12 volt lead acid battery
charge rate is 14 volts. (different amps for different applications)
12.5 volts for shelf voltage. (settled and no load)
various dropping voltage depending on load and time used with load. (should be 10 volts roughly for sonar before quitting, but might need to turn up gain as it drops near depletion)
you can get a free dvm from harbor freight to check any batery and not need to depend on the sonar, but if the sonar has it built-in then the same holds true.