A simple device that you can deploy anywhere on a power network to alert you when your accumulators are nearing or at their (discharge rate) limit:
Due to speaker limitations, the standard version stops working in deep brownouts (below 50% power satisfaction), so for more extensive coverage, there's an EX version:
No solar panels needed! Just make sure the right-hand power pole doesn't connect to anything other than the switch.
Both versions will ping you audibly and with an alert based on how fast your power network is trying to drain your accumulators:
Note: these devices only sense accumulator discharge rate; consider using a complementary autobreaker (or similar) to sense and control the state of charge of your accumulators.