FAQs for Motorhomes and Caravans using Solar Panels for Leisure Battery Charging.
Free Solar Energy.
Solar energy arrives in the form of heat and light every day of the year. How much depends upon the time of day, your latitude and what the weather is doing. We are going to concern ourselves with the light that is arriving and largely ignore the heat. For that reason the solar ‘panels’ we are going to talk about are more properly known as solar photovoltaic modules (pv modules for short). Although the term ‘solar panels’ are often used, that term should be reserved for those devices that directly heat up water – like radiators in reverse. See, you are one up on the salesman already!
The met office has been collecting information about how much light energy hits the surface at various locations within the UK every hour of the day for a long time and so its possible to say for each month how much energy will, on average, arrive. It can be expressed in various forms but ‘sunhours’ is the one that I will use here. It is the measure that the TV weather forecasters use to say how much sun there has been and its actually the equivalent number of hours that the suns irradiance has exceeded 1000 Watts per square metre (1kW/m2) at the earths surface. From memory, in Scotland in December its around 2 hours and in the Hampshire in June its about 8 hours. These are long term averages, in winter you can get many days in a row where this threshold is not exceeded.
Now 1kW/m2 sounds a lot but unfortunately the process of turning this light energy to electrical energy is not particularly efficient so even in bright sunlight you don’t get a kilowatt from a square meter of pv modules. A more reasonable figure for electrical output might be 100 Watts, but that would only be under optimum conditions and with the latest, most efficient and therefore expensive modules.
This brings us on to nominal and real outputs. When you see 50W nominal output what does that mean. Well if you were to take a typical, new module expose it to irradiance of 1kW/m2 by pointing it directly at the sun on a cloudless day with low humidity then you might get 50W from it but even then it would depend on matching its output to a suitable load. By the way it is normal to quote the performance of PV modules at1kW/m2..
So once you’ve paid for the module and its mounting the electricity is free but there isn’t quite as much of it as you were expecting. We will return to how much electricity you can usefully get later after we have considered the other components of a solar system
The three parts of a Motorhome Solar Battery Charging System
Solar Panel PV Modules: already reviewed above.
Solar Charge Controller / Regulator. The regulator is there to prevent damage to the battery from over-charging. It used to be thought that you didn’t need a regulator and if the capacity of the battery is large compared with the output of the solar pv module(s) you may not need one. We will examine the use of the regulator later once we understand the properties of the pv module and battery system in more detail.
Battery. In a solar system you will always need a battery. I was most amused to see an advert for a ‘solar powered torch - needs no battery’. Fine if you only want to operate it in bright sunlight! Of course it had a battery, it was a rechargeable one what the advert meant was that you didn’t have to buy batteries.
We will go into the properties of the ideal battery later but firstly we need to consider something known as ‘autonomy’. In our battery system it can be defined as the length of time you want the battery to last without being charged. This is the one of several fundamental design issues that you should address.
Take the ‘solar torch’ as an example. Lets assume that the battery is big enough to exactly power the torch for as long as it is needed for one night. Then clearly it needs charging every day, but what if its winter and on the next day there is no sun? Perhaps the battery will need to be twice as big and then it will last two days…. and if no sun on the second day?…three times as big? In December 1987 there were over twenty days in a row with no sunhours in the south of England. What then?
Now if the torch manufacturer has given you a plug in battery charger as well then you have a back up to cater for the ‘no sun’ situation but you will have to find a mains socket (and pay for the electricity). Not so convenient perhaps but you can have a smaller battery. The next thing to consider with autonomy is the importance to you of your system. If it powers just your TV (and assuming if the battery went flat you could manage without your daily fix of Big Brother) then the autonomy could be low, if it powers your water pump then I guess you need to set it higher. Some of you might consider I’ve got the priorities wrong but this just illustrates that it can only be your decision.
If we get back into the real world for a moment your van will always have a back up - you can run the engine. It won’t be free energy any more but it’s a back up. My conclusion is that I would want my battery to be able to run my van for no more than two days but as I have said its your decision.
System size
So having thought about autonomy how big a battery do you need? You can’t say yet because you don’t know how much power you need. You have to make an inventory of all the electrical loads. This is the most difficult part of the process and the one most often got wrong. Get it wrong and your system either won’t have the autonomy you desire or will be too big and expensive for your needs.
How to start? Well make a list of everything that will be powered by your battery, next find out what the wattage is then make an estimate of how long it will be on for in an average 24hr hour period. You could make a spreadsheet for this or use the one that I have provided or you could use pencil and paper.
|
Item |
Power (watts) |
Time on (hours) |
Watt hours |
|
Fridge |
20 |
3 |
60 |
|
TV |
6 |
4 |
24 |
|
Fan |
40 |
1 |
40 |
|
Etc etc etc |
|
|
|
The last column is just the previous two multiplied together.
