Battery Isolators

[John61CT]

Actually should be no problem, certainly a good thing if it sits for extended periods.

Many ignition-based combiners have a delay, start isolation feature, based on oil pressure etc, so the joining only happens after warmup. But again, only appropriate if you want the alt to be your only charge source for Starter.

Many just keep the banks always separate, alt is often not a major source for House overall.

But my philosophy is take advantage of any source as it's available.

 

[Weight]

Many circuits in the vehicle disconnect while cranking. My radio does this as well as others.

 

[Reducto]

Actually should be no problem, certainly a good thing if it sits for extended periods.

Don't get me wrong - being able to charge the van battery from your solar panel is a great feature if you're sitting for weeks or months at a time. I rarely sit for that long so it just means a bit of the charge is wasted and I have to hear a loud thump sound as the charge voltage goes above and below the threshold.

 

[John61CT]

So maybe no combiner is needed at all in your case

 

[DannyB1954]

Some people like to go extreme when it really isn't necessary. For a single battery most any continuous duty relay or solenoid will work. Here is one for $14 https://www.amazon.com/CONTINUOUS-S...pID=41weJiwCsLL&preST=_SX300_QL70_&dpSrc=srch
You will need three fuse holders, (one small, like 10 amp to power the coil in the relay), and two larger, (to match your wire size between the batteries). Each battery should have a fuse just in case the wire in between shorts out. You can activate the relay with a manual switch, or wire it to an accessory power terminal.
The size of wire is debatable. For battery charging, larger is better, not because of the amount of current it will carry, but because the larger the wire is, the less the voltage will drop on the far side. .2 volts, (point two), can make a difference in a battery that is fully charged or only partially charged. Others can add their opinion, but I would go with at least 6 gauge wire between the batteries. If you are going to add more batteries later, going bigger now may be cheaper in the long run.

 

[ZacLee]

The reason I personally did not go with this method is because the drained battery is connected at the instant you're trying to pull your cranking amps to start the vehicle. I don't have the background to know how big of a concern that is, but I had been warned away from that method so I went with the battery isolator.

You could add a time delay relay or a voltage threshold detector which would only close the relay if the voltage was high enough (13-14V) to confirm the engine was running.   

I prefer running a higher voltage house battery (24V or 48V) and using a DC-DC converter to charge it from the vehicle alternator.  A higher voltage system does not require the heavy gauge wiring a 12V system typically does and is more energy efficient.  For example, a 48V system would require wiring only a quarter the size needed for 12V for delivering the same wattage.

 

[highdesertranger]

I have used the solenoid type since the 70's and I have never had a problem with one set of batteries being dead and starting the vehicle. highdesertranger

 

[ZacLee]

The current from the starting battery into a depleted battery is limited by wiring resistance and by the relatively small voltage differential between the 2 batteries. A fully charged battery might be 12.6V or so and the depleted battery might be at 11.5V so the voltage difference is small, thus limiting current flow. In practice, the voltage of the depleted battery would rise rapidly when charged so the voltage difference would be even smaller after a short time.

I setup a basic system for a friend's van back in the late 1980s when I was still in high school and he was happy with it. He had a pair of 12V deep cycle batteries (100 ah? the kind typically used in boats for trolling motors) hooked up in parallel. I used a 30A relay that connected the vehicle electrical to his deep cycle batteries when the vehicle voltage exceeded 13.7V. It would actuate the relay when vehicle voltage went above 13.7V and disconnect it when it dropped below 12.8V with some capacitance based time delays to prevent oscillation. He only used 10 gauge wiring and never blew the 30 A fuse. I used a couple voltage comparators and a flip flop to control the relay. A modern version would use a microcontroller with an analog to digital converter to implement more advanced switching logic.

 

[HumbleBeginnings]

You could add a time delay relay or a voltage threshold detector which would only close the relay if the voltage was high enough (13-14V) to confirm the engine was running.   

I prefer running a higher voltage house battery (24V or 48V) and using a DC-DC converter to charge it from the vehicle alternator.  A higher voltage system does not require the heavy gauge wiring a 12V system typically does and is more energy efficient.  For example, a 48V system would require wiring only a quarter the size needed for 12V for delivering the same wattage.

Yes, I went with a battery isolator as you described rather than a simple engine on/off relay.

 

[John61CT]

That is exactly what combiners do.

Ann-Marie Foster, aka Yandina is a marine electrics engineer credited with first commercializing the idea, those units still sold today, not expensive and very reliable.