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AC Switchmode Battery Charger to DC Input modification.

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Newbie level 2
Nov 22, 2010
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I would like to modify this charger to run from a DC source.
I could just just run it from an inverter but that seems like a lot of wasted energy in conversion
12vdc > 240vac > Charger

Also would like to keep it all in the current case if possible.

In the old days of transformer based chargers it was easy enough, find the rectified voltage DC voltage.
Insert new supply here. :)

We have a dual battery charger from milwauke.
Multi voltage charger M12-18C

PCB images attached.

Looks like a dual voltage output.

There is 2 opto's to provide some feedback between the primary and secondary side of the system.

The charging smarts is provided in battery so all the charger needs to do is supply the voltage the batteries needs for charging itself.
Cut off on any error conditions from the battery.

The plan was remove the primary side.
Supply the required DC voltages to the secondary side using off the shelf DC-DC converters.
Figure out what I need to do with the two opto feedback IOs.
Hopefully it is just a straight on/off and not pwm or something more complicated.

Questions before I break the scope out.
Is this likely to work?
Any potential issues I might have missed?

Thanks in advance :)


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Without a circuit diagram, you have little chance of getting your modifications to work. Even if the battery contains some electronics, it is most likely that they just provide a control voltage back to the charger. That 4 pole connector says it all?

Rats, I was hoping I would not have to look up datasheets and trace it.
Unfortunately I could not find a service manual for the device.

Showing my limited understanding on SMPS, would the primary throttle back if the secondary was using less power?
ie Battery almost full.

Depending on the design, the actual charging current is set by the temperature of the battery!, well almost, say 2.2A when the cells are cold then dropped to say 1.8A when the cells get to 50degs C. When the voltage of the cells hit their charged voltage, the current is switched off and the green LED comes on.
My 18V NiCad drill has a 12 - 24V charger, that is what it says on its case. It has the temperature measuring diode in its battery pack, so I understand how it sets the current, but I do not understand how it knows when to switch of my 18V battery, when it could just be a discharged 24V battery. It could do it by measuring the actual internal resistance of the battery, which will start high and get lower as the battery charges, but goes high again if the cell is overcharged?

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