Help with idea

[azazzera]

I have a dewalt 12v cordless drill with a battery charging unit. I was wondering if I could cut off the 120v plug and replace with a 12v plug, whould the unit still charge the battery for the drill. I do not have an inverter yet and thought that this might work. Thanks.

[zesterjester]

First off, I don't know. But I think not. Read the panel under the charger, it will give you info on the voltage in and out. (Usually, it will say 120, or 100-240 etc for input voltage, which is no good for you. Might only be ac, too, there will be a dashed line and/or a solid line to show this) as for cutting the cord off, maybe for nicd, but I don't think for nimh, and definitely not for lithium batteries. The reason being is the charge state is controlled by electronics, it is not a simple two batteries reaching equilibrium scenario. Not positive, so maybe you should ask someone else, but if no one else answers, I'd think of something else.

Like soldering wires onto the battery clips on the drill, and putting alligator clips on the other end.connect this straight to your house battery. Done this, it works.

[tenders]

Small inverters are so inexpensive, and the charger requires such little power - why bother? It is inefficient to go from 12VDC to 115VAC back to 12VDC, but you're talking so little power it really won't matter.

[azazzera]

Don't know under my ide why it would be 12v to 115 back to 12v. I think it would be 12 to 12 v. My only questions are would the 12 v input on a charger designed for a ac input still work. And would it damage anything. I wouldn't have to put a 12 plug on the charger because I already have a standard 115 outlet wired into the 12 v system.

[ndutton]

A few thoughts:

• Supplying 12VDC to a charger intended for 120VAC sounds like a problem. None of the circuitry in the charger will work properly such as the AC/DC rectifier and state of charge circuitry that shuts the charger off when the battery is fully charged.
• Speaking of which . . . connecting the drill battery directly to the boat's 12VDC source won't have the full battery shut off feature either risking damage to the Ni-Cad battery.
• Now, about this:

I wouldn't have to put a 12 plug on the charger because I already have a standard 115 outlet wired into the 12 v system

The blade configuration of plugs and receptacles is unique to voltage and amperage systems. The idea is if the plug fits, the voltage is right. Having a 120VAC outlet in a 12VDC system defeats this safety design. Example - you can't physically put a 120V plug into a 240V receptacle and for good reason.

[Ken Rockwell]

I vote no.

[azazzera]

Thanks for the replies. I had a thought that the circuitry involved would be a limiting factor. I understand the safety concern about mixing systems. I know this is not a reason to use a 120v plug as a 12v but I don't have a 120 system. Other than bad practice, useing a 120v outlet to conduct 12v has no problems. The electrons don't care do they.

[sdemore]

It sounds like you are trying to use 12v to power a 120v circuit. If the drill battery is 12v, why not build a charger that would hold the battery and plug directly into a 12v outlet? You don't need the 120v circuitry for anything, just the 12v going to the battery.

[ndutton]

Thanks for the replies. I had a thought that the circuitry involved would be a limiting factor. I understand the safety concern about mixing systems. I know this is not a reason to use a 120v plug as a 12v but I don't have a 120 system. Other than bad practice, useing a 120v outlet to conduct 12v has no problems. The electrons don't care do they.

We try to provide the best information we can. What's done with the info is up to you. Your boat, your choice.

[tenders]

Small inverters are so inexpensive, and the charger requires such little power - why bother? It is inefficient to go from 12VDC to 115VAC back to 12VDC, but you're talking so little power it really won't matter.

I meant that plugging a $15 WalMart 60 watt cigarette lighter inverter into the boat's 12V system, and then plugging the drill's charger into that inverter, is inefficient (12VDC to 120VAC to 12VDC) for the small amount of energy you're trying to get into the drill battery. But it is an overall small amount of energy, so it doesn't matter much.

Putting 12VDC across the charger's 120VAC plug prongs might do nothing, and might fry the charger. It definitely won't charge the drill. The charger converts the AC voltage into DC voltage using a couple of diodes, then steps down the voltage with a transformer and possibly a transistor.

Another thing that won't work: running AC power tools using inverters with wattage ratings close to the wattage of the tool. My 75 watt electric drill doesn't even start up on a 150 watt inverter. But the inverter will simultaneously power multiple battery chargers perfectly.

