Vehicle alternator + solar questions

I've seen occasional references to setting up your house batteries to charge off the vehicle alternator AND one or more solar panels. This has caused me to ponder several things.

If the alternator is dumping electricity into the house batteries at 13.5 to 14.0 volts, doesn't the solar charge controller see that and assume that's the battery voltage?

And if so, doesn't it then throttle back the solar output?

Depending on the size of the wire you are running from the isolator/solenoid, and how many/big your solar panels are, I could see how you might end up with LESS recharging current rather than more.

Or does the vehicles voltage regulator see the voltage from the solar panels and decide, based on that, to cut back on how much alternator current to send the house batteries?

In short, how well does the solar charge controller and the vehicle's voltage regulator/ ECU play together?

Could these problems (assuming they exist) be avoided by running the output from the alternator to the input of the solar charge controller, instead of directly to the house batteries? Or would that cause the magic smoke to leak out of your vehicles ECU?

Would you need diodes in the lines to keep the current from one source from backfeeding into the other source?

And of course, you would then need a much larger (and more expensive) solar charge controller, to handle the current from both sources.

Answers to these questions, preferably based on real world experience, would be most welcome.

Regards
John

Whenever 2 charging sources are charging the same battery, the charging source with the higher ABSsorption Voltage, the maximum allowed voltage, will continue charging after the charging source with the lesser ABSV saw voltage climb past its ABSV limit and quit.
Multiple charging sources can work together just fine until one reaches ABSV before the other, but when this happens the battery is close enough to the higher State Of Charge, that multiple charging sources are not needed.

With lower amp charging sources both contribute until one reaches their ABSV.

When there is a large charging source, like a fatly cabled alternator, it is the same thing, it is just that the ABSV is reached much faster, and at a lower SOC.

Usually when ABSV is reached, the battery is in the 80% charged rate range, and the amperage it needs to hold ABSV is much much less than it was taking at 50% SOC. Getting more amps to flow into the battery would require more voltage, which is not allowed by the charging source. At this point the battery only needs x amount of amps to hold 14.x volts.

My Solar will act in concert with my alternator, sometimes it also decides to not contribute, and sometimes it is partially powering the stock vehicle electrics like the coil and fuel pump.

It depends on which stage the controller is in, the voltages I have programmed into the controller, the state of charge of the battery, the amopunt of sunlight available, and whether the vehicles voltage regulator has decided that 14.9v has gone on long enough and that 13.7v is safer.

My vehicle likes to revert back to 13.7 or 13.9 well before the battery would like. Nothing I can really do to control this as the VR is inside the engine computer. It does it at night or in the day. when it decides it is time for 13.7 in daytime when the batteries are near full, then my solar partially powers the vehicle electronics. If this happens (13.7) when the battery is still depleted, then the charging amps required to hold 13.7 are much less than at 14.9v, but the solar contributes all it can as it still wants to reach and hold 14.9v for 2 hours.

Lots of little variables effect how, when and how much. Some can be influenced, some cannot. Some are vehicle specific, some are not.

My alternator can do a lot of work, and if the battery is low, the vehicles VR will seek to get it to 14.9v, and it takes 50+ amps and a while to get it up there and hold it there. The amps required to hold it there will keep tapering as the battery charges. Eventually it gets to the point where the amps taper enough that the solar can hold 14.9v on its own. If the battery is in this range, I'll just remove the alternator from this battery with my manual switch and just let the solar do it.

I take advantage of the massive bulk currents the alternator is capable of, but once the battery gets in the 80% SOC range the full potential of the alternator is not needed, and multiple charging sources at this point might, or might not play together very well.

But at this point multiple charging sources are likely not needed. There is no real worry about the solar and vehicles VR confusing each other. They are not going to damage one another. They might not work in concert perfectly, but at the stage where they will confuse each other, in general there is only need for one charging source anyway.

So not Ideal, but nothing to go to tremendous lengths to counteract. Overall charging time is not going to be effected much at all.

Still way better to have fat cabling between alternator and battery being charged as a thirsty battery can ask for huge amperages, and in this lifestyle when the next discharge cycle is coming not far off, it is best to do everything possible to get the battery as close to 100% SOC before the discharging begins, and large alternator currents can help the solar to complete the task, and make the battery happier.

There are so many different ways to isolate/combine an engine/house battery and different solar controllers there are multiple ways for the charging sources to interact and there is no one answer to how they will interact.

While I have a battery monitor I cannot read it from the drivers seat. But I do have two mini voltmeters on my dash, and they are very enlightening as to what is going on charge wise, but in the end it is the battery that decides how much amps it needs to hold a certain voltage. Solar controllers are just basically voltage regulators. They do not always allow the perfect voltage to be held for recharging depleted deep cycle batteries, and this task was definitely not in mind when the Vehicles voltage regulator was designed either.

Short answer - any reasonably smart controller will have a high voltage cutoff which is generally set to lower than what the alternator will put out. When the battery gets higher than this, the controller simply shuts off. I've had my solar setup hard wired to my back battery for a couple of years now and it still plays nicely with my alternator.

They do play well together and having both are a very good idea.

I would not connect them both through the charge controller. Each should go directly to the battery.

Chances are very good that your batteries will last longer with both rather than just one of the other.

Having both really pays off if you are boondocking and parked for an extended time in bad weather. You can just start the engine and drive and be back in business. I went 3 weeks without seeing the sun in the Arizona monsoon. Those who depended on solar alone ran out, those with both were fine.

Bob

Thanks to all three of you, but especially to Sternwake. I was overthinking it, as I often do.

Stern, you mentioned "a suitably fat cable from the alternator". Can you be a bit more specific? I'm thinking 4/0 welding cable might be a bit of overkill, and at today's copper prices . . .

Regards
John

4 awg is a pretty good thickness. I use a parallel circuit directly from alternator to my battery switch that is 4awg jumper cable, but I use both 4awg cables, which is not as good as using the same amount of copper in a single cable.

Welding wire, while nice and flexible, is not really great for automotive or marine use. The insulation is not rated to handle high heat or oils or solvents.

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Also depending on the method used to charge/isolate house battery, full benefits of thicker cabling might not be realized if the stock alternator charging circuit is to be used to pass current to both engine battery and house bank too.

An additional parallel (fused) circuit between alternator and engine battery/solenoid/ isolator can greatly increase alternator amps into depleted house bank.

If depending on the alternator for most of the recharging, then cabling thickness overkill is good for the battery, and harder on the alternator as it will work harder to give the battery all that it can ask for. Happy mediums exist but are hard to define.