solar panel question

[chuckg5]

i'm installing a new 150 watt solar panel on a 42' Vagabond. i'm wondering where the voltage goes when the batteries have been fully charged. The control box had a third outlet next to the panel inputs and battery output. it looks tobe a lite bulb. Now, question is, does a load go on this + an - post, like a lite bulb or 'water heater element' or can this be let alone with no load.
i do not want to boil off water in the batteries, killing them.

[zboss]

Well, it either "shorts" the panel or shunts out the power to somewhere else, depending on the controller design. The controller output you see could be for an accessory load, such as a hot water heater element, or as a shunt, which could also be a hot water heater element (or a pump or a light bulb or anything else really). The voltage at which that load is turned on/off is usually set by some parameter in the controller. So, you set that higher than the expected "full" voltage of your batteries so it only comes on when the batteries are full (or really at any voltage you want it to come on).

What is your controller make and model?

[noelex 77]

Solar panels can be disconnected from the batteries without any damage even when in full sun. This is how the solar regulator/controller works. It turns the solar off and on (very rapidly) to maintain the correct voltage. For example 50% off and 50% on means the panels are. Delivering 1/2 their potential power to the batteries (ignoring switching losses etc). Therefore the regulator can adjust the solar panel output from almost 0 to 100% depending on the needs of the battery.

So the "lost" power does not really go anywhere. It is never produced. The panel only produces power when connected to battery allowing electrons to flow.

If the regulator is set to the right voltages it will do its job and keep the batteries healthy.

The "lightbulb" terminals are not much use and are usually not used on a boat. These terminals can do a couple of things depending on how the regulator is programmed or designed.

The most common function If you connect a load to the "lightbulb" terminals is to supply power only when it is dark. The power comes from the battery, the regulator is just acting as smart switch. This is used to automatically turn on a light like an anchor light. The second most common function is to supply power only when the battery voltage is OK. This can be used to say turn off the fridge if the voltage drops down because the battery is getting flat. You need to be careful with this as usually you can only connect small loads to the "lightbulb" terminals. So you may need a relay for large loads like a fridge.

The better regulators have adjustable "lightbulb" terminals that can be programmed to do smart things like the above and many more. Some are very clever and can say run a pump for 10mins only if the battery voltage is above a certain level. Some can be wired to make use of the excess potential power by automatically diverting it to something useful like fuel polishing. Very smart, but most boat owners just ignore these "lightbulb" terminals.

The "lightbulb" terminals are just labeled "load" on some regulators.

[chuckg5]

Zboss and Noel
Great explainations, many thanks. my current controller is a pwm but for the larger panel a chinese 20 watt mppt, coming all the way from china, custom shipping! via amazon prime