Solar charging while also charging from tow vehicle

[TahoeJoe]

We had a brief break from Winter here in the Sierras, so I took the 5.0 down to the desert by Lone Pine for a warm sunny boondocking trip this past weekend. It was also a chance to do some further testing of my newly installed solar system. That system includes a battery monitor, and I noticed something that happens when I'm connected to the truck, which does charge the batteries through the 7-pin. Basically, I see the charge rate drop significantly as soon as I connect to the truck, and it goes right back up when I disconnect. For example, at one point it was charging at about 5 amps, but then when I connected it it dropped to about 3. I didn't record the exact numbers, but I'm sure it happened every time I connected. What's going on here? Perhaps this is the charge controller being "smart" and throttling back when it senses another charge coming in? I'm using a Renogy 40A MPPT controller...

[alanmalk]

Originally Posted by TahoeJoe

We had a brief break from Winter here in the Sierras, so I took the 5.0 down to the desert by Lone Pine for a warm sunny boondocking trip this past weekend. It was also a chance to do some further testing of my newly installed solar system. That system includes a battery monitor, and I noticed something that happens when I'm connected to the truck, which does charge the batteries through the 7-pin. Basically, I see the charge rate drop significantly as soon as I connect to the truck, and it goes right back up when I disconnect. For example, at one point it was charging at about 5 amps, but then when I connected it it dropped to about 3. I didn't record the exact numbers, but I'm sure it happened every time I connected. What's going on here? Perhaps this is the charge controller being "smart" and throttling back when it senses another charge coming in? I'm using a Renogy 40A MPPT controller...

Can you fill in a couple of small details for us?

Is your truck running when you connect the 7-pin to make this observation? (Assuming "yes"). Do you have a DC-DC booster between the truck alternator and the batteries? (Likely if the batteries are lithium.)

Where is the exact location of the battery monitor shunt (the device that measures the current in and out of the batteries)? It should be the very last thing connected to the wiring prior to the negative post, or the very first thing connected to the positive post.

[TahoeJoe]

Originally Posted by alanmalk

Can you fill in a couple of small details for us?

Is your truck running when you connect the 7-pin to make this observation? (Assuming "yes"). Do you have a DC-DC booster between the truck alternator and the batteries? (Likely if the batteries are lithium.)

Where is the exact location of the battery monitor shunt (the device that measures the current in and out of the batteries)? It should be the very last thing connected to the wiring prior to the negative post, or the very first thing connected to the positive post.

Yes truck was running. I didn't test it with the truck off, but I could. And yes the shunt is the very last thing connected before the negative post on the battery...see attached pic, shunt is at upper right. Only one negative comes out of it and attaches to the battery, and that is the only wire connected to the negative terminal on the battery. No battery booster at all, these are dual 6V not lithium.

[alanmalk]

Originally Posted by TahoeJoe

...
Only one negative comes out of it and attaches to the battery, and that is the only wire connected to the negative terminal on the battery.

(emphasis mine)

That statement eliminates any guesses I may have had. I was suspicious that the truck charge was taking a different path, not measured by the shunt - but not possible since ALL the current is going thru the shunt.

[Effie]

This can be a situation when the voltage from your MPPT is larger than the one supplied by the car’s alternator. So in essence the output from your solar is charging both the Escape batteries and your car battery. If you have a way to see how much current your solar controller is producing (as opposed to what goes into your trailer batteries) you can verify this. Modern alternators have a controlled output to help with gas mileage, so even when you rev your engine you may still have the same situation. When camping I always disconnect the seven pin trailer plug.
The Alabama Hills, one of my favorite places to camp .

[Kirt]

It's normal for the solar controller to cut back when it sees a higher voltage. In this case, your truck running or charging from your alternator.

[MVA]

I would measure the open circuit voltage of the truck charging circuit at the battery and the open circuit voltage of the solar charge controller circuit at the battery. As others have stated, I suspect the truck will be higher. If so, that behavior would be expected.

2 cents

[TahoeJoe]

Thanks all, for the ideas and fodder for further testing, which I'll have to put on hold at the moment...winter has returned and now we are socked in with snow, and no sun to be seen!