Technobabble (in English, not Techno-jargon!)
There is a lot to know about battery power. As with many new innovations, there are some things you really need to know, some things you should know, and a whole lot of stuff that will only appeal to the technology geeks amongst us.
Let’s start with some of the basics:
Batteries these days are generally sold by the kWh.
For example, Powerwall 2 has a capacity of 13.5 kWh, while Enphase batteries are smaller at 1.2kWh each.
Technically speaking, battery capacity is best measured in Ampere/Hours (Ah) and the number or size of batteries based on many factors including peak load, discharge rates, depth of discharge and cycles (see below!).
However, this is all a bit technical for most customers, and with the new battery control equipment (eg Enphase AC micro-inverters and Solar Edge Inverters), and the fact that the battery systems themselves are generally grid-connected (which means that running out of battery power occasionally is not a big deal), the need for a lot of this knowledge is no longer as important.
Therefore, if you buy a 10 kWh battery, you can in simple terms expect to be able to use close to 10 kWh of power per day/cycle (but watch out for the Depth of Discharge!!).
Peak draw is the maximum power you can draw from the batteries at any one time.
In other words, you may, for example, have a 10kWh battery but it may only let you draw 2kW of power at any one point in time – so you would have to use it for at least 5 hours or more to get your 10kWh of energy.
The reason for this is that with most batteries, the higher the Peak Draw, the shorter the life of the battery itself.
This is really important when choosing a battery system. If you wanted to run an appliance that used, say 8kW of power (eg an air conditioner) for one hour, you would need to check not only the capacity of the battery but also the Peak Draw.
Using the above example, you would need 40kWh of battery to run your air-con for one hour! This would therefore only make sense if you wanted to run the air-con for 5 hours every day.
Solar Battery Cycles
A ‘battery cycle’ is one complete discharge and recharge.
For most customers, you would cycle your battery once per day (although some may be able to cycle twice a day if using off-peak power).
Importantly, the life of a battery is measured by the number of cycles you would be expected to get from it. This is important when considering the battery that is right for you.
A battery may have an expected life of 10 years or 3500 cycles (eg 365 days a year times 10 years-ish). If you use all of those 3500 cycles in the first 5 years, the battery is unlikely to last much beyond that 5-year mark.
Depth of Discharge
Another really important number is the Depth of Discharge (look for DOD on brochures).
This means how deeply you can discharge the battery per cycle. For example, if a 10kWh battery has a DOD of 90%, you could use 9kWh per day/cycle. If it has 50%, you can use/draw 5kWh and so on.
Most Lead Acid and Gel batteries have a very low DOD – sometimes as low as 20% (ie of 10kWh you can only draw 2kWh per cycle).
However, the newer Lithium-Ion batteries generally have a DOD of 90% or even 100% which means the amount of energy you can use is approximately the same as that on the label.
In reality, this is normally done by labelling them with the usable capacity instead of the technically correct capacity, but as long as you know your usable capacity, you will be fine.
Also, most of the Lithium-Ion batteries are computer-controlled so that it is not physically possible to discharge them below their DOD – a vast improvement on the older technology where if you discharged them too low too often, you could prematurely destroy the battery.
Always check the actual warranty of a battery.
Most batteries will have, say, a 10-year warranty. BUT, there will usually be a number of Cycles (as per the Cycles paragraph above) specified in the warranty. The warranty will apply to WHICHEVER COMES FIRST – eg 10 years OR 3500 cycles.
A Lithium-Ion (Li-ion) battery has most commonly been used in consumer electronics such as mobile phones and laptops.
They are one of the most common types of batteries in portable electronics due to a high energy density and a slow loss of charge when not in use The Li-ion battery is a rechargeable battery that can have a long life with many thousands of cycles if properly maintained.
This makes it possible for a hybrid solar system to have an expected life of over ten years.
Lithium Iron Phosphate Battery
The lithium iron phosphate battery (LFP, or even better… LiFePO4!) is a type of lithium-ion battery, which has a lower energy density than the more common Lithium Cobalt Oxide Battery, but offers a longer lifetime and better power density (the rate that energy can be drawn from them).
Lead Acid Battery
Lead acid batteries have been around for a long time – since 1869 to be exact – and they are the oldest type of rechargeable battery.
Despite having a very low energy-to-weight ratio and a low energy-to-volume ratio, its ability to supply high surge currents means that the cells have a relatively large power-to-weight ratio.
A battery bank needs ventilation, drainage and constant maintenance to ensure both the battery’s life and safety, making it impractical for a residential installation in most households.
The inverter converts the D.C. power from the batteries to 240V A.C. power that is used in our homes. Some systems have bi-directional inverters that will also convert AC to DC to allow for charging the batteries from the grid.
A hybrid inverter is for use with both batteries and a solar system.
In general terms, a hybrid inverter would normally measure a household’s usage and intelligently determine when to charge batteries and when to use battery power to save the most amount of money.
It is worth noting there is no formal definition of “hybrid inverter” so there are inverters that are promoted as a hybrid that will need to up-graded before use with batteries.
A regulator is designed to protect the batteries from becoming overcharged, which can shorten the life of the battery.
Solar regulators also detect when the batteries are fully charged and prevent the batteries from overcharging. A regulator a quite a small piece of electronics and it often comes with an LCD screen.
With the newer residential battery systems, these are usually part of the inverter.