30A power supply for ex-cordless power tools

Discussion in 'The Projects Forum' started by bobsuper, Jan 26, 2012.

  1. bobsuper

    Thread Starter New Member

    Mar 14, 2011
    I had this idea of putting together a power supply that would make it possible to run cordless power tools plugged in the wall.
    The reason for this is to not waste the batteries' life when wall power is available. (This is a friend's situation...plus his old nicad batteries are getting, well, old)
    And it is a good little winter project.
    So given my limited electronics experience, I thought a linear power supply would do the job, but I want to know opinions of more knowledgeable people..
    The power tools are DeWalt 18v Nicd.

    I figured a microwave oven transformer with new 18 volts secondary going into an appropriate bridge rectifier followed by a humongous capacitor (I figured 4 x 33,000uF rated @ 20 A ripple current) would do the trick.

    Obviously there are things I haven't considered....and I know a switch-mode power supply would be a lot better, but I am trying to keep it simple, albeit inefficient! (switch-mode is a bit outside my range for now)

    What kind of ripple current can I expect for 18 volts @ say 30 amps?
    (how many amps do cordless tools actually draw on average?)
    Any info would help greatly!

    Lucas Riveli
  2. ifixit

    Distinguished Member

    Nov 20, 2008
    Brushed DC motors don't care if the DC is filtered, so you won't need a capacitor, or a regulator. If it is a non-center tapped secondary, then just use a 35A bridge rectifier.

    Keep the secondary winding impedance as low as you can by using a low gage wire that still fits and gives you 18VAC. You will lose a volt or two across the bridge, but the drill should still work very good. It is a challenge to get as much torque as you could get from a battery.

    Try it and see if you like it.

  3. bobsuper

    Thread Starter New Member

    Mar 14, 2011
    Thanks for your reply!
    So the tools will work OK on 60hz pulsed DC? That's amazing...I will try it and see how it performs.
  4. Experimentonomen


    Feb 16, 2011
    I'm running an old Atlas Copco cordless drill off a PlayStation2 power supply, works pretty good for what i do.
  5. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    I'm running an old Sears cordless drill/driver on an old H-P printer desk-top switch-mode power supply. :)


    Senior Member

    Jul 1, 2008
    I have no problem with the concept of powering a cordless power tool from a mains power supply but to extend the life of the batteries?.....Eeh not so much.

    Nicads that just sit on the shelf for extended periods end up needing defibulation. :D

    The average home owner, who buys a cordless tool, uses it for his current project and then lets it sit on the shelf or in the charger for months, will be giving the batteries the kiss of death. Contractors, on the other hand, that use them daily, with drain and charge cycles, get good longevity from them.

  7. Experimentonomen


    Feb 16, 2011
    The trick is to put the battery in the charger when it starts to get tired, not
    when the battery is completely out and the tool doesent have the umphf to rotate a full revolution.

    Completely discharging the battery to less than 1V per cell is what kills them.

    Why batteries die in the charger is because these chargers are built for speed charge and will thus overcharge the battery if left in the charger for too long.

    For long term storage, a battery should be kept at about 80-90% of its full capacity, this is true for atleast lead acid batteries for emergency backup systems and the like.

    Senior Member

    Jul 1, 2008
    A Nicad is not a lead acid battery. Most Nicad (drop in) rapid chargers will switch to trickle charge when the battery voltage reaches a predetermined point. If they're left in there for months it will still kill them. The best thing you can do for a Nicad is use them often.

    Considering the current level of our technology it bugs the hell out of me that upper end chargers don't have a storage mode switch that would mimic use/charge cycles. It wouldn't have to cycle daily but maybe every three or four days or so.

    Come to think of it.... this sounds like a worthwhile project. ;)
  9. bountyhunter

    Well-Known Member

    Sep 7, 2009
    Not if the charger is properly designed. Ni-Cads can tolerate sustained charge rates of c/10 or more indefinitely without harm. NI-MH can not, but Ni-Cads certainly can. You may have the infamous memory effect (short cycle) from a NI-Cad that's been on float charge a long time, but a couple of cycles will fix that.

    It is true deep discharge kills NI-Cad packs but for a reason not often considered: you typically have ten or more cells in series in the pack. The lower capacity cells get fully discharged first and get "reversed" as the rest of the cells push current through the load. Forcing current the wrong way through a NI-cad kills it really quickly.
  10. CDRIVE

    Senior Member

    Jul 1, 2008
    Sometimes there's a world of difference between tech specs and 'real world' application. Prior to retirement I spent over 25 years as owner of a LMR/Marine Electronics shop. The statements that I made in that post were derived from my customers experiences over that time. My policy was "no warranty on Nicad packs" because the manufacturers wouldn't either.

    One of my customers was a large hospital with a very large engineering staff. All the techs carried Motorola HT's that were used daily and charged every night. My service records on these radios consistently indicated longevity of >= 5 years. On the other hand, the ER used the Medcom band with three HT backups for their base station. Because they were rarely used and sat in the chargers for very long periods, they were totally unreliable.

    FYI, the state and county EMC and EOC would periodically send out circulars regarding this issue with instructions to not let them sit idle for long periods. On the marine end, the USCG did the same. Both of these agencies compiled their data from real world use.

    The bottom line: The customer could care less about the technical aspects of Nicads. Cell polarity reversal & memory effects are just technobable to them. I still feel chargers should have a 'cycle mode' for Nicads that are used only rarely.

  11. RFactor

    Active Member

    May 1, 2009
    I will agree with CDrive, NiCads like to be used. I have a lot of experience with them in medical devices. A NiCad that sits in a charger or worse, is unused will fail prematurely. I read somewhere years ago that if you are going to let a NiCad sit for a long period of time it is best to discharge it for storage. Don't know what the accuracy of that is though. At the hospital where I work we had a program of discharging and recharging NiCads monthly.
    For the power tool situation I would use the battery until discharged and then go to the AC power. In my own experience with power tools I have found that it is best to have 2 batteries. One in use, the other charging. Three batteries would be even better if you are doing lengthy jobs.
    The other thing to consider is, is it really a NiCad battery? Many battery drills these days use Ni-Mh or Li-Ion.