Debunk: Alternator DC to DC charger not required for big Lithium Battery

[Checkthisout]

100 amps @ idle stopped in traffic vs 100 amps @ 2000 RPM going down the highway.

In case # 2 the underhood air is cooler and the alternator cooling fan is moving twice as much air for the given output.

 

[scubadoo]

Our 100A rated Mitsi alternator mentioned in the last few posts above spins at c2500 rpm when the 3.9l turbo diesel engine is idling.


3:1 pulley ratio.

 

[justinm001]

The wiring from the alternator to the battery is the original as factory fitted by Mitsubishi.
The alternator delivered c90A by default for about 3 years of traveling trouble free.
For peace of mind I added the Victron Schottky diode isolator about 6 years ago to limit the final maximum charge to 14.1V. It and the associated wiring had the added effect of lowering the maximum current to c80A.
I'm guessing that the slow drop from 80A to nearer 70A over an hour or so is the result of alternator thermal regulation circuitry, battery impedance changes or ???.
It maintains that 70A until almost 100% SOC then rapidly drops to near zero as the battery terminals reach 13.95 - 14.05V.
From 14.0V to the original alternator 14.5V (now 14.1V) occurs rapidly adding no useful energy to the battery. It is already full.

No smoke yet or at least there is still some remaining.

Can you check what the alternator model is and if it has a built in regulator or an external? Sounds like you have some thermal regulator or something controlling its field based other measures. Most alternators just use the field to control the voltage and will run at 100% until they burn up.

 

[justinm001]

Our 100A rated Mitsi alternator mentioned in the last few posts above spins at c2500 rpm when the 3.9l turbo diesel engine is idling.


3:1 pulley ratio.

This is important to remember, engine RPM does not equal alternator RPM in most cases. pulley ratio adjusts the speed. but to @Checkthisout point 2000rpm engine at 3:1 ratio is 6000 RPM fan on the alternator. Most alternators have a max output amperage well below the engines max RPM so the higher the RPM the better cooling without additional loads on the alternator. Idle 700 RPM might only be 50% of alternator output but 1300 RPM might be 100% and 2000 RPM is still 100%.

 

[Zwy]

High resistance in a wire will limit the amps flowing thru to the battery from the alternator due to voltage drop. We had that discussion probably 2 years ago.

Most dc to dc chargers will actually boost the low voltage (circuit resistance) to charge the battery fully.

Now, my favorite saying in these forums. "Just because you can, doesn't mean you should".

 

[scubadoo]

Can you check what the alternator model is and if it has a built in regulator or an external? Sounds like you have some thermal regulator or something controlling its field based other measures. Most alternators just use the field to control the voltage and will run at 100% until they burn up.

No idea.
I have never seen the alternator in 9 years of full-time traveling.
It is buried in there somewhere.

 

[scubadoo]

Duplicate.

 

[OrthoMechanical]

I have a 1/2 ton truck which has a 270-amp alternator and is rated 170 amp at idle. I added a fused wires(dedicated ground wire) from the battery of the truck to an Anderson connector at the back of my truck and ran additional wiring to a Victron 360-watt DC-DC converter mounted in my RV. The wire is high quality 6 gauge. I have used this set up on many trips and have had no issues and am able to charge my LiFePO4 batteries to 14.4 volts. I added this since many of the places where I camp have lots of trees or are in deep canyons and have inconsistent light to charge from solar. I charge when moving from site to site or in an emergency I can run my truck and charge. I disconnect the connector when the truck is not running. The converter does get warm, but I mounted it vertically to improve airflow across the heat sink. I have been very happy with this set up. I have a small trailer, so weight is a limiting factor, so a heavy battery and solar panel load is not an option. However, I would not do this unless you have a large, high-capacity alternator, so for most vehicles it may not be a good option as mentioned in other posts here.

 

[Vigo]

100 amps @ idle stopped in traffic vs 100 amps @ 2000 RPM going down the highway.

