RV Battery Basics
RV batteries store the energy from your solar modules, the tow vehicle or RV’s alternator, or from 230 volts via a battery charger. There are a number of different types of battery:
- Flooded: now used mainly in large property systems, these need routinely topping up with distilled water. If well maintained they may still have the longest lifespan.
- Valve regulated lead acid: misleadingly also called ‘sealed’. These are similar to flooded batteries but sealed in normal use. They have vents that release gas (hydrogen) in the event of a major fault causing excess pressure. They thus need a ventilated enclosure. As with all lead acid batteries, they dislike deep discharging. These vary in price but are generally the cheapest.
- Gel batteries: these have the electrolyte (a conducting solution) in wax-like form. They can be discharged slightly more deeply than those above, and cost more than valve regulated batteries.
- AGM batteries: designed initially for military use in the Arctic, their electrolyte is within a glass fibre matrix. They are sealed, rugged and maintenance free, but heavy. They may be charged relatively faster than other lead acid batteries, but exceeding 50% discharge shortens their life. They are significantly more costly.
- Crystal: these are a newish form of lead aid battery (so far made by only one company). Their main claimed benefit is a yet to be proven life span of 18 years.
All the above need up to 14.4 volts to charge – but deliver only 12.8 -11.4 volts.
- Lithium-ion: made with various chemistries, those used in RVs are LiFePO4 (their chemical make-up – not a trade name). They are about one-third the size and weight of conventional batteries with similar nominal capacity. Whilst 70%-80% of their capacity is available – using that reduces their life: a routine 65%-70% is preferable.
LiFePO4’s ability to accept and deliver huge currents (as well as small) thus combines the benefits of starter and deep cycle batteries. They need specialised charging and individual cell monitoring, and currently cost far more than other battery types. Offsetting this is that, because their output (in RV use) stays at 13-12.9 volts over most of its range and can be routinely discharged more deeply, a 100 amp hour LiFePO4 battery is comparable to a 150 amp hour non-lithium-iron battery. They are however mostly still a specialist product and need specialised knowledge to install. A few companies now claim to offer them as drop-in replacement products.
As with anything heavy, a camper trailer, conventional caravan or fifth wheel caravan’s house battery, or battery bank, should be located in the front part of the caravan, but not on the A-frame. Having a short distance between house battery and tow vehicle alternator also reduces alternator charging power loss issues.
It is advisable, and often now essential, to use a specialised alternator charger to reduce the effect of voltage drop and to ensure the optimum charging regime is maintained; that unit too should be located as close to the batteries as possible, but not in the same compartment.
Whilst some RV makers ignore this, most battery makers insist that battery ventilation is still vital by having a vent at the extreme top and bottom of the battery compartment. There are no RV industry standards regarding this. General practice, however, is to provide a few 25-50 mm holes at the top and likewise at the very bottom. This is usually done via holes about 50 mm in diameter – or by using stainless steel vents available from boat chandlers.
If external, the compartment door should be of a light colour, and heat insulated. This compartment too must be ventilated as above.
Campervans and motor homes need the battery bank either to be close to the alternator, but well away from exhaust system heat. Batteries should be connected to the alternator via at least 10 square mm cable (ideally heavier). It is advisable, and with many post 2014 alternators essential, to use a dc-dc alternator charger. With almost all post-2016 alternators a dc-dc alternator charger is essential and many need a specialised low-voltage version.
Battery makers specify capacity in amp hours (energy used over time). As most RV batteries are 12 volt (and watts equal amps times volts) to convert amp hours to watt hours simply multiply by 12 (or for 24 volt systems by 24).
Batteries can be seen as like (fee-charged) bank accounts. They hold that paid in, less that drawn out. They also lose a bit internally – from 15%-20% for lead acids to 1%-2% for LiFePO4.
As with bank accounts, you cannot store or use more than you pay in, and you incur overhead fees if you have an additional account. Adding another battery is thus like adding a further account. It cannot possibly contain hold more than paid, and it will lose more of that stored because of the overhead in storing it.
Auto electricians say that most RVs owners who run out of 12 volt power ask for an extra battery. Doing so only works if there is enough excess energy to charge it.
A good indication is to limit battery capacity (or increase charge ability) to that which you can fully charge most days year around by noon. This typically requires at least 250 watts of solar per 100 amp hours of battery capacity. There is no upper limit. All alternator and solar regulators control voltage, and batteries self limit their charge. Any LiFePO4 battery over 100 amp hours will happily accept far more than you can ever supply.
If space and weight permits, have as much solar capacity as possible, together with batteries that suit your pocket, available weight carrying ability and your needs. Do not mix battery types nor add new batteries to other than close to new ones.
A few RV owners use a generator to run a CPAP (sleep apnoea) machine at night. It is better and far cheaper to use a generator or solar charge a battery during the day and run it from that.
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