Automatic battery switch

This is a simple circuit to switch 2 batteries in parallel when only one receives charging current.

In European camper vans equipped with a solar panel only the house battery is charged from the panel, while both batteries get charging current when the RV is connected to mains power. In boats exists a similar situation.
This little circuit connects both batteries when the house battery reaches 13.0 V and disconnects at 12.95 V.
The LM134 is a temperature dependent current source; with a resistor at the negative side the switching level is compensated for temperature. If a zener diode is used, the switching threshold is fixed.
With the engine running or both batteries charged otherwise, the relay remains activated until the house battery drops below 12.95 V.

Power consumption is less than 1 milliamp if the relay is switched off. The .5 Ohms resistor or halogen lamp prevents high currents flowing through the relay contacts during starting attempts with a dead starter battery.

 

Hi ( did you get the 50 hp going good) Thanks for the diagram but what component are you showing as a constant current generator 00 top left ?

The bulb idea is bizar ....the normal max current to the aux battery is 25 A ..you wont get that through the bulb and anyone who knows just has a thick cable and battery switch between the two positives for emergancy starting .....lots of 4x4 experience this end .....What you have designed is a Voltage Sensitive Relay...dont we need a diode across the relay coil and some fuses

 

This is just an addition to make better use of the solar panel.
There already is a standard device http://www.acpasion.net/foro/archiv...les/Schaudt_Electroblock_EBL208_SE_Ingles.pdf loaded with fuses and relays. It diverts charge current from the alternator to the house battery, contains a mains charger, powers the fridge and has a solar input.
What they didn't do is keep the starter battery topped up, so even with plenty of sunshine 7 days a week you may end up with a dead starter battery (happened to me!) while the house battery is full to the brim and the solar panel switched off.

The solar panel has 4 Amps short circuit current: the bulb stays cold and has a very low resistance, so the starter battery ends up with almost the same voltage as the house battery.
I intended to put a diode across the coil but forgot it. When testing the circuit the transistor had no problem with it so I left it that way.

 

still dont know what the component layout top left is ...So for a boat or 4x4 with no solar you leave out the bulb........

 

It's an LM134 current source. Looks like a plastic TO-92 transistor but contains a lot more.
Current is set with one resistor to obtain positive temp. dependency of .33%/degree Kelvin. Voltage across the device may change between 2 and 40 V. without influence on the set current.