Here's the almost finished enclosure. Insulation yet to be fitted to the doors. This will be demountable, so the Louvre doors can ventilate the enclosure in the summer.

Relying on the heat out put from the inverters to keep it warm. Roof and sides are permanently insulated with 50 mm styrodur

Given that my DIY skills are sub par, to say the least, is anybody aware of a source for commercially made ‘cupboards/sheds’ for external batteries.  A link would be good if possible.  I ask because mine, in their first cold snap, have completely stopped charging. TIA.

There’s no prizes for design on our thermal cover. It’s 70mm phenolic taped in place, covered by tarpaulin to keep rain off the insulation. The battery temperature didn’t drop below 13C when it was -2C last night. The charge rate is therefore software restricted to 2.7kW which is a pain, not enough to fill in a 3hr cosy slot, but at least it all keeps working.

@judith Here in rural Derbyshire, we have -6 overnight and it's barely above -4 now at 10:30.  Batteries not charging.

Posted by: @alanb

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It has become clear that performance drops off a cliff near freezing. The Skyline system boasts a state of the art battery warming system, so you can install outside. My experience shows this does not work.

Stand by... technical explanation coming 😉 

I'm going to assume that the battery chemistry is LiFePO4 (lithium ferrous phosphate).
These are OK to be discharged below freezing, but can suffer permanent damage if you recharge them at 0°C and below.

Within each battery (which commonly consists of 16 3.2v cells) there is a Battery Management System (BMS).
That offers protection to prevent damage to the cells -

• overcharging: too high a voltage detected on one or more cells in the set, typically 3.6v
• under-voltage: too low a voltage... typically 2.85v
• over-current; more amps being carried than can be handled by the cells, electronics or connecting cables
• cell balancing: the difference in voltage between individual cells in the set is measured during charging, and excess energy either burned off or 'moved'
• over/under temperature; reduces the permitted charge current

A few BMS units have an inbuilt output port which can be used to run a heater.

Either you use it to drive a relay, which turns on the heater,
or else you can directly connect a flat-plate heater.

The flat-plate heater is better. It's sealed and there are some made which accept 48v, which can't then give you a shock if it gets damaged.
You can buy them from garden centres, where they are used for propagation trays,
or pet-stores because they get placed in vivariums for exotic reptiles.

I don't like the sound of insulation being placed in direct contact with the metal case of a storage battery.
You don't know what effect that might have when temperatures rise.

It's far better to surround it with an insulated case.
And I'd also leave open at the bottom, rather than try to hermetically seal the battery units against the air.

A suitable material to consider for a DIY case is the 10mm-thick plastic board which used to clad facia boards.

Yesterday I purchased the timber to knock up some kind of enclosure for my external batteries.  I have also got some foil-backed foam insulation boards.  A thought has occurred to me;  Will the foil not block wifi and thereby comms to the units within the enclosure making it impossible to monitor and control the system?

Yes @morgan  that's a fair point.

Foil insulation is already known to reduce the signal strength of Smart Meter communications and TV reception from indoor aerials.

But what is the battery case itself made from?
Is it steel?

If so, is there possibly a non-metallic portion, behind which the WiFi antenna is positioned?

If you're looking for a more environmentally sustainable way of keeping external batteries warm in winter...

... may I point out that honey bees have perfected this trick over millennia.

The 'winter workers' have a longer lifespan in order to survive during the cold months.
Instead of hibernating, they stay awake in a thermo-regulated cluster.
Their thoracic flight muscles can be decoupled from the wings, allowing them to generate heat by continual flexing.

The bees in the cluster regularly change their position, permitting those on the outside to feed from stored honey.
That maintains a steady 18°C within the hive.

Whilst having bees resident within the battery enclosure could void the warranty, 🤔 
a hive positioned below the battery would surely be capable of keeping the cell temperature above zero.

... just a thought!

@transparent I have some Hive TRV’s surplus to requirements… 😉 Toodles.