RV Battery Basics
RV Battery Basics - RV battery types
RV batteries store energy from your solar modules, the tow vehicle or RV’s alternator or from grid or generator power via a battery charger. There are various types of battery:
- Flooded: confined mainly to large property systems, flooded batteries need routinely topping up with distilled water. If well maintained they have the longest lifespan but are not suitable for RVs (they emit corrosive gasses).
- 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 - necessitating a well ventilated enclosure. As with all lead acid batteries, they dislike deep discharging. These batteries vary in price but are generally the cheapest. Ideal for a quick 'around Australia but bulky and heavy.
- Gel batteries: these have the electrolyte (conducting solution) in wax-like form. Gel batteries can be discharged slightly more deeply than those above, and cost more than valve regulated batteries. They have a loyal following but (in RV Books opinion) AGM batteries have similar benefits but are more rugged.
- AGM batteries: designed initially for military use, their electrolyte is held within a glass-fibre matrix. AGMs are sealed, rugged and maintenance free, but heavy (12 volt ones are about 33 kg per 100 amp hours. They charge faster than other lead acid batteries, but routinely discharging below 50% discharge shortens their life. They are damaged if subject to over 40 degrees C. They are significantly more costly than basic lead acid batteries - but a good buy if travelling extensively off-road..
- 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). LiFePO4 batteries are about one-third the size and weight of conventional batteries with similar nominal capacity. Whilst 70%-80% of their capacity is readily available – using that reduces their life: a routine 65% is preferable. This is nevertheless close to 50% more than any other commercially available battery. They can also deliver and charge at hugely high rates (a 100 amp-hour LiFePO4 can do both at over 300 amps).
Apart from the (typical 18 amp hour) LiFePO4 'jump starters', that ability to accept and deliver huge currents has no value for RV use. Where size and weight is not an issue (as in coach conversions, a 350 Ah and above AGM battery bank will do the same job at a third of the price. Whilst costly, a LiFePO4 is well worth considering for smaller and lighter RVs.
LiFePO4 batteries need specialised charging and individual cell monitoring. They currently cost about three times more than other battery types. Offsetting this, 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 thus comparable to a 150 amp hour non-lithium-iron battery. They are however mostly still a specialist product and need specialised knowledge to install.
Most need a specialised charger, or one that specifically includes a LiFePO4 program.
As with anything heavy, a camper trailer or conventional caravan (for stability reasons), the battery, or battery bank, should be located as close to (and ideally just in front of) the axle/s. It should never at its rear and preferably on the A-frame.
It is advisable, and often now essential, to use a specialised alternator charger to reduce the effect of voltage drop. To ensure the optimum charging regime is maintained; that unit must be located as close to the batteries as possible, but preferably 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 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 - or by using the 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 be as close to the alternator as feasible, but well away from exhaust heat. Batteries must be connected to the alternator via at least 10 square mm cable (ideally heavier). It is advisable, and with post 2014 variable voltage alternators essential, to use a specialised dc-dc alternator charger made for this purpose. 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 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, without increasing the energy generated, is thus like adding a further bank account for the same total income. It will lose more of that stored because of the now increased loses of storing it.
The above is widely misunderstood. Auto electricians say that RVs owners who run out of 12 volt power almost always seek an extra battery. But doing so only works if there is 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 sunny 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 safe limit to charging capacity. Alternator and solar regulators control charging voltage and, within reason, batteries self- limit that charge. Any LiFePO4 battery over 10 amp hours will happily accept higher charge current than you are ever likely to be able to 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 almost new ones of identical capacity and type.
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 truly reliable battery during the day and run it from that.
This topic is covered in depth in our constantly updated Caravan & Motorhome Electrics. The book is buyable (and downloadable) in digital form from our Bookshop - or in a print version (via email) from booktopia.com.au - all Jaycar stores and almost all bookshops throughout Australia.
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