What about if an additional 2 diodes were added in series when a lead-acid (LA) starter battery is desired to be trickle charged from a MP with a LiFePO4 house battery?
So now there are 3 diodes in series, so:
- 3 x 0.3V = 0.9V drop at low trickle currents
- 3 x 0.6V = 1.8V drop at high trickle currents
So now:
- LiFePO4 battery would have to be at least 0.9V higher than LA to start trickle charging
- when LiFePO4 being actively charged at 14.4V, LA would be trickle charged at 14.4 - 1.8 = 12.6V
- when LiFePO4 drops to 13.5V (12.6 + 0.9V) or lower, trickle charging would stop, preventing starter battery from draining house battery & stopping starter battery from constantly being charged even when full
The above scheme would I believe meet the intent of trickle charging to keep the starter battery from dropping below 12.6V.
Notes:
- the above scheme obviously wouldn’t charge the starter battery at 13.2-14.5V, like a real charger would, but I would posit it meets the intent of a trickle charger to stop the starter battery from dropping below 12.6V which should be enough for the starter battery to start the vehicle.
- for systems that have an Orion XS50, 12.6V also has added benefit that it is below default engine detection params for an Orion XS50 preventing a circular loop with trickle charger & XS50 working against each other
As an added bonus this would be ultra cheap to implement, just add 2 diodes (rated for at least 5A current for max 4A trickle charge, like say an 6A05 for discrete, S5A for SMD) in series in your charge wire. Victron could even add this in later versions of their MPs with 2 additional diodes & a physical jumper to bypass 2 of them if secondary battery being trickled charged was a LiFePO4.
P.S. I would still recommend an inline switch (and fusing, of course) so trickle charging can be disabled at any time, if desired.