Alternator and LiFePo battery question

S Davis wrote:

BFL13 wrote:

Teleman wrote:
I have a Redarc 40 amp DC to DC charger but apparently there is a solenoid that sends current from my alternator to the house battery when the ignition is on. I was unaware of this when my mechanic installed the charger. I just finished a 1500 mile road trip and was seeing a lot more than 40 amps on my shunt while driving. Fortunately I have a very robust alternator. I just found out about this solenoid so I will try and straighten it out so only the DC to DC charger is charging the battery.

Not clear how it was done from that. If part of the plan was to isolate the LFP from the alternator in case of way high draw from low SOC LFP (which is in dispute how much of a threat that is, but let's say it is valid for now) then leaving the OEM charging in place was wrong.

Next, the DC-DC itself will draw more than its 40 amps output as its input. Depending on the wiring from engine batt to DC-DC, if long and thin, could be 60 amps. If short and fat could be 45 amps.

So you need to pick the DC-DC output amps size to go with what your alternator can do, which depends on the input amps that you can only guess at.

If the amps are too high you could perhaps fatten that wiring to bring it within spec, or else before buying the DC-DC just assume 50% higher input over the output amps and match that to the alternator amps

The draw on my 50amp Redarc is 50 amps from the truck and around 47 amps charging to the batteries, so at least on the Redarc the rating is the draw on the alternator. This is charging a 280ah LifeP04, I have two but charge them separately.

According to the specs on their website, the Redarc BCDC1250D has a "maximum" charge current rating of 50a. It doesn't specify the input current rating per se. However, it's interesting they recommend fusing the input at only 60a. 60a leaves very little room for the 50a Redarc to compensate with higher input current when the input voltage drops significantly.

That would seem to imply the Redarc may work inverse to the Renogy. Our 40a Renogy produces a constant 40a of charge current (assuming a discharged battery, of course) and can draw as much as 60a to compensate for voltage drop on the input (Renogy also recommends a 60a fuse, but produces 10a less charge current). The Renogy varies the input current to keep the output current constant.

You mentioned you were only getting 47a of charge current (with a 50a input load). It would appear the Redarc varies the output charge current (<=50a) to keep the input current/load at <=50a. If so, this would be the opposite how the Renogy operates.