In case # 2 the underhood air is cooler and the alternator cooling fan is moving twice as much air for the given output.

I hesitate to add 'pointless' detail but another thing is that the performance of a 'centrifugal impeller' is not linear with rpm so its possible that at some point you spin the fan 20% faster and it functions 50% better. The main idea being, don't load the alternator near max for long periods at low engine rpm. If in doubt, rev higher or just don't do it. If one really wants to know, point a cheap ir pyrometer at the back of the alt and get an actual data point.

One idea i've pondered is that i think a lot of the heat issues with alternators have to do with the fact that the rectifier diodes are dissipating a bunch of heat in a semi circle right around the brushes and the rear case bearing, and then in many cases that heat gets sucked THROUGH the rotor/stator gap (possibly heating both?) and expelled out the fan at the FRONT. I am curious if wiring the stator phases out to an external rectifier would allow a much higher current before the 'next weakest link' became a problem. Infrared camera viewing of a heavily loaded alternator would help sort this out for me. I don't know what the max temp of the stator windings' insulation is, but i figure the brushes are probably the weakest link, followed by the stator winding insulation. Just assumptions, really...

 

[hwse]

I can only comment from my direct experience YMMV.

• I have 920Ah of LFP installed in my Class A diesel pusher.
• The batteries are right behind the driver side steer tire.
• The engine is in the rear.
• The wires from the house battery are 2/0.
• They run from the battery compartment to the start battery compartment 30' way, then to the alternator which is another 15'. Total run of 2/0 wire is 2x(30+15) = 90'
• The 2/0 wires have at least 16 ring terminal connections.
• I have a BMK battery monitor installed on the house bank to monitor how much current is being received by the house bank,
• When running with deeply depleted house batteries (-800Ah with resting voltage at 12.5v) the max current received by the LFP batteries was 70Ah with the charging voltage at 13.0v. At the same time, the start battery was receiving 14.4v so there was a 1.4v drop in the 2/0 wire.
• Per ohms law, this indicates that the resistance in my wiring from the alternator to the house bank was 1.4v/70A = 0.02ohms.
• I found the alternator to be no hotter than any other component in the engine compartment.

I have no worries that I am over working the alternator because the resistance in the wiring limits the output current to a reasonable level given the 190A output of the alternator.

That said, this is my experience and if I had a gas rig with the house batteries close to the alternator, I would most likely need a different solution. In that case I would install a Li-BIM 225 which opens the circuit to the FLP for a percentage of the time to allow it to cool.

 

[sunshine_eggo]

I can only comment from my direct experience YMMV.

• I have 920Ah of LFP installed in my Class A diesel pusher.
• The batteries are right behind the driver side steer tire.
• The engine is in the rear.
• The wires from the house battery are 2/0.
• They run from the battery compartment to the start battery compartment 30' way, then to the alternator which is another 15'. Total run of 2/0 wire is 2x(30+15) = 90'
• The 2/0 wires have at least 16 ring terminal connections.
• I have a BMK battery monitor installed on the house bank to monitor how much current is being received by the house bank,
• When running with deeply depleted house batteries (-800Ah with resting voltage at 12.5v) the max current received by the LFP batteries was 70Ah with the charging voltage at 13.0v. At the same time, the start battery was receiving 14.4v so there was a 1.4v drop in the 2/0 wire.
• Per ohms law, this indicates that the resistance in my wiring from the alternator to the house bank was 1.4v/70A = 0.02ohms.
• I found the alternator to be no hotter than any other component in the engine compartment.

I have no worries that I am over working the alternator because the resistance in the wiring limits the output current to a reasonable level given the 190A output of the alternator.

That said, this is my experience and if I had a gas rig with the house batteries close to the alternator, I would most likely need a different solution. In that case I would install a Li-BIM 225 which opens the circuit to the FLP for a percentage of the time to allow it to cool.

great example of when a DC-DC is not needed.

