Lithium Nil1 Nil2 Nil3 Nil4 Batteriesincaravans

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Lithium ion batteries in caravans as well asfifth wheel caravans, camper trailers and motor homesare beginning to replace conventional lead acid, gel cell and AGM deep cycle batteries. Kimberly Kamper has used lithium batteries for caravans and camper trailers since 2012 (and reports it has not had a single failure). Bushtracker claims that 98% of all its buyers specify lithium batteries. The major appeal of lithium batteries in caravans is for those who free camp and/or travel extensively off-road, not least becauseit assist overweight RVs to be lighter.Lithium battery chemistry and working is very different from other batteries. They are almost a different species; for example all can be used as ‘deep cycle’. This article on lithium batteries in caravans, by Collyn Rivers, explains why and how to buy, install and use them.

Lithium battery types

There are various types of lithium battery.

Lithium cobalt oxide (LiCoO2) batteries store the most energy – a bonus for aircraft makers. InJanuary 2013 however, one of themstarted a fire in a fortunatelyunladen Boeing 787. A day or two later, United Airlines reported twomore such fires. All Boeing787s(some 50 plus) were grounded anddeliveries ceased.

These fires rightly made headline news. But what was not reported was that lithium cobalt oxide batteries were very different from the less energy efficient but far safer lithium batteries in caravans.

Lithium batteriesin caravans, camper trailers, motor homes and boats arelithium iron phosphate (LiFePO4). They areclaimed as beingnon-toxic and non-flammable. The graphiteused (for one electrode)can ignite but as graphite moulds are used for pouring cast metals the ignition temperature is way past 1000 degrees C. The industry’s claim that they are as safe as lead acid batteries seems totally realistic. Many feel they are safer.

LiFePO4 batteries areonly a third to a quarter the size and weight of lead acid batteries of similar ratedcapacity (a claimed 105 watt hours/kg).

Unlike a lead acid battery’s 12.8-11.4 volts, LiFePO4 output remains almost constant. It falls from 12.8 volts at about 95% charge toabout 12.7 volts at 20% charge. It then dropsaway steeply.

As lithium batteries are damaged by fully discharging – the discharge is automatically limited, typically to about 20% remaining.That almost constant voltage output virtually eliminates voltage-related fridge issues,andhalogen lights dimming as voltage falls. It is also of major benefit to those countless RV owners who free camp (‘boondocking’ in the USA) yet have units fitted with so-called converters. See accompanying article How to fix caravan converter problems.

Like syrup flowing through a pipe, conventional batteries resist charging current, and also of that current drawn. With conventional deep cycle batteries this resistance is so great that most can only provide a current of 5% of their rated capacity to meet their claimed rating (e.g. 5 amps for a 100 amp hour battery).

If the current draw exceeds that (typical) 5%) a conventional battery loses energy in the form of heat. An ‘800 watt’ microwave oven* for example draws about 1300 watts (110 amps at 12 volts). Twenty minutes use (36.6 amp hours) virtually flattens a 100 Ah lead acid battery. It depletes a similar 200 Ah battery by about 55 Ah and a 400 Ah such battery by about 45 amp hours.

This likewise limits charging – few conventional deep cycle lead acid batteries can be charged at more than 15% or so of the amp hour capacity. AGMs will accept 25%-30%.

Lithium batteries have far less internal resistance. That microwave draw above (f 110 amps at 12 volts) will barely affect even a 100 Ah LiFePO4. Likewise they may be charged at far higher currents – 50% of their amp hour capacity is typical, but 200% - 300% will not harm them.

Whilst this needs to be checked with the supplier, most lithium batteries will readily and safely supply starter battery level current and/or serve as a traditional deep cycle battery (but with less drawbacks).Their charging efficiency is also higher. Resellers claim 92.5-95%. That of deep cycle lead acid batteries is about 80% when new.

Lithium batteries do not vent gas unless charged/discharged at more than triple their amp hour capacity (i.e. 300 amps for a 100 Ah battery). Venting is likely to occur above that- but battery makers claim the emissions are neither toxic nor explosive. Ventilation is still advised to limit heat build up.

Battery Management

Unlike lead acid batteries, for several reasons each (nominally 3.2 volt) cell of a LiFePO4 battery must be individually monitored and automatically corrected: charging current ceases when any such cell is fully charged. As 12 volt have four series (end to end) connected cells, the first fully charged terminates charging of those remaining -thus limiting overall battery capacity. As this has to be at cell level, the management system can only realistically be located within the battery pack.

Apart from cell balancing, charging and discharging voltage and/or current levelstoo is needed. These may be included within that battery management system – or the battery charger. No matter how or where done, these latter are essential as well. As is knowing they are provided.

Charging

There are two main approaches: that recommended by LiFePO4 battery and battery charger makers, and that of DIY (do it yourself) users. Both regard individual cell management as vital and (most) that discharge must be limited to about 20% remaining charge. There is lessagreement about the final state of charge, but both make sense once the relative rationale is understood and accepted.

A 12 volt LiFePO4 will charge safely to 80-85% at an applied 13.8 or so volts. Many DIY users do just that. As discharge must be curtailed at about 20%, usable capacity is 60-65%. This is better than the 50% or so from lead acid batteries but, the industry (in effect) argues, why not charge deeper to utilise what you’ve paid for?

Some DIY users claim that will quickly wreck the battery, yet industry testing at voltages far beyond that show that still provides a handy 2000 or so such cycles of use.By that the industry implies that rated capacity is reduced to 80% of that when new. Some makers claim that longevity is extended by limiting charge and discharge levels.

