Nigel Charles wrote:
> Bob Nuckolls indicates that overvoltage is a problem to batteries
> and can cause them to fail in a short time. I would imagine that
> this problem is just as relevant (if not more so) with the newer
> RG batteries. I have been also told that most modern avionics
> include OV protection. This is cheap to do and improves the
> reliability of the unit which reflects well on the manufacturer.
I'm afraid that seems not true; per the dozen+ avionics service manuals
I have, they have no OV protection circuitry. Most common power supply
is a simple series regulator, which may fail safe...or may not, cooking
every IC in the box. Some newer ones use all switching regulators,
which should fail safe...theoretically of course!
An RG battery is less tolerant of overcharge current than a
flooded-cell, but nothing happens to either until it first exceeds 100%
charge, and then not until it gets hot enough. Main difference is an RG
then can go into thermal runaway, or at least suffer irreversible damage.
> Any aircraft with an all electric fuel pump system should have
> some means of OV protection. The most likely reason for total
> electrics failure is a regulator runaway. Splitting the sources
> of power for these pumps helps. One on a normal busbar and one
> directly connected to the regulator output helps to ensure
> redundancy as well as providing a constant load for the regulator
> should the alternator become isolated.
However, unless the fuel pump is downstream of the OV relay, it won't
pump for too long, subjected to full unregulated output from a failed
regulator! This is one misfeature of OV protection as might be wired
for the 914, compared to the 912 with a mech fuel pump. If there's no
OV relay, the battery and other items on the bus will clamp voltage to a
temporarily safe (average) voltage level, won't pop the pump CB, and
allow more than enough time for one to take corrective action before
harm is done to the battery. Provided one has clear panel warning of
the OV condition.
Regards,
Fred F.
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