EEngr:
This might be more of a steady state phenomena. This APU is an AC device,
driven by a variable frequency drive (an three phase inverter). Although
there is some input filtering on such units, they do draw continuous ripple
current on top of the DC supply. And this ripple current will change in
magnitude and frequency as the APU starter accellerates.
I'm skeptical, in that the ripple current on the dc line will be directly
related to the inverter o/p frequency, which will be at most, in the low
Khz and more likely the low hundreds of Hz. The input filtering must provide
effective filtering for the pwm carrier, which would be typically in the low Khz
range anyway.
Would the battery have a problem dealing with ripple in the low Khz
range ?. Such ripple would be riding on the nominal dc and wouldn't cause
transients above battery unloaded voltage. You need inductance in the line
to produce significant transients above battery nominal voltage and a few feet
of wire would only produce nanosecond level transients. Insignificant..
Some capacitors are noted for having high internal losses (dissipation
factor for the EEs) and, as a result heat up (and sometimes explode )
when driven with a ripple current/frequency over their rating. Although
the total voltages don't exceed the insulation's capability, hot spots
can be generated.
Agreed, but caps for that sort of service are designed for low esr, have
specified ripple ratings and are designed to tolerate hf / fast rise edges.
I really doubt if the cells would be subjected to anything like that high a
frequency, since there would be considerable rf radiation to other systems
as a result...