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Q & A

Solar panel wiring

Our boat has the output from the charge controller feeding direct to the Domestic Busbars which are fed via an isolator and fuse from the batteries. Should the Solar panel wiring go direct to the batteries with a fuse for protection. We don't usually isolate the domestic isolator.

Asked by: John Boggis  | 4.57pm, Thursday 1 February

WW says:

It is better for the wiring to go direct to the batteries- though you might consider taking the negative to the busbar (which is usually not isolatable) and the positive direct to the battery, with an inline fuse a close to the battery as possible (on the positive for preference). This will minimise voltage loss and, of course, mean that when you isolate the domestic batteries the solar panel(s) will still charge the battery bank- and can minimise resistance losses in some electrical systems.
You might also consider fitting a fuse in the supply from the solar panels before it reaches the charge controller (ideally as close to the solar panels as possible) as this can minimise issues if the controller fails or if the output from the solar panels short circuits (which would not cause the fuse after the controller would not blow)

Mark Langley  | 5.27PM, Thursday 1 February

A 30A fuse will be fine for your solar panel connection to the batteries. The purpose of fuses is to protect the cable and prevent fire in the unlikely event of a short circuit.
If your system is 12V (you do not state the voltage) two 100W solar panels are the absolute maximum for your controller (for a 24V system you would have spare capacity). In ideal conditions with flat batteries they could produce 16.6A, but this is unlikely in the UK.

Rupert Smedley  | 8.56AM, Tuesday 6 February

Readers say:

Thank you for the answer. The PVM controller is rated at 15amps to which we have 2 off 100watt panels connected and which we calculated is Ok for the controller which came with the boat. if we do as suggested and include a fuse close to the batteries what should this be rated at. We have just added a 30 amp fuse in the charger positive connection adjacen tot eh batteries would the same rating be correct for the solar panel positive. We have such a fuse holder but the lowest fuse available is 30 amp. In addition we have noticed that on very bright days whilst cruising, the alternator charge light comes on, (and off as we pass under trees) which initially alarmed us till we partially covered one panel with a towel, which appears to cure the problem. We have read this is caused by the solar charge exceeding the alternator output and thus the alternator charge circuit shuts down. Is this correct? If we left it without covering the solar panel would this damage the alternator electrics, diodes etc. In similar fashion we use a Victron multi stage mains charger whilst in the marina on mains hook-up. The solar panels are also live and adding their charge to the batteries. The charger goes through the steps correctly and settles on float, with no ills evident. Is this an acceptable set-up and not doing any damage? When we leave the boat only the solar panels are charging as we don't leave the mains connected. We have once inadvertently started the engine whist this set-up was live and the alternator light came on. We quickly turned the charger off and normality resumed. Hopefully this has not caused any damage.

John Boggis  | 5.54PM, Thursday 1 February

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