OFF-1-2-Both switch for simple dual battery and other questions

[ColoradoChris]

If I missed a thread that already covers this please point me in the right direction.

Actually the marine switches I have found are usually OFF-1-Both-2 (okay that should help future searchers)

A little background and then the questions. The setup is going into a awd van that I use to haul dogs to various venues. Usually just opening the doors etc while we are there in close proximity is enough but I would eventually like to add some fans. I may even put a fridge in at some point. The vehicle is equipped with a dome override and I have replaced the dome lights with LEDs to limit the draw just in case I forget. It has never been a problem, but I don't want it to be either, so I am thinking of a dual battery setup. (with a possible solar panel and mppt controller later down the road if a fridge happens)

Maybe I am missing something but my thought was that using a marine Off-1-Both-2 I could wire 1 as the starting battery, and 2 as the "house/deep cycle" and leave all the accessories and alternator connected just as they are today.

Then use position 1 for starting and leave it there for a while until the starting battery was fully charged by the alternator. Then switch through off to 2 with the engine running to allow that battery to charge during the trip. Once it is charged I could switch to Both for the remainder of the drive to keep them both topped off. Upon arrival, switch to 2 and run whatever during the day on the deep cycle and start all over for the drive home.

I am guessing that "Both" with one battery significantly more discharged than the other would be a bad idea.

If all of the above is correct, and I then decide to add solar (thinking 24v panel to mppt controller) where would I wire the charge controller to? Should it just go to the deep cycle and I use the "Both" position if I want it to charge the starting battery as well, or would it be better to wire it just like the alternator and let the switch direct the power.

And then finally, if I want to connect an external charger, would off be the right setting and connect to each of the batteries individually?

I borrowed a marine diagram showing what I had in mind. I also assume that some ANL fuses near each battery would be a good idea.

Am I off base? Will it work? What improvements could be made for a manual system?

 

[jgardiner]

I would just have an OFF switch to disconnect the house battery and leave the primary always connected.

 

[ColoradoChris]

snip...

Also, the manual switch having an "off" position, adds risk of switching off whilst engine is running what is potentially damaging an alternator. ( this btw, depends on wiring configuration of alternator to a battery)

snip...

Thanks, I learned something today (not an unusual occurrence for me, especially on this site)

Modifying thought process to go from 1 through both to 2 for recharging of the house battery during the drive. I am still not comfortable dumping the depleted house into the mix with the starting battery until it is more fully charged.

 

[dmilamj]

Dude, definitely go with an automatic charge relay setup. I used this one in my van, and I was impressed with the quality of the switch and relay:

With this setup, you don't have to remember to change a switch, and your start battery is always connected to the alternator (just like stock.) Basically, the ACR will isolate the two systems during start (if you connect the ACR start sensor terminal to a crank/start sensor wire in your fusebox), then when the van is running, it will allow current to charge both batteries during driving. When the van is off, and the house battery is being drained by loads, the ACR will disconnect the systems and prevent the load from discharging your starting battery. The cool thing about an ACR is that it does all this without a significant voltage drop or power consumption.

That's just the ACR. The kit I showed comes with a switch so that you can set it up to manually isolate or combine the systems, if you need to. This allows you to override the ACR and combine the house battery and start battery (for instance, if your start battery goes dead, and you need to charge it up from the house battery) or if you want to manually isolate the systems for some reason.

These are so much better than a manual switch that you might forget to switch over at an important moment....

Thanks, I learned something today (not an unusual occurrence for me, especially on this site)

Modifying thought process to go from 1 through both to 2 for recharging of the house battery during the drive. I am still not comfortable dumping the depleted house into the mix with the starting battery until it is more fully charged.

 

[ColoradoChris]

I was less concerned about the additional load on the alternator than I was the effect on the starting battery of suddenly being in parallel with a large discharged deep cycle. Wouldn't the two batteries "equalize" thus drawing a good portion of the charge from the starter battery until such time as the alternator was able to pump enough amps back into both of them to reach full charge? I was trying to reduce the depth of discharge and number of cycles on the starter battery by getting the house battery back up to a more fully charged state before combining them.

Maybe I am over thinking this. It would appear that the ACRs do exactly what I am trying to avoid and lots of them are in use with no ill effects reported.

As far as a free lunch, somewhere way down the list of upgrades is a solar panel and charge controller. But if you check my "build" thread you will see there are still quite a few things I want to get done first.

I do appreciate the feedback and shared knowledge. Thanks again.

 

[4x4junkie]

I was less concerned about the additional load on the alternator than I was the effect on the starting battery of suddenly being in parallel with a large discharged deep cycle. Wouldn't the two batteries "equalize" thus drawing a good portion of the charge from the starter battery until such time as the alternator was able to pump enough amps back into both of them to reach full charge? I was trying to reduce the depth of discharge and number of cycles on the starter battery by getting the house battery back up to a more fully charged state before combining them.

Maybe I am over thinking this. It would appear that the ACRs do exactly what I am trying to avoid and lots of them are in use with no ill effects reported.

Agreed, this is not at all an issue. Some current will flow from the charged battery to the depleted one if the batteries are connected while the engine isn't running (alternator isn't charging), but it's not going to be more than a few percent of it's amp-hour capacity in amps. This is due to a large "dead-region" or cushion (for lack of a better word) between the voltage a lead-acid battery charges at, and the voltage at which it discharges. IOW, a 50%-charged battery will sit at about 12.3V, but it takes about 13.5V to get any real significant charge current flowing into it. (a "dead" battery @ 10.5V will also almost immediately come up to 12V when even a relatively small charge current is put to it). Your concern would have more merit if it was lithium batteries you were dealing with (particularly those of Li-Ion or Li-Po chemistry).