Off Grid Solar Part 3 – Batteries

Research on batteries can be really confusing when you start. Hopefully this post can help you dig down to just the factors that are important when selecting a battery type. First though, here’s the rule; battery technology is moving so fast whatever you select will be close to outdated by the time you install it. So don’t worry about the latest and greatest, just worry about what will do the job for you.

Battery factors to know

Depth of discharge (DOD)

This is a measure of how far you can discharge the battery without seriously affecting it’s lifespan. 30% depth of discharge or DOD means that 30% of the battery’s charge has been expended and 70% remains.

Energy density

How much charge a battery can store for a given size

Charge/Discharge efficiency

Or Faraday efficiency is a measure of how much of the energy put into the battery from your solar panels actually ends up as usable charge

Cycle life

How many times you can charge and discharge a battery before it’s ability to store charge is degraded.

Disposal

When the battery is taken out of service what is the environmental impact of disposal. For example are the components toxic, can the battery be recycled?

Cost

What is the comparative initial investment required to purchase the battery

Common Battery types

Lead Acid – you’ll see this often as SLA standing for Sealed Lead Acid or AGM standing for Absorbent Glass Mat – irrespective of the name the basic chemistry remains the same. These are the cheapest battery option but over-discharging them will cause deposits of sulphate on the electrodes which will greatly reduce the battery capacity and hence it’s usable life. The number of times they can be charged and recharged without losing capacity is also limited to about 1650 cycles at 30% DOD or about a 5-7 year average lifespan.

Lead Carbon – these are basically a lead acid battery but with electrodes made using nano-carbon which greatly increases the resistance to sulphates being deposited and can increase the charge/discharge life of the battery out to 7000 cycles or perhaps 15 years of service at 30% DOD. They are a bit more expensive than SLA.

Nickel Iron (NiFe) – this is a really old, but incredibly durable technology. The electrodes are common metals that are readily recyclable and cheap to source and the electrolyte (potassium hydroxide) can literally be used on your garden when you need to replace it. Unfortunately that is also a factor with nickel iron batteries, the electrolyte needs to be frequently replenished and periodically changed. However, they are pretty much indestructible, able to endure high DOD and have lifespans of many, many years and can last for as many as 11,000 cycles at 80% DOD (note that 80%!). The downside is their charge/discharge efficiency is low at 65% or less and for that reason they take a long time to charge and cannot supply high rates of discharge. Nife batteries are currently quite expensive, even more so than lithium. They also, because of a low energy density, require a large amount of well ventilated space.

Lithium Iron (LiFePO4) – these batteries have a life expectancy of 10,000+ cycles at 80% DOD and have very high charge/discharge efficiencies, meaning they can charge quickly and be relied upon to support high levels of discharge i.e. high instantaneous loads. LiFePO4 batteries also have a high energy density meaning they don’t take up much space for their storage capacity. On the downside they are probably triple the cost of SLA battery packs and are currently minimally recyclable.

Summary Chart

	Sealed lead acid (SLA)	Lead Carbon	Nickel Iron (NiFe)	Lithium Ion LiFePo4
Depth of Discharge	*	**	*****	*****
Energy Density	**	**	*	*****
Charge Efficiency	***	***	*	*****
Cycle Life	*	***	*****	*****
Disposal	***	***	*****	*
Cost	*****	***	*	*

 

 

 

Conclusions

Feeling even more confused? Hopefully not, but let us take you through our journey.

We loved the NiFe battery, it has minimal cost of production, very low environmental impact and is incredibly robust and durable. BUT! they are really expensive and just too slow to charge and discharge, meaning that you have to oversize the battery bank, at least in our solar conditions, to compensate. Perhaps if you had a large budget and lived somewhere that had reliable year round sunshine then they’d be a workable option, but in central Victoria under a mountain – no way!

The SLA battery is certainly attractive price wise, but is plagued by a short life span. The lead carbon battery improves this, but you are still hamstrung by the 30% DOD, meaning that you need between double and triple the storage capacity of an equivalent LiFePO4 battery bank to get the same usable energy. In the end they aren’t so cheap, plus if your defray the cost of their replacement into the mix, then they are something of a false economy.

Lead acid technology however, is readily recyclable and not comparatively toxic, which can’t be said for lithium iron batteries. The lithium batteries though, have a much longer lifespan, maybe as much as 30 years if sized appropriately. Our thinking is that solar panel recycling technology is now coming on rapidly and with the price of lithium we think it’s reasonable to expect lithium iron battery recycling technology will follow the same path.

So in the end we opted for 16Kwh of LifeP04 storage in a modular format so we can add lots of 4Kwh quite readily. It wasn’t cheap and in fact blew a hole in our budget, but we think it was the right choice in the end. We’ll let you know how it goes!

Happy off gridding and catch you next time.