This project monitors the voltage and, indirectly, charge status of a 12 volt lead-acid battery, e.g. a car starter battery or a marine deep-cycle battery. The voltage readings are sent to a Raspberry Pi where you can plot them, set up alerts, send them over the network or do anything else you want. This sort of thing has been done many times before, but you might find my approach interesting. The load is just whatever you're powering from the battery. In my case I'm powering the RPi itself from the battery, through a car-style USB power adapter. But that's not essential to the operation of the circuit, as long as all the grounds are tied together.