Ring RCB320 high speed 12V battery charger

[Phil G4SPZ]

I've just completed this repair, and whilst it doesn't fall within the "vintage" category, it demonstrates the value of the Forum and its generous members.

Last August, my daughter gave me a heavy-duty car battery charger which was thought to be faulty. It looked a handy unit, claiming to be able to jump-start a car from the mains - that would have been useful a couple of winters ago - so I took it home. On plugging it into the mains, it promptly blew the fuse in the mains plug. I tried to connect it to a 12 volt battery, which produced large sparks. So off came the cover.

The circuit is simple, comprising a substantial and very heavy mains transformer with a tapped primary, tappings being selected to provide the wanted output - 3 amps, 20 amps or "80 amps RMS jump-start" - and a centre-tapped secondary feeding two AR501 50 amp button diodes in a full wave rectifier configuration. The diodes were clamped tightly between a small PCB, carring the secondary winding tails, and an aluminium plate that forms a common cathode and feeds ultimately to the positive battery lead. One diode was OK, but the other was short-circuit.

After searching fruitlessly for a source of replacements, I appealed in this thread which eventually led to Chris (a.k.a. "Herald1360" on the Forum) kindly offering me a pair of second-hand Semikron heavy duty rectifier diode modules for the price of postage, which I accepted with gratitude. Yesterday I finally got round to modifying the charger to accept the new diodes.

A bit of ingenuity was required to find suitable space within the case for the large diode modules, which turned out to be rated at 131 amps at 400 PIV, more than a match for anything the transformer could throw at them. In the end I drilled four 4BA mounting holes in the back of the steel case and bolted the diode packs in place. Prior to mounting them, I cleaned up the contacts and fitted the wiring, using fairly heavy-duty insulated lugs and cables, as the connections would not be easily accessible afterwards. As it happened, getting the nuts onto the ends of the 4BA mounting bolts at the bottom edge of the case was awkward, but some bent tweezers did the trick. The cables were cut back to length and soldered into position. The photos show the internal layout of the unit, the new diode modules in position, and the finished charger. I've included the datasheet covering the diode modules in case anyone else finds it of use. Each module has two diodes but they're internally connected, so I had to fit both modules but just used one diode from each. I therefore have spare diodes in place, ready for use in case of a future failure!

The charger now works perfectly. I haven't been able to try the jump-start function in practice as I don't have a car with a flat battery available (yet) but the 3-amp and 20-amp outputs are fine. It's debatable whether the unit will actually deliver its claimed 80 amps RMS (55 amps DC) into a dead flat battery, but I'll probably get the chance to try it out in anger next winter!

Thanks again to Chris for supplying the Semikron diode modules.

Phil

[Herald1360]

Looks like a "proper job" there!

The bean counters would not be amused by the new price of the modules but as supplied they're just the ticket

Be wary about temperature rise of the packs at high current for any extended time- their rating depends on how good their heatsinking is.

[Phil G4SPZ]

Well, Chris, they're bolted direct onto the back of the steel case, where the heatsinking capacity is probably ten times greater than the scrap of aluminium originally provided! I didn't apply any heat sink compound. I doubt I'll ever use the full output in anger for any length of time, but I do intend to test it for a while into a big 1-ohm power resistor when I can find it.

The bean counters at Ring should be ashamed of themselves. The charger is otherwise very well made, with a heavy steel case, rugged end protection, a solid handle, a huge transformer, and then they go and stick tiddly cheap diodes in it, just to save a few bob. I wonder how many of these chargers have already gone to landfill as a result? In fact, if you Google "Ring RCB320..." you get lots of hits saying "...not working", "...won't charge", "...troubleshooting help" and so on, so I think I've answered my own question!

These chargers are still on sale at between £60 and £70. Other makes and models are available...!