The fridge is an interesting one. Its on all the time perhaps but in a compressor fridge the motor/pump runs intermittently. But perhaps it has a control circuit and an indicator light which will be on all 24 hrs so your fridge may need two entries in the list one for the continuous load and one for the intermittent load. The same for the TV which might have a standby . I will leave you now making your list/spreadsheets, next time we can compare notes and we can start sizing the system. Just one final point if you have to measure the current take care its only 12 volts but you could burn yourself and your van if you short out the battery.
If you only know the current (Amps or milliAmps) in your 12 Volt system use the following formula:
Power (Watts) = Amps x 12V
Note 1000 milliAmps = 1 Amp
Battery Size.
Once you know your power requirements and decide on your autonomy you can make a start in calculating how big a battery you need. Note that there is no mention of solar at this stage as its mostly independent of how you intend to charge your battery. So if you have worked out that you need on average 300 Watt hours (Wh) a day and you want two days autonomy that equals 600 Wh. Turning it back into Ampere hours (Ah) by dividing it by 12V gives a battery size requirement of 50Ah.
Unfortunately it’s not that simple. The first thing to consider is if you have put 50Ah charge into a battery you won’t get it all back when you discharge it, this is known as round trip efficiency (RTE), and for a standard sort of lead acid battery its about 80%. One way round this (especially given our limited charging capacity) is to increase the size of battery. Dividing our 50Ahr battery by 80% gives a new size of 62.5Ah.
Now if you repeatedly discharge a lead acid battery until it is empty you will damage it, so its better to plan to only take 90% of its charge, so now you are up to 69.4 Ah.
The above assumed a new battery, but how well will it be doing after five or ten years? Perhaps you better add in a bit for degradation, until you can afford to replace it, shall we say add 10% … now we are up to 76.4 Ah.
So we have ended up with a battery half as big again than that first calculated, but remember its based on the inventory so lets hope you got that right. There are other factors in a solar system, which might influence you to have an even bigger battery.
At this point I should inject a first note of realism. This system is not going to be unattended when the heavy discharges are taking place so there is also the option of rationing power yourself to last until the next charge.
Motorhome Solar Panel PV module sizing.
On average, every day, we need to put back into the battery what we have taken out of it, allowing also for RTE. That way there will always be enough charge in the battery at the end of each day to give you the required autonomy. As well as that you have some more factors to consider.
The first of these is guaranteed output; the manufacturer knows that in any batch there will be a spread of performance and so he quotes a lesser figure than the nominal output to be safe. The second is degradation over time, so with a design life of 20-35 years you should allow a further 20% reduction – or replace them earlier.
Now you only have to point the module(s) at the sun, unfortunately the sun doesn’t stay still during the day nor does it follow the same track summer and winter. Assuming that you don’t invent a tracking system you will have to accept a compromise. The best thing generally is to point it South at an angle from the horizontal equal to your latitude plus 15 degrees. Say 70 degrees in North of England. This gives optimum winter performance, set it to latitude minus 15 degrees if you want optimum summer performance.
It will obviously do wonders for your aerodynamics and fuel consumption with it like that on your roof plus it will only work properly if you drive in one direction! So for practical purposes, whilst driving, it needs to be horizontal, and this will produce a smaller than maximum output.
Instead of doing lots of mathematics we can treat this lower output as if there were less sun hours than there actually are. Tables exist showing sun hours (its called ‘insolation’!) at various locations for each month and for preferred module orientations including horizontal and I have include some of this in a spreadsheet for your use. From that you can now see that in Part 1 my estimate of sunhours was optimistic. The figures for England are the actual logged averages at two locations so the weather is factored in.
Before we put some numbers in place there is one more design aspect to consider – do you really want it to work properly in December in Scotland? Lets be realistic again, if you need £5000 pounds worth of pv modules for two days autonomy are you going to buy into that? Even if you run a van with enough roof space the weight alone is going to cripple the performance. So what we do is pick a ‘design month’, that will be the month when on average enough charge will be put back into the battery to give you the autonomy you desire, months with more sun will give you a surplus less sun a deficit.
So if we assume the sun hours for March at your location is say 2 hours. Now assuming we could get 50W from our nominal 50W PV module and knowing we have 2 hours sun and we require 300Wh we could divide the 300Wh by 100Wh (2hx50W) and decide three PV modules will be just enough. That would of course ignore everything we have said about efficiency and RTE. The real sum would be more like which equals 4.4 so you either need a higher nominal wattage module, use 5 modules or accept a poorer performance.
