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I have the following setup in my van Diagram of the electrical system in the van.

BAT1 is the battery in the front of the van which was build in. It is being charged by the alternator while the car engine is running. This also turns on the 12V 70A relays which usually means that it also charges BAT2.

I ran this for a while. But then BAT2 basically died. This could be because I overdischarged it, but makes me worry.

Sidenote: The two appliances FRIDGE1 and LS1 are examples. In reality there are more.

Today I checked and with mild sunshine at an okay angle, the 2x(12V 100W) solar panels were delivering around 15V 0.1A. So there is a problem. Do you have any idea how I can figure out what that problem is?

The ground is the hull of the van. Is that an issue that everything is connected like this??

EDIT

More information was requested. The black image between the labels Bat and Load depicts a solar charge controller (30A max). I attached a bluetooth reader to it. My readings come as information from the charge controller and a multimeter. I used the multimeter only to measure voltage.

Solar
-15.5 V (can be more sometimes, but not significantly)
-0.1 A
-set up in series as depicted

BAT2
-14.4 V max (while car engine is on), 12.2-12.9 V otherwise
-the 160Ah hour Calcium Battery is still good enough for making a coffee☕, but the voltage drops significantly when doing so.
-I plan to exchange it with a lithium battery. But before that I want to make sure that my setup doesn't break it.

BAT1
-60Ah Lead battery under the hood -12.39V (single measurement with multimeter yesterday)

To precise my questions:

  • Is it okay to connect everything to the same ground like that?
  • How do I "debug" my solar setup to figure out what's wrong?

If there is anything else missing, please clearly tell me what that is.

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  • \$\begingroup\$ At what point you measure the solar panels' output? And were they connected to the charge controller when you did? Have you measured BAT2 directly or is your conclusion based on the loads not doing anything? Do you have a way of checking if the charge controller is still OK? I'm not sure it likes having its charging output connected directly to the alternator's output. \$\endgroup\$ – ocrdu Sep 8 '20 at 16:49
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    \$\begingroup\$ The problem is probably not your ground, but the design of the battery / solar / load system overall. You're going to have to provide a lot more detail for this to be on-topic here. Do you actually have a charge controller for the feed from the solar to the second battery? If not, that's an immediately obvious showstopper problem. \$\endgroup\$ – Chris Stratton Sep 8 '20 at 18:14
  • \$\begingroup\$ the title of your post does not match the content of the post ... the title should briefly describe the problem, which is BAT2 dying \$\endgroup\$ – jsotola Sep 8 '20 at 19:25
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    \$\begingroup\$ Generally, the chassis is used as ground in cars/trucks/vans. Generally it is not a problem. You might have some corrosion in the location where wires are attached to chassis, and that can cause problems, but it probably would not cause your battery to die prematurely. \$\endgroup\$ – mkeith Sep 9 '20 at 5:47
  • \$\begingroup\$ @ocrcu: The charge controller measures the panels' output. I also measured the voltage of BAT2 directly. How would I figure out if the charge controller is still okay? \$\endgroup\$ – Joe Eifert Sep 9 '20 at 7:11
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After doing lots of research, I came up with the answers myself.

Yes, I can and should use the same ground for everything. The current will automatically flow only where it's needed. In my case it's important to use a good relays which limits the amount of electricity flowing between the batteries so that they are not short circuited. It is also recommended to use the chassis as the ground because I'm using fewer wires therefore reducing the weight of my vehicle.

Debugging solar panels can be done by connecting both sides to an ampere meter (multimeter). This won't damage them as long as it's done one by one (and maybe even if it's done in series). The voltage will be normal even in broken panels, but the current will be very low. So this way I can eliminate the broken panel(s). If both of them work, the fault lies at the charge controller.

Sources: https://www.youtube.com/watch?v=IPq_d4ftYnM, Solar panel short circuit, Using the frame of an RV trailer for the ground, Why is the chassis used as ground in automotive electrical circuits

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Your design shows that 2nd batt will only get charge from pv panels when engine is running.

So, connect pv panels to 2nd batt dirrctly with fuse, isolation switch and charge controller.

Then power all secondary loads from 2nd batt and leave 1st batt for vehicle only.

Use a proper split charge relay to charge 2nd batt from alternator when the engine is running.

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  • \$\begingroup\$ 2nd bat being BAT2? If you mean BAT2, then no. It's connected via a charge controller. Yes, the fuse is still missing, I'll add that ASAP. Where exactly do I put the isolation switch and why? The rest of your suggestions are already implemented exactly like you said. \$\endgroup\$ – Joe Eifert Sep 9 '20 at 7:18

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