DIY backup battery

[PaulSS]

That sounds convincing and most of the equipment can often stand variance of between 8 - 18V, so even if it was a bit under 12V it'll still be okay....I think

Now, the only other thing I was thinking was when does the relay give up and open? What am I talking about:

Your backup battery is the power that is being used to close the relay (when the master switch is turned on). When all is working the aircraft power will charge the backup battery and all is good. Now imagine a power failure. Your backup battery will power the essential bus but when the backup battery voltage gets below XXX volts then it won't be able to hold the relay closed and it will open, thereby cutting the backup battery power to the Ess bus.

Do you happen to know what the XXX voltage is because Google hasn't let me know yet and Siri just wonders what the heck I'm talking about

Edited to add: let's pretend you haven't got the essential bus switch, which connects your main battery to the bus.

Edited edited: It would seem that the relay may stay closed until about 7V, by which time most of the gear it's powering would have gone to sleep. Apparently a relay needs a higher voltage to close the relay than to keep it closed (and all sorts of stuff about temperatures etc but that was the long and the short of it).

[PapuaPilot]

Good question Paul. I don't know what the voltage is to keep the relay closed, but when LiPo batteries run out of power they die almost immediately. If the relay were to open it means the batteries are dying.

I am using my LiPo unit to keep my screen alive during the start. If someone wants to use this as backup battery it is very important that they know the current draw and calculate how long it will keep things going.

On certified planes the battery(s) need to provide 30 minutes power for the essential systems. The idea is to give enough time for the pilot to get it on the ground, or at least into VMC.

My backup power source is my main battery. If my alternator dies I have a 16 AH battery that I will use to power my essential bus. To do this I will turn on the essential bus emergency switch (a direct connection to the battery) and turn off the master switch, which eliminates the main bus (non-essential items). The current draw in this situation is about 5 amps and runs my PFD/AHRS, engine instruments, COM radio and transponder. This means I have about 3 hours of power from my battery! I also have my lights and elevator trim on the essential bus, but they do not need to not be on unless it is night. Worst case scenario would be 2 hours of battery power. Even if my main battery dies I would still have my iPad and everything that the Garmin Pilot app shows, plus my handheld COM radio.

[PaulSS]

My requirement is just to keep the MGL ASV-1 powered to provide an ASI and altimeter, as a backup instrument, in case everything else gets lost. This is a UK LAA requirement for those who wish to use electronic instruments in place of mechanical ones. It is not satisfactory to just use the main battery, despite the insignificant power required by the ASV-1. With the MASSIVE current draw of 73mA I reckon your battery pack will keep the instrument powered for about 20 hours. Even if the relay opens at 7V, I still think we'd be good for 10 hours and by then I think (a)I'd have become a great glider pilot and (b)may need a 'rest break' as they say in the colonies

The Eurofox doesn't use the 'normal' master switch/contactor set up and, instead, the key switch connects the battery direct to the aircraft systems....it's a beefy key switch as you might have guessed. This means there's no convenient method of grounding the relay as you've done in your machine. Of course, a switch could be added but (a)my OCD didn't want another switch in the panel and (b)I wanted an automatic system. So, I basically put another relay in the circuit to act as a switch to earth....I think it'll work

I love these forums for this kind of stuff. It's great to see clever people sorting out snags and then helping the rest of us to try and get to grips with other 'challenges', so thanks again for all your help.

My thinking for a backup instrument backup battery attached.