Major update September 2018
Lithium-ion batteries for solar homes supply huge power but that more relevant is their cost of storing and supplying electrical energy. Here’s why.
Energy and power
Energy is the ability to perform work. Power is the rate at which energy is used. Consider stacking cans on shelves. Virtually anyone can over time, lift a hundred 1 kg cans from floor level to a shelf two metres high. Doing so requires a knowable amount of energy but very little power. A weightlifter heaving that same 100 kg through two metres in a second or so uses the same amount of energy. The power required, however, is hugely more.
A car’s starter motor draws massive power (about 4.8 kW) – but only for a few seconds. The energy used is about that of a 5-watt LED globe over an hour. It depletes the starter battery by 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. Pic: (of Olympic weightlifter Svetlana Podobedova) courtesy Wikipedia
Stackers and lifters
Deep cycle lead acid, gel cell and AGM batteries are shelf stackers. If enough of them (or big) they have more available power. For a typical stand-alone system of 4.5 – 7.5 kW or so, a lead-acid deep-cycle battery bank provides enough power for all typical domestic loads. Gel cell and AGM battery will cope with major power tools as well.
Truly large such batteries can cope with arc welders and start large air compressors but, unless such loads are routine, a generator is a far cheaper approach.
Lithium-ion batteries are the (costly) weightlifters of the battery world. They can, if needed, release huge amounts of their stored energy. But unless that ability is needed that they can do that is of no benefit.
Lithium-ion batteries for solar homes – size and weight
Lithium-ion batteries about one-third the volume and weight of most other rechargable batteries, a benefit where weight and space must be considered. Their makers claim they can be more deeply discharged. The LiFePO4 version was not in general use until 2012, so their longevity is currently unproven. There are indications that they will last at least 10 years.
Lithium-ion batteries for solar homes – lithium battery types
Lithium cobalt oxide (LiCoO2) batteries store the most energy, but fires in two aircraft (in early 2013) resulted in grounding all then in service and production of the aircraft ceased until the issue was resolved.
The lithium-ion batteries used for RVs and solar homes (LiFePO4), however, use a very different technology. Their makers claim they are effectively non-flammable unless exposed to temperatures above 10000 C (well above that of molten steel). In addition, they are non-toxic.
Typical lithium-ion LiFePO4 (12 volt) battery. (Larger capacity such batteries are rare). Pic: SmartBattery.com
Lead acid type batteries have slow internal reactions. This resists charging, and of current drawn (without shortening their lives). Peak draw is best limited to 10% of their capacity. This can be an issue in small RVs – but less so (or not all) with home and property systems.
This huge traditional battery bank (at Yarrie Homestead ) uses 24 two-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%, as a result, it may do so for 10 years or more. This likewise limits charging. Most battery makers regard a battery as worn out when its capacity is reduced to 80% of that when new.
The voltage available from lead acid batteries falls as current draw increases and also with remaining charge. A well-rested such battery is typically 12.7-12.8 volts when fully charged and about 12.2 volts at 50% charge. Discharging routinely below 50% is not recommended for RVs and preferably not below 80% (remaining) for home and property systems.
Lithium-ion batteries for solar homes – characteristics
Lithium-ion batteries in solar homes (etc) 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. These batteries 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. Then voltage drops rapidly.
Lithium-ion batteries charge safely at almost any rate that solar input allows. During testing a 1000 amp hour lithium-ion battery bank is typically charged and discharged at 500 amps. It could, however, be at 3000 amps if required – hugely more than in any typical home or property solar use.
A LiFePO4’s lifespan is now known to be affected by its routine depth of discharge. Most makers claim 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 is less important: it can be disregarded for such batteries in solar home use.
Lithium-ion batteries for solar homes (sizing)
Optimum battery size is very much related to the extent one seeks to be solar-dependent and the amount of solar input available. This will vary from area to area, but the general concensus is that the financial optimum is 90%-95% of the year. A great deal depends on whether air conditioning (for cooling or heating) is required at night. Almost all solar homes and properties thus use some generator back-up.
It is for applications like the above where lithium-ion batteries excel – in that (given enough solar capacity to recharge them the following day) they can be discharged to as little as 20% remaining without unduly shortening their life.
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 rates of discharge that will never be even approached in solar home usage.
A LiFePO4 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. For ease of installation the 48-volt lithium-ion batteries typically used in solar homes are of four interconnected 12 volt batteries.
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. Unless this is done correctly a LifePO4 battery can be wrecked Such correction has to be at individual cell level. A management system that does this is essential. Some LiFePO4 batteries have this function located within the battery. Bigger systems may have it internally. Alternatively, it can be within a separate unit 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. As such management is so essential know for certain it is provided.
Some users of lithium-ion batteries for solar homes (and likewise with RVs) use an approach substantially different from that commercially. They limit charging to 3.4 volts/cell (13.6 volts for a 12 volt battery), corresponding to about 80% charge. Many, however, 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.
Lithium-ion batteries for solar homes – temperature effects
No existing battery likes working at extreme temperatures. Lithium-ion (LiFePO4) is no exception. Their preferred working range is -180 C to about + 400 C. They work best at about 250 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.
Lithium-ion batteries for solar homes – pricing
In many countries (including Australia) all LiFePO4 batteries and associated systems are imported. Some are sold via several levels of distribution, each thereby adding overhead and profit. Lithium-ion batteries were initially expected to fall in price as they became increasingly accepted. The global demand for lithium however escalated. Furthermore there is only a limited amount of lithium base 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. In essence, day-time input is stored for a few hours and then drawn on at night. Grid-connection is nevertheless retained, 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. These may be 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. Such systems must not be connected to the home’s fixed grid wiring.
As the existing fixed wiring is not changed, this can thus 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.
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 a major (IDC) lithium battery conference in Sydney (May 2016) stated that any appreciable increase in output is now unlikely.
Lithium-ion batteries for solar homes – further information
Please see also my just published https://www.successfulsolarbooks.com/knowing-the-best-batteries-for-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 (homes and properties). Solar That Really Works! is for boats, cabins and RVs). 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 and is applicable worldwide.