Here’s a writeup on electricity and overlanding cars. Key words are auxiliary battery and charging of this, solar cells and power consumption. And a little about cables, fuses and connections.
Table of Contents
About current, voltage and power (for beginners)
Cars usually have 12 Volt systems, some have 24 V. Current is measured in Ampere, A. The product of current and voltage is power, Watt, W. Watt is how much effect is used by the battery right now. How much you use over time is watt-hours, or kilowatt hours, KWh that is used in the household. You can also use Ampere hours, Ah, as a measure of consumption. This is fine for cars because the battery capacity is stated in Ah. Typically, a car battery is 70 – 105 Ah.
If consumption is stated in Watts, you can find the current in amperes by dividing by 12 (or 24 if the car has a 24 Volt system).
Example
If you have a cooler that uses 48 Watts this will mean 48/12 = 4 Amps.
If it stays on for10 hours, it will then use 4 x 10 = 40 Ah, Ampere hours.
Auxiliary battery and cooling box
If you have a cooling box or other things that use a lot of electricity in the car, you should have a separate auxiliary battery for this as not to risk emptying the starter battery overnight. If you are going to lie still for several days without access to electricity, it becomes even more important.
Cooling boxes are of two different types. The cheap ones are thermoelectric, but they use a lot of power. Then compressor coolers are far better in terms of power consumption. They are also usually larger, but cost 10 times more.
Typical examples, 12 V:
Compressor cooler / freezer, 60 liters uses 0.50 – 2.5 amps. Average 1.5 A, 18 W
Thermoelectric 20 liters, 3 – 5 amps, Average 4 A, 48 W
That is, a compressor cooler uses about 20 Ah, ampere hours and a thermoelectric about 50 Ah overnight (12 hours). Then you are well off to drain the battery.
The solution is thus a auxiliary battery. The battery should be mounted as low as possible to keep the center of gravity down. Here are two examples, a 90Ah battery on a traverse under an LC80, and two 105 Ah mounted inside on the floor of an LC78.
Be sure to fix the batteries securely. They are heavy, and should you crash or roll, you will not have the batteries flying through in the car. I use 10 mm bolts through the floor with a backplate on the underside, and eye hooks and straps over the batteries. In addition to being held in place by angles on the floor.
Battery types
You can use a regular starter battery as a auxiliary battery, but it is not recommended. A starter battery shall supply a lot of power over a short period of time, and should not be completely discharged. An auxiliary battery, on the other hand, should deliver low power over a long period of time and can be completely discharged. The starter battery don’t like that.
So you should opt for an auxiliary battery that is made for this. Since the battery usually is inside the car, it must be a closed battery that does not release gas or can leak acid. This applies to most modern batteries, and most used are gel or AGM batteries that do not contain liquid acid. Lithium batteries used in electric cars and tools are also possible. They are compact, lightweight and unfortunately very expensive.
Charging the auxiliary Battery
In order for the auxiliary battery to be charged, it must be connected to the alternator when the car is running, but for the starter battery not to be drained when the car is parked, the auxiliary battery must be disconnected from the car’s charging circuit. This can be done with a manual switch, but it’s better that it happens automatically so you do not forget it. There are two types of separators used, a relay with voltage sensor or a DC-DC charger.
Relay / Solenoid
A charging relay connects the auxiliary battery to the car’s charging circuit when the charging voltage has reached a certain level, eg 13.3 volts. This is an affordable and reliable solution that covers most needs. Prices from USD 30 and up.
I tried one of these, but experienced that the relay did not connect on several occasions so the auxiliary battery was not charged. Probably due to worn starter batteries that took a lot of charging current, or voltage loss in wires. It would be better with a relay where you could adjust the closure voltage. I changed instead to a regular 80A relay which I connected to the alternator’s charging relay (charging lamp). Thereby the relay switches when the alternator starts charging. This was not perfect because sometimes the relay switched on too early, and then the fuse for the auxiliary battery blew because the starter pulled power from this.
A starter on a diesel engine draws 400 – 700 A during start-up, and without a large relay and thick cables it is not possible to start from the auxiliary battery. See below.
The solution was to connect an additional timer relay so that the main relay switches after a few minutes. It works fine, but the solution is starting to get complicated so I would have been better of by changing to a DC-DC charger in the first place. See below.
DC-DC charger
A DC-DC charger is a more sophisticated charger that converts the input voltage to a voltage that is adapted to the charge status of the battery. This means that you can get higher or lower voltage out of the inverter than you put in. This again means that the charger can always choose the most efficient charging voltage for the auxiliary battery. In practice, this means that the charging time is somewhat shorter. They are more expensive than a relay, from USD 200 and up. I doubt they are so much more effective that they are worth the price. In practice, you drive far enough for the battery to be charged. But there is a possibility that the battery will have a longer life.
Solar panels
If you want to lie still for a while without mains access, solar panels is the thing. You get packages with 110W power from just over USD 100 and up. The package consists of panel, fasteners and regulator. Dimension panel typically 100 x 65 cm. This is a size that is easy to mount if you have a flat roof, or on top of a roof tent. I mounted such a package on LC78 and have used it one summer in Norway.
110W corresponds to 9A charging current. You should be quite lucky with the weather and parking to get this in Nordic countries. I measured the current to typically 2 – 5 A. This is enough to power the cooling box. If you go south it will be more sun, but then the problem arises that you do not want to park in the sun. So then you are just as far.
The solution is to have a loose panel, or preferably a flexible mat with a long cord so you can park in the shade and have the solar panel in the sunshine.
The controller that comes with such a cheap package is a so-called PWM controller. If you are going to upgrade, you should invest in an MPPT regulator that is more efficient, about 20%. It works in the same way as a DC-DC charger as mentioned above.
