You want to have your avionics powered up so you can see the oil pressure. You need to provide a way so that the voltage doesn't go too low, causing the screen to reboot.
There are several ways to do this.
- The TCW product can do it.
- I installed a second battery to power my essential bus during the start. My E-bus powers everything needed to run my screen / engine sensors / electronic ignition, and is separated from the main bus by a power diode. My starter is on the main bus, so when the main bus voltage drops lower than the E-bus the second battery takes over and keeps the screen alive.
Initially I used a 4 AH sealed lead acid battery (mounted in my tail), which weighed about 5 pounds with wiring. I removed it last year and replaced it with a LiPo battery & charger/maintainer circuit board that I assemble from parts I found online. The entire unit weighs less than 1 pound so I shaved off 4 pounds.
If anybody is interested in building their own backup battery I can give you the information. It uses 3 LiPo batteries and a charger/maintainer circuit board. I also added a relay that turns it on with the master switch. I spent about $45 for all the parts and build an equivalent to the TCW part. I used Tenergy batteries and circuit board, not inexpensive Chinese stuff.
The one I made can power my E-Bus for 15-20 minutes, but it really only needs to work for a few seconds during the start.