Building a Linear Power Supply for DeWalt 18V NiCd Cordless Tools—Capacitor Sizing?

Hi,
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?
Any info would help greatly!

Lucas Riveli

The problem with nicad batteries is memory. That is if you only use them a little and then recharge them, pretty soon they are dead much sooner than using them until they completely die and then recharging them. My brother killed a fairly new nicad for his laptop by charging it every day after only 30 - 60 minutes worth of use.

Better to have your friend get a 2nd battery so that when the 1st goes dead he can be using the 2nd while the 1st is recharging.

Thank you Mike,
my friend does have other batteries,

However I still want to know if such a power supply is reasonably sound :)

anyone?

Hi Lucas.
Do you have means to measure the off load current that the power tool Draws.. I am assuming the 30 Amps you quote is the full load current. Also with a fully charged good battery can you measure the on and off load voltage and report back here with your findings.. The reason for asking is that the power tool will use electronics to control the motor speed and it is important that the voltage never goes much over 18 Volts in this case or damage could occur. Also the mains voltage varies it is also important to allow for this in the design

Regards Ian

Hi,

your comment on huge capacitors got me thinking - so apologies for throwing in some lateral thinking

As the finished result is to be static and size and weight don't seem to matter much (the ex-microwave transformer) - a possible solution is three 6V lead acid batteries (or suitably matched 12V and 6V) to give an 18V DC output, capable of many tens of amps (very forgiving of load spikes and stress) and able to accept a background trickle charge from a smaller transformer with little or no regulation.

Think of the baterries as several thousand Farad electrolytic capacitors!

Just a thought!

Jeff

Hi
That is an idea jeff. Maybe it would be a good idea to use a little circuit to disconnect the charging circuit while the drill is in use. Otherwise the trickle charger may take a bit of a beating unless it is current limited . A series resistor would most likley do the job in this application so as to keep things simple..
Ian