 

[hwse]

great example of when a DC-DC is not needed.

I know how hard it is on an alternator if it runs wide open because I only have about a 6' run of wires from the alternator to batteries on my boat and it would put out the full rated output for +6-hours if I let it and that is in a very hot engine room. In that case, I have external regulator that limit the field current and also has a thermostat that shuts off charging when the alternator's case goes over 200º. I also have an exhaust fan mounted on the fan end of the alternator and a cool air blower on the opposite end to maximize the cooling.

Charging with an alternator CAN cause the loss of magic smoke but as long as you do it right, it works great to maximize the charging current from your engine.

 

[sunshine_eggo]

Charging with an alternator CAN cause the loss of magic smoke but as long as you do it right, it works great to maximize the charging current from your engine.

Truth. Few do the analysis, and simply installing a DC-DC is just "easy" mode. It's become the default.

 

[hwse]

High resistance in a wire will limit the amps flowing thru to the battery from the alternator due to voltage drop. We had that discussion probably 2 years ago.

Most dc to dc chargers will actually boost the low voltage (circuit resistance) to charge the battery fully.

Now, my favorite saying in these forums. "Just because you can, doesn't mean you should".

The voltage drop is what limits the output but it is the current that creates the drop. When the battery is low, the current will be max and on my coach is self-limiting to about 60A. once the battery gets closer to full, it quits taking so many amps and the voltage drop is less.
To put it a different way, if my LFP batteries need to be charge, I will get 60A and the voltage at the LFP will be 13.0 to 13.4v. If I am coming off the campground and the battery is full, the current to the battery will be nil and the voltage will be 14.4v. No current = no voltage drop.

 

[hwse]

Truth. Few do the analysis, and simply installing a DC-DC is just "easy" mode. It's become the default.

The engineer in my needs to know the "why" so that I can control the 'what".

 

[justinm001]

The voltage drop is what limits the output but it is the current that creates the drop. When the battery is low, the current will be max and on my coach is self-limiting to about 60A. once the battery gets closer to full, it quits taking so many amps and the voltage drop is less.
To put it a different way, if my LFP batteries need to be charge, I will get 60A and the voltage at the LFP will be 13.0 to 13.4v. If I am coming off the campground and the battery is full, the current to the battery will be nil and the voltage will be 14.4v. No current = no voltage drop.

Just to confirm do you have any lead acid batteries in this system?

I'm confused about a couple things. Is the voltage drop from wire length limiting the batteries ability to pull current or is it simply the wire size limiting the current? If it's the wire size isn't it getting severely hot?

If it's the wire length and voltage limiting the current it seems like your entire system is relying on the alternators regulator to be at that specific voltage. If this failed and voltage ramped up even a little it could cause all kinds of issues.

I was under the assumption that lfp pulls all the power it has available at all times until it's full.

 

[sunshine_eggo]

Just to confirm do you have any lead acid batteries in this system?

I'm confused about a couple things. Is the voltage drop from wire length limiting the batteries ability to pull current or is it simply the wire size limiting the current? If it's the wire size isn't it getting severely hot?

Can be both. When wire length is the predominant component of resistance, the higher resistance limits current, and it stays well within wiring limits. Additionally, a long wire has a lot of surface area for cooling.

If the alternator "sees" 14.4V when the battery is only at 13.6V due to voltage drop, it's going to limit output current.

If it's the wire length and voltage limiting the current it seems like your entire system is relying on the alternators regulator to be at that specific voltage. If this failed and voltage ramped up even a little it could cause all kinds of issues.

If the alternator regulator fails, you're going to have problems anyway. A suitably sized fuse can protect the system and allow normal operation.

I was under the assumption that lfp pulls all the power it has available at all times until it's full.

Generally speaking, compared to lead acid it does, and it does almost all of its charging in the 13.2-13.8V range. but when the equipment that's feeding it has limitations, there is less current available.

