Operating temperatures. Where should I site it and does this matter?
Again, it is the classic case of unknown unknowns… if you did not know it mattered you would not ask about it. So now we DO know that it matters then why does it and how does this influence what I do?
Batteries may be made of up different chemistries, you may have heard of lead acid or aqueous vitreous or lithium ion. If Lithium-ion which TYPE of lithium-ion and if you have heard of that does it differ from lithium IRON batteries or are they the same thing? Am I just making this stuff up now? Even if I am not does operating temperature make a difference to any or all of them? Yikes… Read on people…
First and foremost, pretty much all home batteries like the same ambient temperature as you do to work most efficiently. Think of Goldilocks and the Three Bears…. They did not like their porridge to be too hot nor too cold…they wanted it Juuuust right… Your battery is the same about its environment. You should remember that a battery is simply a self-contained chemical reaction. It will always be influenced by heat in some way – either through too much or too little.
So, what are the rules of the game in simple terms?
Rule number one – do not charge the battery in freezing conditions. This means that if you live somewhere that the temperature drops to zero or lower (Great Britain) and think of putting the battery somewhere that is not heated in some way – e.g. a normal loft or a garage then this is preferably a no-no. The battery CAN be there in most cases but will not charge when too cold and some have warranty statements that you should not expose to certain low temperatures. So simply put use your utility room, airing cupboard, larder, under the stairs etc unless the battery specifically conditions its own temperature such as the Tesla Powerwall II or states that you can place it in un-sheltered areas.
For those more curious boffins out there here is a short scientific explanation of why:
If you charge the battery in freezing conditions then the lithium ions become sluggish and will essentially coat the graphite anode inside the battery rather than intercalate (merge with and consequently charge) with the anode. This means that by plating the anode rather than merging with it they are essentially increasing the internal resistance of the battery and reducing the capacity of it as they have blocked the outer layer stopping other ions from inserting themselves into the internal structure of the battery’s anode. I bet you are now really glad you read that bit…
Rule number two – do not position your battery in a place where it can repeatedly have to charge and discharge in an elevated temperature. You might be surprised to know that your loft in summer, especially if you have dark roof tiles, can achieve some really high temperatures up to 50 Co. The more you do this, and the higher the temperature, the more you degrade the storage capacity of the battery chemistry by structurally changing the composition of the electrode and its surface, electrolyte composition, the rate of active materials dissolution and phase change of materials within the battery. This is a cumulative effect and varies by temperature at the time, as such it is better to be avoided in the first place. The key thing to remember is this: you bought a certain capacity battery so why would you want any less if it is avoidable?
Essentially you can start out with a useable rating of 5kWh and end up with a greatly diminished capacity after cooking your battery. Then there is lifespan to consider – how many times can you charge the battery – as we all know batteries degrade over time – hence there is a specification on your battery stating charge cycle e.g. 6000 charges
To be extremely clear if a normal battery likes to operate at 20Co how much loss of cycle life do you get when charging and discharging at higher temperatures? Roughly speaking for a typical home battery:
30 Co – the cycle life is reduced by 20 percent.
40 Co – the cycle life is reduced by 40 percent.
45 Co – the cycle life is reduced by 50 percent.
So, if you want your battery to last then do not cook it and do not freeze it – after all you may have paid for a 10kWh capacity 6000-cycle battery and you want as much of the 10kWh and 6000 cycles as you can get.
Written by John Rowlatt