This does not and cannot work. Most such devices put out about 12.6 volts. A few put out 13.8 volts (offload) but even a full 13.8 volts is not remotely enough bring a 12-volt battery to much over 30%-40% charge, and that is lower than the battery should really be discharged to anyway.

A (fortunately) small number of people obtain 12- volts by tapping it off one of the two 12-volt series-connected batteries. This is a seriously bad thing to do because from there on, the lesser charged battery will be restricted in re-charging by the other one coming up to full charge first. Only disconnecting and charging each battery separately from a three-stage, 12-volt charger remedies this condition. The equalising system described below overcomes this problem.

First Define the Problem

Enquirers almost invariably seek a better way of charging the 12-volt battery bank. But this, with every respect to the enquirers, is seeking a solution before adequately defining the problem.

Here, that problem is not necessarily ‘how to charge a 12-volt battery from 24-volts’, but rather ‘how to ensure a reliable and affordable 12-volt supply when the vehicle’s system is 24-volts.’ Charging a 12-volt battery bank is only one of several solutions.

Alternative Solutions

Instead of seeking ways of charging a 12-volt battery bank from a 24-volt system, consider not doing so at all! Instead, use a couple of 12-volt batteries in series as a 24-volt house battery. Charge this in parallel with the starter battery via an isolating relay and, if solar is used, use two series-connected 12-volt solar modules to supply 24-volts via a 24-volt solar regulator.

Then, and only then, reduce the 24-volt output to 12 volts via a 24-to-12 volt dc-dc converter (of which Jaycar has a good range), or via a 24-to-12 volt equalising system (such as that from Redarc).

Dc-to-dc conversion simply involves a box that accepts a dc voltage and converts it to another that is higher or lower in voltage.

Equalising systems work by drawing current off one of two series-connected 12-volt batteries and constantly equalising their overall state of charge by restoring the current drawn from the other of the two batteries. Whilst it is still rare in motorhomes, the technique is common in large sailing boats where a 24-volt supply is required for winching, yet most yachting instruments require 12-volts. Each of the two methods (i.e. 24-to-12 volt conversion, and equalising) has various advantages and very few and minor disadvantages. There is also the inverter/ charger method outlined later. All work well. Which to choose is also discussed later in this article.

Voltage Conversion

The 24-to-12 volt conversion enables the use of lighter 24-volt wiring (and/or lower voltage drop) with appropriately scaled converters sited close to whatever is to be driven. These can be obtained with outputs ranging from an amp or so but there is a typical maximum of 25 amps continuous output and a peak of about 35 amps: thus, whilst providing flexibility there is a limit to the current that can realistically be pulled. Up to a point it is worth considering several small units, but the technique does not really lend itself to driving big inverters that may be required to supply over 100 amps if, for example, powering a microwave oven.

Equalising

Equalising systems require heavier 12-volt wiring running from where the unit is sited (close to the 24-volt battery bank) to everything running from it. Various current ratings are available but these relate only to the equalising current (used to rebalance). There is no limit to the peak current than can be drawn - excepting that which the battery can supply (i.e. the equalising unit does not of itself impose limitations) so obtaining more current than you can presently access may involve only heavier cabling and a larger capacity battery bank.

The two techniques work in totally different ways, but choosing which to use is mostly a matter of the maximum current you wish to draw. The equalising technique is overkill if you are powering one or two lights plus a small TV, but if that draw is in excess of 25 or so amps, then the equalising approach is likely to be a better bet. It is also more flexible.

The dc-dc conversion approach is very much cheaper for small systems but the costs become comparable at 25 or so amps – and from thereon equalising is cheaper.

Inverter/Battery Charging

Another very effective approach and simple to install, is the inverter/three-stage charger system that I outlined in a recent issue. It is initially more costly to implement as it requires a 24-volt input inverter as well as a three stage charger, but it does also provide 240-volts.

Further, the three-stage charger can be used to charge the batteries wherever there is a 240-volt mains supply. Whilst more costly and rather more complex, of the three methods outlined, this latter approach is the one that I personally prefer.

Vehicle electrics are covered in detail in Collyn’s book ‘Motorhome Electrics’. It is advertised elsewhere in this issue and is available directly from the CMCA (and also from the NZMCA).

Collyn Rivers W8054.