In this situation when you turn the ignition one click the batteries are connected together, then when you try to turn the truck over the starter motor tries to pull all its power from the auxilliary battery because it will always take the path of least resistance. The main problem arises when for some reason the starter battery is low on juice (left the headlights on) and the auxilliary battery has a good charge. ![]() This system is normally fine, however there are certain circumstances under which it can cause a few problems. ![]() This means that when you turn the key in the ignition, the batteries are immediately connected. The most common application for a 12v SPST-NO relay is to have a 12v coil which is activated by a cable connected to your ignition. On any type of relay the coil can be designed to be activated in a number of ways. Single pole in that it only acts on the live cable, single throw in that it either makes or breaks the circuit and does not change from one circuit to another, and normaly open, in that it the circuit is normally broken, unless the relay coil is energised and the the circuit shuts. If you want to automate this process, then you need single-pole single-throw normally open relay/solenoid. Of course the simplest solution is to just have a manual isolator which connects or disconnect the two battery banks but with this you need to remember to connect the two when the truck is running, and disconnect them when you have stopped the engine. ![]() But you want the two batteries to reconnect when the truck is running so that you can charge the auxilliary battery (and the starter battery) from the truck's alternator. The most common usage and what I suspect you want is that the two batteries are generally disconnected so that you can run loads from the auxilliary battery without discharging the starter battery and compromising the ability of your truck to start. I'm not sure I know exactly what you want to achieve but I have a fair bit of experience in low voltage DC electrics so I should be able to help.Ī solenoid in this case merely disconnects the two sets of batterioes, but unless I missed it you didn't say what you wanted to trigger the disconnect or connect action.
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