Lithium-ion Batteries in Caravans and Motorhomes - updated December 2018
Lithium-ion batteries in caravans and motorhomes deliver a lot of energy and pack a lot of power but need specialised knowledge to be used safely and reliably. Here's how and why for the LiFePO4 variety used in homes and RVs.
Energy and Power are Not the Same
Whilst lithium-ion batteries have many virtues but their promotion - relating to 'power' can mislead.
That needed from a battery is both energy and power - but for typical RV use its mainly energy that's required.
Energy enables work to be done. Power relates to how fast that energy is used. A marathon runner mostly needs energy. A competition weightlifter vitally mostly needs power. A rower usually needs both.
The energy needed to start a big 4WD is tiny - about that drawn by a 5 watt LED in an hour - but when starting an engine, that energy is used in a second or two. As with a weightlifter, it demands a lot of power.
Most RV ‘domestic’ loads (apart from a microwave oven and a power winch) need only small amounts of power. They are more marathon runner-like. The amount of energy required is usually more important as that determines battery size (and solar capacity if solar is used).
LiFePO4 batteries are widely (and correctly) promoted as combining both high energy and power, but their massive potential power has no value unless truly needed. If weight is no issue, a 300 amp hour AGM battery bank is more than adequate for any RV load. And far cheaper.
For RV use, the real benefit of LiFePO4 batteries is that they are only a third or so the size and weight of conventional lead acid deep cycle batteries and the even heavier AGMs. All supply that massive high peak power yet double as deep cycle batteries.
Lithium-ion Batteries: Almost Constant Voltage
Whilst less promoted (and the benefit less understood) unlike almost all other rechargeable batteries, LiFePO4 voltage remains almost constant regardless of the state of charge - typically 13.1-12.9 volts in RV use. They may also be routinely discharged (to about 20% remaining). This results in a 100 amp hour LiFePO4 having the usable capacity of 130 -140 amp hour of most else - plus an almost constant voltage. Your RV's lights will not flicker every time the fridge cycles on and off.
Even a small (18 amp hour) LiFePo4 battery can briefly release many hundreds of amps. This is adequate for starting even a big 4WD's diesel engine - and will run your microwave oven for a few minutes. Do not, however, be fooled by those sold as '18,000 milliamp/hour - that's 18 amp hour in marketing-speke!
LiFePO4 batteries are generally rugged but may suffer damage or are ruined if totally discharged. To preclude this they are protected by an associated battery management system.
Lithium-ion batteries: Cell Monitoring is Essential
Charging voltage too is critical: the upper safety limit of a LiFePo4 cell is about 4.2 volts. Another risk is that if an unbalanced cell is totally discharged, remaining cells may reverse that cell's polarity. Subsequently attempting to recharge (warns EV Australia) 'carries a significant risk of catastrophic failure'. In practice, the obligatory cell management system safeguards against all this.
Control of charging and discharging voltage and current is also essential. This may be done by the battery management system or by the battery charger. LiFePO4 cell management is essential - but not necessarily supplied.
Lithium-ion Batteries: Charging
Some LiFePO4 users have strong views about charging. All rightly regard cell management as vital. Most rightly agree that discharge be limited to 10%-20% remaining. Not all, however, agree about their being fully charged: or more specifically - how close to 100% charging is safe. Most agree a 12 volt LiFePO4 is safe to charge to 80%-85%, at a constant 13.6 volts. Many DIY users do that. If then discharged to 10%, usable capacity is 70%-75%.
The LiFePO4 industry, however, generally maintains that virtually 100% charging is safe. Most commercial LiFePO4 chargers do just that.
Regarding charging efficiency, LiFePO4 is way ahead. Vendors claims of 95% are probably true - that of lead-acid batteries is about 80%.
Lithium-ion Batteries: Usable Lifespan
The battery industry defines its products’ lifespan as cycles that can be drawn before capacity falls to 80% of that when new. This lifespan closely relates to the ongoing depth of discharge of conventional batteries. Many users and vendors, however, claim that LiFePO4 lifespan (in RV usage) is barely affected by discharge rate or depth. They usually claim about 2000 cycles if discharged to 20% remaining (almost regardless of load). They suggest life is extended by limiting the charge to 90%.
Lithium-ion Batteries: Battery Chargers
Some LiFePO4 makers advise that, given battery monitoring/charge control, all required is a two-stage charger. Not all agree. RV Books’ opinion is that LiFePO4 chargers are safer.
Lithium-ion Batteries: Alternator and/or Solar Charging
Some LiFePO4 users claim ‘normal’ alternator charging is fine, but that cannot be: there has been no such thing as a ‘normal' alternator since 2000. From thereon alternator outputs began to vary, from 12.7 volts to plus 14.7 volts. Many have voltage that varies with load and/or temperature. Some now have voltage that varies from plus 15 to 12.3 volts. Or even none at all at times. Only a very few LiFePO4 batteries are promoted as ‘drop-in’ replacements.
Redarc and Sterling produce alternator chargers specifically for LiFePO4, or with a LiFePO4 option. Such chargers include under/over voltage protection and cell balancing. Some also accept solar input. Redarc stresses that their LiFePo4 chargers should only be used with batteries that they approve.
Lithium-ion Batteries: Buying LiFePO4
Currently, all lithium-ion batteries are imports. Some have several levels of distribution – each adding a profit margin. Prices for identical batteries, but of different brand names, vary hugely. Their prices may fall but a major decrease is less probable as lithium sources are limited.
The essential battery management system is not necessarily supplied. Unless totally sure of what you are doing, buy only LiFePO4 batteries with that management system inbuilt. Advise the vendor of the intended usage. Obtain written assurance they are suitable for that usage.
Should I Use Lithium-ion Batteries in Caravans and Motorhomes?
When buyers can obtain truly direct drop-in replacements, LiFePO4 batteries in RVs make every sense. In the meantime, RV Books advises caution. Buy only from a truly reputable source.
Installing LiFePO4 Batteries
Apart from advising that you should locate the LiFePO4 charger close to the battery bank (not the alternator), the installation of alternator chargers is far too complex to explain in article form. It is however fully covered in our associated Caravan & Motorhome Electrics.
This area constantly changes - this article is thus routinely updated.
Our companion website solarbooks.com has many articles on caravan, motorhome as well as home and property solar systems.