I see a lot of very basic questions in forums about 12v DC electronics and often see them replied to with some very complex answers.
12v DC electronics is actually a very precise science and it’s really quite easy to understand once armed with some basic information. There’s nothing stupid about not understanding it. Most of us go through life never having to think about it. It’s only when we’re thrust into (or maybe gently led) into the world of off grid living that we have to understand a few basic rules.
Please note: information provided here is also applicable to bricks and mortar dwellings. You do not need to be a licensed tradesperson to undertake 12v installations. If necessary, cabling for 12v DC systems can run freely through dwellings. Basic 12v DC systems can dramatically cut home electricity bills. It doesn’t matter how many thousands of dollars you are told emphatically that you must spend, by solar power contractors; you CAN cut out the middle operator and install very inexpensive systems yourself.
Renewable energy is vital to our future, but already it has become an industry rife with operators who provide misinformation for the purpose of profit. Much of that misinformation pertains to the validity of more basic systems. Electrons are electrons – it doesn’t matter how you convert them from photons, just as long as you benefit.
Ohms Law Basics – It’s really simple. Honestly!
The main things we need to concern ourselves with are:
1. Power – Measured in Watts and abbreviated as P. This is the amount of work that an appliance or light globe does.
P and I are very variable for the purpose of calculations. V doesn’t vary much in a DC circuit. There are variations based on a batteries charge or depth of discharge (DoD), but it’s reasonably negligible, so we use 12v as the basis for our calculations. There is an exception to this rule that I personally apply when doing calculations involving inverters, but that will be explained in due course.Some caravan RVs use 24v systems. They aren’t as common as they are in large commercial vehicles, but the same rules apply, substituting 12 with 24.
The most important equation that we can remember is:
E / V = I
In plain English: Power in Watts, divided by voltage in volts, equals Current in amps.
It’s important to be aware of how much current is being drawn, as that tells you how much you have left in your battery.
Example: 10 watt LED globe / 12v DC supply = 0.833 Amp current
Power is important for the purpose of calculating current. It’s also important to know how much power devices consume, if you are using an inverter or generator. If the appliance consumes more power than is provided by a source of electricity from an inverter or generator, they just won’t work.
If you have a hair drier that produces 1,300 Watts, plugged in via a 1 KVA (1,000 Watt) generator, the generator will cut out. The same goes for inverters.
Appliances that heat or cool via compression (air conditioners) consume a lot of power. That’s why items such as electric heaters, hair driers, electric kettles etc, are very impractical in terms of most small off grid 12v DC power set ups.
Heating appliances are too powerful for small, practical inverters and generators, whilst large inverters / generators draw far too much current to be practical . In the case of inverters, they run batteries down quickly. In the case of larger generators, their power capacity is inversely proportional to their size, weight, noise level, fuel consumption and cost.
Many forums advocate the use of MPPT solar chargers, claiming that you get more charge from less sun. I won’t get into the technical nuances of solar regulators, but will say that an MPPT regulator is far more effective if your array of panels exceeds 600w. A good quality PWM charger will suffice for smaller arrays and cost you much less money than a quality MPPT unit. I’m not saying that MPPT regulators don’t employ better technology, they certainly do. I just personally feel with smaller arrays, the cost can far outweigh the benefits. In some instances, high quality PWM regulators such as those made by Victron, are better than lower end MPPT regulators. On the other hand, low end PWM regulators are often bloody useless.
The charge that a panel generates is generally measured in Watts.
A 100 Watt solar panel produces approximately 6 Amps of current in full sun. That means that 6 Amp hours of charge would be absorbed by a battery in one hour of full sun, with the sun closest to the earth’s surface ie at midday. Given the movement of the sun from east to west from sunrise to sunset, a 100 Watt panel will produce around 30 Amp hours of charge on a cloudless sunny day.
If you have a 100 Ah battery, given that it’s not advisable to discharge a battery below 50% DoD, no more than 50 Amp hours.should be discharged in a day. To regain 50Ah per day, you, would need a 200 watt array.
It’s a commonly held belief that your array should be double in Watts, what your battery bank is rated in Amp hours.
100 Ah battery bank – 200 Watt array
200 Ah battery bank – 200 Watt array
I have a 240 Ah battery bank and a 460 Watt array. According to the above guide, I would have a 280 Watt array. However, I don’t draw anywhere near as many Amps from my battery in one day as is generated by my array. My array produces an average of 85 Amp hours in a day in good weather. I draw around 50 Amp hours from the battery bank, per day on average.
However, I have a generator for topping up the batteries when necessary. Array size is to some extent, a case of “horses for courses.” An array can certainly be too small in electronic terms, but not too big. On the other hand, it can be too big in terms of size weight and cost. It’s about finding a balance.
A 240v powered intelligent charger is, in my opinion, an essential part of any 12v set up. Regardless of how effective your solar charging is, there’s always going to be a need to plug into a 240v supply and put the battery bank through a very reliable, continuous charge cycle that doesn’t depend on available sunshine. This might be when the sun just isn’t providing and charging takes place by means of a generator. It might be simply when you’re on grid and plugged into a 240v supply, which is a great opportunity to give your batteries a good solid charge.
Many intelligent chargers will analyse your battery and if necessary, put it through a regeneration cycle, which has the potential to repair a battery that has been damaged to some extent, by being over discharged or being discharged too quickly.
Most intelligent chargers allow selection of the charge rates up to 50 Amps. The charge rate can be explained as simplistically as the amount of Amp hours (litres of fuel) that are put back into the battery (fuel tank) in 1 hour. It isn’t quite so simple due to the nature of multi stage charging. To explain further: One would expect that a 25 Amp charger would charge a 100Ah battery from 50% DoD in in 2 hours. in realty it will charge the battery to around 75% full in around an hour. 25 Amp hours added to an existing 50 Amp hours, taking it up to a 75 Amp hours or 75% of capacity. At approximately 75% full, the charger will automatically switch stages from “bulk” to “absorb” charge. The charging current is reduced in order to prevent the battery from over heating and over charging. As a result of the reduced current, absorb charging can take considerably longer than bulk charging; often around 3 hours to charge the last 75% of a 100 Ah battery.
