Selecting House Batteries for Your Motorhome
Billions of dollars are now being spent annually on alternative energy research. Some of us have been living almost entirely on renewable energy for years. However, all this research is making our lives a little easier. New and improved products are emerging thick and fast, fuelled by the now much larger market created by the green wave.
Battery technology is one of the areas that is receiving a lot of attention. In reality, the lead-acid battery has not changed much in the last 100 years. Sulphuric acid in contact with lead plates has been starting our cars since we put away our crank handles in the 1920’s.
In recent years we have seen the emergence of a number of new battery technologies. The vast majority of the focus has been on the energy-storage-to-weight-ratio issues. Clearly lead-acid is not going to win this race. Electric cars, notebook computers and even the latest high powered battery tools are all driven by lithium-ion batteries. Currently there are no readily available lithium-ion battery systems available for motorhomes, and even if there were, an entirely new charging system would be required to correctly charge them. For now we must be satisfied with lead based batteries for our energy storage needs.
This does not leave us without choice. There are a number of recent developments in lead based battery technology. Absorbent Glass Mat batteries have been slowly improving in the last few years and sales of the so called Gel battery have now exceeded the numbers of flooded lead-acid batteries sold for use in motorhomes.
We recently replaced the 6 year old, flooded (Trojan brand) battery bank in Hobohome and I was amazed at the wide selection of replacements on offer.
I finally decided on a pair of 12 volt 230 amp hour (ah) gel batteries. These batteries are larger and heavier than the Trojans, they cost over $500 more and they are rated only 5ah above the batteries removed – so why did I select them?
Before I answer that, let’s first discuss battery capacity. Battery capacity is measured in amp hours. One amp supplied for one hour will deplete a battery by one amp hour. One hundred amps supplied for two hours will deplete a battery by 200 ah.
From this, you would expect a 225ah battery to happily deliver 225 ah of power every day – right? Wrong! Flooded batteries (like a car battery) absolutely hate being deeply discharged. Deep cycle batteries have been designed to better withstand deep discharges, but it is true to say that if you want your flooded batteries to live beyond a few cycles, you had better not ask them to deliver much more than about 50% of their rated capacity. Suddenly our 225 ah batteries are only really able to provide us with 112.5 ah of usable energy. The life of a battery can now be calculated and is a factor of the number and depth of discharge cycles. The deeper the discharge, the fewer cycles a battery will be able to deliver (for a motorhomer a cycle represents a day of charging and a night of discharging. So for full-timers like us, 5 years is 1825 cycles).
Hobohome’s New Batteries
Back to our new gel batteries – why did I select them? Simply put, they are able to supply far more usable energy by withstanding deeper discharges. It is not unreasonable to ask the gel batteries to provide over 70% of their rated storage capacity. That is over 160ah. If the average daily discharge was this deep (and ours is closer to 30%) the batteries expected life would be about 1300 cycles or 3.5 years. At our normal 30% discharge, the batteries should still be delivering power to Hobohome in 8 years. So the long life combined with the gel batteries ability to deliver more power on the days when the sun won’t shine, made them the best choice for our energy storage needs.
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