Batteries are obviously critical for any offgrid system, and they are really expensive. They’re also the part that you’ll likely have to replace first.
Unfortunately there’s a huge choice, not only of brands, but types. And the operational constraints, reliability, life expectancy, maintenance and cost differences between types is enormous.
The following tries to summarise key types and advantages and disadvantages.
HOW MUCH CAPACITY DO YOU NEED/WANT?
If you don’t have a backup generator, the recommendation is for 3 to 5 days storage to cover off extreme weather events.
But installing that much capacity would be super expensive, and would not necessarily cover all eventualities. The bushfires a few years ago produced enough smoke that many Victorian solar systems produced hardly any power for months in summer (and required water tanks to be isolated to prevent contamination). And with severe bushfire frequency likely to increase…. A standby generator is a must.
So with a standby generator, you can reduce the size of your batteries, but to avoid running the generator frequently in winter one to two days storage should be provided. eg. 2 x daily electrical kWhr.
We’ve ended up with about 1.5, for our full load list, but it includes contingency and a lot of nice to haves/infrequent usage things that we just won’t use when the weather is bad, to minimise running the generator. So with loads well within our control, we should easily have two days, just not on paper.
As usual, to avoid too much expenditure, really think about how you will use your system, and what if any compromises you might be prepared to make.
DEEP CYCLE LEAD ACID BATTERIES
Note: Automotive batteries are not suitable – they are designed to give short burst of electricity to start a car.
For decades, this is what there was. Tried and true. 20 odd years ago, when I installed solar on our shed, this is what I used. They have thicker lead plates (electrodes) than automotive batteries and draw power more slowly and evenly.
There are three main types:
Flooded lead acid batteries require maintenance – topping up with distilled water, and need to be kept upright so the electrolyte (dilute sulfuric acid) doesn’t spill. Some of this dilution water will be electrolysed to oxygen and hydrogen gas so they need to be well ventilated to ensure the hydrogen does not accumulate (extremely flammable), reducing the volume of water in the battery, which then needs to be replaced. High ambient temperature will also cause some water to evaporate, so more frequent check of water levels should be made in summer. If the water drops to low levels the battery will be permanently degraded, if it runs out it will fail irreversibly due to sulfation.
Conversely, overwatering can lead to a drop in performance from reduced electrolyte concentration (filling batteries to too high a level).
If like us, you go away for a year, you may come back and find you need to replace your batteries, as we did. Personally, I wouldn’t get these again. Don’t like the severe consequences of lack of maintenance.
- AGM, Absorbent Glass Mat
Slightly more expensive, these batteries trap the electrolyte and hence do not need to be topped up. Some hydrogen does still escape, so ventilation is still required. Typically they are a little more robust, and should last longer than flooded batteries. If I were selecting lead acid batteries (Victoria), this would be my pick.
These batteries trap the electrolyte in a gel, and perform well in high temperatures, but not so well in low temperatures – much better suited to Queensland than Tasmania. They vent less hydrogen, so are safer. These batteries need careful charging.
Useable Capacity – Depth of Discharge
Despite the stated capacity of a lead acid battery, the amount you can take out of a lead battery is severely limited if you want to get a reasonable life out of them.
As a rule of thumb, routine use should only be about 20-30% of capacity, with an occasional draw to maybe 50%. From boondoctor, the following chart shows the laboratory lifecycles achievable in ideal conditions. So for 20% DOD, you get around 3000 cycles or around 8 years, assuming you draw down this much once every day, or 5 years at 30% DoD.
Given that your batteries should be sized for 20 to 30% use in winter, when they will be required supply more power to your house for longer (when the sun isn’t shining), then depth of discharge should be much lower for most of the year (if you’re in the southern states of Australia). Unless, of course, you increase load in summer to provide air conditioning, evaporative cooling etc. This reduced depth of discharge would extend the battery life relative to the chart above.
On the other hand, this is ideal performance. Temperature effects, discharge rates and maintenance will all tend to reduce the number of times you can cycle the battery before they need replacing.