Usable amp hours

A few sums show that one is in effect buying usable amp hours – you can use a lot of amps over shorter time, or less amps over longer times.

The most common industry approach/recommendation is to charge at substantially constant current to about 90% of full charge. If fuller charge is required that final part is at a tightly controlled fixed voltageof a typical 14.6 – 14.65 volts. Achieving thisrequires very accurate control: 95% charge is about 14.4 volts, 98% charge is about 14.8 volts. This is not hard to do given adequate measuring technology, but many DIY users underestimate how hard it can be to measure voltage (current is harder) accurately and consistently - let alone control it.

Establishing the totalamount of charge (rather than a voltage assumed related to it)is harder still. It can be done by charging and discharging at tightly controlled constant current over measured time (so-called coulomb counting) but the results may only be true for a specific battery. Those from other makers (or even production batches) will inevitably have minor variations.

It thus makes sense (to me at least) for most DIY users to limit charge ata measured 13.8-13.9 volts (which may well be 13.6-14.1 volts – about 80-85% of full charge). A LiFePO4 battery is truly safe at that. There is general agreement reautomatically terminating discharge at 20% remaining charge

Battery chargers

Most lithium battery makers advisethat, for charging from 230 volts, most basic two-stage chargers are all that is required. Theywarn against any that include automaticdesulphation- because the associated 16 plus volts is too high - but most chargers with that function have it optionally selectable. Also unsuitable (say LiFePO4 makers) are chargers with a final lower voltage stage as that may preclude the battery reaching full charge.

Alternator and/or solar charging

It is commonly claimed (for LiFePO4 charging),that ‘normal alternator charging’is fine. That cannot be - there has been no such thing as a ‘normal’ alternator since 2000. About then, alternator outputs began to vary, from 12.7 volts to plus 14.7 volts. Many have voltage that varies with load.

Companies such as Redarc and Sterling produce alternator chargers specifically for LiFePO4 (or with a LiFePO4 optional regime). They stress that their units must only be used with LiFePO4 batteries with management systems that include under/over voltage protection, cell balancing, and able to handle the charge current. Some also accept solar input.

As with dc-dc alternator chargers, the LiFePO4 charger should be located in the caravan etc, as close as feasible to the caravan battery bank. As these chargers are safely able to pump 40 amps or more into a LiFePO4 battery bank, it will help immensely to replace the existing cable from the alternator to the rear of the tow vehicle by one of approximately 13.5 square mm, to take it to the caravan via an Anderson plug and socket – and then 13.5 square mm to the charger and then to the battery.

Unless that is done, the charging current will be unnecessarily restricted.

Battery temperature

Whilst this varies from maker to maker, LiFePO4 batteries have a preferred working range of -18 degrees C to about + 40 degrees C. Their available capacity, charging and draw rate is reduced to about 60% at the lowest temperature. Several (US and Canadian) makers advise that they will perform at lower temperatures than quoted above. They say that whilst they current may be very limited, applying a smallish load (such as the headlights) for a minute or two will warm them rapidly up a bit prior to charging or discharging heavily.

Battery life

Over time, the battery industry has developed a more or less de facto standard of defining the products’ useful life: this is the number of cycle of charge/discharge that causes capacity to be reduced to 80% of that claimed when new.

With lead acid deep cycle batteries, that is very much a function of the depth of discharge (but is also shortened by chronic undercharging and discharging at loads higher than about 5% of the amp hour capacity (e.g. 5 amps for a 100 Ah hour battery).

If discharged to 50% remaining at that 5% rate, the lifespan is a probable 1200 cycles, but only 200 or so if discharged to 30%.

A LiFePO4 is far less affected by the rate of discharge. Claims vary but that for most makers is about 2000 cycles if discharged to 20% remaining (virtually regardless of load). Some makers suggest that this can be extended by limiting the maximum charge to 90%.

Storage

This area is contentious. Possibly because lithium batteries are generally limited to 50% charge whilst in transport, it is often claimed that such a level of charge is also best for storage. On the other hand the University of Texas (that discovered phosphate as cathode material for rechargeable lithium batteries) is widely quoted as stating that storing a fully charged battery has minimal impact on its life span.

Lithium pricing

Currently, no lithium batteries are manufactured in Australia. All are thus imported and may have several levels of distribution – each adding overhead and profit. Some are claimed to be rebadged versions that can be bought otherwise cheaper. Lithium batteries are expected to fall in price as they become increasingly accepted. It is not an area for eBay specials unless one is certain of what one is doing.

A sense of proportion

Whilst lithium technology isa major advance in terms of weight and volume, the energy stored in a LiFePo4 battery - 120-165 Wh/kg (watt hours per kg) is trivial compared to that in fossil fuels. Most such are around 12,000 Wh/kg (or 2400-4800 Wh/kg allowing for the 20-40% efficiency of that which they fuel).

Battery reality is that the energy storable (for size and weight) increased less than twice between the mid 1800s and the advent of lithium (in the 1990s) increased that to five-six times that originally. That really needed is at least a further 10 times and ideally 100 times. Lithium technology is thus part of the answer, but far from the whole of it.

Updates

As this area is changing rapidly this article will be updated every three months, or sooner if deemed warranted.

Copyright ©(2015) Caravan and Motorhome Books, Church Point, NSW 2105.

Collyn Rivers' main books are Caravan & Motorhome Electrics, Solar That Really Works and Solar Success. The use of lithium batteries in home and property solar systems is covered in our associated website: successfulsolarbooks.com

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