Charging a 6 volt battery with a 12v charger

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Charging a 6 volt battery with a 12v charger

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Hmmmm......I've just agreed to do a job for a mate which involves getting his BSA C15 back on the road. One of the main problems is that the battery is flat. I only have a 12v charger and don't want to buy a 6v charger just for this job (supposing I could even FIND one locally!)

Can anyone tell me what resistance I need in the circuit to drop 12v to 6v (actually 14.2v to 7.1v) to enable me to get started. I was thinking along the lines of a couple of bulbs in series with the charger......?

I know that:

Volts=Amps / Ohms
Watts=Volts x Amps but I'm now getting confused, 'cos I've had too much Black Sheep (dribble).

Help!!!!

redrooster wrote

Hmmmm......I've just agreed to do a job for a mate which involves getting his BSA C15 back on the road. One of the main problems is that the battery is flat. I only have a 12v charger and don't want to buy a 6v charger just for this job (supposing I could even FIND one locally!)Can anyone tell me what resistance I need in the circuit to drop 12v to 6v (actually 14.2v to 7.1v) to enable me to get started. I was thinking along the lines of a couple of bulbs in series with the charger......?I know that:Volts=Amps / OhmsWatts=Volts x Amps but I'm now getting confused, 'cos I've had too much Black Sheep (dribble).Help!!!!

noV=IRnot really feasable really. you need a 6v charger. Can you borrow one, your mate will one anyway. Will a C15 start with a flat anyway??Kev

I think it will fry the plates in the battery with 12V going through it....

This is easy, just put a 12 volt 5 watt sidelight bulb in series with the battery and charge thru' the bulb. this will give a gentle but safe charge.

Vicvomitt wrote

I think it will fry the plates in the battery with 12V going through it....

Read it again......

oil slick wrote

This is easy, just put a 12 volt 5 watt sidelight bulb in series with the battery and charge thru' the bulb. this will give a gentle but safe charge.

Will this be enough resistance to drop the voltage, then? I'm easily confused, you know.

KFZ wrote

Will a C15 start with a flat anyway??Kev

No, I don't think so - it has points ignition so will need all the volts it can get.

The 12 volt 5 watt bulb would normally draw about half an amp if connected direct to the 12 volt battery, with the 6 volt battery in series it is not possible for more than about half an amp to flow. (actually slightly more will flow because the bulb will be dim and therefore the filament resistance will be slightly lower than at full brightness!) This will be sufficient to charge the battery overnight to a level sufficient to start the bike. It wont however provide enough to start it straight away.Note that if the battery has been flat for a while it will be kn4ckered. If the battery is taking the charge ok then the bulb will glow dimly.I have used this technique many times and it does work fine.I used to run my british bikes without batterys, they always shook to bits! used to convert them to 12 volt and use a capacitor and zener diode to regulate at 12 volt. They always started ok.

FWIW my battery charger cost less than a fiver and is switchable between 12v & 6v, so if you did go down the new charger route it needn't be very expensive.

Foxy wrote

FWIW my battery charger cost less than a fiver and is switchable between 12v & 6v, so if you did go down the new charger route it needn't be very expensive.

Ahhh! May investigate that further, then. Ta for that.

Vehicle Wiring Products sell a 6/12v charger for bikes for about £12 plus postage. I got one 2nd hand from Newark autojumble last month for £2, so thye are available if you're lucky. If he's paying for your work, add the cost to your expenses.The Ceefer shouldn't need a lot of battery power as you'll be starting on a kick, or you could use the standard Beeza method of getting a push.

Bike Breaker wrote

If he's paying for your work, add the cost to your expenses.

I've already priced for around £150 worth of work that needs doing, and need to keep the cost down.....

Doesn't it have an "emergency" setting on the ignition? This would bring all the coils in the alternator into play (usually only half are active during normal running and the rest being brought online when it's turned to the lighting position) and is designed to cope with a flat battery.

What Daytona said.... i think its usualy all the way round, If the bats been flat for a while its probably dead anyway.

Daytona wrote

Doesn't it have an "emergency" setting on the ignition? This would bring all the coils in the alternator into play (usually only half are active during normal running and the rest being brought online when it's turned to the lighting position) and is designed to cope with a flat battery.

Haven't got it in the garage yet, but will look into that. Seems to ring a bell!

Beg ,Borrow etc another 6V battery and charge the pair in series  SIMPLES

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