Charging From A Vehicle
Many Australians say, there’s no such thing as a free lunch. I hope it’s not entirely true, because I’ve had hundreds and I don’t want to get a surprise bill, but the general gist of the euphemism is true.
If you’re going to gain from something somewhere, you’ll pay elsewhere. Charging a house battery from a vehicle is a bit like that. A vehicle’s alternator is a readily available source of a charging voltage, but a load on an alternator is inversely proportional to fuel consumption by the engine that drives the alternator. It’s not a huge consideration, but it is one to be aware of and should not be over looked.
From a personal perspective, we don’t charge our caravan house battery from the car’s alternator. We have a significant solar array on the roof of the caravan and that generally suffices as far as our charging needs go. We also have a second deep cycle battery system in the car that powers our Engel Freezer. That is charged from the alternator via a simple battery isolator. The isolator ensures that the Auxiliary battery doesn’t charge if the car battery is below a nominal voltage. That prevents draining the car starting battery. It also ensures that charging ceases when the Aux battery reaches a certain voltage, thus preventing over charging. Battery isolators are a compromise in that they don’t offer multi stage charging as outlined above. They essentially cut off charging prior to the battery reaching full charge, where a multi stage charger drops the charging current in absorb charge mode, then continues to maintain the charge in float mode.
Multi stage charging is easily achievable via a vehicle’s alternator with a DC to DC charger. This type of device is an “intelligent” multi stage charger that’s powered by a 12v DC supply from the vehicle as opposed to a 240v mains supply. They are usually connected between the vehicle and a caravan or trailer via a significant cable of sufficient diameter and Anderson plugs. They usually charge at a rate of around 15 Amps, some up to 25 Amps.
Decent DC to DC chargers are reasonably expensive. They’re potentially an extravagance unless you spend a lot of time towing as opposed to staying in one place for long periods. However, the up side is that a lot of the better models also double up as a solar regulator, which makes them quite cost effective. In saying that, even the good ones aren’t necessarily the best solar regulators.
Given that I prefer to hang around in places for a while and travel as infrequently as possible, it’s not worth me spending a lot of money on a DC to DC charger. A simple battery isolator serves my purposes well, given that we rarely travel for more than 4 hours in a day. This means that my battery is essentially bulk charged on the road, then plugged into a separate solar regulator for the later stages of charging.
Fuses, Breakers and Cable Ratings
The 3-Way Fridge Trap
Points to Remember
- Batteries and battery banks are measured in Amp hours
- Amp hours are like litres in a fuel tan
- P (Power) / V (Voltage) = I (Current) – That’s how to calculate how much “fuel” you’re using
- A deep cycle battery is designed to deliver a low currently continuously over a long period of time, whilst a car / cranking battery is designed to deliver a high current in a short burst to start a motor.
- You’re battery or bank should never fall below half full (50% DoD)
- Voltage indicates how full a battery is – 12.2v is about half full.
- A group of solar panels is called an array. Measured in Watts.
- In full sun, 6 Amps is generated by every 100 Watts in an array.
- Solar arrays are connected to solar regulators. Good solar regulators are intelligent multi stage chargers. They ensure a full charge and prevent over charging.
- As a suggestion, an array should total in Watts, twice the size of a battery bank in amp hours. For example – 200W battery bank requires a 400W array. Not a hard and fast rule. 6
- A good 240v intelligent charger is important for putting your batteries through a good continuous charge cycle when mains or generator power is available. They are also a very good means of regenerating batteries that have been over discharged.
- Charging from an alternator / vehicle is very convenient if you travel regularly. Always bear in mind that gained energy is paid for elsewhere. Charging from a vehicle will increase fuel consumption.
- DC to DC chargers provide multi stage charging from a vehicle. They are reasonably expensive and might be unnecessary for people who don’t move on from a location regularly.
- Battery isolators can be used in place of DC to DC chargers. They are very inexpensive, but only provide a bulk charge. They will never completely charge a battery because a solenoid cuts switches charging out at a nominal voltage in order to prevent over charging.
- Battery isolators also prevent a car / cranking battery from being discharged when a house / aux battery reaches a nominal DoD.
- A house or aux battery can be bulk charged by a vehicle via a battery isolator whilst driving and then switched over to solar charging for final stage charging with a reduced current.
- Inverters transform 12v DC from a battery into 240v AC for powering domestic appliances.
- When an invertor is left on but no appliances are being used, the invertor is “idling.” It does not use run at it’s rated power (wattage) when on but not powering an appliance. Idling draws a very small current.
- Invertors are not magic. They are only as capable as your battery bank. Large invertors are often a waste of money. They can provide a 240v supply to appliances of a high wattage, but they’ll suck batteries dry in a hurry.
- Lithium batteries are expensive, evil and the core of world domination by a US power giant. (Shit that’s going to cause some arguments – good job I enjoy being insulted by people who I’ve insulted because they just spent a fuck load of money on lithium batteries) Fuses, breakers and correct cable ratings can save your equipment or possibly your life by preventing a fire. They’re designed to ensure that a circuit does not carry a greater current than it’s supposed to.
- Fridges that run on LPG gas are supposed to run off LPG gas when a 240v AC supply is not available. If you run one from a 12v DC battery, you will discharge it very quickly, so don’t do it! If you do, don’t whinge about it online, because now you know not to and it’s your own fault if someone calls you a goose.