CHARGING BATTERIES -
A NEW WAY

A newish way of charging ‘house’ batteries, initially proposed for motorhomes that have 24-volt alternators and starter batteries but 12- volt ‘house’ systems, is rapidly gaining acceptance in wider fields.

The big problem with 24/12-volt systems is that the 24/12-volt converters generally used (including split battery equalisers) are intended to supply 12-volts for running lights and appliances. They are neither intended nor suitable for battery charging because their voltage output is not high enough.

Many motorhomes already have, or will have, high quality three-stage battery chargers and an inverter so I have for some years proposed that people replace that inverter with a 24-volt equivalent (12-volt inverters are readily saleable) run from the vehicle alternator whilst driving, and used to power the three-stage charger that in turn charges the house batteries (only).

Electrically knowledgeable people rightly comment that converting 24-volts to 240-volts and then back down to 14 point something is hardly efficient. And they are right - but it’s a lot less inefficient than trying to charge a battery with insufficient volts! It also results in an optimally and fully charged battery: close to impossible with a conventional charging system and conventional lead acid batteries. And it works even better with gel cells or AGMs (each needs a specialised charging regime but all good three-stage chargers include such functions).

Taking This Further

There are also increasing problems with vehicles that have computerised anything. Vehicle computer systems rely on stable battery output for voltage references and this precludes anything that increases charging voltage (eg. three-stage regulators, 14.7-volt regulators etc). Doing so would almost certainly invalidate warranty – and would equally certainly be blamed for almost anything going wrong, even with no warranty applicable. And possibly rightly so.

With trailers, and indeed with motorhomes, voltage drop on charging circuits is an issue, even though using heavy enough cable can solve that one. But even if voltage drop were overcome, deep-cycle and specialised batteries (AGM, gel cell, calcium/calcium etc) really do need specific charging regimes. Paralleling across a starter battery works to a point – but, as my recent article on battery charging points out, it does not remotely compete with three-stage charging. It is also worth noting, that many up-market three stage chargers take their voltage reference directly from the battery, and adjust the charge rate accordingly so voltage drop (within reason) is automatically allowed for. There is thus a good case for the inverter/charger system generally and connecting an inverter to the vehicle electrics is no different from adding a sound system or a couple of spotlights. Further, if you did not have an inverter before, 240-volt availability will prove useful for other tasks – such as running the odd power tool, providing a wider and cheaper choice of portable TVs etc.

Inverter Size

The alternator’s size, and also the draw taken by other ancillary devices, will limit the amount of alternator output available for charging. It’s hard even to generalise, but with a standard 12-volt alternator about 25 amps (12.5 amps for 24-volts) should be safe. With inherent inverter and charger losses this will still give you 20 or so amps charge – almost certainly far more than you had before. The inverter must of course be big enough to drive that charger. Three-stage chargers draw about 10-20% more energy than they put out so a 300-watt inverter will be fine for 20 amps charging.

If you have an existing inverter that is much larger, this may still be used as long as the battery charger does not draw more than 20 or so amps.

Whatever the size inverter you can if you wish connect it so that it is used for this purpose only, or for this and other purposes as well. Either way this will need a voltage sensitive relay (such as the Redarc unit) to prevent the inverter loading up the system whilst starting. But these are common anyway. If you intend to use the inverter also for other purposes, you will need to include automatic change-over switching so that the inverter is normally connected to the house batteries and cut across to the alternator (or starter battery) only when the engine is running. This is not nearly as complicated as it sounds!

Safety Issues

A few people who presumably spent their formative years around diesels have expressed concern about having 240-volts in a car - presumably overlooking that petrol engines have 20,000-35,000 volts across the secondary of their ignition systems. Nevertheless there is a safety issue if a chain-store cheapie is used.

Transformers are used in inverters to increase the extra-low voltage input to 240-volts output. A transformer consists of a lump of magnetisable material that has (for example) one 12-volt winding, and one 240-volt winding. The cheapest way of doing this is via one continuous winding with a tap taken off at 12/240ths of the number of turns.

This saves a lot of money, but such non-electrically isolated inverters result in one side of the 240-volt output being connected to one side of the battery. This is not inherently as dangerous as it may seem, but as electricians will understand there are safety-related issues if connected to an MEN circuit. The matter is already the subject of an Australia-wide (Construction Industry) Safety Notice.

A well-designed and safe inverter has two electrically and mechanically separate windings. Each is very well insulated from the other. One is for the extra-low voltage input; the other is for the 240-volts output. These transformers (and the inverters that use them) are known as ‘electrically- isolated’. This will always be made prominent in the technical literature and promotional material. If you are unsure about this any electrician can readily check and advise: it takes only a second or two.

The above highlights the risks that people take when attempting to save money buying electrics. There is a 3:1 range in inverter prices for what may seem to be similar products. People who pay the higher prices are not all fools! In some instances, part of the higher price may be due to excess or multiple mark-ups, but most are related directly to performance, quality and safety. With inverters, and also battery chargers, by and large you get what you pay for.

Buy a Trabant, you get a Trabant. I’d hesitate to use a non-electrically isolated inverter even as a door stop! It is probably worth noting that, for safety related reasons, the only inverters that leading electronics distributor Jaycar Electronics now sells are ‘electrically-isolated’.

Installation

As long as the inverter is not connected into fixed mains-voltage wiring (i.e. the battery charger is plugged directly into the power outlet on the inverter) the work may be legally done by any competent person. Nevertheless, I strongly advise that this particular work be done by a licensed electrician, or at least checked by one after you have finished.

Fuel Cell Breakthrough

On a totally different subject, one of the biggest problems with fuel cell development is their need to run on hydrogen. Ideally, they use hydrogen as their direct fuel, but as this is not readily available, most use diesel, petrol, methanol or LPG and reform this internally to produce hydrogen. Unfortunately, and as with earlier car engines, the process builds up performance-reducing carbon (this sound familiar?). The losses inherent in the reforming process also further limit efficiency.

A new system, researched by materials scientist, Scott Barnett, uses a catalytic process that causes the otherwise carbon-forming process to produce harmless carbon dioxide instead. Barnett’s experimental system is fuelled by iso-octane: i.e. essentially petrol before the additives are added.

If proven successful, the main benefit will be an increase in efficiency.

(The above has been widely reported in the science press – an excellent but more technical account is in New Scientist, 9 April 2005).

Further information on battery charging, inverters etc is included in ‘Motorhome Electrics’. This book is advertised elsewhere in this issue of The Wanderer and is obtainable directly from the CMCA NHQ and also at the CMCA’s major Rallies.

Collyn Rivers W8054