18v Battery Conversion AC to DC

Old 11-03-05, 06:55 PM
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18v Battery Conversion AC to DC

A simple question for the experts....I need a AC-DC converter to simulate a 18v battery pack (2400mah)14 cells in the pack. Any ideas?

120V AC -> 18 v DC (able to support a power tool) Although the battery pack reads ~20 v on a multimeter

Would it be better to be a little over or under?

I was considering using one of my old dead battery parks inorder to plug into one the tools and inserting a disconnectable plug/fitting to accept power from the converter.. Not sure what converter to use and why the dewalt battery packs have 3 copper tips on the battery and if this can be simulated via AC/DC converter.

Last edited by ToolBox; 11-03-05 at 07:35 PM.
Old 11-03-05, 07:26 PM
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Regarding the third terminal I think it must be involved in the temperature of the battery pack which is of no concern to my set-up and is easily bypassed.

Anybody have any ideas on a converter or if this will even work?

Old 11-03-05, 09:27 PM
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The battery may be rated for 2400 milliamp hours.
We don't know what the drill pulls.
It may pull 4800 ma and drain the battery in half hour, or 2400 ma and drain the battery in one hour.

If you can find out what your motor pulls under load, we may come up with something.
Like some cars, the battery is rated for 60 amps but it can supply 250 amps to your starter for a shorter time.

I would try to stay with 18 volts or lower, but you will need a power supply with a current rating above what the motor pulls.
That maybe 3 amps to 10 amps or something.
Old 11-04-05, 04:50 AM
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NiCd and NiMH batteries (commonly used in power tools) when fully charged are between 10 and 15% over 'nominal' voltage. Your 18V pack will quite reasonably be up to 21V when fresh off the charger; your power tool should be able to safely deal with 20V, though it might overheat more rapidly if you provided it with a 20V supply.

As GWIZ mentions, we don't know what the drill pulls. Most power tool battery cells are capable of '10C' discharge, meaning that if the cell is rated at 2000mAh, the cell is capable of safely being used at 10 x 2000mA = 20A. Your drill could conceivably use up to 20A, though it probably uses less. Under load, the voltage of the battery will drop, so that same pack which measures 20V fresh off the charger, might measure 16V under a 10A load.

What you need to do is to find a power supply which can provide the full current required by the drill, with a voltage that sags roughly the same as the battery pack under the same load.

I would start by borrowing a good 'lab' power supply, one that lets you adjust its output voltage and which measures its output current. You would need something capable of at least 20V and 10A, possibly more. Then use this power supply to run the drill, determining how much current is required under full load (the largest drilling load that you are going to use). In particular, experiment with letting the voltage 'sag', say by adjusting the supply for 16V, and trying the drill out running a load, to see if the performance is sufficient and to see what the current requirements are. After some time playing with the system, you will end up with a specification for 'I need an '18V' power supply with a maximum unloaded voltage of A (probably about 20V), a maximum current capability of B (5-20A), and a minimum voltage under full load of C (15-18V).'

Old 11-04-05, 08:37 AM
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Probably cheaper to just go and buy a corded drill then to try and power a cordless.
Old 11-04-05, 10:17 AM
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I just forwarded some questions to dewalt customer service regarding:
maximum unloaded voltage, maximum current capability , and a minimum voltage under full load.

we'll see how that goes...

Actually I have more than just a cordless drill on these 18v packs..... 3 saws and cuttout tool.

Hitachi actually makes a ac/dc converter for there 18v tools which might serve as a alternative if available and the cost reasonable

Thanks to everyone for their advice.... I'll let you know dewalt's response to my questions..
Old 11-04-05, 09:52 PM
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DeWalt customer service.....Worthless BUT that's what I expected...

Looking into that v18 hitachi ac/dc converter right now
Old 10-16-10, 11:22 AM
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I know this is an old thread but google indexed it so I decided to update it and join the forum too.

Click here - Convert your battery charger and or a worn out battery.


A very easy way is to buy 1 or 2 used 19v 90 watt laptop chargers. Some Dell, Toshiba and some other brands use a simple 2 wire + / - wire end. The diagram will show that a "normal" tip is - outside and the inner female part is +. Use a cheap multimeter to confirm. You could even use two in parallel to double available power.

You could also use a "good" or decent battery pack with one laptop power charger which would supply extra power when needed. If you hook up the multimeter and use the tool and monitor voltage drops that along with performance compared to a new battery will indicate whether you are getting enough power. Don't forget - cordless tools are designed with tolerance for volt drops -as this is a normal part of battery consumption.

I am getting ready to do this and will post a how to when I do.

Shopping: craigslist (post a wanted Ad: " will pay $$$ for laptop battery packs"), ebay, thrift shops, etc.

Last edited by mrbios; 10-16-10 at 11:28 AM. Reason: add info

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