Voltage Drop Calculator

This free voltage drop calculator estimates the voltage drop of an electrical circuit based on the wire size, distance, and anticipated load current.

www.calculator.net

@ 45' length (one way), 2/0 has a 1.89V drop @ 270A thus, the alternator will see 14.4V when the LFP battery is at 12.51V, which is nearly empty, so it will start tapering its output to less than 270A once the battery hits 12.51V. This assumes all connections are perfect.

 

[hwse]

[Q1] Just to confirm do you have any lead acid batteries in this system?

[Q2] I'm confused about a couple things. Is the voltage drop from wire length limiting the batteries ability to pull current or is it simply the wire size limiting the current? If it's the wire size isn't it getting severely hot?

[Q3] If it's the wire length and voltage limiting the current it seems like your entire system is relying on the alternators regulator to be at that specific voltage. If this failed and voltage ramped up even a little it could cause all kinds of issues.

[Q4] I was under the assumption that lfp pulls all the power it has available at all times until it's full.

Sorry but these questions cannot be answered in a simple way, so this is going to get a bit technical and have some math [shudder].

Q1 Yes, start battery bank is two Group 31 start batteries.

Q2 You have the cause and effect backwards.

• The battery does not "pull current". It is a sponge that soaks up current that is pushed from somewhere else.
• Also, the wire does not limit the current. 2/0 wire as an ampacity of over 300A so the 70A is no problem at all. The copper wire inside the insulation is 5/8" diameter and has 133,000 strands of pure copper wire.
• The voltage drop is caused by the current that is passing through the wire.
  • Based on Ohms Law, the total resistance of my 90' of wire and a bunch of connections is 0.02 Ω. I know this because back at the engine, the alternator output voltage is 14.4v but up at the house batteries at the other end of the 40' motorhome, the voltage is only 13.0v while 70A are being pushed into the LFP batteries. Ohms law V=Ir so r = V/I. The V is the total drop in the circuit between the sourse (alternator) and the load (LFP battery). In this case 14.4v - 13.0v = 1.4v.
    Therefore r = 1.4v/70A = 0.02 Ω.
    Because of the extremely low internal resistance of LFP, the voltage at the LFP battery will always be the battery voltage and it does not change much with the addition of large amounts of current. It only changes once the battery gets full.
  • The battery wants all the current it can get so if the voltage was not regulated, it would pull the full 190A that the alternator can produce (minus what is being used by the chassis). So, if the alternator was not voltage regulated, the voltage drop in the wire would V = Ir = 190A x 0.02 Ω= 3.8v. At which point the alternator would be putting out 13.0v + 3.8v = 16.8v. It is a darn good thing that alternators have regulators to control the power output.
  • Because 16.8v is much higher than the 14.4v set voltage of the regulator, the regulator reduces the field current to the alternator so that it puts out fewer watt of power which reduces the amps sent to the battery until only 70A are getting to the battery which brings us down to 13.0v at 60A and 14.4v at the alternator.

Q3 Here we are at cause and effect again. We rely on the alternator's regulator to control the power output (W) to not allow the voltage at its terminals to go over 14.4v. If everything is fully charged and all batteries are at 14.4v and there are no other loads, the regulator will reduce the field current to the rotor to 0A and no power will be generated. Fortunately, the designers of alternators know of the dangers of overcharging. Because of this, the regulator is designed to fail open rather than closed. If the regulator fails, it will not produce any power and charging will cease. The current to the LFP is therefore controlled by the non-changing resistance of the circuit and the max voltage of the alternator. At a worst case, if the LFP was completely dead at 2.5v per cell or 10.0v the max possible voltage drop would be 14.4-10.0 = 4.4v. with 0.02 Ω of resistance in the circuit the current would be 4.4v/0.02 Ω = 220A. This is more than the max output of the alternator so the alternator would top out at 190A at a voltage of 13.8v at the alternator. Even this is not a concern because this is in the very steep part of the charge curve and because of that, the voltage of the battery will rapidly increase so that within minutes it would be up around 13.0v and we are down to our happy 70A.

