DIY 6V battery charger

[Jules Richardson]

In a fit of madness, I just bought a project car that's old enough to have 6V electrics.

Am in need of a battery charger as the vehicle's sat for a while since the last run; it looks like ones that are switchable between 6V and 12V aren't too expensive, but I thought I'd ask about making one too (just in case I have something in the junk pile).

I'm pretty sure that my noddy 12V charger just runs the transformer output through a hefty rectifier - I don't think there's even a smoothing capacitor in the thing, it's just a lumpy waveform, presumably with a peak voltage that's comfortably above 12V (I'll have to hang a meter off it at some point). Anyway, that seems to do the job just fine for 12V batteries, so presumably the same would work at a lower output for 6V.

I'm not sure if there's a cunning way of simply halving the AC input to the transformer on my existing charger so that I get half the output; I'd assume that transformers are built for a specific voltages though and under-driving them will be Bad? :-)

Ultimately I'll probably get the engine starter rewound for 12V, but that's a long way off.

cheers

Jules

[Dave Plowman (News)]

Both Lidl and Aldi occasionally have small modern chargers on sale for about 15 quid. They do both 6 and 12 volts - and will charge at least a

12v car battery overnight, and switch to a maintenance charge when it's full. My local branch (Clapham Junction) of Lidl still had some in stock the other day.

Your old charger probably has a large series resistor to limit the current

- changing that to a suitable one would probably make it ok for 6v.

[harryagain]

You can connect an electric fire element in series on the 12Vdc side. You will need to check the current flow & monitor, most old battery chargers have an ammeter.

[meow2222]

Running a transformer on lower v is fine. But you wont want half the v_in, to get half the v_out you need more than half v_in due to rectifier voltage drops.

12v lead acids should be limited to 14.4v for a fast charge, 13.6v for a sl ow charge is better for a charger. So 7.2v and 6.8v for a 6v battery.

If you have the right transformer you could work with a peak voltage of 6.8

-7v. If you used a 4 diode bridge rectifier with each D dropping 1v, you'd need a transformer with peak V_out of 8.8-9v, or 6.2-6.35v rms. It would wo rk with a 6v transformer if you pick the right diodes. A thermal cutout on the transformer would be good practice.

You can always charge the battery off a 12v charger via a big resistor, but charge current wont drop as the battery tops up, so it should be kept an e ye on and disconnected once charged rather than left.

6v to 12v conversions of old cars usually leave the starter wound as is. De spite getting double the rated V they normally work fine.

PS metering the existing charger wont tell you its peak v output, not with a standard meter.

NT

[Brian Gaff]

And get very warm as well? Brian

[Brian Gaff]

Bit worried about just getting the starter rewound though. What about relays, flashers, ignition voltage, lamps etc?Could also have the fastest wipers on the road and a washer that washes the screen of the following car as well. Brian

[Andrew Gabriel]

IME, they just rely on the supply impedance of the transformer. They probably use a leakage induction transformer, to artificially increase the supply impedance without generating resistive heating.

[Dave Plowman (News)]

Could well be. I'm basing that on an ancient one I have here - complete with selenium rectifier. ;-)

[Theo Markettos]

Since I presume you're in 110V land, one thing you could do is find a 220V transformer with an middle tap on the primary for 110V (so US folks would connect the middle tap to live, and rest of the world across the whole

110-0-110 coil).

Connect 110V live to the 220V input, and neutral to the neutral. Don't connect the secondary. The 110V tap on the transformer is now at 55V in autotransformer mode.

You'll probably want to watch the charger current, as when undervolting it you don't provide any more current. If there's any kind of SMPSU it will try and boost the current to compensate, which could overload the transformers. There's also no isolation here, so treat the 55V as carefully as if it were mains.

Theo

[The Natural Philosopher]

actually the prime requirement is to limit the current to less than the ten hour capacity of the battery, and let the voltage sort itself out.

That's the same as lithium ion.

The general algorithm is take the capacity in ampere hours and divide by (at least 10) so a 70Ah battery should be current limited to 7A or less.

