Can I connect my alternator to my MPPT?

When driving between camping spots I already have an alternator turning and charging the engine battery.
I know not to connect my alternator to the coach deep cycle batteries since this would probably overcharge the engine battery.

Thinking about a DC to DC battery charger, I had the thought that the MPPT is essentially just a DC to DC charger isn’t it?

Can I safely wire the alternator to the PV input of the MPPT? I expect I would have to unplug the solar array for this to work.

Drooling Piston said:

I know not to connect my alternator to the coach deep cycle batteries since this would probably overcharge the engine battery.

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How would connecting the alternator to coach batteries overcharge the engine batteries?

Unless you have a very large rig with a 24 volt starting battery and alternator with a 12 volt house battery, you won't even get enough voltage from the alternator to charge your house batteries.

MPPT charge controllers can reduce the voltage coming from your PV array, but they can't raise the voltage. Most charge controllers need something like 5 volts more than the battery bank's charging voltage to even start charging.

MisterSandals said:

How would connecting the alternator to coach batteries overcharge the engine batteries?

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If the alternator is throwing a bunch of amps to a low battery, while the engine battery is full I expect it could/would overheat or overcharge the full battery.

The full battery will stop accepting energy the same as it always would when full and the engine is running.

Electricity always takes the path of least resistance. If the coach batteries have the least resistance then that's where it will go.

With that said, A good DC to DC charger is a buck/boost device. As an example my truck has "smart" charging. The last time I used my DC to DC charger the input was 12.8-12.9 and 14.00 output with 35 amps output (the cost to boost was 5 amps with a 40 amp charger.) It is very unlikely that a MPPT will have that capability. Food for thought.

Just An RVer said:

Unless you have a very large rig with a 24 volt starting battery and alternator with a 12 volt house battery, you won't even get enough voltage from the alternator to charge your house batteries.

MPPT charge controllers can reduce the voltage coming from your PV array, but they can't raise the voltage. Most charge controllers need something like 5 volts more than the battery bank's charging voltage to even start charging.

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In this scenario, if the alternator is putting out 13.2V would a PWM charge controller work? They are smaller and half the price of the Renology 20amp B2B charger.

If your battery bank is all lead acid batteries, you might be able to charge directly from the alternator. If you have lithium batteries, you will absolutely need a DC-DC charger to charge from the alternator.

You mentioned your alternator supplying 13.2 volts. Battleborn, for example, recommends bulk charging at 14.4 volts and a float charge of about 13.6 volts. Without a DC-DC charger to boost the voltage from your alternator, the LiFePO4 battery will never charge.

Yes, a DC-DC charger is more expensive than a solar charge controller. They do different jobs.

You'll see people here talking other devices instead of a DC-DC charger such as in this thread:
... but it requires more equipment than just a solar charge controller. It also requires a thorough understanding of how your equipment works.

Please don't take any of this as being dismissive. I mean this in the most supportive way, but while you're getting started, I suggest you use the gear as it was intended. There is a time for experimentation and innovation once you understand how everything works. But if you don't really understand it, you can spend a lot of money for nothing, or worse, set fire to your RV.

The whole issue is when the tow vehicle battery if fully charged it will shut off the alternator even if the coach batteries are not charged, the alternator does not “see” the coach batteries , the solution is to use a B2B charger, this is designed to charge a battery from another battery, really made for using a vehicle with a lead acid battery to charge a coach lithium battery, it is equally useful to charge a lead battery from a lead acid battery. With the voltage drop from the tow vehicle to the coach, there will not be enough voltage to top off the coach battery as it usually be in a lower state of charge than the starting battery. A B2B charger (battery to battery) charger is a buck/boost converter so it is designed to deal with different voltages. There are ones that can charge a 24 volt battery from a 12 volt battery

Reference: Wikipedia buck/boost converter, but read first buck converter and boost converter.
Efficiency in conversion 95-98%
One that impresses me....Victron

Drooling Piston said:

When driving between camping spots I already have an alternator turning and charging the engine battery.
I know not to connect my alternator to the coach deep cycle batteries since this would probably overcharge the engine battery.

Thinking about a DC to DC battery charger, I had the thought that the MPPT is essentially just a DC to DC charger isn’t it?

Can I safely wire the alternator to the PV input of the MPPT? I expect I would have to unplug the solar array for this to work.

Click to expand...

I've done that, sort of. I haven't built my system yet, but, while testing, I've reached the point where I have an undersized (20AH of Tesla NCA cells) 24V "house" battery, a slightly larger (38AH LiFePO4) backup 12V battery, and of course a Group 65 starter battery in the vehicle, connected to the alternator. My test is making a pot of tea with an electric kettle every morning, and it turned out that I can't do that from the 20AH test battery, especially after it's become somewhat depleted overnight, before the sun comes up. So, when I want tea before the Sun comes up, I start recharging the 24V from the 12V, using a cheap Chinese boost converter, with variable voltage and current, set to about 30V, or a couple of amps (depending) paralleled with the solar panels (I have a fistful of Schottky diodes, and added one to the output of the boost converter, just in case, although I think it's not necessary). But now the 12V will need recharging, so, after the Sun comes up, I recharge it from the 24V, using a buck converter (carefully adjusted to 14.4V, and current limited), to the charge connection on the BMS. But, what if there is no Sun? Well, I already had a situation with my starter battery, which used to be trickle-charged by a small panel on the dashboard, but, now that I'm setting larger panels on top of the vehicle, including over the windshield, for tests, isn't enough, so I looked at the voltages, thought about it, and tried running a cable from the buck converter to the starter battery (I already knew that the vehicle voltage regulator stops at 14.4V), which worked OK, and one day, when the backup 12V charge connection and the starter 12V battery voltages were not too dissimilar, I just tied them together. It seems OK, for testing at least. Once I have those connections, if I start the vehicle, and disconnect the output of the house battery 24V to 12V buck converter, I'm charging on alternator. I think. And when I connect the boost converter's 12V input, charging the house battery, via the solar input of the controller.