Q4 LFP's do not pull power. They absorb what is pushed to them by all of the power generating sources. In the case of the alternator, it can only push a max current of 190A and a max voltage of 14.4v at any current less than its max power output.

I am sure that the power wise out there have noticed the linguistic error I have made. I used the US standard nomenclature of referring to alternator output in terms of amps, but this is not electrically correct. The output is 190A @ 12v. An alternator is a power generator and power is measured in Watts. Therefore, the output of my alternator is P = Iv or 190A x 12.0v = 2280w. This means that the max current it can produce at 14.4v is 2280/14.4 = 158A so that is what would be sent to the battery not the 190A above. This is why people often think that their alternator is not working properly when it does not produce as many amps as it says on the box. Drop the voltage down to 12.0v and run it cold and it should be close.

 

[RV8R]

Hmmm ? ,,, No DC2DC ,,, IMO there are “Pros & Cons” & YMMV as do the many variables & quality in DIY projects.

The Following Story is relevant to this thread but involved AGMs not Lithium;

About a week ago we arrived home from a 2 week Van Trip. During our van trip which had no DC2DC & is pretty much 100% charged by alternator, our “Van Use” changed ,,, more Mountain Hiking “Shasta & Lassen” & not as much driving each day. Further, our Honda 2200 Generator was taken off our equipment list as we never use it. Also, we do not have solar.

I designed our van system to directly charge relying upon a fairly high SOC about 75% or 80%. Our AGMs can take 83 amps & the manufacture’s ideal charge current is 50 amps. I have a 80 amp “slow to blow” fuse feeding the system & a 100 amp ( spare backup ) & #1 AWG. My point is the design use was to control the charge rate by internal resistance of the 250Ah AGMs & if it got outta hand, the fuse would blow, or we would run the Honda to top up so the non-DC2DC could operate below the 80 amps. For almost 2 years the theory of design worked well. Then with our new more hiking / less driving trip & the effect if accumulative non 100% SOC top offs, I saw the largest amperage spike we have ever encountered ,,, I would typically see 60 or 70 amps ,,, but not 88 amps. These spikes settle down quickly & the charge rate is usually around 50 amps after a few minutes.



So, I sat in the van with my Wife & we agreed the thing to do was to “Limit the Charge” & then be able to use 80% of the battery capacity ,,, not 25% of the battery.



So just today I got my new Kisae 1250 wired up & operational in my Promaster Van. The unit can be user programmed 3 stages & all chemistries. Now I get the “right voltages” for Bulk / Absorption / Float & I have limited the current to 50 amps.

I have no solar, but the unit is also a solar charger ,,, if desired;







Here is a few screenshots of it @ various stages today;







So “Can“ You Do It Without a DC2DC ,,, Yup

and ,,, YMMV


Pros & Cons

 

[hwse]

...
@ 45' length (one way), 2/0 has a 1.89V drop @ 270A thus, the alternator will see 14.4V when the LFP battery is at 12.51V, which is nearly empty, so it will start tapering its output to less than 270A once the battery hits 12.51V. This assumes all connections are perfect.

All correct except that the alternator has a mechanical limit as to the max power it can generate. It is not capable of producing more than 190A at 12.0v so that limits the max current to 158A at 14.4v which is well withing the ampacity of the 2/0 wire. Then we need to add to that limit that this power is only at the rated rpms which is typically spinning faster than what the engine will run at highway speeds. I do not think I could ever see much more than 125A under any circumstances and that would be for a very short time because the battery will not stay below 13v for more than a few minutes.

 

[hwse]

Hmmm ? ,,, No DC2DC ,,, IMO there are “Pros & Cons” & YMMV as do the many variables & quality in DIY projects.

