DIY backup battery

A few of you have asked about the backup battery I made for my plane. Its purpose is to provide power to my screen, instruments and electronic ignition during the engine start. Without a backup power source there is a chance of the G3X screen going black (rebooting) as well as my electronic ignition not working.

The picture shows a schematic of my main power & batteries. The backup battery is the blue box and consists of 3 LiPo 18650 batteries, a protection circuit board and a relay.

The unit is able to provide 1.5 AH of power with a maximum current of 9A.
-That is 1.5A for an hour / 3.0A for 30 minutes / 5A for 18 minutes / 9A for 10 minutes.

When fully charged it PCB tops & balances the 3 batteries at 12.15 volts (4.05 x 3), which is sufficient to keep any 12V circuit powered.
-The batteries are 1500 mAh and are rated for a 15A draw.
-The PCB completely takes care of the batteries. It is rated up to a 9A draw. I would suggest keeping it under 7 amps if you want to use use it as a continuous power source.

The PDF file lists the components that I used and gives tips for the construction of the unit. I used Tenergy brand batteries and PCB. Don't buy cheap parts. I bought the batteries, PCB and relay for about $45.

Undoubtedly this unit could be adapted for other backup or emergency uses as long as you keep within its limits. You could also make two units to double the current or the time.

I have been using it for over a year and it has worked perfectly. At my condition inspection I isolated the P+, turned it on, checked the voltage and it was right at 12.15 volts. I am very happy with the unit.

Brilliant post, thanks PP :)

I love the shopping list and the very easy to follow instructions.

It took me a little while to figure it out but then I realised you're using the backup battery to close the N/O relay when the master switch is turned on and it grounds the relay. In my hastily-drawn diagram on the other thread I was using the main electrical supply to close the relay and connect the battery to the equipment, which means the battery would continue to power it even when the master is switched off. Obviously you need to be able to turn off the backup battery and your system does than nicely.

This is an excellent diagram from any backup battery system and I will certainly be making use of it.

Edit: One question: you talk of 3 x 4.05V batteries and yet the ones you've listed are 3.6V. Am I missing something? You're going to tell me it's all to do with 1/square root stuff aren't you :eek:

I believe the best answer is the nominal voltage of a LiPo battery is 3.6 to 3.7 volts. When they are fully charged they will be close to 4 volts.

The PCB has over voltage protection that is set at 4.25V. If that ever happens it will discharge the battery to 4.15V. I think in normal charging it stops at 4.05V.

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 :eek:

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).

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.

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 :D

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.