I have a solar charge controller permanently connected to my starter battery and use a solar panel, usually of 80W, to top up the battery when not driving frequently. (It was also fitted to allow me to run accessories for extended periods while parked, and comes in handy when fitting and testing accessories too.)
I recently installed (Hella DayLine) LED daytime running lights (DRL's). These have a controller that senses the increased voltage when the alternator is running and automatically switches the DRL's on.
As I suspected might occur, when I solar-charge the starter battery the DRL's think the engine is running and switch on, and as the solar controller cycles the voltage the DRL's keep switching between on and off every few seconds.
In the interests of avoiding unnecessary wear on the DRL controller I would like to disconnect the DRL's automatically when the solar panel is connected.
Questions
1. Is the solution as simple as placing a relay in parallel with the solar controller input, wired to disconnect the DRL's when energised? From what I can determine an automotive relay will draw less than 0.5A and in my limited grasp of circuit theory the remainder of the panel's output will be available to the solar controller, the solar controller hopefully not somehow becoming confounded because it's in parallel with a relay.
2. If so, would it be helpful to place a resistor in series with the relay (of less than or equal to the relay's internal resistance)? The theory being that it would reduce the current consumed by the relay and also protect the relay from over-voltage from the panel. The internet is telling me that an undisturbed mechanical relay will remain energised at half its nominal voltage.
I understand that the DRL's may still switch on at the moment the panel is disconnected and then switch off again once the battery voltage has settled, but as this would only occur once per charging event it's not of concern.
No, I don't want to "simply" keep pulling the fiddly and spring-loaded DRL fuse every time I plug in the solar panel. It's only a matter of time until fuse and/or spring launches itself out of reach and out of sight. Besides, with the starter battery being sealed I've gotten to like being able to charge it without opening the bonnet.
Think connecting the DRLs from the battery through a relay which is wired so that it is energised by the ignition circuit would work - similar to how driving lights are wired up with relay energised from the high-beam circuit. Relay contacts would be placed in series between battery and DRL controller, completing the circuit when engine is running. See Dorian's post in the thread '12 volt/240 volt automatic switching' in Techies' Corner regarding a diode placed in reverse polarity across the coil to suppress voltage spikes when voltage to the coil is switched off (as noted by 03_Troopy in the same thread some relays come with a diode internally fitted).
I'm sure others with more expertise may suggest alternatives or shoot this one down in flames!
Regards
Joe
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Hino Rainbow motorhome conversion towing a Daihatsu Terios
Joe's answer is spot on, the only small difference I would make would be to feed the switching power to the relay from the ignition (probably actually accessory) through a switch to the relay, that would give you the ability to disable the DRLs if you wanted to.
