If you have similar batteries with different capacities the biggest battery will always have most crank-power, if both are charged 100%.

The problems with big batteries are:
-Weight
-Price
-They stress your charging system
-They usually need more time to get fully charged

On the other hand driving in cold weather (-10°c and below) or with a highly modified engine you might (or will) reach the limit where a standard 19Ah battery can’t deliver the crank-power you need. It also depends on a lot of other factors like your charging system, oil, compression, how easy the bike starts, type of starter etc.
If you are on the low side with cranking-power you will use a lot of your battery’s capacity to start and it will require a long time recharging

A battery has full capacity at 25°c, at -18°c it’s only 40% left. This means that my 19Ah battery only have 12Ah left. That’s not much when it’s cold…
As a rule of thumb you should have 14.2-14.4V at 25°c and increase the voltage with 0.3V for each 10°c the temperature drops. (-15°c => 15.1-15.3V)

There are a lot of things that can be modified on the airheads-charging systems to get better charging.
My charging system is original (except a Thunderchild diode-board). I haven’t rebuilt it because for my use it works. It’s always easy to start the bike if it’s hotter then -10°c (or -15°c with synthetic oil) and that’s enough for me. The numbers might be a bit better now after I fitted the ND starter.
If you increase the voltage (which is good for your battery) you will also increase load on your generator. But again; different needs requires different solutions.



Hmm, eight years ago I had approximate 8 km (7-15 minutes) to work. In January/February I had to drive some extra kms 1-2 times a week to keep the battery up. The rest of the year it worked flawless.
I guess it works as a rule of thumb on a healthy system operating in normal temperatures.