The Following Story is relevant to this thread but involved AGMs not Lithium;

About a week ago we arrived home from a 2 week Van Trip. During our van trip which had no DC2DC & is pretty much 100% charged by alternator, our “Van Use” changed ,,, more Mountain Hiking “Shasta & Lassen” & not as much driving each day. Further, our Honda 2200 Generator was taken off our equipment list as we never use it. Also, we do not have solar.

I designed our van system to directly charge relying upon a fairly high SOC about 75% or 80%. Our AGMs can take 83 amps & the manufacture’s ideal charge current is 50 amps. I have a 80 amp “slow to blow” fuse feeding the system & a 100 amp backup & #1 AWG. My point is the design use was to control the charge rate by internal resistance of the 250Ah AGMs & if it got outta hand, the fuse would blow, or we would run the Honda to top up so the non-DC2DC could operate below the 80 amps. For almost 2 years the theory of design worked well. Then with our new more hiking / less driving trip & the effect if accumulative non 100% SOC top offs, I saw the largest amperage spike we have ever encountered ,,, I would typically see 60 or 70 amps ,,, but not 88 amps. These spikes settle down quickly & the charge rate is usually around 50 amps after a few minutes.
...
So, I sat in the van with my Wife & we agreed the thing to do was to “Limit the Charge” & then be able to use 80% of the battery capacity ,,, not 25% of the battery.
...

So just today I got my new Kisae 1250 wired up & operational in my Promaster Van. The unit can be user programmed 3 stages & all chemistries. Now I get the “right voltages” fir Bulk / Absorption / Float & I have limited the current to 50 amps.

I have no solar, but the unit is also a solar charger ,,, if desired;
...

So “Can“ You Do It Without a DC2DC ,,, Yup

and ,,, YMMV


Pros & Cons

Yep.
All of this is in full agreement with my last comment in post #35. My analysis and system design is based on my coach and its charging and battery system and YMMV [your milage may vary]

I suspect that you do not have a 90' round trip from alternator to battery. You also might not have a 190A @ 12v alternator. I checked out my system before installing the LFP and found that it limited itself to 60A with FLA. I also knew that this would go a bit higher with LFP but not by much. I could see that the alternator was limiting the output because I had full 14.4v at the start batteries and was only getting 60A to the house batteries. I also do not need to worry about pumping too much current into my LFP batteries because they each can be charged at 200A for a total charge capacity of 400A. With my system, if I have low batteries and a short trip from one boondock to another, I fire up the generator for the drive so that I get the 60A-70A form the alternator plus the 108A from the Magnum. It takes a while to recharge a 920Ah pack that is down on its SOC. I would really like to find another 100A 12v charger to add on.
On my system, the biggest DC2DC would amount to a "why bother" charge.

 

[RV8R]

Yep.
All of this is in full agreement with my last comment in post #35. My analysis and system design is based on my coach and its charging and battery system and YMMV [your milage may vary]

I suspect that you do not have a 90' round trip from alternator to battery. You also might not have a 190A @ 12v alternator. I checked out my system before installing the LFP and found that it limited itself to 60A with FLA. I also knew that this would go a bit higher with LFP but not by much. I could see that the alternator was limiting the output because I had full 14.4v at the start batteries and was only getting 60A to the house batteries. I also do not need to worry about pumping too much current into my LFP batteries because they each can be charged at 200A for a total charge capacity of 400A. With my system, if I have low batteries and a short trip from one boondock to another, I fire up the generator for the drive so that I get the 60A-70A form the alternator plus the 108A from the Magnum. It takes a while to recharge a 920Ah pack that is down on its SOC. I would really like to find another 100A 12v charger to add on.
On my system, the biggest DC2DC would amount to a "why bother" charge.

I didn’t really read in-depth, but yes my van is 15’ #1AWG red ,,, 2’ to 3’ of #1 black to chassis return path. Can’t remember the voltage drop, but it was minor ,,, 220 amp alternator. Still the typical non-DC2DC voltage was around 14.2v & it wants 14.7v

We do not use a lot of electrical energy & we are self sufficient for about 2 weeks then we need fresh water (we carry 44 gals). We basically run on gasoline & propane ,,, Well & Coffee ?.

