Solar + Split Charge Relay blows a fuse on starting the van

May 21, 2020
18
26
falkirk
Funster No
70,993
MH
Class B
Exp
Since 2018
Hi all,

This is a just to share - I have the problem and a solution. Other people on this site have had similar issues on more complex vans, but want a cheap and robust solution that I can install easily.

When I bought my 1995 van, the habitation charging circuit was just a relay that was closed then the ignition was turned on so that the alternator could charge both batteries. But if the vehicle battery was a little flat then too much power was drawn from the habitation battery and the main habitation fuse blew. I replaced the relay with a split charge relay which solved the problem. Now I have installed solar the problem is back. The split charge relay senses a charging voltage from the solar unit and connects the two batteries (it doesn't know the difference between the alternator voltage and solar voltage [because electrons are all so alike]). That's handy because I have been doing lots of battery-draining short trips lately and the solar might help keep the vehicle battery topped up, but bad because the starter motor is pulling current from the habitation battery and blowing the fuse again.

My simple option is to add a normally closed relay to disconnect the solar controller to the battery when the ignition is on. The split charge relay will then disconnect the habitation battery from the vehicle battery. The very small downside is that the solar panels are not working when the engine is on. Amazon have a 100 amp changeover relay for six quid which should do the job. I'll confirm that it works next week.
 

Lenny HB

LIFE MEMBER
Oct 18, 2007
29,919
52,077
On the coast in West Sussex
Funster No
658
MH
Hymer B678 Dynamic Line
Exp
Since 2008 & many years tugging
Sounds like you are using a VSR (Voltage Sensing Relay) for your split charge relay and you have just found out why you shouldn't use one if you have solar. Always best to use a relay switched via the D+ on the alternator.
 
Apr 27, 2016
3,089
3,005
Manchester
Funster No
42,762
MH
A class Hymer
Exp
Since the 80s
When I bought my 1995 van, the habitation charging circuit was just a relay that was closed then the ignition was turned on so that the alternator could charge both batteries. But if the vehicle battery was a little flat then too much power was drawn from the habitation battery and the main habitation fuse blew.
If the relay was triggered by the ignition (not the D+) then that is always a risk. The habitation battery is connected to the starter battery while the starter motor is drawing its heavy working amps, blowing the fuse. Triggering from the D+ closes the relay after the starter motor has finished, and the engine is running.

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Apr 27, 2016
3,089
3,005
Manchester
Funster No
42,762
MH
A class Hymer
Exp
Since the 80s
My simple option is to add a normally closed relay to disconnect the solar controller to the battery when the ignition is on.
Depending on the specific make and model of controller, you could be heading for further problems. Some solar controllers need to be connected in the correct sequence: battery first, then solar panels. Disconnecting the battery might put it into reset mode, where it automatically detects the battery voltage, and decides if the system is 12V or 24V. If the solar panel is connected, the output voltage might rise and the controller will erroneously detect a 24V system. This might lead to overcharging a 12V battery.
 
OP
Arnothill
May 21, 2020
18
26
falkirk
Funster No
70,993
MH
Class B
Exp
Since 2018
Update.

The relay successfully isolated the solar feed when the ignition turned on. And without that voltage the vsr disconnects the habitation battery from the vehicle battery. I am so clever.

The only problem is the vsr takes 10 minutes to disconnect. And I will forget. .

So the relay is out and I spoke to Sterling who suggested a diode arrangement. I know about voltage loss etc. but the diodes will be here in a few days. Plan C if the diodes fail is a battery to battery charger but they are expensive. Once I get a good solution I will post it.
 
OP
Arnothill
May 21, 2020
18
26
falkirk
Funster No
70,993
MH
Class B
Exp
Since 2018
Update.

All working perfectly.

I have added diode so that the current can only flow from the vehicle to the habitation battery and not the other way round. Now the habitation battery cannot contribute to starting the van and so the fuse won't blow. A relay bypasses the diode when the ignition is off so that the vehicle battery can benefit from solar.

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OP
Arnothill
May 21, 2020
18
26
falkirk
Funster No
70,993
MH
Class B
Exp
Since 2018
I have some Schottky diodes and the alternator voltage drops from 13.9v to 13.5v. I am ok with that.
 
OP
Arnothill
May 21, 2020
18
26
falkirk
Funster No
70,993
MH
Class B
Exp
Since 2018
Hi,
Who is John? But whatever.

I found that adding solar caused a fuse to blow when starting. And I have a £20 solution that works perfectly for me although the internet wanted me to spend a lot more. It isn't that I want to penny-pinch, it is just that I want to solve the problem properly for myself.
 
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