i can upgrade the cells

and the best part about using these lithium cells is theres only 5 of them.. just five, not 15, so its so much easier and less work to test for bad cells and replace when theres just merely five

on ebay i can get 18650 size cells at under $3 a pop at 3.6ah capacity... i mean.. wow, screw the idea about having a corded option, with an upgrade in cells i can just really overhaul the battery packs to have an absolutely enormous capacity for their size.. awesome...

in fact, im rebuilding a motorcycle into a cafe racer, bike will be kickstart only and id only need four of these batteries to replace my heavy lead-acid motorcycle batter... heh

 

[audioguru]

A lithium battery might catch on fire (an extremely hot fire that burns hotter if you spray water on it) if it tries to produce the very high current of a cordless tool. New cordless tools use lithium batteries but probably have a current-limiting circuit.

I saw an RC model airplane show. One airplane had a brand new expensive lithium battery. The battery burst on fire, the airplane was landed quickly and the burning battery was put in a can of sand. It burned for at least half an hour. The battery costed $500.00.

 

[jason41987]

hmm.. these are the same type of batteries the pack uses now, just more capacity

 

[audioguru]

hmm.. these are the same type of batteries the pack uses now, just more capacity

Please attach the datasheet for the batteries you want to use.

 

[ChrisP58]

Not all cells are created equally. And there is more that you need to factor into your decision than just size and capacity. And, in this case, the higher capacity may actually be a bad thing.

That is because one of the ways that the cell manufacturer increases the capacity is to use thinner foil, allowing them to put more surface area inside the cell. But the foils are not just the electrodes. They are also the electrical path for current to get in and out of all that surface and the cell can and top terminal. Making the foil thinner reduces the cross section of the conductor, as well as increases it's length. This increases the internal resistance of the cell, and reduces it's ability to deliver high operating currents like those found in power tools. Also, there is usually only one contact point along the of each electrode foil and the can and top terminal.

Cells that are optimized for high current use will have thicker foil, and redundant pionts of contact between the jelly roll and the case terminals. So they are able to deliver those high currents with less voltage drop, and without overheating internally.

 

[jason41987]

seems the only way to increase the max discharge on these battery packs is to have multiple packs wired in serial, wired together in parallel... i believe these power packs put out roughly 40 amps maximum... which is a tall order

 

[KeepItSimpleStupid]

If you really want to do things the right way would be to tag the pack with an ID and make the charger smart.

battery temperature and charge equalization are two things you need to be concerned about. Modes to remove memory effects etc. It's really tough to get good performance of ANY battery pack unless you can charge the cells individually or able to equalize the charge between them.

Lithium ION and Li-Po batteries always require a circuit to protect the batteries.

 

[jason41987]

these battery packs do have circuitry for this sort of thing... but right now im trying to figure out of these are 18650 or 26650 batteries inside.. i may have to open the pack and measure the size of the batteries

 

[KeepItSimpleStupid]

I'm not sure if the equalizarion type circuits have been adopted for small battery packs yet. There was an article in Circuit Cellar that went over some of the design details like putting a FET between each battery in series.

 

[jason41987]

im not so much conerned with the longevity of the battery itself, but getting more of a charge out of each pack

 

[jason41987]

i opened the battery pack.. found the batteries used inside it now and these are 20a rated batteries... so i guess thats my target, try to find the highest capacity with atleast a 20 amp max discharge

 

[MrAl]

Hi,

There is also a less expensive way to go. That is, put a nice heavy gauge wire on your cordless drill and connect it to a lead acid battery. Lead acid batteries are a lot cheaper. I have been doing this for years now with my drill as i got tired of buying new batteries for it. The drill itself isnt bad, it was just the TWO battery packs that came with it died after about 6 months and hardly any use. The next set i bought replaced all the old cells, and i built a very smart charger for them using a uC chip. But the life there was only 5 years anyway, so i decided that was enough and went to lead acid.
You'd have to see if your drill runs ok on 12v or else you'll have to rig up a series set of LA batteries. My drill was 10v so 12v worked out nice.

I also considered making a 40 amp "wall wart" type power supply for the drill for times when i could easily plug into the wall. Well the cheapest route there seems to be rectifying the AC to DC then using a buck converter, but the inductor that has to handle 40 amps comes out a bit high priced and i wasnt sure if i really needed this anyway so i didnt create the wall power supply after all.

But the lead acid battery solution worked very very well. You always have a wire going from the drill to the large battery which you might put on the floor, but it's not too much of a problem. I also have a shoulder case now that i can keep the battery in when using it so it's almost as portable as it was when new.