Predicting the life on a lead acid battery is extremely difficult – there are so many variables that impact longevity, although many of them are in your control.
The quality of the battery will also have a major impact – try to get a chart like this specifically for any batteries you are considering buying.
Undercharging lead acid batteries can lead to sulfation/battery degradation and reduced life. This can be a problem in winter, when batteries will not likely to be always fully charged on a given day. Regular application of overvoltage “equalisation” voltage can fix this issue. The over-charge helps to remove lingering sulfation and equalise individual battery cell voltages in a string.
However, the equalisation charge can’t be done too frequently or for too long, as over-voltage conditions lead to gassing (evaporation) of the water component of the electrolyte. In flooded lead-acid cells, this gassing can be reversed with battery watering, but for sealed technologies (eg. gel or AGM), excessive gassing may cause irreversible electrolyte loss. All of which will also reduce battery life.
And you can’t just really rapidly charge the batteries (using your generator for example), because the last part of the charge takes a long time – so while you can fully charge, your generator will need to run for quite a while.
Discharge Rate & Temperature
The instantaneous or available capacity of the battery is strongly affected by the rate of discharge. At high discharge rates, drawing a large current, the voltage plummets, reducing the capacity of the battery.
From PVEducation, capacity also falls by about 1% per degree below about 20°C. However, high temperatures are not ideal for batteries either as these accelerate aging, self-discharge and electrolyte usage. The graph below shows the impact of battery temperature and discharge rate on the capacity of the battery.
The BatteryCentre puts the impact of discharge rate into context – the graph is to running “flat”. Since most solar batteries will be only discharging when there is insufficient sun, discharge will occur over say 12 to 20 hours. Only 70-80% of the battery capacity will be available for discharge – that’s OK because you need to design for only 20-30% of capacity use. But it does mean that if you’ve designed for 30% of nominal capacity, you’re actually using more of the available capacity. Fast discharges reduce battery life.
Typically more expensive, newer lead acid batteries are sealed, and so don’t need water added. This is what we have in the shed now. However, you will have significant cabling that wires the batteries together and the terminals and connections should be occasionally checked to ensure good connections. See also Charging.
Round Trip Efficiency
Overall round trip efficiency is around 80% – you get back about 80% of the power you put into it.
End of Life
Lead Acid Batteries are at imminent risk of failure when their state of health falls to about 80% of their rated capacity. Source: www.batterytestcentre.com.au
Since they use lead plates, they’re heavy and much much larger than lithium for equivalent energy storage.
As a rule of thumb, the heavier the battery, the better it is likely to be (and more expensive) – lead anodes and cathodes will be thicker and more robust.
Lead acid batteries are relatively cheap and reliable if you maintain them, treat them gently and don’t overdraw, and ensure they get fully charged/equalised correctly.
Rough cost/useable Ah, assuming 20% DoD, 80% roundtrip: $13/Ah useable. ($2/Ah rated). additional costs for storage room, and cabling linking all the batteries together, and any maintenance equipment required to check cell voltages, equalisation, state of health etc.
There are a range of different chemistries in Lithium batteries. Lithium ion batteries have been involved in fires etc, since the chemicals promote run away thermal reactions. Many off grid style batteries use LFP technology (lithium iron phosphate), and these do not suffer from the same problem.
I’m not going to go into each type of lithium or equivalent battery, because the technology is still changing rapidly. The battery test centre does provide info on some of the different types/
The battery test centre is a great resource for performance of a range of available lithium batteries. In partnership with ARENA (Australian Renewable Energy Agency), the test centre at Canberra Insitute of Technology is putting some batteries through their paces. The batteries are fully cycled 3 times per day (extreme testing conditions) to test reliability and loss of capacity with cycles, and the ambient temperature is varied to mimic batteries exposed to outdoor temperature. They produce a report every 6 months or so. Well worth a read before you pick a brand. So this testing covers off many of the issues described above – battery life under different temperature conditions with extremely fast charge and discharge – much more extreme testing than would ever be experienced in an off grid deployment.