[zesterjester]

connecting the drill battery directly to the boat's 12VDC source won't have the full battery shut off feature either risking damage to the Ni-Cad battery.

I'm sorry, just to be clear, I meant removing the battery from the tool, then wiring the tool straight to the house battery. No battery except the boat battery.

[Peter]

The charger converts the AC voltage into DC voltage using a couple of diodes, then steps down the voltage with a transformer and possibly a transistor.

Not quite - the step-down transformer comes first, as transformers only work with AC. Following the transformer is the rectifier (the diodes), then a filter to smooth things out, probably a regulator and then the charging circuits.

As tenders stated, putting 12 V DC into the 120 V AC cord will not charge the battery - it will simply run a DC current through the primary of the transformer. Likely will not damage the primary, as it is designed to run at 120 V AC.

Peter

[JOHN COOKSON]

Somebody clue me in here.
If there is a step down transformer that steps 120V to 12V wouldn't 12V be stepped down to 1.2 volts?

TRUE GRIT

[sastanley]

I meant that plugging a $15 WalMart 60 watt cigarette lighter inverter into the boat's 12V system, and then plugging the drill's charger into that inverter, is inefficient (12VDC to 120VAC to 12VDC) for the small amount of energy you're trying to get into the drill battery. But it is an overall small amount of energy, so it doesn't matter much.

Putting 12VDC across the charger's 120VAC plug prongs might do nothing, and might fry the charger. It definitely won't charge the drill. The charger converts the AC voltage into DC voltage using a couple of diodes, then steps down the voltage with a transformer and possibly a transistor.

Another thing that won't work: running AC power tools using inverters with wattage ratings close to the wattage of the tool. My 75 watt electric drill doesn't even start up on a 150 watt inverter. But the inverter will simultaneously power multiple battery chargers perfectly.

I like tenders idea..I think it is the safest solution and probably less hassle than building your own, and probably a LOT cheaper than irreparably hacking up the battery charger. I have one of these little 12v cigarette lighter inverters for powering the Crock Pot in my wife's trunk, but I used it on the boat a lot too for powering things like a laptop charger, or doing exactly as you hope, charge battery powered tools using their 120v charger on a 12v circuit..my boat has no 120v AC power either.

Another caution is to be sure the circuit supplying the 12v to the cigarette lighter is up to snuff and rated for the loads of the inverter you might plug into it (proper sized wires/fuse/etc.)

https://www.amazon.com/dp/B01H2XD2DY/ref=cm_sw_r_cp_dp_T1_Y6znzbS27XQTR

[Peter]

Somebody clue me in here.
If there is a step down transformer that steps 120V to 12V wouldn't 12V be stepped down to 1.2 volts?

TRUE GRIT

A transformer only works with AC - not DC.

The physical principle is electromagnetic induction - Faraday's Law - which states that a changing magnetic flux through a closed loop induces an emf (electromotive force) around that loop. The magnetic flux is the integral of the magnetic field over the loop.

The AC current passes through the primary which sets up a changing magnetic field. The primary and secondary are "flux linked", which means the field produced by the primary goes through the loops of the the secondary, producing magnetic flux in the secondary. When the current in the primary changes, the magnetic field changes, hence the flux changes, and a voltage is induced in the secondary.

If the current through the primary is constant - i.e. DC - then there is a magnetic field set up by the primary which goes through the loops of the secondary. But, because it is a constant field, the magnetic flux is constant and hence no voltage is induced in the secondary.

A voltage will be briefly set up in the secondary when the primary is energized, because it goes from zero field to some finite steady field, but once we have constant current in the primary, we get no voltage in the secondary. This turn on time will be a few milliseconds, depending upon the inductance of the coil and the resistance in the primary circuit. The time constant is the ratio of the inductance of the primary divided by the resistance of the circuit. Typical values for inductances are milli henrys, and the resistance of the primary will be on the order of a few Ohms, hence the millisecond time scale for the turn on.

Hope that helps.

Peter

PS - the flux linking is accomplished by winding the primary and secondary around a core made of "mu metal" sheets. The mu metal is chosen to "focus" the magnetic field - it is energetically more favourable for the field to be in the mu metal than in air. It is put into sheets to reduce losses due to Eddy currents.