Major update June 2016
Lithium-ion batteries for solar homes assist owners to free themselves from, or reduce the need for grid power. Here’s how to know if they are needed.
Responses to earlier versions of this article showed that many readers were being unintentionally misled by vendor claims to the effect that lithium-ion batteries generate far more power than conventional lead acid and other batteries. Such claims are correct. Lithium-ion batteries can produce far more power than most other forms of battery. But what was not understood (unless technically-minded) is that power and energy only seem to be the same. They are very different concepts.
Energy is the ability to perform work. Power is the rate at which energy is used. An excellent example is of stacking cans on supermarket shelves. Virtually anyone, no matter how slight, can over time, lift a hundred 1 kg cans from floor level to a shelf two metres high. Doing so requires a known amount of energy but very little power. A weightlifter, however, who heaves that same 100 kg through two metres in a second or two uses the same amount of energy. The power required however is hugely more. (Power was initially defined by the ability – of a brewery horse! – to lift weight over time.)
Another example is a car’s starter motor. It draws massive power – about 4.8 kW – but only for a few seconds. The energy used is only about that of a 5-watt LED over an hour – and depletes the starter battery by only 2% or so. The alternator replaces it in about two minutes.
Massive power – but the energy expended is much as that of a supermarket shelf packer over an hour or so. Olympic weightlifter Svetlanas Podobedova. Pic: courtesy Wikipedia
Deep cycle lead acid, gel cell and AGM batteries are by and large the shelf stackers – if enough of them (or bigger ones) they also have more available power. For most typical stand-alone systems above 3.0 kW or so, an associated (and correctly scaled) lead acid deep cycle battery bank will provide enough power for all typical domestic loads. A gel cell battery will do more so, and an AGM battery yet more so. That battery bank however cannot cope with multiple arc welders used for any length of time, nor start large air compressors etc. Both require a great deal of power (as well as energy). Here, lithium-ion is a better choice.
Lithium-ion batteries (of the same capacity) are the weight lifters of the battery world. If of identical capacity they the same amount of energy (but more of it can be used without them prematurely wearing out) but can, if needed, release far more of that stored energy for a short time. If that ability is not needed then lithium batteries’ unquestioned ability to release high power for a short time is of no benefit.
Lithium-ion batteries have other benefits that, depending on usage, may or may not be of value. They are about one third of the volume and weight of conventional lead acid and similar batteries. This saves transport costs, and eases installation in applications where space is limited. The makers claim they can be routinely more deeply discharged, but as the LiFePO4 version in general use has only been around in quantity since 2012 or so, such claims may well be true, but as also for longevity, are as yet (mid 2016) unproven.
Lithium battery types
There are various types of lithium batteries. Those of lithium cobalt oxide (LiCoO2) batteries store the most energy but fires in two Boeing 787s (in early 2013) resulted in grounding all fifty 787s then in service. Production of the aircraft ceased until the issue was resolved.
The lithium-ion batteries used for RVs and solar homes (LiFePO4) use a very different technology. They are non-toxic and, their makers claim, effectively non-flammable unless exposed to temperatures above 1000 degrees C (well above that of molten steel). They store about 105-120 watt hours/kilogram.
Typical lithium-ion LiFePO4 (12 volt) battery. (Large capacity such batteries are rare). Pic: SmartBattery.com
Lead acid battery characteristics
Lead acid batteries have slow internal reactions that resist charging current, and current being drawn. Unless of large capacity, this limits their ability to routinely provide heavy current (without shortening their lives) for only brief times. As a general rule the peak draw should be limited to 10% or so of their capacity. This can be a limitation in the smaller RVs – but less so (or not all) with home and property systems.
This huge traditional battery bank (at Yarrie Homestead in the Australian outback) uses twenty four 2-volt lead acid cells. Pic: courtesy Peter Wright.
Routine deep discharge also reduces their lifespan. A good 100 amp hour deep cycle lead acid battery discharged daily and routinely to 50% at 5.0 amps is likely to last for five years. If routinely discharged only by 30% it may do so for 10 years or more. This likewise limits charging. Most battery makers define product lifespan as the number of cycles of a (not necessarily specified percentage of charge/discharge) before capacity is reduced to 80% of that claimed when new.
The voltage available from lead acid batteries is far from constant. Voltage drops as current draw increases. Battery voltage falls with remaining charge. A well-rested such battery is typically 12.7-12.8 volts when fully charged and about 12.3-12.4 volts at 50% charge. This is only an issue if deeply discharged – and doing that is not recommended for home and property systems.
Lithium-ion battery characteristics
Lithium-ion batteries in solar homes (etc) will typically fall from 13.1 volts at about 95% charge to 12.85 volts at 10-20% remaining charge. The voltage then drops steeply. This almost constant output virtually eliminates voltage-related issues. They can also provide massive currents with only minor drop in voltage. Such close to constant voltage precludes meaningful measure of remaining charge – until only 10%-20% remains – when voltage drops rapidly.
Low internal resistance enables lithium-ion batteries for solar homes to charge at virtually any rate that solar input allows. Only 5%-10% of the energy is lost in doing so. Whilst not probable in a home solar application, it is possible to charge a lithium-ion battery without harm at many times its amp hour capacity. Virtually any amount of solar capacity can be used. During testing a 1000 amp hour lithium-ion battery bank is typically charged and discharged at 500 amps – but could be at 3000 amps if required. This is massively more than in any typical home or property use.