Then ensure that the off charge voltage is never more than 2.2v/cell for lead acid, or 4.2v for lithium ion or IIRC 3.3v for LiFePo.

Nickel is different. There you can go for capacity/3 for max charge current, and look at the voltage, as soon as the voltage starts to FALL under constant charge current - by tens of mV for NiCd, or a few mV for NiMh, the battery is essentially fully charged and a bit more.

One way to achieve a reasonable trickle charge is to have your charger at the limiting voltage and stuff a resistor in series. So a 7.2v supply and a resistor is OK ish for a 6v lead acid..all that overcharge does is generate hydrogen and oxygen in explosive quantities...:-)

Today its all most easily achieved with a combination of voltage and current sensing and a LV supply fed through a chopper and a choke to deliver relativity smooth dc to the battery and ensure it never goes outside the limits.

For homee use a 7.2v regulator or suitably agricultural dimensions, and a resistor of about 0.5ohms or so will deliver a peak current of 15A into a short (fuse it at 5A) and a couple of amps into a flat battery.

Suitable regulators would include an array of e.g 2N3055 transistors in darlington emitter follower configuration, fed from a more or less stable zener reference diode of around 8.4v to allow fr the 0.6v base emitter drops times two

If running off a putative 18v DC source, that equates at fuse blow levels to a dissipation of 55W or so. Twin 3055s will do that on a big heat sink.

At 1-2A they are just keeping themselves warm really.

0.5 ohm resistors of the 5-10W flavour can be made out of old fire elements, or bought.

And indeed regulators designed to go on heatsinks are available.

The LM317 can be configured very simply to be a 6cv battery charger - see application note - from almost any DC source under 25V.

[Jules Richardson]

I expect I'd lift a coil, alternator, flasher unit etc. from something a bit more modern. I don't think there are any screen washers, but you're right that the wiper motor's something to scratch my head about. For dashboard lights and instruments I could probably use a DC-DC converter and keep those at 6V.

I'm blessed with lots of time to worry about it all, anyway - beyond doing some mechanical work in the next few months so that I can move it around the property under its own power if needed, I'm not planning on starting serious work on it for a few years, once the kids have stopped being so expensive :-)

cheers

Jules

[Jules Richardson]

Yes, that was part of my logic, that I bet there's still a 220V charger at my parents' place in England which is sitting there doing nothing, and that if brought over here and run on 110V it might become a handy 6V charger. But like NT said, it's not quite as simple as that.

Problem now solved anyway, because it seems that lots of old tractors also run on 6V, so I stopped by my local farm supply store yesterday evening and they had a charger for a reasonable price which is switchable between 6V and 12V.

It also means that when I acquire an old tractor that uses 6V electrics, my wife doesn't have to kill me quite as much because at least I'll already have a charger for it... ;-)

cheers

Jules

[Bill Wright]

Mine are in their thirties and they are still just as expensive.

Bill

[meow2222]

Metal oxide rectifiers have plenty of inbuilt resistance, quite enough for a lead acid charger

NT

[meow2222]

If you get an old fashioned battery with metal bars on top you could perhaps attach some 6v loads to a halfway tapping on the battery. As long as enough charge happens to charge both halves it would work.

NT

[meow2222]

You can do that. You can do various things with lead acids. The ideal regime is voltage limited though.

I've been charging NiMH at 1C, and theyve all been fine.

That does slow charging down, as i falls all the way to zero as the battery charges.

H&O are seldom produced in explosive quantities when a battery is gassed. Thats mainly an issue in small battery compartments, which should be vented

You can. Lead acids arent really fussy enough to need regulation off a fairly stable mains.

NT

[Jules Richardson]

Yes, I've since heard of someone doing that, and rigging the system so that the engine starts on 12V and then drops to 6V once running. I'm actually surprised that the 6V starter will take it, though.

Also surprised that 6V batteries are reasonably cheap - I imagined it was one of those things where it was a niche market and so they'd be very expensive, but they're roughly half the price of a 12V unit.

cheers

Jules

[Dave Plowman (News)]

They do. Everything has to be changed except the starter when going from 6 to 12v.

[robgraham]

+1 Rob