Updated April 2016
Taking 12 volts dc from 24 volts dc can ruin the batteries in a 24 volt system. Here’s how to do it effectively and safely. There are two main ways. The choice depends mainly on the peak current needed: for example the plus 130 amps for a typical microwave oven limits choice to the equalising system described below.
12 volts dc from 24 volts dc – via an equalising system
Simply tapping 12 volts from one of two series-connected 12 volt batteries (i.e. 24 volts) damages both. This is because the battery from which 12 volts is tapped, is prevented by the other (untapped) battery from ever fully recharging whilst so connected. Even if both are disconnect and recharged every few days, battery life is shortened.
Tapping 12 volts dc from 24 dc can, however, be safely done via an equalising system. (Makers include GSL and Redarc).
An equalising system enables 12 volts dc to be supplied from one of the two 12 volts dc batteries in a typical 24 volt system. It does so by constantly equalising the two batteries state of charge. This is often used in boats that require 24 volts for engine starting and anchor winching, but 12 volts for most else. It also works well in fifth wheel caravans, big motor homes and coach conversions that have 24 volt alternators.
These units have various current ratings that relate only to the equalising current used to rebalance. The drawable 12 volt peak current is limited only to that which the battery can provide. Equalising thus enables high current draw. Its overall efficiency (of 80-85%) is however lower than that of the dc-dc conversion units described below. Further, it cannot supply the high current needed to drive microwave ovens etc.
There are various ways of connecting an equalising unit. That most commonly used is shown below.
Obtaining 12 volts dc from 24 volts dc. The equaliser is located close to the battery bank and connected via heavy cable. Here’s how it is done using the Redarc unit.
The GSL equalizer shown here can also be used as a 12 volt battery charger.
12 volts dc from 24 volts dc – via dc-dc conversion
Another way of obtaining 12 volts dc from 24 volts dc (including 24-volt batteries) is via a suitably rated 24-12 volt dc-dc converter. They are made (by Redarc, GSL and others) in varying capacities – from 1 amp to 60 amps or so. Their output is typically 13.65 volts offload, dropping to about 12.6 volts on full load.
Obtaining 12 volts dc from 24 volts dc can be done by using a dc-dc converter. Pic: Redarc
Dc-dc conversion thus works in totally different ways. Choosing which to use is mostly a matter of the maximum 12 volt current you need to draw. Equalising is overkill for powering only one or two lights but, if maximum is realistically limited to 50-60 amps.
12 volts dc from 24 volts dc – via an inverter
This is a ‘do it yourself’ approach to obtaining 12 volts dc from 24 volts dc. The concept is to use a 24 volt input 230 volt inverter. This, apart from supplying 230 volts ac, drives an efficient multi-stage 12 volt battery charger. The charger can also be used wherever there is a mains 230 volt supply. It requires an efficient inverter and charger, but the best of both now approach 95% efficiency.
If used whilst driving, that 230 volts is being generated may be an unexpected hazard in the event of an accident. There is no known reporting of a consequent accident, but is an issue to consider. The method is described here as it is commonly used.
12 volts dc from 24 volts dc – further information
The design and installation of systems to obtain 12 volts from 24 volts dc is too big a topic to fully cover in article form. All you need to know (including that of the electrics) generally is covered in my book Caravan and Motorhome Electrics. (It is even used by auto electricians as their working guide.) My book Solar That Really Works tells all you need (and more!) for buying or installing solar in boats, RVs and cabins. Solar Success does the same for home and property solar systems. See also my all-new Caravan & Motorhome Book or if your interest is camper trailers – the Camper Trailer Book.
If you liked this article you will like my books. All are technically competent but written in down to earth plain English. Reader after reader confirms the cost of my books is repaid many times over by their getting their systems right first time. The books are unusual in that I have both an engineering and writing/publishing background of over 60 years. For my bio Click on About the author.
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