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Battery storage for solar PV: Which battery to choose?

Posted by Chris Rudge on 30 March 2016 at 12:55 pm

Battery storage bolt-on options for existing renewable energy generation systems are now becoming the next 'must have' for people looking to either reduce their energy bills or increase their energy security at times of power cuts.

I have been providing different types of battery systems for customers over the last 10 years and have seen a huge upsurge in interest in the last 6 months. There has always been some bafflement over the best battery technology to use with different installations, which has escalated recently now that Lithium-Ion batteries are coming down in cost.

In this blog  I'll outline the features and cost benefits of  some currently available battery types offered for renewables.

The main battery types currently offered to customers wanting a battery add-on are based on either Lead Acid or Lithium-Ion.

Lead Acid

These batteries have been available for a very long time and until recent years, would have been the only option available for anyone to use. Originally provided as what we refer to as 'flooded', you may be familiar with the need to keep these topped up with distilled water regularly in your car. Really big versions were made for fork lifts, submarines, commercial backup supplies, etc. Though this is still the case, as flooded batteries are quite robust when scaled up to commercial size, most Lead Acid offerings' for domestic and small commercial are either Sealed AGM or Gel types.

Without going into the internal technology, which you can easily discover on Wikipedia, the practicality for non commercial users mean that Sealed batteries do not need regular maintenance such as topping up with water. Sealed AGM's will usually be able to take a far higher charge current than Flooded, thus making them ideal for the Solar PV market where potentially a lot of power is available to be stored during peak times of the day. Likewise, Gel batteries have a good storage capability, but cannot be heavily charged.

Both types will have a lifespan of no more than 5 years and you will need to keep in mind that you'll need to replace them at this point or before. As either type contain a good amount of lead, they will attract a good price at the recycling centre. Typically around 20% of their cost as new. Lead Acid batteries are about the only battery type that can be recycled in this way. So disposal is straightforward through recycling and produces some income!

As with anything, the less you pay for your batteries, the lower the performance. No matter what the salesman tells you, a typical lifetime of an AGM or Gel battery is around 800 - 1000 charge discharge cycles to 50% depth of discharge. This is around 2.5 years if cycled to 50% every day of the year. For many households this will not be the case, thus the batteries will last much longer

This reminds me to mention that all lead acid batteries do not fare well if deeply discharged, which will considerably reduce their storage capacity and longevity. As part of the battery management which will be installed with your add-on storage system, it will be set up to ensure the batteries will not drop below 50%, this is critical for any system, as well as carefully charging the batteries according to their individual characteristics.

Another big advantage with any Lead-Acid battery type is that it comes at the lowest cost of any current storage type. You may find this aspect appealing, even though you will get much fewer charge/discharge cycles. You will know without heavy use you will get 5 years of life and a small payback for selling them at the end of this time. You can then ask your installer what the cost of other more efficient technologies are, which may have dropped in price by that time. See the chart below to get an idea of cost effectiveness at the moment.

Lithium-Ion

These fairly recent storage devices have been hailed as the holy grail of battery technology and you will find it fitted in most portable devices, such as your phone, due to their far higher efficiency and space saving over all other types available to us.

Even more recently, Lithium-Ion has been offered to the renewable energy market in easy to use packages making it far safer than ever before to use. As Lithium-Ion has a really low internal resistance, allowing them to be charged really quickly if needed, they can also discharge very fast if given the opportunity and will overheat and cause a fire very quickly - the very reason for Hoverboard fires recently.

All approved Lithium-Ion battery sets contain a battery management setup which will shut them down with excess charge/discharge or fire, so these are nice and safe to use in the house.

The huge advantages over Lead Acid is the smaller space required, higher lifetime cycles and that you'll be able to discharge to 80% or more daily if you wanted with little lifetime effect.

Most Lithium Ion batteries will come with a 10 year or 6000 cycle warranty and will work across a far wider temperature range, far far better than Lead Acid. The Lithium Ion battery really is pretty much a fit and forget battery.

So, though Lithium Ion costs much more on initial purchase, the lifetime will be at least 3 times more, and easily in excess of this if you keep the discharge down to 60% or less. See the chart below for cost effectiveness against Lead Acid.

One of the drawbacks with Lithium Ion - there had to be one - will be that the recyclability is practically zero. Once the battery set is depleted you'll need to find a way of safe disposal, which you need to pay for as they are listed as a pollutant. Disposal will be in at least 10 years, so as Lithium is a limited resource, the situation may have changed by then, but right now it's a difficult disposal issue.

Aqueous Hybrid Ion batteries

For the real Eco warrior, the recent release of the Aqueous Hybrid Ion Battery is a major leap in clean power. Being based on a harmless saline solution with other natural products such as coconut husk, the Aqueous unit will never be a hazard in its own environment.

They can be deeply discharged and each 'module' will store 2kWh of power.

As you will imagine, this natural solution does come at a cost. It is more than either of the above per kWh, although currently not excessively so. As production ramps up the price will come down to make it a far more economic solution.

Aqueous hybrid ion battery charge and discharge rates are far lower than either Lead-Acid or Lithium-Ion, so are more suited to low capacity loads such as lighting and electronics. Though the 2kWh of storage is available in each cell, it will need around 5 hours to charge or discharge that amount. Obviously stacking more in parallel will double the power and storage at the same time.

This chart easily shows that though Lead-Acid wins on initial system outlay, for system economic longevity, Lithium-Ion wins hands down!

Table of batteries

The Aqueous hybrid ion, which I have scaled to be representative of the others, is also great on long term economics, but you need to be careful the installation does not have a big current draw.


Photo: torsteinsaltvedt

 

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About the author: Chris Rudge is a qualified electrician who specialises in renewable energy.

If you have a question about anything in the above blog, please ask it in the comments section below.

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Comments

4 comments - read them below or add one

terryh

terryhComment left on: 26 October 2016 at 2:45 pm

Interesting article. I have recently investigated battery owner storage and would agree the cost are very high indeed and not enough is known about about various systems at this point.

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Kay Bickley

Kay BickleyComment left on: 22 October 2016 at 2:08 pm

We are currently monitoring the export on our two solar arrays (one single and one three phase) before purchasing battery storage. With the single phase we can monitor using the solaredge kit and for our three phase we are using an Owl Intuition monitor. Initial quotes for batteries were steep and I would like to have some good data on our peak season export and general overnight base loads before we dive into this market. We were early into the market with our electric cars, but I feel there is merit in waiting for more choice in the battery market place. Are other countries further ahead than us in battery storage and is it worth learning from their experiences?

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Rudge Energy

Rudge EnergyComment left on: 11 April 2016 at 8:50 pm

Hi PeterFT .. I deliberately aimed the blog at widely available battery types that are of a tried and tested technology offered by battery installers across the UK.

Looking around for suppliers of Lead Crystal, none of the whoesalers I use have any intention of stocking them. In addition, though the website I came across has some bold claims for performance, I'm really not sure they stack up, being a variant on the standard calcium traction battery. I also noted there is no warranty stated.

Personally, I would not risk using them until independant testing has been carried out. At the moment, I'd spend your money on known technologies.

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PeterFT

PeterFTComment left on: 5 April 2016 at 10:04 am

No mention of Lead Crystal batteries. Any reason why?

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