So you’re not sure what power you need in your van, how to charge your mobile, whether you can use a hairdryer and how long you can watch TV for. Hopefully I can explain everything you need to know in simple terms. Read on…
The very first step that most don’t do and which is what confuses everything is working out what power you need. Forget batteries and solar panels for now, let’s sit down and look at exactly the kinds of power you are going to use while away. This is assuming you won’t get Electric Hook Up (EHU ) very often.
What power do you need?
So, firstly what boiler do you have? Hopefully a gas one, if so use it on gas. Same with a heater. They both use tons of power so if you can use them on gas, do it and save your batteries. Most heaters might be about 2000 watts. The relationship between watts and amps is simple: divide the watts by the voltage and you get amps. If we always use 13 volts as default then, 2000 watts, divided by 13 = 154 amps. Your battery firstly could not possibly supply that amount of power but even if it could if you drain a battery that quickly it will overheat and the wires could easily melt, or the batteries explode.
Even if it’s a small heater at 1000 watts, that’s still 77 amps. So you can see that running high drain devices is a definite no no.
So, the 12 volt devices in your van that you will use are typically:
- Water pump, 2-3 amps
- Lights, at most 1/2 amp if they are LED, 1 amp if they are standard filament bulbs.
- Laptop through a charger, maybe 4 amps
- Fan assist for gas heater, 1-3 amps depending on speed
- Radio, 1-4 amps depending on volume
- TV, depends on type but could be 2-3 amps
Now to work out simple amps usage, just take for instance a laptop. You use it for 2 hours a night, if the charger says 4 amps then that’s 8 amps used. Nice and simple.
The water pump, only used for seconds at a time so maybe uses 3 amps 20 times a day at 6 seconds each. So = 20 x 6 is 120 seconds or 2 minutes. 3 amps divided by 60 minutes is 0.05 amps per minute. So in 2 minutes you have only used 0.05 amps x 2 = 0.1 amp. Hardly even worth factoring in. If you’re not used to maths just take your time reading it and consider the numbers only, it becomes second nature after a while.
If your radio is on for 3 hours using 1 amp, that’s 3 amps, so that’s simple. So let’s say a typical usage in a day is:
- Water pump, 15 minutes = 0.75 amp
- Lights, 2 of them for 5 hours a night at 1/2 amp each, 5 amps in total
- TV, 5 hours also at 3 amps, 15 amps
- Laptop, 1 hour for updating Facebook etc at 4 amps, 4 amps.
Total used .75 + 5 + 15 + 4 = 24.75 amps used. So, you now know what your power usage is throughout one typical day, let’s call it 25 amps.
That’s 12 volts, what about mains power in the van?
There is one device that complicates things and that’s an inverter. An inverter is a box that you wire into your leisure batteries, and it inverts the 12 volts from them to mains power of 230 volts. It’s very clever in that it allows you to use mains devices without having EHU, however it does use some power to invert the voltage, typically about 1/2 to 1 amp. Given that you are getting mains power without being connected to the mains that’s a small price to pay.
So, your TV might not be 12 volts. It may be mains voltage but what’s happening there is every TV is probably about 12-19 volts anyway. So when you plug it into the mains a transformer drops the voltage down from 230 volts to 12-19 volts. So for you to use an inverter means 12 volts is turned into to 230, which is then dropped to 12-19 by the TV! There’s a lot of wastage in there so if you can use a 12 volt TV do so, if not, well you just have to put up with it.
However, if you do take this route the transformer or “charger” as most call them will have a sticker on and it will say something like ‘Input voltage: 230 volts, .26 amps. Output voltage: 15 volts, 4 amps. If your TV plugs directly into the mains without a transformer it will still have a sticker somewhere saying this. The simplest way to work out roughly what power you’re going to use is add an amp onto what the sticker says for the output voltage. So your TV is going to use roughly 5 amps when plugged into the inverter.
Many people will tell you not to use an inverter because it’s wasteful. It is, but travelling at anything over 55 mph is also wasteful as is leaving food on a plate but we all do it as a considered choice. I have used dedicated 12 volt items that used more power than a similar mains device run through an inverter. My feeling is if it isn’t a massive cost and the benefit you get is worth it, just do it, only you can decide what’s best for you.
Obviously this is a limited example, you might use a variety of other electrical devices, each one of them will have the current draw (ampage) on a sticker attached to it so you can easily add it to your list. Once you draw up your list you know what power you are always likely to use and that’s the starting point, the only starting point that will help you decide what batteries, charger, solar panels etc that you will need.
This has by far been the most difficult part for a beginner so it all gets easier after this you’ll be glad to know.
So, what batteries do I need and how do I charge them?
OK we worked out that you are going to use about 25 amps per day. Now, how long do you go away for? 2 days at a time? 3 days? A week? Let’s start with a weekend away.
The first thing is leisure batteries, like all batteries they are weird. Whatever they are rated at, you should usually never use more than half that or you are shortening the battery’s life. So, if you have a 110 amp battery, you’ve got about 60 amps to play with. You can use up to 80 amps, but you must recharge the battery fully almost immediately afterwards or you risk damaging it. So lets stick to 50%. It’s generally called 50% DOD, or Depth Of Discharge. That’s how much your battery has been discharged or used.
So if we have 60 amps to play with, and you use about 25 amps per day, you’re only going to get a little over 2 days usage from that battery. That’s perfect for a weekend. No worries at all as long as your battery is charged when you set off, you can easily last the weekend.
If you want more than a weekend there are several choices you have:
- Get a slightly bigger battery – amp for amp this is really a costly choice and not the best one. If you are needing to replace an existing battery do try to replace it with the biggest you can get. Typically for most people this is a 120 ahr battery. I have two 240 ahr batteries as my usage is very high, if you work from the van and need tons of power you might consider this route. You would need to back such a big battery bank up with a generator and solar power and a decent mains charger and maybe a battery to battery charger. 110 ahrs batteries now cost around £120 for lower end ones.
