#145859  by Mark
 Wed Oct 31, 2018 3:23 pm
I was working on the motorhome yesterday, putting in a battery isolating switch for the house batteries.
I realised I'd disconnected them before isolating the solar panels. Fortunately the sky was overcast or I may have done some damage to the controller (I may still have, but haven't yet hooked things back up to test :? ).
This was despite my having a bit of tape on the battery cable saying "STOP".
Clearly I need a more foolproof system.

I'm wondering if there would be anything wrong with hooking up a relay to switch the solar connection out if the house battery gets disconnected. Would there be a risk of supply spikes etc with both (battery and solar) cables being disconnected simultaneously?

If there is no problem, I guess I need a relay that will handle 10amps (315W panel @ 39V) and hold it closed while the house battery is On.
That means the relay would be activated almost 24/7. Any problems with that?

Is it as simple as that?
 #145874  by Mark
 Thu Nov 01, 2018 7:41 am
:D
Then I'd just have a bigger bit of tape to ignore :roll:
 #145876  by mattn
 Thu Nov 01, 2018 8:12 am
Realistically the risk of disconnecting the battery is way overstated, and I would suggest mythical on an controller worth owning.

On the panel side, there is no difference between a disconnect battery and a full charged battery. No current flow because of a full battery is electrically the same as open circuit and the panel produces open circuit voltages.

On the battery side of the system, the controller normally senses battery voltage, it will have done one of two things - not powered down and still be running the same as it was, or reset and be waiting to see what battery gets connected. Sometimes a transient will cause the controller to get confused. Any half decent controller will handle transients caused by the disconnect intelligently, however pressing the reset button is a sure way to fix this.

Much of the myth is historical and from lower quality components, before the days they made them idiot proof. The rest is safety around disconnected meaning loose wires with 30+Volts capable of supplying 200W of energy.
 #145881  by Mark
 Thu Nov 01, 2018 8:52 am
Hi Matt
Thanks for that very clear explanation of how things work: that all makes sense to me.
Since they have already made the controllers "idiot proof" I get comfort from the fact that this has already been required before this particular idiot came along ;)
So, I can cross that job off my list.
 #145882  by scubadoo
 Thu Nov 01, 2018 8:53 am
Matt beat me to it.

Kiss comes to mind Mark.
Just keep it all just as it is. Added relays etc.just means more to go wrong and extra unnecessary current draw unless the latching types are used.
The chances of any solar controller circuitry damage from disconnecting the battery positive before the solar input even at full output is remote at best. Sure it could as has occurred. It has also occured using the "correct" method. Any quality regulator will have decent protection inbuilt. Manufacturers have instructions to cover their a#s. My manual also informs me that my controller should not be submerged in water. :roll:

There are valid voltage sensing reasons to reconnect the battery feed prior to applying solar input but again physical damage is extremely unlikely to occur if the process is reserved.

Here we are sitting here at Ngawi and our neighbour is convinced that discharging a battery to 49% SOC will mean certain destruction but at 51% it will last forever and that our lithium battery pack is about to explode and destroy our motorhome at any moment. :roll:
 #145884  by Mark
 Thu Nov 01, 2018 8:56 am
scubadoo wrote:
Thu Nov 01, 2018 8:53 am
Here we are sitting here at Ngawi and our neighbour is convinced that discharging a battery to 49% SOC will mean certain destruction but at 51% it will last forever and that our lithium battery pack is about to explode and destroy our motorhome at any moment.. :roll:
Well then, I hope he's parked a good 3m away from you :-T
Thanks for confirming that the risks are minimal.
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