When I 1st got into the DIY van build hobby, the electrical came down to one big question really, “How are You going to Charge Your Batteries??”

 

[Checkthisout]

This is important to remember, engine RPM does not equal alternator RPM in most cases. pulley ratio adjusts the speed. but to @Checkthisout point 2000rpm engine at 3:1 ratio is 6000 RPM fan on the alternator. Most alternators have a max output amperage well below the engines max RPM so the higher the RPM the better cooling without additional loads on the alternator. Idle 700 RPM might only be 50% of alternator output but 1300 RPM might be 100% and 2000 RPM is still 100%.

Right. The engine doesn't idle at 1000 RPM, I was merely demonstrating a concept.

The faster the alternator spins for a given output, the better because cooling will be better for the given output.

Can maybe put an RPM triggered relay that prevents auxiliary battery charging below 1500 RPM or something like that.

 

[hwse]

I didn’t really read in-depth, but yes my van is 15’ #1AWG red ,,, 2’ to 3’ of #1 black to chassis return path. Can’t remember the voltage drop, but it was minor ,,, 220 amp alternator. Still the typical non-DC2DC voltage was around 14.2v & it wants 14.7v

We do not use a lot of electrical energy & we are self sufficient for about 2 weeks then we need fresh water (we carry 44 gals). We basically run on gasoline & propane ,,, Well & Coffee ?.

When I 1st got into the DIY van build hobby, the electrical came down to one big question really, “How are You going to Charge Your Batteries??”

If your alternator output on the dash is not getting to the same voltage when charging your house batteries as it does when they are full, then you are taking too much to the house and it will cause your alternator to to overheat. In that case, a DC2DC or a LiBIM225 is a good idea.
I get 14.4v max and that is from idle upwards. Because the big diesel is a low revving engine, its alternator can handle a high ratio pulley, so they typically work much better at idle than does the alternator on a gas engine at idle.

I will freely admit that I am an energy hog. My 40' motorhome is all electric, so I have no propane for cooking or heating water. It is all done with electricly and a diesel hydronic heater that is run on 12v. My wife and I also work out of our coach, so we have three large high-powered computers, 5-monitors and two printers also that run most of the time. I go through about 400Ah per day so fast charging is very important.

 

[RV8R]

If your alternator output on the dash is not getting to the same voltage when charging your house batteries as it does when they are full, then you are taking too much to the house and it will cause your alternator to to overheat. In that case, a DC2DC or a LiBIM225 is a good idea.
I get 14.4v max and that is from idle upwards. Because the big diesel is a low revving engine, its alternator can handle a high ratio pulley, so they typically work much better at idle than does the alternator on a gas engine at idle.

I will freely admit that I am an energy hog. My 40' motorhome is all electric, so I have no propane for cooking or heating water. It is all done with electricly and a diesel hydronic heater that is run on 12v. My wife and I also work out of our coach, so we have three large high-powered computers, 5-monitors and two printers also that run most of the time. I go through about 400Ah per day so fast charging is very important.

Yup

I don’t have an alternator problem, I put in a DC2DC to limit the current to not fry the AGM batteries.

”Energy Hog” ?? Well “Battery Energy” maybe. At our house we have grid electricity & natural gas. The natural gas can be used to; Cook, Hot Water, Fireplace, Furnace, Clothes Dryer, BBQ. We also have an electric heat pump for heat & AC.

Van ,,, Well we carry propane. Just a different energy that battery electricity ,,, still energy. We use propane to cook & heat the van. 1 - 20lb regular bbq size propane tank holds as much energy as 116 - 100Ah 12vdc lithium batteries ,,, So if you are an energy hog ,,, I am an energy hoarder. ?