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1. 10:43 AM - Re: Interesting thread on overvoltage event (nuckollsr)
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| Subject: | Re: Interesting thread on overvoltage event |
After the server upgrade at matronics, I'm still wrestling with some email issues
. . . but these too will pass . . .
In the mean time, I subjected a fully charged ETX36 lithium battery to a 40v, 15A
constant current source.
Didn't take time to record and plot the data but my suspicions were correct, these
batteries WILL grunt some level of over-charge current.
In this experiment, it took several seconds for battery voltage to rise above 16.0
volts. Risks to accessories in an airplane fitted with a PM (read small)
alternator and legacy ov protection (16.2v) are small. Indeed, if the system loads
are a significant fraction of the alternators maximum output, one might well
never trip a legacy ov protection system.
We're in the late stages of bringing a new suite of OV protection devices easily
calibrated-to-task. A 16.2v trip point was derived from a series of bench tests
on various ov scenarios . . . for airplanes with 60A or larger alternators.
Trip dynamics is generally based on a simple RC integrator calibrated to trip
in 50 mS after a step up from 14.4 to 20.0 volts. This generally produced
a trip response tolerant of the Mil-Std-704 envelope for NORMAL bus excursions
while minimizing nuisance trips.
The next generation of ov discriminators will be software based and easily tailored
to the target system. It's a certainty that trip point can be lowered for
systems with PM alternators.
In any case, this experiment confirms the idea that legacy ov protection is still
okay, even with lithium paired with small alternators. Electronics internal
tot he ETX battery allowed 'overcharge' energy to be grunted by the battery.
However, we can and will do better.
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=468639#468639
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