Charging conundrum

[doug748]

Apologies for this non boaty post but I suspect someone will have the answer out there...
I want to gently charge a 6volt motorbike battery of 4AH capacity. The book of words says that the normal charge rate is 1amp though a more rapid charge can be given in an emergency - but it will shorten the life of the battery.
I have a domestic (Halfords style) charger of 1970's vintage. It is switchable 12 to 6v and the box says it delivers 4 Amp RMS.
......Will this wreck the battery? It certainly sets it fizzing.
......If so, is there a simple circuit I can add that will gently charge this small lead acid battery without frying it?
Many thanks.

 

[ghostlymoron]

try sticking a 10w lamp in the system. I used to do this to enable charging a 6v battery from a 12v charger. What bike do you have - I didn't think there were any 6v around anymore.

 

[VicS]

No ammeter I take it.

The simplest solution may be a series resistor to limit the current. Experiment with 6 ohms downwards but you'll need an ammeter. Most cheap multimeters have a 10 amp range.

I've got some lengths of resistance wire I would experiment with.

Could try some car bulbs in series. Try a 21 watt flasher bulb and take it from there.

 

[matelot]

If its fizzing you either are fully charged or have a duff cell. Peer down the holes and see if one cell is fizzing or the lot.

I always used to use Optimate battery chargers on my bikes but I dont know if they do a 6v version. optimate

 

[William_H]

You really need an ampmeter to confirm what you are doing to your battery. However because the current is in a pulse (half sine wave) shape a digital ampmeter (multimeter) willl get very confused. You need a mechanical (analogue) meter to smooth out the variations and give you an average current measurement.

In your case the simple answer is to fit a series resistor which will limit the current. So resistance equals voltage divided by current. Roughly 12 volts divided by 1 amp will give 12ohm resistor. That is 1 amp into a short circuit or about 2/3 of that figure into a the resistor in series with a 6 volt battery.
Less current into a battery means it takes longer to charge but gives you more safety in that it will not be so badly overcharged if inadvertently left on charge. I would charge a 4AH at max about 1/3 amp. even batter 1/10 amp would do the job from flat in about 40 hours and would be very gentle on the battery.

At this kind of rate you might like to use one of those transformers fitted into a power plug rated at 9V. Note you must not exceed the current rating and you will still need a resistor in series.

Note also that the power dissipation of the resistor will be the voltage drop multiplied by the current. So dropping 6 v at 1/2 amp will have the resitor dissipating 3 watts. That is fairly hot so buy a 5w rated resistor.
good luck olewill

 

[doug748]

Thanks for the response everyone. I attempted to reply last night but we had a power cut!
I shall put a more than adequate resistor in there and rest easy.
The bike is an old Triumph so hence the 6volt, converting to 12 is probably not worth the effort and expense.