I did an autopsy (cut the lid off) on the PowerSport YTX20L-BS 12V 20Ah AGM battery that failed open, causing total loss of electrical power and engine stoppage (electronic ignition and fuel injection) leading to forced landing and damage to my RV-4 (separate threads elsewhere to come).

Battery was mounted forward of firewall, so failure may be heat and/or vibration related. Battery is designed for sports vehicles, so should be vibration resistant. Could also be a manufacturing defect. Battery was installed 29 Jul 2023. In service for 45 hours with an estimated 50 cycles (starts).

Note that the battery did provide power to start the engine with no problems 10 mins before the failure. Looking at engine monitor log, about halfway into the flight spikes in voltage started to show up and got bigger end bigger. So a somewhat gradual failure.

Each of the six cells are interconnected via "bridges". I assume zinc electrodes to lead electrodes. As you can see in the second picture there is a crack in the lead bridge to the second cell.

When I short the crack with a metal pick or connect the two electrodes with wire clips the battery shows full voltage and can easily deliver a 12 amp current. When not shorting the crack, voltage drops to zero with even the lightest load (12V 0.16A fan).

I'll post a separate thread speculating on why the alternator did not continue supplying the needed power.

In case you think this is a one-in-a-million case, an autopsy was done in 2016 on an Odyssey PC680 battery with similar failure mode -- again internal cell-to-cell interconnect failure (third picture). Fortunately that guy had magnetos on his Rotax.

My design philosophy was that the battery was the backup for the alternator and the alternator the backup for the battery. In my case that did not work out.

For any engine completely dependent (EFI and/or electronic ignition only) my recommendation is to have a backup battery behind the firewall. Connect the engine bus to either the main battery or the backup battery with a reliable DPDT switch. (Perhaps even with an additional parallel switch). When in normal position, charge the backup battery with a diode from main battery/alternator. When in backup position the engine bus is connected to the backup battery and completely isolated from main battery and alternator (main bus). That will survive a main battery short and also alternator over-voltage event (with a OVP device installed).

Finn