The Springers Solar guide to 12V electrics, part 1: POWER UP!
Where would we be without batteries? Perhaps hand cranking our cars, eating canned food and drinking hot beer on our tours of the countryside. We often forget about them but we certainly know it when we have a flat battery. To get the best out of our batteries, we need to understand a little bit about them and how to look after them.
For a start, all batteries are not created equal. There are starter batteries for getting the vehicle engine started. These are designed to do this job well but aren’t so good for keeping our fridges going. Deep-cycle batteries are the go-to batteries for all those things that make life on the road a bit more civilised. Fresh food, cold beer and a fully charged mobile device are the standard requirements for travellers in the 21st Century.
Eddie Springer from Springers Solar gets technical in this video all about RV batteries
SO WHAT ARE DEEP CYCLE BATTERIES?
These are batteries that are designed for continuous discharge and can be recharged over and over again while powering all our needs. There are different types of chemistry in deep-cycle batteries including gel, lead carbon, Lead Crystal calcium and AGM (Absorbent Glass Mat). The most common deep-cycle battery is the AGM. They are good value for money and have a long service life, if looked after.
If you have a full battery and typically wait until it’s flat before recharging it, you will only get a limited number of cycles before the battery becomes unusable. If you were to use only 50 percent of the battery’s capacity before recharging it, the number of cycles would increase substantially. Again, using 30 per cent then recharging will give you even more cycles.
This table (above right) showing the relationship between depth of discharge and number of cycles is useful in understanding how to get the best life out of your deep-cycle battery.
HOW DO I KNOW HOW MUCH CHARGE IS LEFT?
This is relatively easy to work out but first of all you need a way to monitor the amount of charge left in your battery. A multimeter is the simplest solution, but not particularly practical when on the road.
A simple analogue or digital voltmeter gauge will do the trick, although its readout will normally reflect the current voltage of the battery.
Battery voltage is only a guide though – we need to know a little more about the battery. Is the battery currently being charged or discharged or is it at rest? The state the battery is in will affect the voltage reading you get. The voltage chart shown above right is for batteries that are at rest, ideally for more than two hours with no load or charge source. A battery connected to a solar panel that is in the sun will show a false high voltage as it is currently under charging conditions. Similarly, a battery that is under heavy load will show a false low voltage as it is currently being discharged.
With that in mind, a fully-charged battery at rest will show 12.8V. As the charge is used, each 0.1V decrease means that another 10 percent of the battery has been used. For example, a battery showing 12.6V will have been used by 20 percent, leaving 80 percent charge left. A battery showing 11.65V is dead flat. Monitoring the current voltage of the battery will provide you with an idea of how much power you are using and how much is left. The simple chart above shows the relationship between the volts and the percentage of power
left in the battery.
CHARGING DEEP-CYCLE BATTERIES
Deep-cycle batteries have thicker lead plates than cranking batteries so are more difficult to charge. The standard alternator in the modern vehicle will struggle to get deep-cycle batteries to 100 percent. To do this, you will need a more sophisticated charger. A DC to DC charger is what you will need. These will supply a charge in three main stages to get the deep-cycle batteries of different chemistry back up to 100 percent.
WHAT SIZE BATTERY DO I NEED FOR MY RIG?
This is where things get interesting. Working out what you need in batteries will require looking at your usage patterns for all your electricals. Light bulb strength is measured by watts, as is most electrical equipment. Think of your microwave or laptop charger.
If you have a 12W light bulb that runs on your 12V system, divide the wattage (12) by the volts used (12) and this will give you the number one which equates to one amp. If you run that 12W light bulb on 12V for one hour then you will require one amp per hour or 1Ah. Now you have something you can work with. Batteries are measured in amp hours (Ah).
If you have a 120W light bulb on 12V running for one hour, you will use 10Ah of your battery. If you have a 100Ah battery, how long could you run that 120W bulb? Now we have to consider the depth of discharge and the number of cycles you want out of your battery. After three hours of running your 120W light bulb you should really consider putting charge back into your battery if you want the maximum number of cycles.
THE BOTTOM LINE
The more batteries you have, the more devices that you can run, but the more charge you will have to add to them. More batteries, along with the ability to recharge them, will cost more, weigh more and take up more space. Power on the road means choosing what is important to charge and what is not. The hair straightener and the toaster are two things that should really be left at home.
As batteries get more sophisticated, size and weight can be reduced for the same amount of power. Lithium is a great example of this, but more on that next issue.