Note that the factors used are 80% RTE, 95% solar guaranteed output, and 90% solar degradation. We have already catered for battery degradation and only using 90% of available charge in the battery design.
Motorhome Real Voltages, Current and Power.
The simple calculations that we have carried out to size our solar system do not accurately model the dynamic system. This need not concern us but a little more understanding is needed before we talk about regulators.
As I’m sure you know the normal relationship between electrical current, voltage and resistance are defined in Ohms law , and power is , combining these two gives . You can now see how the resistance has an effect. The difficulty is the internal resistance of the battery changes with its state of charge and the internal resistance of the solar module and its current output are also non-linear. Also note in passing that the maximum current output of a module is not at the same voltage as its maximum power output. We are also going to have to talk about what the actual voltages are and here is where that awful word nominal comes in again. What we think of as a 12V PV module has a typical open circuit voltage of about 18V or more in sun. A 12V battery in a good state of charge would be a little over 13V a flat battery might be 9V with a high internal resistance. So lets look at what might happen for real.
Motorhome Solar Panel PV Modules: When a PV module is connected to a semi charged battery its output voltage will fall and the battery voltage will rise to meet it. What the actual voltage is at any given time can be modelled – but lets not bother, suffice to say that it will vary over time as the battery is charged, and with the amount of sun, temperature etc. The highest voltage will be when the battery is fully charged (or paradoxically if the battery is totally discharged or if no battery was connected and there is only a small load).
The Battery: With the battery fully charged any excess voltage will cause the battery to ‘gas’ – give off hydrogen, and whilst gassing once in a while is good for a battery it permanently removes some of the electrolyte. In a car alternator there is a regulator which limits the voltage to under 14V and hence limits the gassing. In our PV solar system the partially charged battery acts as if it were a regulator holding the voltage down.
The electrical equipment: Equipment for your motor caravan is designed for 12V, allowance is made for it to work over a range of voltages say 11 to 14 but whether it will survive 18V is doubtful unless it has been designed to a military specification, certainly lamps would have a very short life at that sort of over-voltage. So you must never run a solar system with no battery connected and if there is a danger that the battery will become fully charged (and that is what you would like isn’t it?), then a charge controller to regulate battery charge is a must.
Motorhome Charge Controller / Regulator. The good news is that simple regulators are in expensive so I would always recommend fitting them with any solar panel above 10W. They act in a much simpler way than an alternator regulator. When the voltage goes too high they disconnect the PV module(s) from the system and therefore the battery cannot become ‘overcharged’ nor can other equipment be damaged. It also normally tells you what is happening with a couple of indicators so crudely acts as a monitor for your system. I like the idea of one regulator per module as you can really tell what is happening then. By the way there are much more complicated ones that allow ‘step charging’, ‘load management’ and battery state of charge. The more complex modern Charge Controllers have improved technology which allow multiple patterns to re-charge the batteries, depending on there state of charge.
A few problems
When a PV module is in deep shadow or at night it produces no power but it still has a fairly low resistance so current can flow from the battery back through the module discharging the battery. To prevent this a blocking diode that passes current in one direction only is always fitted, sometimes it comes with the module other times you must buy it separately. Free Solar Energy Solar Panels are fitted at the factory with blocking diodes.
When the charge controller switches the power from the PV module off and on, transients, (short term large changes in voltage) are produced. To avoid damaging electrical circuits these need to be minimised, using the latest charge controller technology minimises these affects by using different charging patterns. Thick wires with multi cores and good connections help in reducing the size of these transients and the battery helps absorb them.
The Spreadsheet, Circuit Diagram and further notes.- click hee to download the load calculator.
Motorhome Battery and Solar systems
Motorhome Design stages using the spreadsheet
Motorhome Battery Size.
Battery size is given by: Replacement Ah multiplied by autonomy and any degradation factors.
The first bit of the spreadsheet lets you input all your power requirements in terms of current and hours used, and your required autonomy and calculates the size of battery required. You then input the size you are actually going to fit!
This last stage is important, all design should incorporate reality checks, and if all you can fit is one 100Ah leisure battery then say so. That is not ignoring that you have calculated that you need 150Ah, it’s just being practical.
Note that I tend to work in Ah with batteries and Wh with the rest of the system
Motorhome Solar Panel PV module sizing.
