A typical solar system will supply around 10-30 amps to a battery bank, while an alternator can provide 45-200 amps. To maximize charging efficiency when the engine is running, you want the alternator to supply as much current as possible. However, in some boat setups, the alternator may sense the combined voltage from the battery bank and solar controller, detecting a higher voltage than the battery bank's actual level. This can cause the alternator to enter 'float mode' prematurely, reducing its output to low amperage, so the batteries don’t receive a full charge.
If this happens, turning off the solar panels with a switch allows the alternator to deliver maximum power to the battery bank. You can tell if the alternator is going into float mode too early by checking the amp meter or battery monitor if available. This switch is installed between the solar panels and the controller because the controller is powered by the battery, not the panels; this setup keeps the controller running smoothly without frequent reboots.
It’s safe to have multiple power sources—solar, generator, shore power, or alternator—connected at the same time. This issue typically arises only when the batteries are close to full charge, it's midday, and the solar system is producing at peak output. It's also worth noting that LiFePO4 batteries, with their higher charge curves, may be more prone to this issue compared to lead-acid batteries.