BATTERY CHARGING AND BATTERY CHARGERS

A recent spate of questions on the CMCA and other websites shows there is considerable interest in the many ‘smart’ chargers now on the market. This is yet another area where prior knowledge is helpful as ‘smart’ is a very loose term that, in this area, may be more applicable to the vendor than the product. I’ll return to this theme later in this piece, but first it helps to have a general idea of how a battery is charged and the most common way of doing so. Then, the need for better ways of battery charging are obvious.

Conventional Charging

A lead-acid battery is charged by applying a voltage across it that is higher than the voltage it already has across it. The higher the difference between that applied voltage and the battery voltage, the greater the charging current that will flow, and the quicker the battery will be charged.

Standard vehicle charging systems (and low-price battery chargers) generate a more or less fixed voltage (typically 14.2-14.4 volts). As the battery charges, its voltage rises towards that 14.2-14.4 volts. This voltage is fixed, so the voltage difference between the battery and the charger falls and as a direct consequence, the charging rate falls. Eventually, the battery voltage approaches the charging voltage and charging ceases altogether.

In other words, the charging rate begins to fall soon after a battery begins to charge, and continues to drop until charging ceases. Batteries charged this way (which is most of them) take many hours to charge even a per cent or two beyond 70% - in fact, says one battery maker, few RV batteries exceed 65%.
The 70% charge is not a problem for starter batteries because the starter motor is designed to work at this battery level. But it is a problem with conventional house batteries charged from the same source. These too are limited to 70%, and may take forever reaching even that.

This limitation has been recognised for the better part of 100 years. Since the early 1900s, serious battery charging has been done by various methods of maintaining a constant rate of charge through most of the charging cycle. In semi-technical terms the battery is charged at constant current, not constant voltage.

By far the most common of these methods is ‘three-stage’ charging. There are minor variants of this but all work generally as outlined below. Whatever the variants the charger ensures that neither it nor the battery charges beyond safe levels.

Stage One

Since the charge rate depends on the difference between the charging voltage and the battery voltage, the first stage of three-stage charging is to keep this voltage difference constant so that, as the battery voltage rises, the charging voltage rises and the charging current remains constant.

Stage One typically takes the battery to about three-quarters of full charge, and at a rate that does not exceed 25% of the battery’s amp/hour capacity. This corresponds to an instantaneous voltage between 14.4 and 15.0 volts (depending on battery type).

Stage Two

Since battery charging is an electro-chemical process that has (in the case of deep cycle batteries) very long reaction times, the charging voltage is now reduced for a few hours to enable the charge, that is now concentrated in and around the plates, to become homogeneously distributed through the plates and the electrolyte.

To enable this to happen, the charger moves automatically to a so-called ‘Absorption Stage.’ In this stage, charging current is reduced to about half that of the previous stage.

If at any time during this stage, a heavy load is drawn from the battery, the charger will switch back to the Stage One ‘boost’ condition. Absorption is typically maintained for several hours.

Stage Three

Following the Absorption stage, the charging voltage drops once again to a level that eventually counterbalances the battery’s internal losses. This is called the ‘Float Stage’ and, depending on ambient temperature and battery type, will be from 13.2-13.3 volts (AGM/gel cells) to 13.6-13.8 volts (conventional lead acid batteries). Batteries may be left ‘floating’ indefinitely as long as their electrolyte level is checked from time to time.

At this stage of charging a battery will typically be 95%-97% charged (it is very rare for a battery to become 100% charged).

Here again, as with the absorption stage, the charger will revert to boost if the battery voltage drops because of a heavy applied load.

Equalisation

Some chargers also provide an ‘Equalising Cycle’. This cycle fully charges the battery and then continues to pump current through it (using up to 16 volts to do so). Equalising is the lead-acid equivalent of giving the rig a good thrash down the expressway to blow the carbon out. Equalising is good for batteries but should only be done two or three times a year.

