How to make a cheap isolated dual-battery setup for $50

[Smith]

Many thanks for the diagrams, they're fantastic and REALLY help. I'm just about to install my second battery and found this thread...now i'm making provisions to add a solar array the cap...i'm noticing an inverse relationship between amperage flow and cash flow....

Few questions:
- Does the setup assume that the solenoid is not engaged while cranking the engine? Meaning the 'on' source from the Truck's Fuse Block is not actually 'on' until the engine has started? I'm guessing the vast majority of automakers disengage all electronics during ignition except for the starter?
- What would the difference be if I used a 'battery isolator' instead of a 'solenoid'? I see WARN makes one...
- What would be the best quick-disconnect for the 8GA solar power? I see the weatherpack typically only accomodate up to 10GA...
- Is there a problem with running accessory lights, radio, etc from the solar to battery connection or should I run a separate wire from the battery out to these devices?

[evldave]

Many thanks for the diagrams, they're fantastic and REALLY help. I'm just about to install my second battery and found this thread...now i'm making provisions to add a solar array the cap...i'm noticing an inverse relationship between amperage flow and cash flow....

Few questions:
- Does the setup assume that the solenoid is not engaged while cranking the engine? Meaning the 'on' source from the Truck's Fuse Block is not actually 'on' until the engine has started? I'm guessing the vast majority of automakers disengage all electronics during ignition except for the starter?
- What would the difference be if I used a 'battery isolator' instead of a 'solenoid'? I see WARN makes one...
- What would be the best quick-disconnect for the 8GA solar power? I see the weatherpack typically only accomodate up to 10GA...
- Is there a problem with running accessory lights, radio, etc from the solar to battery connection or should I run a separate wire from the battery out to these devices?

#1 - I'm not sure. I've never checked mine to see if it disengages while actually starting the truck. If it doesn't disengage, you could hypothetically be 'mini jump starting' your battery if you just turn it on and don't start the truck right away.
#2 - Depending on the type of isolator, they are essentially two diodes that prevent current from running back between batteries. For your typical cheapo isolator, they will use regular diodes, which will result in a 0.7V drop across the diode - not great for charging batteries! More advanced isolators don't have the voltage drop so they work great too (but are generally more expensive than a cheapo solenoid)
#3 - hmmm, not sure on waterproof 8ga quick connects...you could downgrade to 10ga wire and use the weatherpack...10ga should handle 120W solar no problem (that's only 10A or so, which means you could even get away with 12 or 14ga, but remember, bigger wire = less voltage drop over distance )
#4 - as long as the accessories are connected to the battery and not the charge controller, you should be ok. Technically you could connect them to the charge controller, but it's better to have them ties to the battery (as use fuses!). Don't connect accessories directly to the panel - the output of the panel changes with sunlight and you could under or overvoltage your accessories. That would be bad.

I'm sure someone else will chime in if I'm wrong here

[dwh]

All diode type isolators have a voltage drop across the diodes. Good ones just have less voltage drop.

The voltage drop doesn't really matter until the battery approaches a full charge. Then, with the diode isolator, the battery will always be, in evldave's example, .7v below full charge. That's a significant amount.

The voltage drop though a wire is a little different. It varies depending on load, so as the battery approaches full charge, the amp flow gets less, and so does the voltage drop. So even if there is a voltage drop though a wire at high load, eventually, the battery will reach full charge voltage as the amp flow reduces and the voltage drop goes away.

[dwh]

I'm sure someone else will chime in if I'm wrong here

Well...you didn't mention beer, and that's just wrong.

Now, do one with a SunSaver Duo charge controller.

[bansil]

Thanks!

[Smith]

To better explain an above question...would this setup work to contain the solar setup to the cap of the truck? The battery would be my 2nd deep cycle battery under the hood, charged from the alternator via solenoid when the truck is running.

I'm only confused if the wiring of the positive is OK to just run both the solar array and the battery to the bussman fuse block to run powered items or if a separate line is needed directly from the battery?

Solar Setup.jpg

[dwh]

That'll work.

The wire from the battery to the fuse block will likely be larger than the wire from the charge controller to the fuse block, so once the power gets to the fuse block, it won't be restricted flowing to the battery.

It's all just one big "12v bus".


EDIT: Think of it in terms of compressed air. You are running a line from the air tank (battery) to a manifold (fuse block) and you have your compressor (charge controller) hooked up to the manifold instead of directly to the tank. Doesn't matter - the pressure (bus voltage) in the whole system, including the tank, is going to rise when the compressor kicks in.

EDIT AGAIN: You could even feed from the charge controller into the bus, by going IN through one of the fuses on the fuse block if you had a spare slot.

[oliverlove]

Quick question, do you guys upgrade your ground leads as well?

[dwh]

Quick question, do you guys upgrade your ground leads as well?

It's a circuit. The ground wires should be equal size to the hot wires.

[NW European]

Thanks for this thread -- a much more entertaining way to look at this 2nd battery system with the pictures!

Maybe it is just late at night, but I cannot get my head around one thing – Let’s say that I was a lazy bastard, and wanted to use the 200A+ solenoid so that I did not have to get my jumper cables out of the trunk when my starting battery was dead. If the starting battery is dead, and I have a powered switch in the dash to “turn-on” my solenoid….how would it work when the battery on those circuits (assuming OEM fuse block to supply this switch power) is dead? Won’t I have to get my jumper cables out anyway? And if so, then why not just stay with the 80a solenoid set-up so I can have more beer money?