Alternator and LiFePo battery question

[Teleman]

What percentage of an alternator's rated output is it safe to run at a steady charge rate? 50%?

[otrfun]

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.

[BFL13]

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]

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]

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]

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.

[Teleman]

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

It was an oversight. I was unaware of the solenoid that connected the alternator to the house battery. Obviously my alternator survived the high current draw which topped 100 amps at one point. The DC to DC charger is very close to both batteries with 6 ga cables recommended by Redarc for their length so that is not a problem.

[BFL13]

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

[pianotuna]

Trackrig wrote:

pianotuna wrote:
Duty cycle is usually 1/3

Example 120 amp alternator--continuous load 40 amps.

Don - Is your rating what you feel a standard gas engine alternator should be capable of? I ask because I have Leece Neville 165 amp heavy duty alternators on a lot of equipment with diesel engines Nodwells and Tucker Snocats). 33% of 165 amps would be about 55 amps. When running them in the late fall or winter with all of the lights, heaters and engine fans on, I can easily pull 90 to 100 amps. If I'm using the electric winches, I'll run more than that. I haven't had an alternator fail yet, but your post made me curious.

Bill

It is for the Ford alternator on the E-450. Full rated output is 130 amps. Your alternator is far more robust.

I believe Mexicowanderer may have designed the Ford 130 amp unit.

That said, I did get peak loads of greater than 70 amps as I have dual charging paths with 50 amp self resetting circuit breakers. The meter would not go higher than 70--but the breakers would switch off and on.

I'm still on the OEM alternator. On a cold winter night, with the dash heater full on with the headlights, the "house" bank will send power back to the engine. Even so the fan ran slow.

I'm planning on adding a 20 amp dc to DC unit and hope to do that this summer, after I get the SiO2 batteries that I've been wanting for so long.

[Trackrig]

pianotuna wrote:
Duty cycle is usually 1/3

Example 120 amp alternator--continuous load 40 amps.

Don - Is your rating what you feel a standard gas engine alternator should be capable of? I ask because I have Leece Neville 165 amp heavy duty alternators on a lot of equipment with diesel engines Nodwells and Tucker Snocats). 33% of 165 amps would be about 55 amps. When running them in the late fall or winter with all of the lights, heaters and engine fans on, I can easily pull 90 to 100 amps. If I'm using the electric winches, I'll run more than that. I haven't had an alternator fail yet, but your post made me curious.

Bill

[Teleman]

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.

[time2roll]

I would run 50% maybe 60% however that is total draw not just battery charging.

Don’t need a “proper” charge with LFP like is needed with lead-acid. Just get it close to 14 volts and it is charged.

[theoldwizard1]

A typical automotive alternator can NOT properly charge a LiFePO4 battery. The only way to properly get a full charge LiFePO4 battery from an automotive alternator is use a DC-DC charger.

[Lwiddis]

I don’t use my TV to recharge my TT Lithium battery.

[pianotuna]

Duty cycle is usually 1/3

Example 120 amp alternator--continuous load 40 amps.