How to charge the second battery
Once again there are so many options, and the optimum solution depends on your specific requirements.
Most people start off with connecting the 2nd battery to the alternator and hoping this will charge the 2nd battery. Depending on the exact setup you may be able to charge the battery to some extent, but highly unlikely you will get more than 80% of the possible capacity of the system. Remember the previous section where we spoke about the voltage drop, and the final voltage available at the 2nd battery.
The cheap solution is to pre-charge the battery at home, with a 220V charger. Then relying on the alternator to keep the system going for a typical long weekend trip. And this works surprisingly well for many people. But once you take a trip of more than a few days the limitations of this approach becomes very apparent. Then you start thinking of better systems …
The deep cycle battery is best suited for campsite use, but requires a charge voltage of 14,5V. This is more than most alternators now deliver. This created a market place for the DC-to-DC (dc2dc) chargers. The principle of this charger is simple, it takes the voltage from the alternator and pushes the volt value up, by reducing the current value in turn. You now get a situation where your alternator puts out 13V at idling, but after the dc2dc charger your 2nd battery sees 14,5V. An added benefit is that most dc2dc chargers actually limit the charge current, thus you can now more accurately choose a cable size that matches your installation.
Most dc2dc chargers are also “clever chargers”. The “clever chargers” monitors the charge level of the 2nd battery. Once the 2nd battery is almost fully charged the charge current is reduced, to protect the battery.
So while you are driving, your alternator, via the dc2dc charger, charges your 2ndbattery at 20A (different models have different pre-set values, but this is a realistic figure).This is a very good system and has many benefits, but still does not address the fundamental question of power while you are parked.
As long as you are parked your fridge draws power from the battery. With an average current draw of about 60A.h per day you need to drive at least 3 hours per day to keep the battery charged, or find some other way to keep it charged … More about this later.
Fridge power draw
National Luna (NL) provides a current draw figure of “an average of 2,5A” per day for its 52 liter Weekender Twin unit. Basic maths says 24 hours times 2,5A = 60A.h -
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BUT, NL briefly mentions “..consumption is measured in a controlled environment ... Actual power consumption may vary ...”. Actual tests show a current draw of 5 to 6 A. Due to the on/off cycles the average of 2,5A is possible in moderate climates. During summer in Kgalagadi or Namibia, with day temperatures of more than 40 degrees Celcius, and night temperatures mostly above 30 degrees Celcius the fridge hardly ever switch off !! At 5A for approximately 20 hours you suddenly need 100A.h to power your fridge !!
The actual power draw of your fridge will depend on many factors:
- Insulation – the quality of the insulation has a big impact on the power draw.
- Where the unit is installed – A unit installed inside an air conditioned SUV will draw a LOT less power than a unit stuck in the back of a super hot canopy.
- Use – Opening the fridge too often increased the cooling load, and thus increase the power draw.
- Loading of goods – Try to load cold goods into the fridge, or load the goods when you can use mains power to cool it down, thus saving your battery capacity.
- Operating temperature – The colder the thermostat setting of a fridge, the higher the rate of heat flow through the insulation.
Never over look the most obvious though – use the 220V in the camp site when available.
What is a “relay” ?
When switching a circuit on or off, a spark is generated at the switch. This spark actually burns away the switch contacts over a period of time (number of uses). The solution to this problem is to limit the current that flow through the switch – but the current flowing to the load is a constant. As such the circuit is now split in two:
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Think of this as the electrical circuit for a set of spot lights. By switching the spot light switch, you are passing a small current through an electromagnetic coil, this closes the heavy duty switch between “30” and “87” and power is now sent to the spot lights. The numbers used in the illustration are above are the standard numbers found on relays.
Typical use of an inline fuse and a relay to power a load:
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The current flow will determine the type of relay to be used. For spot lights and other small items standard relays are used. For connecting two batteries a much higher current flow is allowed for and a heavy duty colehersee type relay is used.
The benefit of a relay is that it will function automatically and no additional user input is required. That said, there are still a few users that want full manual control. They would then use a typical “marine switch” to connect or disconnect batteries.
The use of relays are VERY handy for spot lights, hooters, vhf radios, etc, and combining this with a nice waterproof box creates endless opportunities:
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