Some vendors provide this facility without banging a promotional drum. Others use heavy percussion – and may even describe their products as ‘four stage’ chargers.

Equalising is also provided routinely in many high quality solar regulators.

Presettable

Different battery types require different voltage/current settings so all good quality three-stage battery chargers have programs for standard lead-acid, sealed lead acid, gel cell, AGM batteries etc.

The Need for Caution

The above more or less defines a three-stage charging cycle. If the charging regime does not generally follow the above it is not a three-stage charger.

Chain-stores are however now cashing in on the growing awareness of the need for a better way of battery charging. Many use the term ‘smart’ charger and imply that this is synonymous with three-stage. It is not!

A lot of people are being fooled, and particularly those convinced it is possible to obtain Volvo performance at Lada prices - if one only knew what to buy.

Often I will suggest what I know to be a fair-priced good quality solution that I know will fix someone’s problem. And time and again someone will post to the effect that ‘there’s no need to pay the $350-$1000 that Collyn is talking about – XYZ Hardware has the equivalent for a third of that price.’

But 99% of the time it hasn’t. If it had, companies like SEA, Selectronic, Ample Power, Xantrex etc would rapidly be out of business.

And if the chain-store ‘solution’ is adopted there may now be two problems. For example, I repeatedly advise not to buy a cheap modified square-wave inverter unless you know exactly what you are doing. A CMCA Member recently followed website advice to the contrary. He saved $500 by buying an ultra-cheap inverter. This ‘bargain’ inverter instantly destroyed his $1100 laser printer.

Beware then that unless a charger states quite specifically that it is ‘three-stage,’ and spells out that those stages are more or less described as above, it is odds on to be a conventional (i.e. tapering charge) device, with a sensor that drops the charge to a fixed float level (typically 13.6 volts) once the battery voltage reaches 14.4 volts.

Its operation is more or less identical (or often totally identical) to that of a cheap conventional charger, and with similar less than useful characteristics. That is, it is unlikely to charge a battery beyond 70% and, as the charge rate tapers throughout, it takes a long time to reach even that. Further, the float voltage is usually fixed and is far too high for gel cells and AGMs – and even normal lead acid batteries in hot places. It is less likely to overcharge, but I would not leave a battery permanently connected.

When comparing prices bear in mind that a three-stage charger brings a battery up to charge much faster than does a conventional charger. As a rough guide, a 7.5 amp three-stage charger will outperform almost all 15-amp conventional chargers and many a 20-amp cheapie. The apparently much higher prices of three-stage chargers are thus less than they appear. A three-stage charger is also capable of bringing a battery close to 100% charge. This is virtually impossible from the cheapies: to build them to do so would risk severe battery damage.

Price is a very good guide. Well-designed and well-made three-stage chargers cost from $350 (for a 5 amp unit). The hardware store ‘smart chargers’ cost $150 to $250 for 10-20 amp units. In terms of actual performance, a three-stage charger does not cost that much more!

Solar Regulators

Good quality (plus $275) solar regulators have a three-stage charging regime similar to that of three-stage battery chargers. It is in fact often possible to run a conventional charger through a solar regulator. This requires the charger to have a higher than usual voltage output – but some have.

Whilst the above is feasible there are possible complications. It’s best to stay clear of it unless you know about electrics. (For example, adding parallel capacitance will increase charger output voltage but this is a no-no if routing through a solar regulator unless that regulator can be switched from PWM to on/off operation).

Batteries and battery charging are covered in greater depth in ‘Motorhome Electrics’. This and Collyn’s other books are advertised elsewhere in this issue and are available directly from the CMCA Head Office, and also through the NZMCA Head Office in New Zealand.

This article, as with all of Collyn Rivers’ published writing, is protected by the Commonwealth Copyright Act. It may not be reproduced in any form without express written permission of the copyright holder – Caravan & Motorhome Books, Broome, Western Australia.

Collyn Rivers, W8054.