Hemp wrote: Hi folks.
I'm deep into a café-ish build on a 1980 750 twin and trying to finalize my battery design. I'm opting for a kick start only bike with a smaller battery rather than capacitor. There's chatter about some people using smaller scooter batteries, but I'm considering small lithium battery pack. There's lots of good small lithium packs available but I'm trying to wrap my head around the charging system to see if they are compatible. My questions are...
For a kick start only, does the battery have to have rated output amperage equal total system load(lights, ignition etc) or only enough output to energize the ignition during start up? In other words once the bike is started will the R/R and stator handle the operating load?
If the smaller battery can only be charged safely at a couple amps will the R/R prevent too much current from charging the battery? Lithium packs usually have built in protections that disconnect over a certain amount of charging amps. My fear is at higher rpms(amps) a smaller lithium battery's built in protection(2-5 amp cutoff) will disconnect from the charging system and the battery will get depleted under load while not charging.
Any thoughts?
The excited field type charging systems usually tend to have a higher output available than the permanent magnet charging systems. This is because they are regulated in such a way that they only put out as much as the bike needs, thus the output of the alternator is regulated. The permanent magnet type alternator, like what I think is on your bike ('80 750 twin) is often made to put out about as much as the bike will need. This is because the regulator has the task of getting rid of the extra power that the alternator creates. This results in a lot of heat dissipation, and also needlessly robs excess power from the crankshaft, so they don't want the alternator to be too powerful.
The permanent magnet alternator should be powerful enough to power the bike, even at idle, albeit at a lower than optimum voltage. The 3-phase alternators tend to be able to put out more power than the 1-phase alternators.
The battery has two purposes. It provides power when the bike is not running, or idling slowly, and it acts like a capacitor to smooth out the power pulses coming from the charging system. The pulses must be smoothed out in order for the regulator to work properly. If there is too much pulsing, the regulator will tend to cause over-charging. In extreme cases, such as if the battery becomes disconnected while going down the highway, the voltage has been known to get so high it melts the light bulbs and burns out other components.
So the real question is whether the battery you are using has enough capacity to smooth out the pulses. Power-wise, it's not really an issue since capacitor have no real power storage compared to a battery, and most/all permanent-magnet KZ's can go batteryless with a capacitor.
The only way to know for sure is to measure the true RMS voltage on the battery during high output times. A typical voltmeter only measures average, and the regulator only regulates to average voltage. But pulsing DC has much higher RMS voltage, and that is what does the damage.