Depth of Discharge
Depth of discharge or state of health reports from earlier this year show the following results:
After 10 years equivalent cycles the Sony battery retained 80% of its capacity. Note that all the other batteries did not complete this many cycles for a variety of reasons.
This chart shows that all lithium batteries are not equal… After around 5 years (ca 1800 cycles) 85+% capacity remained, although it suggests that you don’t want to buy a GNB lithium battery, and the LG doesn’t look so good either.
Or perhaps not a DCS either….
Lithium manufacturers suggest different maximum DoD, often between 90 – 100%
Lithium battery life is usually assumed to be around 10 years – these test show that it should be a lot longer than that, if you get the right battery.
Round Trip Efficiency
Round trip efficiency is also quite variable by battery manufacturer. Redflow and Sonnen don’t look so good…
Lithium batteries typically have higher round trip efficiency than Lead Acid batteries.
Undercharging is not an issue with lithium batteries. They can be charged significantly faster than lead acid batteries, however over charging or charging too fast can still lead to loss of performance and failure. Battery management systems are typically designed to ensure that these conditions do not arise. Many also account of any extreme temperature issues to ensure no thermal runaway.
A number of the batteries in the test have failed or had issues. They are much more complicated and require good battery management systems and must be compatible with the inverters they run with. Lead acid batteries tend to be reliable until they fail altogether, while the additional lithium complexity tends to make them more likely to be intermittently unreliable.
End of Life
Lithium Batteries will continue to work until their capacity is no longer useful.
Small space required.
Just to make things tricky, lithium batteries tend to be quoted in kWhr rather than Ahr. Annoying! To convert between them, divide kWhr by the battery voltage and multiply by 1000. Power (Watt)= Voltage x Current (Amps).
Rough cost/useable $39 / Ah.
Much more expensive than lead acid, and potentially less reliable intermittently, the batteries are more flexible in charge and discharge rates, and are likely to last at least twice as long. They are typically warranted to have at least 60% capacity after 10 years. They don’t need such careful operation to maintain their health.
SO AFTER ALL OF THAT, WHICH ONE TO CHOOSE?
From the Battery Test Centre “As the required charge/discharge rate increases, the capital costs of lead-acid batteries (on a $/kWh basis) increase, and the economics begin to favour lithium-ion chemistries.
A similar situation occurs when the battery is to be frequently used. The slower capacity fade and the higher efficiency of lithium-ion cells does not impact the capital cost (on a $/kWh basis), but the levelised cost (or total cost of ownership) will tend to favour lithium-ion batteries.”
We’re on our third set of lead acid batteries in 22 years (last ones installed a couple of years ago are AGM sealed). First set lasted around 15 years in ideal circumstances – very low load, not even 20% DoD, only used on weekends, very few cycles. Second set lasted around 4 years – maintenance issue with running out of water while we were away.
We really thought long and hard about AGM Lead Acid Batteries.
Battery technology is changing and improving all the time. Costs are dropping and government subsidies are likely to come in. While the lead acid batteries would not last as long, upgrading to lithium in 5 to 10 years would likely be much cheaper than purchasing them now. But in the end, we opted for the piece of mind of lithium, and the ability to run AC etc without thinking about it. We wanted a set and forget system.
We’ve chosen BYD lithium. Relatively low cost for lithium, performed well (apart from a couple of issues) in the performance tests and will support high draw currents if we’re running our AC, power tools etc. We don’t want maintenance as we will likely be travelling, nor do we want to be thinking about protecting our batteries when we turn on our appliances. We don’t want to worry about equalisation charges, run our generator for a long time at low load etc etc. We want something that is set and forget.
We also don’t want the hassle of swapping out big heavy batteries in a hurry, as we’ve had to do in the past. Or providing the larger ventilated (and hence hotter and colder) storage space.
We’ve gone for 2 x BYD with 2 x Sunny Island inverters, so hopefully if something has an issue we can rely on 50% of the system until we get it sorted. Our system is fully expandable.