I'm working out a charging system layout for my latest project and trying to work out how small a battery I can get away with. It's to go on a 12V AC/DC 350 bullet. I know they fit a 5.5Ah battery but I'm wondering if this is really overkill. I know you can do without one altogether but I like the security of being able to limp to a safe location, or at least have if the charging system fails.

The headlamps are AC, it wont be having flashers and I'm intending to use LEDs everywhere else (so a side, stop/tail and some dash backlights) which will draw next to nothing (calculate less than 1A). Just the ignition to account for.

I have a diddy little battery on my 612 bullet that will run all the lighting for 2 hours and fits inside an old tea-caddy air filter housing along with most of the wiring. Thing is that has a self-exciting ignition so it's only the LED lights it's running.
What's the current draw of a 12v points ignition?

Primary winding resistance of a Lucas coil is I believe around 3.5 Ohms, as it is a 12 Volt system Ohms law states that V/R=I

Which gives us 12/3.5 = 3.4 Amps
Before fitting a magneto to (612cc) Slo Poke - well, the Bullet Whisperer did, on my behalf, but never mind - I ran a 4 Cell EVO2 Ballistic lithium battery.

Ran the lights (halogen headlight, LED tail and stop light (no flashers)) and points ignition no problem. About the size of a can of Red Bull.
The alternator can usually supply the lights and the ignition without the batteries help, hence some folk just fitting a big capacitor instead of a battery.
But the alternator may struggle with indicators and the stop light as well as the headlight, especially when the revs are low. So braking while indicating and approaching a corner at night can be quite exciting. :shock:
Fitting LED ones will help, but a wee battery is much easier.
I still wouldn't worry about a battery if I had a decent alternator, stick a Sparx 005 or Lucas RM21 on there instead of the Indian copy, single phase generic reg/rectifiers are ludicrously cheap now so that you can carry one as a spare, and fit a separate capacitor. Going all-DC means you could even replace the H4 headlamp bulb with a LED too, even less drain on the system.


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