by Collyn Rivers
A rechargeable battery stores energy in electrical form, but none does this remotely efficiently. A conventional deep-cycle battery of 2012 stores about twice as much energy per kilogram as one of 1912. By way of comparison, an LPG gas cylinder stores 133 times more energy (per kilogram) than a typical lead-acid battery (which is why they go off like a bomb in a bush fire). Further, you can routinely empty a gas bottle, but do that to a battery and it will last only a few months. Reality is that a gas cylinder holds about 250 times more accessible energy!
Inefficiencies of Scale...
The least inefficient commercial battery, the lithium variety, stores about four times more energy than an AGM or gel cell battery in terms of volume and weight, but is approximately four times the price. The battery of most current interest to RV users is the deep-cycle lead-acid unit, of which AGMs and gel cells are variants.
A battery is charged by applying a pressure, which is vaguely like pumping up an RV’s tyres. In the battery’s case the ‘pressure’ is voltage, which needs to be greater than the voltage it already has. The greater the voltage difference, the faster the battery will charge.
A conventional 12 volt deep-cycle battery needs up to 14.7 volts to charge it quickly and deeply. Originally, this was done by connecting it across a car’s dynamo (and from the 1960s onward, the alternator). This worked reasonably well when their output was 14.4 volts, but decreasingly less so, after 1990, as alternator voltage was progressively decreased (currently to a typical 13.8 volts). By 2014 it may be reduced to as little as 12.7 volts.
The amount of energy a deep-cycle battery can contain and supply is shown in amp hours, e.g. a 100 amp hour battery can typically supply 5 amps for 20 hours. But, like pouring beer into a glass, the faster one pours, the less that’s usefully enters the glass. If you draw 50 amps, it might do that for only one hour, not two. Likewise, such batteries cannot be charged at a rate greater than 10-15% of their amp hour capacity.
Whilst users are usually scared of overcharging, this only happens if you buy a cheap hardware store charger and leave the battery across it for too long. Apart from that, it is almost as rare as a fish riding a unicycle to encounter overcharging in RVs (except by people who use solar modules, but not a solar regulator).
Discharge of the Lights Brigade
Few RV deep-cycle batteries achieve anything like their intended lifespan. Most are killed in their infancy by chronic over discharging: typically until the lights go dim and the beer warms up in the fridge. The more deeply a battery is discharged, the shorter its lifespan, but that relationship is far from linear. One can draw them down by 20% thousands of times, but draw them down by 80% and they are wrecked after 100 or so such cycles. Most battery makers suggest that 50% discharge is an economic level (they will withstand that some 500-1000 times).
In essence, one is buying a heavy box that holds a certain number of cycleable amp hours and you can choose to use slowly over time, or quickly over decreasingly less time. It’s mainly a monetary issue – not a Protestant ethic. AGM and gel cell batteries will withstand more frequent deep discharging, but they too will suffer if chronically over-discharged. Battery life is also shortened (particularly with conventional deep-cycle batteries) if they are not routinely fully charged between discharges.
Until 2000 or so, battery charging from an alternator was very much a compromise. It was much like pumping up that tyre from a pressure tank that had only slightly higher than the desired RV tyre pressure. The closer the tyre was to reaching the tank pressure, the slower the rate of increase.
This was substantially fixed, in upmarket chargers, by (in effect) constantly increasing the charging voltage as the battery’s voltage increased. This enabled the charging rate to be more or less constant and was very effective. Prior to its adoption, it might take 10 hours to charge a fully flat battery to 70% of full charge, and another 10 hours or more to approach 100% charge. In practice, few deep-cycle batteries were charged much beyond 70-75%. Those with a head for numbers will thus realise that (if the battery maker’s 50% advice was taken), that a battery could only supply about 25-30% of its amp hour rating. That’s why so many got flogged so often and many still are.
The constant current approach initially resulted in slower initial charging, but from that 50% onward it made a huge difference. That final 25-30% charge was achieved in only an hour or two. Later, such chargers included a very clever technique – called Multiple Power Point Tracking (MPPT) – that enabled optimum charging to be independent of the incoming voltage (from alternator and solar). The technique is usually known as dc-dc alternator charging and it is so effective that it is pointless to seek alternatives.
The choice of battery is very much a function of probable usage. None (except a lithium battery – of which more anon) will withstand constant flogging. For such use the best choice is the so-called ‘traction’ battery (typified by Trojan) and used in golf carts, small fork lift trucks, etc.
Cheap and Cheerful?
If you are marginally anal – to the point that you will truly top up the electrolyte every month or so and mostly draw down by about 30% – then the traditional ‘wet’ deep-cycle battery is by far the best and cheapest choice. These are used in big property solar systems and may last as long as 12-14 years. The sealed conventional deep-cycle is the best choice as long as you do not discharge too deeply – nor apply loads that exceed about 15% of their amp hour capacity. For example, a 100 amp hour battery should not be used for loads exceeding 15 amps – a microwave might draw 130 amps and will ruin that battery very quickly.
For general use, however, the AGM battery makes a lot of sense, as does the newly revived gel cell (now sold by Exide), both of which enable higher current to be drawn.
In theory, the lithium battery (as used in many power tools) has a lot going for it. It is light and compact, can be charged far faster than the others and (is claimed by vendors) to be almost totally discharged routinely without harm. Some resellers claim life spans far exceeding all other batteries, but not necessarily supported by the manufactures’ published specifications. Their main drawback is that vendors do not appear to agree about their charging (except perhaps for solar use) but none that are known of will supply one under warranty if it is to be charged other than by their own chargers, which effectively precludes alternator charging. My view is that, like the early days of LEDs, they are best left to the pioneers – whose most common characteristic was an arrow between the shoulder blades!
Essentially, batteries are a convenient but dreadfully inefficient way of storing energy. For most, well over 20% is lost in the charge/discharge cycle. The lithium battery is currently the most promising, but what is really needed is an increase in terms of energy storage/weight of at least ten times. In the medium-to-long term we might see a move to LPG fuel cells to supply an RV’s ‘domestic needs’. Truma’s Vega unit, which is now on sale in Europe, weighs just 7 kg and produces 5 amps at 12 volts continuously, but requires ultra-clean LPG, which might hinder local sales. But imagine that...
Engineer/author Collyn Rivers writes and publishes books, written in understandable English, on various aspects of RV and solar technology and usage. His website (and many of his articles) can be found at www.caravanandmotorhomebooks.com
For the full article download Issue 20 of iMotorhome eMagazine by clicking on the link below.