It's unconventional, I suppose, and I can think of some ways it could go horribly wrong, and there are almost certainly more ways I haven't thought of, but, I might wind up with some variation of this setup in my final design, probably with more breakers, and fuses, and a low voltage disconnect which I have in my box of stuff, to be installed. In case it helps the reader to know, I started tinkering with electronics about 55 years ago, and, I started tinkering with solar in my van a little over a year ago, which is to say I don't try to go too fast, and think about what I am doing, when I am departing from convention, which takes a lot longer, one way or the other, in my experience.

You want get all the volts to the house batteries because of distance. AND the alternator could send 80-120Amps up a too-small wire. Sure, RVs come with the 7-way wired to charge the house batteries and it works- just not well.
Just buy the DC-DC and no probs

The problem is that its takes voltage to push amps, so the closer the voltage differential means lower amps- longer charge times. At 13.2 you would likely see very little amps.

12VoltInstalls said:

You want get all the volts to the house batteries because of distance. AND the alternator could send 80-120Amps up a too-small wire. Sure, RVs come with the 7-way wired to charge the house batteries and it works- just not well.
Just buy the DC-DC and no probs

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That is what I decided to do. I ordered the Renology B2B charger. I plan to put another lug at the starter, over the current battery cable, run a 6 gauge wire down the frame of the TV and put a 50amp DC plug at the bumper. I will put another mating plug at the trailer tongue and run a 6 gauge wire the 35 feet down the frame to the batteries.
This raised another question, This B2B charger wants to be "woke up" by a wire from the alternator. How have you folks done that?
I suppose if I knew I would NEVER forget to unplug this new 50amp plug every time I stop, I could just jumper that signal from the main 6 gauge wire.
Or use the wire that used to try to charge the batteries. It is already in place, in the trailer light plug. I may have to trace it & cut it on the trailer side, somewhere along the way to remove it from the rest of the system, splice on a wire and run it along the frame, with the 6 gauge to the battery compartment.

Drooling Piston said:

This raised another question, This B2B charger wants to be "woke up" by a wire from the alternator. How have you folks done that?

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You need to run a wire from something that is energized when the ignition is on (and not energized when the ignition is off). Some vehicles have IGN leads - the Transit van has them in the back for things like this.
The Victron Orion Smart DC-DC senses increased voltage (possibly configurable but it did not need to change anything) when the alternator starts producing 14-ish volts. And turns off roughly 20 seconds after the alternator stops. Very handy. I ran an IGN wire because i was skeptical but it worked perfectly right out of the box. I installed the 30a model (there are other sizes including a hard to find 18a).

MisterSandals said:

You need to run a wire from something that is energized when the ignition is on (and not energized when the ignition is off). Some vehicles have IGN leads - the Transit van has them in the back for things like this.
The Victron Orion Smart DC-DC senses increased voltage (possibly configurable but it did not need to change anything) when the alternator starts producing 14-ish volts. And turns off roughly 20 seconds after the alternator stops. Very handy. I ran an IGN wire because i was skeptical but it worked perfectly right out of the box. I installed the 30a model (there are other sizes including a hard to find 18a).

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Well that does sound like a smart DC-DC. I thought "smart" was regarding switching between charging stages. When my B2B arrives, I will have to study the manual to see if it has this same feature.

My old 1997 Dodge doesn't have an ignition wire already ran to the back, but I do believe the "charging" wire in the 7 way light plug goes dead when the key is off.

Oh.... I thought smart charger usually refers to the ability to select battery type AND to have the ability to intelligently provide the right charge profile to the battery type?
Or maybe it’s also bluetooth?

I think the ‘signal’ ignition-on feature is universal though

MisterSandals said:

The Victron Orion Smart DC-DC senses increased voltage (possibly configurable but it did not need to change anything) when the alternator starts producing 14-ish volts. And turns off roughly 20 seconds after the alternator stops. Very handy. I installed the 30a model (there are other sizes including a hard to find 18a).

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The Renology unit I ordered arrived. It does NOT have the smart sensing function to know when the engine is running and then turn the unit on or off. It will require a separate wire and or switch.
I’m considering returning this unit and looking at other B2B. I looked at the Victron unit like u bought. What I don’t see in the description is the battery charger having multiple stage charging to include a floating or maintenance charging.
If I leave home with fully charged batteries, will this unit recognize that? Or once it senses the truck is running will it go into charge mode? What have u found with ur set up?

Drooling Piston said:

If I leave home with fully charged batteries, will this unit recognize that? Or once it senses the truck is running will it go into charge mode? What have u found with ur set up?

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Unfortunately my experience with it is limited to installing it in a friends van and performing basic start and stop charging based on alternator sensing.
She was heading out for a trip and has been delighted with its charging.

I asked her about some details but she knows almost nothing about batteries.

Sorry, I wish I knew more. It just worked out of the box. If it wasn’t Victron I would have insisted on knowing more about it.

I don’t know how important this really is. But I liked the sound of this “Intellegent charger” automatically switching between different stages of charging.
Almost all chargers decrease their amperage output as the batteries get closer to full charge.
The floater/maintenance mode is a must have for an AC charger that may stay plugged in for a few months, but since this won’t be plugged into the truck and charging for more than a few days at a time, it probably won’t be any big deal.