High performance/heavy duty alternator battery charging

The beauty of running a smart charger from the inverter strapped to your truck battery is that the trailer battery gets charged using the smart charger's profile for bulk constant amps to 14.x then absorption.

That is way, way faster than the unmodified, as is, truck charging with its 13.8v and immediate tapering to low amps.

I can easily run a 35amp Vector from the truck that way. I did it because I wanted more amps to the large battery bank and the Honda 3000 maxes out at 130amps with non-PF corrected chargers. Using the truck as a second inverter/gen, I was able to add another 35amps for a total of 165amps.

Why all the amps? A big battery bank and short allowed gen time means you need amps to get a 50-90 done in the time allowed.

When searching for a way to charge the batteries on my trailer, to run the trailers inverter/charger to power the 120vac mini fridge, I tripped over these..
They make 10, 20, and 50 amp versions. Basically a charge pump from one 10-15 volt input to a 12 volt battery. Intended for the batteries on lift gate equipped trailers. It *should* work to force feed up to whatever its rated amperage into your batteries overcoming slight shortages in alternator voltage or wire gauge.

I did a brief experiment using my inverter to power my Schumacher to send 12 amps into another battery. It was using nearly 25 amps from one battery to send ~13 amps into the other.

What a waste, even if the alternator is providing the juice to power the inverter.

My Voltage regulator is controlled by my engine computer. It arbitrarily decides that 13.7v is plenty and the amps needed to hold 13.7 can be 1/3 the amps required to hold 14.5. While I have seen over 100 amps from my alternator when cold with depleted batteries, it never lasted very long and 13.7 always came well before the house battery approached 80%, and my house batteries are on a shorter circuit path than is my engine battery.

I've kind of given up on getting anything more out of my alternator. The fatter cabling I added ensures I do get some high initial amps but 13.7v does not do nearly as much.

I have no clue what a truck with 2-125a alternators could do. A 90a Iota seems ambitious but don't know.

Mine just has one 105a alternator, had the inverter on the battery posts with 18" 1 gauge copper, not on the alternator, and it could do about 50a worth of Vectors (900w?) before losing it at higher amps when I was trying that out. You could see the truck's voltage needle wavering, then give up and go down to the left when you maxed it out.

I think a 90a Iota needs a 20a circuit for input so not sure how you do that with an inverter receptacle set. Gotta go for that PF corrected PowerMax :) (They have PF in sizes 75a on up to 100a)

I was thinking Prowatt 2000 and 90 amp IOTA.

Specialty three stage alternator & regulator would also work well.

I am assuming 2x 125 amp alternators as specified by OP.

smkettner wrote:
Put 100 to 250 watts of solar on the TC roof and the issue might just go away.

Otherwise for real charging control from the alternator... use an inverter (powered by the truck 12v system) to drive the TC charging system.

Remember that the inverter first needs to be big enough in watts to run the charger's VA draw. EG a 35amp Vector charger wants 600w so a useful charger will need say a 1000w inverter. Usual inverter rule applies to keep its wires to truck system short and fat. But not so short the inverter is into the engine compartment or sits on the fender or the inverter will get too hot and shut itself off. Let it hang down the fender away from engine heat.

Next, your truck's 12v system needs to run that without the truck's own voltage sagging below 13.8. You can do more with that if you turn off the climate control etc! :) Anyway, you find the charging amps limit real quick when you see the truck's voltage needle swing down to the left and you then have to go with fewer charging amps until you find out what the truck's system can handle.

BTW I found no difference revving the engine from just leaving it at idle, but I gather there are differences between trucks on that.

Wo.ody,

Exactly, what is your goal?

For most battery types The recommendd peak charging current (Recommended by Xantrex) is 30 percent of the 20 hour amp hour rating.

So if you have say 3 group 29's at 105 AH each, (315 total) the peak (Maximum) Charging current should be no more than 94.5 amps.

Trojan, however, recommends a much slower charge rate (like 10%) and Lifeline, on their AGM's a much higher one.. ONLY LIFELINE.

The alternator can limit it's total output, However it has no control over how much goes to the batteries as opposed to headlights, HEVAC, and so on.

Wo.ody wrote:
-How do you determine the max input amperage (less 15%) your system can handle?
-What component in the electrics determine what amperage is sent to the TC battery series?
-Can you send too many amps (too hot so to speak) back into the battery series?

Hi Woody,
- The amount of current that you charging system can "handle" will be based on the output rating of your alternator, the size of the cabling from the alternator to the battery (bank), the state of charge of your battery, and the battery's internal resistance.

- There are several "components" that all work together and affect the charge amperage (e.g. alternator, wiring, fuses, battery chemistry & SOC), but in terms of the electrical control, the electronic device that does that is called a voltage regulator.

- No, you cannot "send too many amps back into the battery"… unless of course the voltage is too high. That said, you most definitely can have too many volts, and if you do then you risk overcharging your (chassis & house) batteries.

In order to give you specifics about an appropriate alternator/charging plan for your truck & TC, you will need to let us know more about your system, and your wants & needs; What alternator(s) are you currently using? What brand and model batteries do you have? How quickly do you need to charge your batteries? How far do you typically discharge your battery before recharging? What other loads do you anticipate operating from the alternator? What's the output voltage from your truck's alternator at high-idle? Do you have budget constraints?

MrWizard wrote:
As the engine and alternator warm up , the max voltage drops to about 13.6

I respectfully disagree (acknowledging the differences between vehicles and the fact that I have an aftermarket alternator)...
Here are a couple of photos of my in-dash volt/ammeter I snapped this past Friday afternoon (12/20) as I was heading out for a weekend camping trip with my family. Ambient temp was around 60ºF and the the voltage never once dropped below 14.6V during the course of the 1 hour drive to our destination.


This pic (45.4A @ 14.9V) is with the engine running, shaded solar panels, battery bank at ~80% SOC, and no other loads operating in the camper


This one taken 40 minutes later indicates 76.4A (15.1V). The additional current is from the refrigerator running on 120Vac via the inverter (battery now at ~87%)

smkettner wrote:
Put 100 to 250 watts of solar on the TC roof and the issue might just go away. Otherwise for real charging control from the alternator... use an inverter (powered by the truck 12v system) to drive the TC charging system.

I agree that solar is a great way to charge RV batteries and probably a better first-step than a fancy-shmancy alternator charging scheme.
However I'm not in favor of charging while driving by adding an inverter and battery charger to the mix… that's just too lossy, complicated, and expensive… especially if the batteries are deeply discharged and you're hoping to charge quickly.
When paired with an appropriate voltage regulator & wiring, a high output alternator can generate 2X the current of any RV battery charger I have ever seen, and it will do so more reliably and for fewer dollars. Besides, if the engine is running then the alternator is already spinning, so why not use it?

Cheers,
-Mark

Put 100 to 250 watts of solar on the TC roof and the issue might just go away.

Otherwise for real charging control from the alternator... use an inverter (powered by the truck 12v system) to drive the TC charging system.