My wiring diagram for solar panels & 4WD

I have seen various wiring diagrams for solar charging/dual battery set-ups for caravans, but not for 4WDs. Recently I drew up a proper diagram of the circuit I use (finally got sick of the hand-drawn version). I thought I’d throw it over to the forum for comments and/or constructive criticism.

I think that many people must use a similar set-up, with the auxiliary battery, solar regulator and fridge housed in the 4WD rather than the camper/caravan.

The solar panels are portable, designed to be left out in the sun while the 4WD and camper-trailer stay in the shade. The 15 m of 13.5 cable leading to the regulator should result in a voltage drop of only 0.3 volts when running at 8 amps. I’ve found on several trips that 15m is enough cable, but I wouldn’t use less.

It helps to have the regulator as close to the battery as possible because voltage loss over 15 m is more tolerable at the 19 V the panels can put out than the 14 V from the regulator. You wouldn’t have the regulator in the engine bay however, and I like to be able to see the screen from the rear of the 4WD while in camp, so that’s where it is. To minimise voltage drop I have thick 20.3 cables leading from the regulator to the auxiliary battery under the bonnet. By my calculations this 7m length will have a voltage drop of only 0.09 V if the regulator is pumping out 8 amps. The size of the cables is probably overkill, but the loss figure doesn’t include losses from fuses and connections.

Note that the lead from the BAT+ terminal of the regulator to the auxiliary battery is for sensing the battery voltage. Don’t be tempted to “simplify” the wiring by combining this with the thick cable to the battery.

When we were camped near Exmouth in June 2008, we measured an average of 38.6 Amp-hours of power coming in from the 128 W solar panels over a 10-day period. With, say, 9 hours per day of sunshine, this is an average of about 4.3 amps coming from the panels (although the peak reading was about 8.5 amps). We used 31.0 Amp-hours per day (mainly running the Engel, as a fridge). We therefore had power to spare, but the weather was mostly bright and sunny. I didn’t record the temperature, but the Bureau of Meteorology says the average maximum for that area in June is 24.8C, and minimum 13.1C, with 9 hours per day average sunshine. The excess of power seems to be enough to cope with hotter weather we have encountered further north. The size of the auxiliary battery (105 Ah) means that it is discharged only to a small degree, which is better for battery life and means it is able to cope with a couple of days of cloudy weather.

The inverter is rated at 300 W continuous, so the 25 amps are too high to run off the PL20 regulator (20 A max). It is therefore connected “directly” to the battery, which means any power it uses is not recorded by the regulator. This is generally small (charging cameras, mobiles and torch batteries), but can be surprisingly large if running the laptop (1.5 A at the time – better to use a 12 V cigarette lighter adapter). If I were to install the optional PLS2 shunt adaptor, the regulator could then measure all power both in and out to the auxiliary battery, and so calculate its “State of Charge”. I can’t see the point of this in my case as the PLS2 would still miss any gains picked up through recharging when the 4WD goes on little adventures.

Most of the equipment runs from a fuse box, with the negative bus hooked to the LOAD- terminal of the regulator, and the positive bus connected to the auxiliary battery +. Plasmatronics recommends a diode be placed across the load to prevent damage to the regulator from reverse voltages that some equipment can produce when turned off.

The nerve centre (the 39 l Engel fridge) has 2 m of 7.7 cable, so the voltage drop at the maximum of 3.5 amps should be 0.03 V.

The VSX battery isolator connects to something that only has power when the engine is running (e.g. the wiper motor), so the alternator will charge the auxiliary battery only when the engine is on and the starter battery is fully charged.

Before you get stuck in, there are some minor idiosyncrasies in the circuit that I won’t apologise for. Down the bottom right you can see that the negative cable from the auxiliary battery ends in a “Post”. This is a simple bolt attached to my shelving system, which I have used because the cable lug needed for the 20 cable is just too big to fit easily onto the Plasmatronics Regulator. The geometry also made this a convenient place to connect the Inverter.

I have also drawn the circuit with more detail than some would like, particularly around the Fuse Box. This is done so that it serves as both a circuit and a map (for when I’m trying to work out what’s going on).

So, any thoughts? Are the cable gauges overkill? (Probably, but 4 years after this installation, I can’t remember the cost). I’m sure someone will tell me there are not enough fuses/circuit breakers. What else?


(click below, then "download", under the thumbnail)
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Reply By: glids - Thursday, Mar 01, 2012 at 10:15

Thursday, Mar 01, 2012 at 10:15
G'day GOM,

Can't see too many problems with the circuit, other than:
* The 25A fuse for the inverter is 'on the limit'. If you ran the inverter at full 300W rating, the fuse may blow. You will obviously get away with the 25A fuse for lighter loads (charging phones, laptop etc) but should go one size higher (30A) in case you use full inverter power.
* The 30A Maxi-Fuse may also be on the limit if all circuits are in use. This fuse should only be protecting the cable to the fuse box as all loads have their own fuses. A 50A fuse would be more appropriate. Again, you will get away with the 30A unless you load up the inverter and other loads.

I see no problem with your large cable sizes. If you can afford them, and physically fit them in the installation, larger cables are always better.

I have the Plasmatronics PL20 in my caravan setup with 3 x 64W UniSolar panels. I have the PLS2 shunt, but my setup is independent of the vehicle charging so I don't have the problem you alluded to. Great setup.

AnswerID: 479062

Follow Up By: GOM - Thursday, Mar 01, 2012 at 12:02

Thursday, Mar 01, 2012 at 12:02
Thanks glids.

Your comments on the fuse sizes make good sense, I'll beef them up a bit as suggested.

There was a lot of good discussion on circuits for caravans, initiated by Drew T (thread 57352). I'm hoping this posting will be equally useful.


FollowupID: 754548

Follow Up By: wombat100 - Friday, Mar 02, 2012 at 10:41

Friday, Mar 02, 2012 at 10:41
Hiya Glids

What settings do you have set on the PL20??
We have 2x 68w (136w) uni-solar panels- batterys are 95aH and 82aH inparallel.
What is a PLS2 shunt??

Cheers J&D
FollowupID: 754642

Follow Up By: glids - Saturday, Mar 03, 2012 at 09:20

Saturday, Mar 03, 2012 at 09:20
Hi wombat100

I'm not sure what you mean by 'what settings'...
There are a number of settings required in the PL20 if you want to monitor the battery condition, or 'state of charge'. These settings are different for each installation, things like battery capacity (I have 2 x 100Ah, so 200Ah) etc. I don't plan on going through my PL20 and record everything for you as (as I said) my data is meaningless for other installations. If you have a PL20, follow the instructions in the manual - or you can download a copy from the Plasmatronics site.

The PLS2 shunt is a device that is installed in the main +ve feed from the battery BEFORE ANY LOAD (OR CHARGE) CONNECTIONS. It measures 'drain' current(s) and charging current(s), and allows the PL20 to display the 'state of charge' of the battery. It can only provide an accurate state of charge if ALL incoming and outgoing currents pass through it, hence GOM's comment about charging from the vehicle - although I expect even that could be measured if the PLS2 was in the right position.

FollowupID: 754701

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