Connecting solar panel wiring to TWIN leisure batteries

[Andy from Sandy]

Hi,

I need to connect a new 250 watt solar panel control unit wiring to the TWIN leisure batteries on my 2016 Autotrail Dakota. Exactly what do I connect them to please?

The existing heavy 12 volt cable in the battery compartment from the motorhome splits into two and plugs into two battery looms, each with a 25amp in line fuse to protecting each battery..

I was going to connect the new solar panel black and red cables to ONE of the batteries, with a 30amp fuse in the red cable but reading the many posts on this site I don’t think this is the proper way - because there are two batteries.

Any help would be really appreciated please.

 

[Lenny HB]

What you are planning to do will be fine.
If you are in a wiring mood you could strip out the wiring and rewire the batteries with decent size cables Autotrail are notorious for using undersize cables.

 

[tonyidle]

Bear in mind that you will need a solar regulator between the panels & the battery.

 

[Andy from Sandy]

Hi,

I do have a solar regulator thanks, and have now researched the project more and can answer my own question . Here is what I have found, in case it helps anybody else.

My 2016 AutoTrail Dakota motorhome has ONE positive wire connecting to EACH positive battery terminal (via a fuse near the battery in the next wet locker compartment, to avoid any explosive gas from the batteries). The other end of the two posative wires go to the main distribution board inside the motorhome, where they effectively join together to the same terminal. I thought this was strange but perhaps it’s cheaper for AutoTrail to fit two thinner wires than one thick one? Not having them joined at the battery end as well is not a problem for normal leisure battery charging system. More on that below.

The two negative battery terminals are connected to the distribution unit with a pair of thinner wires and joined together, exactly like the posative wires are. No fuses needed in the negative wires.

If I were to simply connect the new solar charger regulator to one posative and one negative battery terminal on the SAME battery, the second battery, which is next to it, would be charged through around six metres of single thinner cables going to and from the distribution unit!!! This would result in the second battery always being charged to a lower voltage than the first battery due to the resistance of the long connecting cables.

Here is the answer. Install a heavy duty red cable to link the two posative battery terminals to each other and do the same with a black linking cable between the two negative battery terminals. No fuses are needed because the two batteries are right next to each other and there is no risk of the two linking cables shorting out and causing a fire. These two links don’t affect the normal leisure battery charging system because if you think about it the like-coloured pairs of long wires to the distribution board are joined to each other at the distribution board end anyway.

Connect the posative wire of the solar charger regulator to the FIRST battery posative terminal via a 30 amp fuse (in my case) and the negative wire from the solar charger to the negative battery terminal of the SECOND battery. This seemingly strange arrangement ensures that the small resistance of the two battery linking cables you have just installed is balanced out and enable the two batteries to be charged by the new solar panels to EXACTLY the same voltage. It is always important that both batteries are charged to exactly the same voltage because if one is lower than the other, the resulting voltage to the motorhome will be the lower of the two batteries - and you don’t want that.

I hope this helps somebody else, as it has helped me.

Please see the battery connection images in this page as a reference.

 

[funflair]

That sounds like you have done a proper job

Martin

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[DBK]

Unless I'm misunderstanding your wiring scheme, how are you able to measure the charging current? My system has separate cables and fuses for each battery but there is a shunt in the system for the BM-1 battery monitor so the output from the solar charger is connected "downstream" of this shunt. This means the battery monitor shows what is happening to the batteries, that is whether they are being charged or discharged.
Your system may be different of course but i can't see how connecting the controller directly to the batteries allows you to work out what is happening.

 

[Andy from Sandy]

Hi DBK,

I’m using a Victron Energy MPPT 12/24 20 Amp solar charger with BLUETOOTH monitoring built in, via an app on my phone. MPPT type chargers are about 98% efficient, as opposed to ordinary chargers, which are only about 70% efficient. See

I’m also using two high tech and high efficiency MiPV 110w flexible solar panel kits, minus their standard solar chargers at
.

The result of both bits of kit should be a system that can keep the leisure batteries topped up even using the TV, charging all mobile devices and running the heater fans in winter. If two panels is not enough then I will add another couple of panels and upgrade the solar charger. I’ll let you know in a few weeks

 

[Andy from Sandy]

That sounds like you have done a proper job

Martin

Thanks Martin, I hope so.

 

[gus-lopez]

 

[DBK]

Hi DBK,

I’m using a Victron Energy MPPT 12/24 20 Amp solar charger with BLUETOOTH monitoring built in, via an app on my phone. MPPT type chargers are about 98% efficient, as opposed to ordinary chargers, which are only about 70% efficient. See

I’m also using two high tech and high efficiency MiPV 110w flexible solar panel kits, minus their standard solar chargers at
.

The result of both bits of kit should be a system that can keep the leisure batteries topped up even using the TV, charging all mobile devices and running the heater fans in winter. If two panels is not enough then I will add another couple of panels and upgrade the solar charger. I’ll let you know in a few weeks

A monitor on the controller won't tell you what's going into the batteries. It will only tell what is going out of the controller and it might all be being consumed by the load and the batteries could still be discharging. If this is the case you will know it is happening because you will see the battery voltage going down but being able to monitor what is going in or out of the batteries gives you more information.

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[emjaiuk]

You may have done this already, but the optimum setup is that all pairs of red&black wires, ie pos &neg, should be the same length as each other.



Malcolm

 

[DBK]

You may have done this already, but the optimum setup is that all pairs of red&black wires, ie pos &neg, should be the same length as each other.



Malcolm

I wouldn't have thought that was necessary on the solar charging side, the currents involved are too low.

 

[Andy from Sandy]

A monitor on the controller won't tell you what's going into the batteries. It will only tell what is going out of the controller and it might all be being consumed by the load and the batteries could still be discharging. If this is the case you will know it is happening because you will see the battery voltage going down but being able to monitor what is going in or out of the batteries gives you more information.

Good point, thank you, so I might have to get a charge/discharge meter as well. Obviously there is one on the standard system already so I would have to look at both meters to see the combined results. I know Vetek do one if these but they are not cheep. Are there Ant you can recommend please?

Isn’t this discussion forum fantastic. Thank you guys.

 

[DBK]

The NASA BM-1 Compact is the one I've got but there are a few other makes including Victron. There are also ones you can get at a fraction of the price from eBay but I haven't read any reports of them on here.

 

[Andy from Sandy]

The NASA BM-1 Compact is the one I've got but there are a few other makes including Victron. There are also ones you can get at a fraction of the price from eBay but I haven't read any reports of them on here.

Thanks for that, I’ve got some more homework to do now. I’m just waiting for slightly better weather before I can stick the flexible PV panels to the roof and complete the wiring.

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[arto stu]

I have a similar set up on my boats solar... the solar charger has a built in shunt and knows what charge it is putting in to the batteries... doesn't know what is being taken... doesn't need to, it just measures the voltage and supplies what it can... the BM uses its shunt to determine drain out and charge from alternator.... note to self... must get some solar for van