Alternator and LiFePo battery question

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.

BFL13 wrote:
One thing with the DC-DC is you can set its output voltage and do the same with your solar controller's to get them about the same so they add their amps while you are driving.

With ordinary alternator charging in newer vehicles, the alternator voltage gets lower than the solar's, and you mostly only see what the solar is doing.

On my 2019 Chevrolet 2500HD the charging voltage stays at a constant 14.4 it never changes, so newer vehicles are not all the same.

S Davis wrote:

BFL13 wrote:
One thing with the DC-DC is you can set its output voltage and do the same with your solar controller's to get them about the same so they add their amps while you are driving.

With ordinary alternator charging in newer vehicles, the alternator voltage gets lower than the solar's, and you mostly only see what the solar is doing.

On my 2019 Chevrolet 2500HD the charging voltage stays at a constant 14.4 it never changes, so newer vehicles are not all the same.

My 2003 2500HD Chev starts at about 14.6 then settles at around 14. That is when idling and as it gets hotter around the engine. I will check for when on a long drive. My 1991 Class C holds at around 14.5 steady while driving though.

Another alternator thing with LFP is "load dumping" if the BMS cuts off the battery while the alternator is running. If the engine battery is FLA, that maintains a load, which can save the alternator. The DC-DC input is from the battery posts not from the alternator directly so that should be ok--I am not too clear on that issue so if I have it wrong do correct the info!

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.

@ortfun

What size wiring from your alternator? I have about 10’ of 1/0 DLO cable that might make a difference. The Redarc does boost the voltage to 14.5 so I think that is the 3amp difference.

The wire from the alternator to the batteries and from the house battery to the inverter is all 1/0 ga. The very short wires from the batteries to the DC to DC charger are 6 ga as specified by Redarc. The problem is the alternator is tied directly to the house battery. This is so it can be used as a boost if the starting battery is too weak to start the engine. I need to disconnect the alternator from the house battery so it only feeds the starting battery, then passes through the DC to DC charger to the house battery
I will miss the super fast 80-100 amp charging lol, but I don't want to risk burning out the alternator.

S Davis wrote:
@ortfun

What size wiring from your alternator? I have about 10’ of 1/0 DLO cable that might make a difference. The Redarc does boost the voltage to 14.5 so I think that is the 3amp difference.

50a in/47a out, is excellent. It's obvious you have very little voltage drop.

We ran approx. ~20 ft of 2 gauge from our truck's battery to the 40a Renogy in our truck camper. 43.5a in/40a out. Going with larger cable would have only reduced our current loss about 1.0 - 1.5a and cost us considerably more. 2 gauge offered us the biggest bang for the buck.

The premise of my post was not so much voltage drop or efficiency, it was how Redarc appears to put a priority on stabilizing input current, whereas Renogy favors stabilizing output current. Interesting difference.

Teleman wrote:
The wire from the alternator to the batteries and from the house battery to the inverter is all 1/0 ga. The very short wires from the batteries to the DC to DC charger are 6 ga as specified by Redarc. The problem is the alternator is tied directly to the house battery. This is so it can be used as a boost if the starting battery is too weak to start the engine. I need to disconnect the alternator from the house battery so it only feeds the starting battery, then passes through the DC to DC charger to the house battery
I will miss the super fast 80-100 amp charging lol, but I don't want to risk burning out the alternator.

then disconnect the solenoid controller, and leave the emergency start,

personally I think you will be fine, boaters run up in the 90's for hours, with significantly less ventilation.

also these alternators at least some are computer controlled, i would think they would reduce amperage at high temp.

i also still have not read of someone burning out an alternator on here from charging.

The Redarc also has an MPPT controller. I'm planning to add 600 watts of solar. I upgraded to LiFePo to charge the big batteries on our E-mtbs. Between the driving and solar I think we'll be fine.

Teleman wrote:
The Redarc also has an MPPT controller. I'm planning to add 600 watts of solar. I upgraded to LiFePo to charge the big batteries on our E-mtbs. Between the driving and solar I think we'll be fine.

I am going to add a second dedicated alternator and plan to have three of the 50 amp Redarc, two on the truck for two 280ah LifeP04 and one in the truck camper to charge one 280ah LifeP04. I talked to Redarc and they said you could parallel up to four.