backchannel: case study - direct charging LFP in a Rav4
from this thread on /r/overlanding
TL;DR
a small DC-DC (like 20-30A) would be simplest, in the sense that there would be less homework.
more info and caveats about the DC_DC approach
- Quick googling suggests Rav4’s alternator is likely 100A, hence the sizing of the DC-DC mentioned above.
- Li profiles are often overly aggressive voltage-wise and no-float charging comes as a surprise for some.
- there are scenarios where DC-DC is preferable or even mandatory
more info and caveats about the “isolator” (combiner) approach
LiFePO4 can charge quite nicely from a combiner:
- data summary from >40 rigs, including rigs where the owner later changed to DC-DC
- data from 50 direct charging runs on my rig
HOWEVER, the setpoints on voltage sensing relay mentioned in the other thread will not allow the batteries to disconnect normally after charging stops:
CUT-IN VOLTAGE: 13.3V
CUT-OUT VOLTAGE:12.8V DC
Compare the cut-out voltage to LFP’s resting voltage to see why this happens. Workarounds in the link above.
I do not necessarily suggest direct-charging LFP for the general public installing their first alternator charging setup. I do suggest testing it for people who already have a combiner installed, or for people who have working knowledge on how relays, LFP, and alternators work.