Camper Trailer Battery

Hi Peter,

Is there any advantage supplying 24 volt via 2 x 80 watt solar panels through a MPPT regulator to a 12 volt battery. What I am trying to acheive max power supply to the battery via the solar panels.

Allan

Hi Allan,

yes, there are a couple of advantages having a 24V rated panel on the input of the true MPPT solar regulator.

The DC/DC converter inside the MPPT regulator works most efficiently (around 95%) for input voltages which are 2~3 times the output voltage.
E.g. if the battery was sitting on 13V during charging, then the optimum input voltage band would be 26~39V. So this coincides nicely with the max power voltage of a 24V panel which is somewhere between 32 and 36V.

The second advantage is a 50% voltage drop reduction in the panel wires.
That's because power equals voltage times current. So for constant power, but double the voltage, the current drops to half.
In your situation the max solar panel current drops from ~8A down to ~4A when you configure the panels in series instead of parallel.

You can select cheap 3mm (rated 10A, 1.13mm squ) auto wire for this.
At full insolation, this wire pair only dissipates about 0.5W of power per metre of distance between solar panel and regulator. Because full insolation usually is only achieved for short periods, the average power loss is less than that.

Any more questions, just get in touch with us using the links provided below.

regards, Peter

Allan,

There is advantage in running the panels in series provided you are using a suitable MPPT controller and it is mounted at the battery, not at the panels. With the 2 panels connected in series, the voltage is doubled, if in parallel the current is doubled. Losses in wiring leading to the battery are proportional to the square of the current, so doubling the current quadruples the losses. These losses may be negligible if the wiring is short and uses heavy wire.

A suitable MPPT controller such as Peter suggests will convert the available voltage to that required by the battery, effectively swapping any excess voltage for extra current, and allowing the panels to operate at their maximum output. It is important that wiring losses between controller and battery be minimised, so that the controller can deliver the optimum charge regime at the battery.

In your case, as others have said, the controller must also be capable of delivering the higher voltage (>15) required for your calcium doped battery. The voltage from the alternator will almost certainly not be high enough to fully charge a calcium battery.

I'd be a bit doubtful that your present panels and battery could handle the 2 fridges and other camp needs. Perhaps the present battery has suffered as a result? I've added a dc-dc charger to our rig (200 Ah storage, 160W panels, 1 fridge etc) and find it excellent. If you can accommodate a second battery, suggest two cheap 100Ah ones in parallel might be better than one expensive 100 Ah one. Add a 20 or 30 amp dc-dc charger and all electrical problems solved!

Cheers

John

Excuse my lack of understanding.

You have suggested the ah drops from 8 to 4 ah in series, and if the fridges use 6 amps per hour wouldn't the battery go flat loosing 2 amp hour.But if it was hooked up in parallel you would be producing aprox 8-10 amps per hour therefor the battery would maintain charge? Very Confused.

If I did have x 100 amp batteries fitted would the panels be able to keep up the charge to the batteries?

Thanks heaps

Allan,

There's a lot of confusion around this topic. You are not alone!

The rate of flow of electrical charge is measured in amps. In vehicle electrics we commonly express the total amount transferred in a a period of time (say an hour) as so many amphours.

An 80W panel in full sunlight will typically produce about 4 amps, so in one hour it can deliver to the battery about 4 amphours, or in a typical clear day with say 7 hours of sunshine, about 28 amp hours.

Meanwhile the fridges are cycling on and off, each drawing about 3 amps while running. If the fridge motors ran continuously they'd each consume about 3 amphours every hour. In fact the motors don't run continuously, they cut in and out and are probably only running about 1/3 of the time. The average over an hour is then about 1 amp each, so they are each consuming about 1 amphour every hour. If they do this for a day, the 2 fridges will require about 48 amphours. You have 2 solar panels, each providing (in this simplified hypothetical story) 28 amphours, which just covers the hypothetical requirements of the fridges, with a small margin.

The key to optimising your charge rate is the MPPT controller. This device takes the available energy and converts it so that the panels operate at their optimum and also provide optimum charging conditions for the battery. This is achieved by converting voltages. Each of your panels operates best when delivering about 17 volts. The battery requires around 15V. The controller trades those excess volts for additional current. With your 2 panels in series, they'll deliver about 34V and again an MPPT controller will convert this to 15V and trade the excess for extra current.

A non-MPPT controller does not have this voltage conversion ability. Such controllers simply pass on whatever current (amps) is available from the panels. Here, two panels should be in parallel so that their current, rather than voltage, is doubled.

Hope that makes sense. You may find Electricity for Camping a useful read.

Cheers

John

Thanks John for you patience,

Now I can understand why you would have the panels in series.

Would the system benefit from 2 x 100 amp batteries ? as I do have space to sit another battery alongside the other. Would the solar panels keep up the charge to both batteries ?

Cheers

Allan,

as John explained it already, just a few more words finishes the lecture for today ;)

What makes all things electrical tick is power, which is the product of current and voltage.

A true MPPT solar regulator can transform the incoming current (solar panel current) to a higher outgoing current (battery charging current).
The transformation factor is the ratio incoming voltage/outgoing voltage.

E.g. for a panel current of 4A, and an in/out voltage ratio of 2.5 (solar panel working voltage of 35V, battery charging voltage 14V), the battery charging current would be 10A.

And yes, the solar panels will keep up the charge on an even greater number of batteries.
But whether this will work or not, really depends on the amount of solar power coming in, versus the amount of power the loads consume.
The batteries merely act as energy storage for times when there's no solar coming in.
And yes, 2 smaller batteries in parallel are always better than just the one (or just one big one), for a number of reasons:
Less depth of discharge when keeping all other things equal (this results in longer battery life).
More batteries are less likely to experience excessive charging currents when used in connection with an alternator.
A more balanced weight distribution.
More vibration/shock resistance of two smaller batteries compared to one large one.
Cheaper shipping rates of smaller batteries - couriers won't charge excess weight fees.
Easier handling during installation, better portability.

And most importantly, the energy storage capacity (Ah) increases which lets you ride out longer periods of little or no sun.

cheers, Peter

Very good read John, I will be printing this article and locating this in my folder in the Cruiser.

I would recommend anyone with any questions to read this article first, Electricity and Camping could save yourself and others a great amount of time.

Tanks again
Allan

Allan,

If you have space, I'd go for more storage capacity. Peter has outlined the advantages.

You ask if your panels can deal with the extra battery. Yes, a pair of batteries connected in parallel will simply act as a single battery with double the capacity - the increased capacity doesn't dictate increased charging requirements. BUT These requirements remain directly related to the energy balance. If you take more out of the batteries than you put in, obviously things will come unstuck! Having extra battery capacity gives you a bigger buffer. I think with your 160W of panels and 2 fridges your energy budget is pretty tight. As I mentioned, although we carry a similar amount of solar generating capacity, we fitted a dc-dc charger to provide better alternator charging. Coupled with 200 Ah of storage, this has been very satisfactory.

Thank you for your kind remarks concerning the blog. It was originally written specifically to answer the sorts of questions that come up here very often. That it has the highest hit rate of any EO blog indicates just how fraught this this electrical stuff is. It's overdue for a rewrite, but the fundamentals haven't changed.

Cheers

John