Hadn't seen DG80x installations, but sounds just like the DG400.
Last time at Schempp-Hirth, they were using LiFePO4 for avionics but not the engine.
If LIFePO4s are still not suggested for the engine, you could put in two of them for avionics and use two SLAs only for the engine. Plenty of gusto for either circuit.
This would require separate charging points for the two types, and keep the Phosphates off the engine's charging circuit.
Jim

On Monday, January 4, 2016 at 2:23:51 PM UTC-8, Chris wrote:
Responding to a request for more detail:

DG-808B Solo engine
Four 6v,12 Ah lead acid battery (150x50x110 mm)
Batteries are 'permanently' mounted in bays slightly larger than current battery size .
Batteries are connected parallel/series to create 12v system.