minimum battery amp hours

purplellama

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i have a 1982 yamaha xs650, heavily modded, kickstart only. it came with a tiny battery the size of a deck of cards. you turn the bike on to start it, and this battery has just enough juice power it up to start. in theory. in reality i think it's not quite enough. i've always had difficulty kicking it over, but if i jump it, it starts right up.

i don't have space with the current configuration for a typical battery, which is why the previous owner used this one, which fits in a hollow under the seat. the battery is rated at 12 volts/0.8 amp hours.

two questions.

1 - i'm wondering if anyone knows of similar small battery that would work in this application.
2 - can anyone recommend a minimum amp hour rating? would a higher rating solve my starting problem.

any help would be hugely appreciated.

thanks.
 
You don't say what type of ignition system you have, this will have some impact on battery requirement.
A magneto CDI, like Powerdynamo etc, requires no battery at all to start and run.
Stock points and TCI with twin coils, as well as single point, Boyer, Pamco etc all require a decent battery voltage to work properly.
I would look for a battery large enough to maintain at least 12 V when current is flowing through the coil. With the stock Yamaha coils, this should be about 3 amps, if I remember correctly.
If the current draw of your coils drop the battery voltage below 12 volts, I would look for a larger battery.
If size and weight is more important than cost, I would recommend the smallest "Super B" battery. It weighs less than 1lb (450 g) and has more than enough juice to run the electric starter on a Suzuki 125 4-stroke single with a 143 cc 11:1 piston. The stock battery for this little bike was 12V 6Ah.....
 
purplellama,

You left out a couple things to help us answer your question:

1. What's the ignition system?
2. What's the charging system?

It's not just the available current from the battery that is important, but it's charging rate as well. A .8 AH battery should not be charged with more than 10% of it's rating, or .08 Amps. If the battery voltage drops to 10 Volts, then a stock charging system would provide at least 2 or 3 Amps to charge the battery and that is 37.5 times the maximum charge rate for the battery, so each time the battery is recharged, the excess current is dissipated by heat in the battery so the battery eventually gets cooked and fails.
 
Stock points and TCI with twin coils, as well as single point, Boyer, Pamco etc all require a decent battery voltage to work properly.

The original PAMCO works down to 3 Volts. The coil requires at least 6 Volts. The new E-Advancer needs at least 8 Volts.

Here is a video of the original PAMCO working from a lantern battery:


Some electronic ignition systems with electronic advance have a 200 RPM minimum crank speed, making starting with a low battery difficult and more so when kick starting. The PAMCO E-Advancer has no such limitation and in fact has a special algorithm to retard the advance below 200 RPM to make starting easier.
 
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