Solar panel charging can be run simultaneously (in parallel) with the alternator.
220 V «shore power»
Sometimes you have access to 220V (or 110V) at campsites. Then it is good to be able to connect for consumption and charging of batteries. I then use a regular battery charger connected to a regular caravan socket through the wall. Here is the full diagram for my LC78 with batteries and charging:
Schematic aux battery charging:
Charging cable for consumable battery should be 6 or 10 mm2 / AWG 7-10 to get a low voltage drop. The large fuses of 50A are placed as close as possible to the positive terminals of the batteries
Power consumption
An important question is, how long do you have power if you lie still? A compressor cooler uses an average of 1.5 A per hour. I.e. 36 Ah per day. If you have a 105 Ah auxiliary battery, this corresponds to being able to lie still 105/36 = 3 days.
This is theoretical since you can not drain the battery completely empty. Besides, you probably use a little light etc. But LED lights use very little power, so at least interior moderate lighting does not mean much.
In practice, 2-3 days on a 105 Ah battery.
If you have a solar panel, the calculation will be a little different.
My 110W panel provides 2-5 A in practice. Let’s say 3.5 A on average. Over 10 hours, this becomes 35 Ah per day. That is, when it is reasonably sunny, a 110W panel is delivering just enough to power the cooling box. On clouded days, you probably have to drain the battery a bit too, but if the weather is reasonably good you can lie down for as long as you want.
Dimensioning of fuses and cables
In general, use as thick a wire as possible when laying wire in a car. There are two reasons for this. It means the lowest possible voltage drop. The higher the current in a given wire cross section, the lower the voltage at the other end will be due to loss in the wire. And most importantly, avoid the insulation melting due to overheating, with subsequent short circuit and in the worst case fire.
All wires must also be protected with fuses. The larger the fuse, the thicker the wire.
When cables and fuses are dimensioned for domestic use there are clear rules for what is allowed. I have not found anything similar for cars, so the numbers in the table below must be seen as indicative and given without guarantee. It is partly from personal experience, partly what can be found by a little math. Permissible power for domestic installations is stated for comparison.
There is a difference between domestic and car installations in that in houses cables are laid inside walls, while in cars they are more open and have better cooling.
AWG is an American way of stating cross-sections. It is often used to specify connection cross-sections, e.g. in connectors.
Cable layout and components
Cabling
When laying cables it is important to mark them well, preferably at both ends. If possible use different colors and make drawings. Write on the drawing where the different components are. You do not remember it after a year when there is a fuse that you stuffed under the dashboard somewhere that is blown.
Keep order. Try to collect fuses and relays in one place. Bundle wires and fasten with strips. Mark all fuses and relays.
Try to use the same components everywhere, such as relays. Then you avoid many varieties as a spare.
And one thing, never use quick splice connectors, especially not externally. They are guaranteed to start corroding, and in the worst case the wire you have connected to will break. Get a soldering iron and solder everything. Use shrink tubing to insulate, and if not, use good tape. Externally insulate with vulcanizing tape first, then plain tape. I use Scotch 88 tape. Expensive, but it lasts forever. The cheap tapes become stiff, loosen and do not withstand oil. Scotch 88 can be purchased from electrical wholesalers, eg Elfa or rs components. There you also get good quality switches switches etc.
Vulcanization tape self-vulcanizes to a lump of rubber that makes a waterproof connection. But it is not mechanically strong so therefore Scotch 88 on the outside.
Picture shows fuses from the battery on my LC78 when I bought it, and after I cleaned up by mounting all the fuses in a holder. All lines that are controlled by ignition go via an 80A relay which in turn is distributed on smaller fuses. Thereby it is sufficient with one relay instead of 4.
Fuses
The most common type of fuse today is flat pin ATO up to 30 A For higher currents Maxi fuses can be used. These are easy to get hold of, but get very bulky. They go up to 100A.
MidiOTO and megaOTO are smaller. They also have better termination for thick cables with M5 and M8 bolts. MegaOTO is available from 80 to 500A, midiOTO goes from 30 to 200A.
Here are 50A main fuses for the consumer battery. Or rather automatic fuses. They are handy as they also act as a switch.
Contacts
From left: quick splice connector. To be avoided!, Waterproof 1mm2, Flat pin 30A.
Below: One half Anderson connector.
Waterproof connectors are available with 1 – 6 pins. Low cross section, max. 1 mm2 cable. Ie only for low currents, a few amps.
Flat pin connectors are available with 2 – 6 pins. Maximum cable cross section 6 mm2. Withstands 30A current.
Anderson contact. These are smart as they are asexual. That is, both parts of the connector are equal. (Only one part is in the picture). Used where there is a lot of current. Available in different sizes, 30 – 400 A.
Source: Elfa or rs components.
Start from the auxiliary battery
Something everyone fears is running out of battery far from people. If you travel with others and have jump cables, this is not a problem. If you are alone, it will be worse. If you have a large auxiliary battery, this may save the situation if there is power on this. It is possible to lay cables for start so you can only turn a switch, but it requires 30-50 mm2 / AWG 0 cable and a switch or relay that can withstand 300A.
You should anyway bring jump cables, and if they are long enough you can connect them directly to the auxiliary battery. If they are too short, you must remove the battery.
Welding
If you have jump cables it is also possible to weld. You simply bring some welding electrodes, connect 2 batteries in series so you get 24 Volts, and use the jump cables. It can be discussed how important it is to be able to weld on a trip, but such a solution weighs only a few hundred grams. You can get your car welded anywhere in the world, but you have to get to the welder. Remember to bring a short cable to connect the batteries in series, and a piece of welding glass that you can use to make a welding mask. Simply a piece of cardboard that you cut a hole in and tape the glass onto.
