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Batteries for drill

  • hybridfiat Friday, May 27, 2011 8:25 PM Reply
    Id like to use 6 of these batteries in a 12v drill I have that requires a new battery. The new battery would cost $130 and I want to use 2 x 4 in series to make a high amp 14.8v powerpack. The drill runs happily on a spare 18v I have so I dont see the extra 3v as a problem.
    Can anyone see an issue?
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  • viamer Sunday, May 29, 2011 1:19 AM Reply
    I made similar thing, 2x3, 11.1 V, but with similar SKU.20392. I got a lot of problems with protection circuits. After all I disassembled batteries and removed protection PCBs. It works well now but I had to output balancing connector, buy balancing charger \SKU.35190 and discharge alarm SKU.10787. Another problem is quite high internal resistance of ordinary Li-ion. 2x4 battery will give high rotating speed without load, but the maximal force under high load will be the same as 2x3, because of voltage drop. Maximum current is limited by battery internal resistance even without protection circuit. It is maybe about 10 A per battery. I thought "No, my little screwdriver will never sink 20A!" but it does, under load. Now I think I'd better buy Li-Po battery for RC models, they has excellent discharge currents for the same capacity and price, and they already has balancing connector.
    Excuse my poor english.
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  • ItalianBoy Sunday, May 29, 2011 1:49 AM Reply
    Use high power LiFePO4 batteries such as the A123. They are 3.3V each, so you could use 5 of them in series to give your drill a little boost. They are not so picky about balancing (a 4.3V zener diode in parallel with each is enough to prevent damage) and supply 20A without problems. They come in 18650 1100mAh and 26650 2300mAh.
    Pros: cheap, high power, easy to keep in balance, longer lifespan, very flat discharge rate
    Cons: less capacity

    I made a 6s1p pack for an 18V cordless drill with the 18650 and they have no problem feeding it even under heavy load. I added a fast charging plug and am now using a modified laptop power supply with a DC LED driver in series (that provides current limiting - I also added a schottky diode to prevent reverse current) to fast charge the pack in 1.5h and it also gives you the option to use the drill while in charge. Sweet!
    Oh, all for 30$ and 2h of work.

    RC battery packs have at most 50-100 cycles of life. Very bad.
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  • viamer Sunday, May 29, 2011 5:39 AM Reply
    Good idea, ItalianBoy! But, remember, you should prevent over-discharge.
    I heard about short life of Li-Po's but today I have read interesting things on internet:
    - Li-po battery withstands 2-3 years and 400-600 cycles when discharging at 2C current and charging balanced without overheating. (>1000 cycles on Wikipedia)
    - Big charge & discharge currents reduces life for all Li batteries.
    - After 2008, technology of Li-po batteries was significantly improved. Hi-current batteries (10C-40C) were designed.
    In RC helicopter, battery works about 15 minutes - average current is 4C. Overheating is very probable for such discharge rate. It can be easily overdischarged, usually there is no limiting circuit. IMHO thats why it dies after 50 cycles.
    Excuse my poor english.
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  • ItalianBoy Sunday, May 29, 2011 5:46 AM Reply
    LiFePO4 are much tougher than LiPos and do not suffer that much from overdischarge. However, as soon as you feel that the drill loses power, just charge the pack and you'll be fine. I had though about making a LiPo pack, but then considering the price and complications I though it wasn't worth it.
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