One aspect of solar battery storage that appears to be under-reported and not transparent in most solar storage providers’ literature is just how long will Lithium ion batteries last before they need to be replaced.
There are a few abbreviations you will come across in some of the material included in this post:
Calendar life: How long the battery will last, influenced mainly by state of charge and operating temperature.
Cycle life: The number of charge/discharge cycles a battery can undergo before it is no longer usable.
DoD: Depth of discharge, meaning how far the battery has been discharged from full charge.
SOC: State of charge, meaning how much charge the battery currently holds.
SOH: State of health, meaning how much usable capacity the battery has left over time.
EOL: End of life, the point at which the battery is no longer usable.
Ah: Amp hours, meaning a measure of total energy; for example, drawing 10 amps current for 2 hours is 20 Ah.
Watt: A watt is a measure of power. For example, to draw 1 amp from a 12 volt battery is 12 watts of power.
kW: A kilowatt is a thousand watts.
kWh: A kilowatt hour is a measure of energy. For example, 1 amp from a 12 volt battery for 100 hours is 1.2 kWh.
Let me start by quoting 7 reasons for using Li ion batteries instead of lead acid batteries, as quoted in 7 facts and figures comparing lithium ion vs. lead acid batteries
Lead, which is the heaviest non-radioactive metal, has been the standard in batteries for decades. Why should you consider a lithium battery conversion? Here are seven features explaining the disparity between lead acid and lithium-ion batteries.
1) Weight: Lithium-ion batteries are one-third the weight of lead acid batteries.
2) Efficiency: Lithium-ion batteries are nearly 100% efficient in both charge and discharge, allowing for the same amp hours both in and out. Lead acid batteries’ inefficiency leads to a loss of 15 amps while charging and rapid discharging drops voltage quickly and reduces the batteries’ capacity.
“I disagree to some extent with Item 2 above in that Li-ion batteries have about 95% efficiency. That is, if you draw 100Ah from the battery, you will need to put in about 105 Ah to recharge it.”
3) Discharge: Lithium-ion batteries are discharged 100% versus less than 80% for lead acid. Most lead acid batteries do not recommend more than 50% depth of discharge.
4) Cycle Life: Rechargeable lithium-ion batteries cycle 5000 times or more compared to just 400-500 cycles in lead acid. Cycle life is greatly affected by higher levels of discharge in lead acid, versus only slightly affected in lithium-ion batteries.
5) Voltage: Lithium-ion batteries maintain their voltage throughout the entire discharge cycle. This allows for greater and longer-lasting efficiency of electrical components. Lead acid voltage drops consistently throughout the discharge cycle.
6) Cost: Despite the higher upfront cost of lithium-ion batteries, the true cost of ownership is far less than lead acid when considering life span and performance.
7) Environmental Impact: Lithium-ion batteries are a much cleaner technology and are safer for the environment.
Though they are used to power the same applications, that is where the similarity between lithium-ion and lead acid batteries ends. Lithium batteries deliver higher-quality performance in a safer, longer-lasting package.
The following link indicates the characteristics of the Sunverge batteries being employed in the AGL VPP.
Solar battery storage
If you examine the “Market Comparison” data for the Sunverge 11.6 kWh battery you will see it has a capacity of 11.6 kWh and a usable energy of 9.9 kWh. This means they are designed to have a DoD of 9.9 / 11.6 or 85%. It also states a warranty period of 10 years. I would expect that the original design concept was for the batteries to be discharged once a day, meaning 3,650 cycles over the ten year warranty period. This in itself suggests to me that a cycle life of about 5,000 cycles would be a reasonable estimate, and that the design would allow for the full 85% drawdown each day. If this is correct, then using off-peak or controlled load to charge the batteries at night, as well as during the day, would lead to a much shorter cycle life. The option of controlled load grid charging may well be appealing to those who have a heavy night time and breakfast load.
Cycle life is dependent upon a number of variables; mainly depth of discharge and temperature, but also rate of charge and discharge, under or over-charging etc.
The following very important link will show you very clearly just what effect depth of discharge (DoD) and temperature will have on your batteries. It is very important, in my opinion, to locate your batteries in as cool a place as possible.
Li ion battery life Technical Sheet
The following advertisement is definitely worth reading, if perhaps for no other reason than to suggest that the Sunverge batteries may be well chosen for the VPP, given this manufacturer is proud to claim a total of 2,500 cycles.
PV series solar storage
The following link to a supposedly high quality German battery manufacturer claims 5,000 cycles for 80% DoD. At least they make clear what the specifications are, something lacking in most material.
BMZ Energy Storage System
I have requested a graph of DoD vs cycles from Sunverge, but they told me to ask AGL. This I have done and will post the graph should it appear. It has to be born in mind that life cycles are very dependent on DoD and temperature, as you will have seen from the third link above to saft batteries.
The following link is very informative and descriptive, and well worth reading.
Lithium Ion Battery Test Centre
The following three links are also well worth reading if you have come this far. There is an abundance of material on the web, but it appears educating people about the real facts of solar battery storage is something you have to do yourself.
Batteries for medical consumer hobbyist
Solar Choice battery storage product lifespan comparison tool
Battery and Energy Technologies