- Buy a generator – you would need to justify the outlay by using it regularly for long trips away and factor in the weight and the cost of the gennie and ongoing cost of the fuel. If you go away for very long periods at a time then this could prove a good option, but it would be an option mainly for winter. The rest of the year solar panels would be likely to keep you going. A 2 Kilowatt (2000 watts) is a good size, but this depends on what batteries you have. Mine is 2 Kw and feeds a 50 amp charger. Typical gennie prices might be as low as £600 for a 2 Kw.
- Book onto a campsite for EHU – the best answer if you like campsites, you don’t have to worry at all about batteries then. Some sites allow up to 16 amps mains which is a lot of power. 8 amps is also common and gives you almost 2000 watts so you could do almost anything with that except a high power electric heater or boiler. Most devices say how many watts they use so check all your devices.
- Add a second battery – you’re paying a lot and adding a lot of weight and only adding 2 more days of power. If you only go on a few trips a year this could easily be your answer, as more expensive options simply aren’t going to pay for themselves. So, if you don’t often use your van much get another battery of the same size and type and that should do you fine. Bear in mind if you rely on the inbuilt charger that your van came with, it will probably not be able to cope with 2 batteries, you need to get this checked out. Even after short trips you must put them on charge as soon as you return and periodically charge them if they are not used for long periods. 110 ahrs batteries now cost around £120 for lower end ones.
- Add solar panels – initial cost is high, but the weight is less than a battery and most of the time they permanently provide plenty of free power, although next to nothing in winter. Even if you have a generator solar panels will continually give you free power on most days outside of winter. Pound for Pound these are by far the best choice if you need more power for long periods of time. A 100 watt panel is about 5-6 amps depending on it’s own voltage which is usually about 16-21 volts. You need perfect conditions to get that 5 amps and it’s almost impossible to get those conditions. You are likely to get 3 usable amps from a 100 watt panel even in good conditions and you will only get that most of the day in high summer. In winter you will most likely get nothing at all. Therefore, add as many as you can of as high a capacity as you can. My panels were £120 each but then another £240 for the controller which you need to take the voltage from the panel and send it to the batteries. It was also about £80 for cables, fixings, mounts etc.
- Fit a Battery to Battery charger – these are specialist chargers which charge your batteries as you drive. A decent one will cost about £300 and you need about 5-6 hours driving to charge a typical 100 ahr leisure battery fully. They are very cost effective if you drive long distances, or if you drive shorter distances but frequently, but if you tend to drive somewhere close by then stay for a while they’re not worth the cost. A charger should be about 20% of the battery bank. So, if your battery is 100 ahrs, your charger needs to be 20 amps. if you have 2x 100 ahrs batteries your charger needs to be 40 amps. 30 amps would do but obviously would be slower to charge. They are absolutely simple to fit, connecting in between your vehicle battery and the leisure battery with strong cables. Mine is a high end one and cost £340 but it has paid for itself many times over in the last 12 years.
Some simple examples based on whatever your usage worked out at
So, you go away for a weekend or long weekend now and again, check your existing one, if it’s something like 85 ahrs (remember you can only use half) then you don’t even get 2 full days out of it. So replace it for a 120 ahrs and you will survive fine for a weekend, but if you already have something close to 120 ahrs but find you’re running out on the last day, think about a second battery. Remember if you have 2 leisure batteries wired in parallel they need to be the same size and type and age.
If you go away for longer trips you might want to consider a small generator. Second hand ones can be bought cheaply but even a new one isn’t bank breaking. It might be years before it pays for itself but if in the meanwhile you get lots of pleasure then it’s worth it. It saves you the cost of buying a second battery, but you must fuel it too and that will cost around £5 – 6 per use. Remember the gennie won’t just charge your batteries, most 2 Kw versions will put out 8 amps so if the gennie is plugged into your EHU socket you can also use those mains devices which can’t run on your inverter, such as a hoover, hairdryer, microwave etc.
Again for longer trips you can consider solar panels. The whole setup of 400 watts ( which could get you up to 15 amps) would be a comparable price to a generator, however there are no ongoing fuel costs, no maintenance costs, no extra weight and bulk inside the van and no humping it about when you need it especially in the rain.
Monocrystalline panels tend to be slightly more efficient than polycrystalline and sometimes cheaper. The more you have the more difference there is so if you’re only going to have one panel of 120 watts a poly will do fine, if you’re going to have 4 panels of 150 watts it would pay to choose mono. The difference is small though so it should not define your ultimate decision.
If the panels voltage is higher than the batterys voltage (which they usually are) you need to get an MPPT controller and not a PWM. So battery voltage is about 12 – 15, panels usually run at about 16 – 21 so I would always advise an MPPT. Also, if your panels are say, 4x 120 watts that’s 480 watts, which at 21 volts is 20 amps, (480 divided by 21) so your controller needs to be a 30 amp model as it needs to be larger than the potential maximum power coming from the panels.
Are flexible solar panels worth the money?
You can get thin, flexible solar panels which are glued to your roof and some you can even walk on without damaging them. I have read in many places that because of the heat build up they often fail, and also that they are less efficient so you need much more to get the same power as less panels. I’ve read 1 or 2 anecdotes from people who say they work perfectly, but there is enough doubt in my mind that I bought rigid panels with plastic mounts to put on the roof. The decision is yours alone. Research it for yourself and see what people say and see what you think.
I hope this brief introduction has helped, you’re welcome to email me or leave a comment if it has and if you have points to make or more questions to ask feel free to do so.