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In my case, a VFR ship is just fine without all the crazy backup systems people add. The SkyView, handheld GPS, and Dynon PocketPanel all have backup batteries. All that I am really missing is a T into my external antenna for a battery powered handheld radio. |
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I now have both SD8 and plane power 60 A sitting in my hangar. Both of them were sourced from the local field.
I realized that I don't have a gear assembly on the back of the engine. It was capped off. Looks like PP might be the only option now. Otherwise; SD8 might be totally sufficient for my needs. If anybody has a vacum gear assembly with housing by chance for o320A1A (Narrow Deck) then please let me know... |
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- Two batteries are more reliable than two alternators - if and only if you incorporate the one of the various simple power distribution schemes to have them independent from each other. Most two alternator schemes have at least one common failure point that would take out both units. The easiest way to do this is to have two identical batteries that combined provide the cranking amps you want. That way nothing is just along for the ride "in case". - Dynon and others have backup batteries and such, but this does not support things like XPDRs, radios or electric boost pumps. - Do not use a BNC 'T' to connect a handheld to an existing antenna connected to any other radio. You will be feeding RF energy into the panel radio - and a little bit can do a lot of damage. Use a BNC double female connector and first disconnect the antenna lead from the panel radio and then attach to your handheld. On the original question, a 70 amp alternator breaker mounted on the firewall in between the alternator and the battery side of the starter solenoid protects from the unlikely but "real bad outcome" of discharging a battery to a failed alternator - or a broken off alternator lead that is shorted to engine ground. If you have a 60 amp alternator use a 70 amp breaker. If the breaker trips you will not be able to reset in flight - but if it does something bad has happened and you don't want to reset it. Two batteries like the PC-625 provide at least 3 hours of IFR flight. http://www.aircraftspruce.com/catalo...clickkey=37667 Carl |
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Hate to tell you, but a CB will not protect your equipment from an overvoltage event. All it will do is open if you have a catastrophic short to ground (with current supplied by the *battery*), or, if you rate it too close to the alternator capacity (which is almost always higher than its rating), it will 'nuisance trip' if the alternator tries to meet heavy loads while recharging a 'soggy' battery.
The only way you get overvoltage protection is with a dedicated overvoltage circuit that is designed to either open the alternator field, or open a contactor between the alternator and its load(s). If the alt goes overvoltage without dedicated protection, the voltage will go up until {voltage divided by the load resistance = alternator amps capacity} stops the voltage increase. This will take a looong time to happen in electronics time units, if at all. edit: What I mean is that if the CB is rated higher than the alt's max capacity, as it should be to prevent nuisance trips, then it will *never trip* due to an overvoltage event, because the alternator 'self-limits' its output, by nature. It cannot produce more current than it can produce. :-) If its rating is, for example, 60 amps, then it will likely be able to produce 65-70 amps in the real world. And it will do that, if asked, at 14 volts, or if you have a regulator failure, at whatever voltage it can get to & still produce that maximum current. So, if you supply a CB that's safely above the alt's capacity to avoid nuisance trips, then current never rises high enough to trip it. The CB has basically no interest at all in voltage, until it exceeds the insulation capacity of components inside. Then you'll get internal arcing; not a trip. |
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