The second part of the spreadsheet calculates how many modules you need. You can alter the design month, the latitude (basically S England, Borders, N Scotland) and the nominal power of the modules. The spreadsheet calculates how many modules will be needed and again as a reality check you state how many fitted
Energy Balance
The table headed energy balance shows the surplus or shortfall in energy each day. (Please remember this is all based on averages). It does not take note of the size of battery fitted nor the state of charge of the battery, just how much energy you will get from the sun compared with how much you intend to use. The effect of course of a shortfall will be that the battery is discharged; shown on the spreadsheet by red numbers and a minus sign. A surplus (black figures) will charge up the battery but you cannot charge a battery up fuller than full so some surpluses may be wasted.
Effect of Battery capacity and Module output.
We calculated battery size based on the autonomy we required and assumed a fully charged battery as the start point. It follows then that at the end of each day that there is a surplus the battery will be fully charged and some energy will have been wasted by the regulator. Typically summer months will have a surplus, winter months a deficit. This is shown in the next table which takes the battery size into account. For deficit months, when the battery won’t be fully charged at the days end it shows how long the battery might last. With no modules and a battery the same size as suggested it will equal the value set by you as the ‘autonomy’. Altering the size of the battery or the number of modules will within reason model the performance of your system.
A Balancing Act
Once you have an idea of how may modules you need and how big a battery, you will realise that you need a bigger van and a deeper wallet. Being pragmatic you realise that it’s time to compromise and fit what you can get in and afford. Before you spend your hard earned cash though you need to consider one more thing. Let’s say that the sums tell you need 5 modules and 200Ah worth of batteries. Perhaps you can’t afford both but lets just say you could physically fit them. If you fit all the batteries but not all the modules this is known as a ‘battery heavy’ system. If you fit all the modules but have a smaller battery this is known as a ‘battery light’ system
A ‘battery heavy’ system gives the required autonomy but to keep the batteries charged you are either going to need good sunny weather or another energy source, which as it won’t be running for ‘free’ will increase the running costs, but remember batteries are cheap compared to modules.
A ‘battery light’ system is more easily kept charged up even in poor sun but you may not get your required autonomy and it will still be expensive.
Personally, I favour battery heavy systems: because I move around a lot and hence charge up my battery anyway from the engine, plus I don’t go out as often in the winter and anyway, batteries are cheaper than modules. I haven’t any room for more batteries however, and have loads of room on my roof so a compromise (what again!) will be called for.
What I’m saying here is you need to compromise and as motor home users you know all about that. What you are able to do is feed your solutions back into the spreadsheet to see what the effect of your compromises will be.
Mounting Modules
Modules are a sandwich of glass, silicon crystals, and a mounting panel, normally aluminium, they contain electrical joints and are liable to be damaged by impact, twisting or vibration. Using Free Solar Energy UK's specialist framed Solar Panel Mounts and Elevated Solar Mount to maximise 30-40 degrees elevation south facing - any risks can be mitgated with solid robust mounting, with Sikaflex to remove the need to screw mount to your Motorhome. Free Solar Energy Uk Framed Solar Panels are made with low iron toughened glass tested at the factory with a 1 meter drop down test with a steel ball - now thats robust!
Impact: don’t park the van under conker trees!
Circuit
The basic circuit is shown above with two modules and two leisure batteries.
Points to note:
The existing circuits eg battery charger, split charger and fuses connect at the point arrowed.
Wires need to be as short as possible and as thick as you can afford. You aren’t really concerned with current carrying capacity but with voltage drop so go for cable rated at a minimum of 40 amps say 10mm2. Check however that the cable will fit the terminals of the regulators and modules. As its overall voltage drop you are concerned about you can, if you need to, cut the cable down to fit the terminals (within reason).
The fuse only needs to carry the maximum charging current but if too small will drop voltage, so size it to protect whatever cable you fit.
If you have more than one battery they should be identical, ideally purchased at the same time. Connections between them should be as short as possible and as thick as possible. Buy the best batery connectors you can afford - good make and thinkness. If there is an imbalance one battery will do all the work and the overall capacity will be affected. i.e old battery and newer battery of a larger Ah size.
As charge controllers are inexpensive and carry indicators of performance you may fit one per module. In that case fit them immediately after the blocking diodes before the join.
Blocking diodes may be included in your panels or the more expensive Solar Panel Charge Controllers may have this blocking feature - in that case there is no need to fit extra ones .
In the event that there is no convenient chassis earth make a direct connection. Note on most Motorhomes being of fibreglass / composite a earth to the chassis is not alwatys possible and it may be just as easy to run 2 quality solar UV safe cables back to the solar panel charge controller on your Motorhome, then from solar panel charge controller back to your batteries.
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