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Summary of charging, split charging and battery monitoring.

If you're reading this then it's probably fair to assume that you have (or will soon have, or maybe want) a DC electrical, battery based, system with 1 or more battery banks, 1 or more chargers (be they alternators, solar panels, wind turbines or AC powered chargers) and probably an engine to start (which can probably also recharge one or more of the battery banks).

Let's go back to basics and start from the beginning. Instead of looking at what has been done in the past, what other people have done, how some people do it etc, let's, instead, look at what would be the ideal system. What is actually required, irrespective of whether it has been done before, irrespective of whether conventional wisdom says it is possible.

Firstly we have an engine to start. We must be able to start the engine even if the battery bank that supplies other loads is flat. Otherwise, if someone leaves the lights (or whatever) switched on and flattens the batteries, we cannot start the engine to recharge the batteries. Obviously this has to be avoided. So one of our first priorities is to be able to start the engine, always.

There are a few ways of dealing with this problem.....

One solution is to fit a device that disconnects all the loads from the battery bank before the battery discharges to a state of charge that will prevent it restarting the engine. SmartGuard is one such device.

This solution works well. It will disconnect all equipment from the batteries, before the batteries are discharged to a level where the batteries may not start the engine. This is a good solution where a small battery bank (say 1 battery) is used. It allows installations such as vehicles to use the lights and radio but always ensures sufficient power remains to restart the engine, which is one of the high priorities. This system has become the installation of choice for vehicles with a simple system where it is required to use some power for purposes other than actually driving the vehicle but where the cost of a more complicated twin battery installation cannot be justified.

If only one battery is fitted, then there is little other choice. There really is no other solution.

There is, however, a serious drawback with this type of installation. And this is that an engine start battery, correctly sized to start the engine (i.e. the one that came fitted in the vehicle from the factory) has very little reserve power available. If the battery is discharged to only 60% it is very doubtful that it will start the engine under all conditions (i.e. cold weather, tired heater plugs etc). Therefore the disconnect device (such as SmartGuard) has to disconnect all the other loads well before this level of discharge is reached. This means that very little power is available to run other equipment.

However, if only the one, originally fitted battery, is available then there is really no other choice. We have to make sure sufficient power remains to start the engine. In these installations SmartGuard will ensure the engine will always start.

An often used solution is to simply increase the size of the battery. This will "sort of" work. The problem is that the battery still has to be an engine start battery in order to provide the enormous currents required during starting. Deep cycle batteries are not good for this use. They simply will not produce anywhere near the currents that a similar sized engine start battery will. Obviously fitting a bigger battery (or installing 2 thus doubling the available power) will provide more power for the other loads, but still we have the problem that this is an engine start battery.

If 2 batteries are going to be fitted then it clearly makes more sense to use 2 different types of battery. An engine start battery to start the engine, and a deep cycle battery to power the loads. An engine start battery can produce enormous currents to start the engine (which a deep cycle battery cannot). A deep cycle battery can happily be discharged to much lower levels of charge (that would very quickly destroy an engine start battery) thus providing far more available power as we do not have to reserve any power in this battery to start the engine.

So in conclusion, it seems to make far more sense to have 2 separate batteries. Each one being the best type for it's intended purpose (i.e. engine start battery for starting engines, deep cycle battery for deep cycling). It also being acceptable to discharge the deep cycle battery to a much lower level of discharge giving more available power.

But now we have 2 separate batteries, for 2 different purposes, we have to keep them separated, which makes charging them a problem.

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Page last updated 02/04/2008.
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