convert that old 12V battery charger to 24V

I did this over the weekend. For the 12/24V switch, I used a big knife switch, since I did not have a more 'normal' 25 amp switch laying around. It cost me about $3 for the rectifier and I suppose a SPDT switch for 25A would run about $5.

I try to explain how the charger's component parts work for those who are not electronically inclined. This is an old Sears 25-amp charger, about the size of a lunchbox, which is not a regulated type of charger, just an old-school one. I bought it from a pawn shop 'as-is' for $5 and found the rectifier diodes to be shorted out. I bought a bridge rectifier of 50A/100V rating from a surplus store and wired it back like the original, using only half the bridge. The mounting was simple because the monolithic block the rectifier is built into is insulated and just mounts with one screw through a hole to the rear wall of the charger to dissipate heat. I used some silicon grease to help the heat to be conducted away. Later I decided that since the voltage was available from the transformer due to the full wave center tap arrangement, I could go with a full bridge and get 24V for charging the deuce on occasion.

Most people would prefer a heavy duty toggle switch for the 12/24V selection, but the switch I used was already on hand, and is also effective in preventing people from wanting to borrow the charger! Only low voltage is exposed, not really a concern.

Now that is cool! Thanks for the info.

I just got done converting one of my battery chargers to a 12V/ 24V system. Lucky I had 2 600v 40A rectifiers laying around from a previous project.
Pictures to come.

cool, it is nice to see there is some intelegence out there!

Thank's now I dig for my old charger!

As promised.

Picture #1 shows the additional 2 rectifiers added, I used 10 Gauge primary wire to make the connections (good for 30 amp) I paralleled the wire for a combined load of 60 amps. The rectifiers are rated at 40 amp each.

Picture #2 is the charger that I used.

Picture #3 Shows the voltage when set on the 6 Volt 10 Amp setting

Picture #4 Same as above but on the 12 Volt Setting 10 amp

Picture #5 12 Volt 40 amp

The voltages shown where taken while the charger was unloaded.

I hooked the charger up to my XM818 yesterday. The batteries were low enough that I tried to start the truck a couple days ago and ran them down before it could start.

The weather had not been above freezing for a week before I tried to start the truck, not the best conditions to try to start it after it had been sitting for more than a month.

Results were as follows with the batteries in that condition.

On the 12v 10 amp setting the charger was putting out 27.65 volts and 6.3 amps.

On the 12v 40 amp setting it was putting out 28.90 volts and 24.5 amps.

After and hour on the 10 amp setting the amp draw had dropped down to a little under 4 amps.


Future upgrades include a heat sink for the 2 additional rectifiers and a toggle switch to select from 6/12 Volt or 12/ 24 Volt.

Update.

I finally got around to completing this project. Tonight I installed a selector switch for 6/12 or 12/24 volt operation and installed a volt meter in the front panel.

Slick, thanks for the detail.

Sorry to revive an old thread, but I thought I would add my 2 cents to this useful thread.

A few years ago I bought a battery charger, which went belly up about a year ago. I recently bought an M756A2 to experiment with. It has a 24 volt electrical system, so I dug the dead battery charger out of my scrap pile and took it apart and found that its simple electronics section was toast. Instead of repairing it, I decided to upgrade it so that I could charge the batteries on the M756A2.

So, I removed all of the electronics and installed two KBPC5010 bridge rectifiers on the heat sink, and wired them to the transformer. I then added a 10 AWG wire pair going to the KBPC5010 bridge rectifier that was connected to the 26 volt section of the transformer. I also cut the battery terminal clips off the 12 volt DC wire and soldered on 50 AMP Anderson connectors to both DC leads. I then made a battery terminal clip with a 50 AMP Anderson connector on it, so that it could be plugged into either output connector.

The charger had a switch to select one of 4 outputs: 6volt, 2AMP 12volt, 10AMP 12 volt, and 60AMP 12 volt. The switch still works and works even better now that the KBPC5010 bridge rectifiers handle the output. I can also charge 12volt and 24volt systems simultaneously if I want.