A LiFePO4’s lifespan was initially claimed (by some) to be not affected by the depth of discharge. This claim is no longer generally made. Claims vary but that made by most makers is about 2000 cycles if discharged by 80%. Sonnenbatterie claims 5000 cycles of use (approximately seven years if discharged by 70% once a day). The rate of discharge too is less important. It can be disregarded for lithium-ion batteries for solar homes use.
Lithium-ion batteries for solar homes (sizing)
Optimum battery size is very much related to whether one seeks to be totally solar-dependent. It is feasible but less so if extensive air conditioning is required. Those that do tend use some generator back-up. It is more feasible for battery-backed solar grid-connect systems.
A typical 4-10 kW lithium-ion battery bank for solar homes is about 65 cm wide, 130 cm high and 50 cm deep (small filing cabinet size). A 10 kW version weighs about 250 kg.
This graph shows the typical (per cell) voltage during discharge. That discharge voltage most probable for a home is much as that blue line. Ignore the red line – it applies to massive rates of discharge that will never be even approached in solar home usage.
A LiFePO4 lithium-ion battery consists of inter-connected cells – each of a nominal 3.2 volts. A 12 volt battery thus has four such series (end to end) connected cells. The 48 volt versions, more commonly used as lithium-ion batteries in solar homes, have sixteen such cells. For ease of installation lithium-ion batteries in solar homes (and RVs) often have multiple interconnected batteries – each of smaller capacity.
Battery management systems for lithium-ion
Unlike lead acid batteries, each cell of a LiFePO4 battery must be individually monitored and automatically corrected to ensure all are at much the same state of charge. A LifePO4 battery can be wrecked if this is not correctly done. Such correction has to be at individual cell level so a management system that does this is essential. Some LiFePO4 batteries have this function located within the battery. Bigger systems may have it internally, or built-into a separate unit with a multi-core cable connected to the battery.
Apart from cell balancing, control of charging and discharging voltage and/or current levels is equally required. This too may be included within that battery management system. If it is not it must be provided by the battery charger. Such management is essential (as is knowing for certain it is provided).
Many users of lithium-ion batteries for solar homes (and also in the RV area) currently use and defend an approach substantially different from that recommended commercially. This relates mostly to cell charging voltage. Many DIY users limit charging to 3.4 volts/cell (13.6 volts for a 12 volt battery). This is about 80% charge. Many argue strongly that exceeding that is risky. Battery makers and (now) alternator-charger makers however disagree. Most advise charging to 3.6 or 3.65 volts a cell.
No existing battery likes working at extreme temperatures. Lithium-ion (LiFePO4) is no exception. Their preferred working range is -18 degrees C to about + 40 degrees C. They work best at about 25 degrees C. USA and Canadian vendors advise that lithium-ion batteries in solar homes will work at lower temperatures but that charge current must be initially limited – applying a small load for a minute or two will warm the battery sufficiently to remedy this. As a 10 kW/h version is the size of an office filing cabinet it should be readily feasible to locate them where it is above freezing.
In many countries (including Australia) all LiFePO4 batteries and associated systems are imported. Some are sold via several levels of distribution, each adding overhead and profit. Lithium-ion batteries were initially expected to fall in price as they became increasingly accepted but the global demand for lithium has escalated and there is only a limited amount of lithium base material available. Some experts have stated that, because of that limitation, lithium-ion is an interim technology.
Lithium-ion in grid connect systems
Currently, most vendors and installers supply lithium-ion based system that supply the whole home or property via the existing cabling.
A good DIY approach (and my own currently) is to time-shift the solar input so that excess day-time input is stored for a few hours and then drawn on at night – but retaining grid-connect for short high current loads (e.g. electric stoves, vacuum cleaners, big power tools etc). The concept is to use individual systems (each of a time-controlled battery charger, LifePO4 battery and stand-alone inverter) to feed suitably grouped loads – such as computer/ entertainment-related, refrigeration-related, garden lighting etc. Lights and appliances in each group are connected via power boards, that plug directly into the stand-alone inverters, and using double pole change over switches that totally isolate the solar network from the home’s grid wiring.
As the existing fixed wiring is not changed, this can be done legally in most countries by non-licensed electricians. Doing it this way requires only sufficient battery capacity for about 14 hours use and equating to about 50% discharge. (As the LiFePO4 charging debate is not yet over, it may be prudent for DIY installers to take that 80% approach, increasing it later if then deemed safe).
A sense of proportion
Battery energy storable (for size and weight) barely increased between 1870 and the advent of lithium in the 1990s. The latter resulted in a 3-5 times increase. It is now 120-165 Wh/kg for LiFePO4. There initially seemed to be scope for major development but experts at major (IDC) lithium battery conference in Sydney (May 2016) stated that any appreciable increase in output is now seems likely.
Please see also my just published Knowing the best batteries for my stand alone solar. (Please select from Articles).
Copyright © (2016) Successful Solar Books, PO Box 356, Church Point, NSW 2105 – Australia.
Collyn Rivers’ main books are Solar Success (for home and property systems), and Solar That Really Works! (for boats, camper trailers, caravans and motor homes). These books provide all you will need to know to specify, design and self install stand-alone solar systems. Solar Success also has a major section that shows how to slash existing electricity usage by 30%-50% If you do this before sizing a solar system you will save huge times the cost of that book.
Caravan & Motorhome Electrics covers every aspect of the electrics of small cabins, boats,camper trailers, caravans and motor homes. It is also used by auto-electricians in Australia and New Zealand (and is applicable worldwide).