That's a tidy install @windsor-tg. Li-ion batteries only tend to de-rate charging capacity below 0degC. How often does your garage get below 0degC? You can partially or wholly insulate the garage if this is a concern, or fit a small thermostatically controlled frost protection heater. Don't put anything around or on the batteries however.

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Thanks @allyfish. I am not really sure how often the garage gets below 0C but even though my garage is attached to the house, I have an up & over door on the garage, and the three outer walls are all single brick so the garage is not too well insulated. I have a portable thermometer so I could position this on top of one of the batteries to see in winter if the temp drops below 0C. 

I don't know if you can see it clearly on the photo I attached before but you may see a double socket on the LHS of the inverter. This socket has attached a digital thermostat with temp sensor and I have placed the sensor against the casing of the inverter. Plugged into the temperature switch are 4 small USB fans, which I have positioned just below the inverter at the back, so when the inverter temp reaches 39C, the temp switch turns on, and the fans start to cool down the inverter.  

I am wondering if I can use this temperature socket for plugging a heater, or alternatively buying a thermostatically controlled frost protection heater. The combined width of the batteries on the wall are 3 feet - would something like this 2-feet heater on Amazon be suitable, and is it best to place a heater that faces the batteries, or placed underneath? 

I has this person on the SunSuynk forum recommend doing something similar to what he did which was building a cupboard around the batteries (without a back), lining the sides with silver bubble sheets (what you put behind radiators), and then placing a couple of heated pads. Previously, he had wrapped the batteries in a hot water tank jacket strapped on with thin rope.

This person said that it was important to keep the batteries warm in the winter as it allows then to charge and discharge at full 2500w power. Below 12c, SunSynk batteries halve it to 1250w and approaching 0c, they turn off altogether. Would this be true? 

Many years ago (late 70’s or early 80’s) I had a Westland Garador fitted to the front of an extension built at the side of the house. This up and over door was not for car access but the area was my workshop and storage space. Come the winter I noted the greatest loss of heat was through the Garador and about that time, took delivery of something packed in a large box lined with about 25 mm expanded polystyrene sheets. The sheets could have been made to measure for the horizontal ribbed ‘panels’ in the door, so I lined it with the sheets - they not only deadened the ‘clang’ when shut but made a noticeable difference to heat loss from the cavity walled brick built extension. I held the sheets in place with judicious use of gaffer tape and it was all intact still when I sold the house some years later. Regards, Toodles.

thanks @toodles. The inside of the garage door has some form of insulation, not sure what it is but I have attached a photo of the inside of the door.

I couldn’t see any thermostat mentioned in the description of the tubular heater you mentioned but there are models of heater available for frost protection so that by placing one beneath your battery, you might have it set to ‘frost protect’. At any other time, the heater would be ready but not consuming power if it is warmer. Regards, Toodles.

@windsor-tg Sunsynk battery data sheet.

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Lithium Batteries | Sunsynk

Seems to indicate no de-rating between 0 and 50degC. 'Operating temperature' might be inside the battery, so you could assume 40degC maximum garage temperature. A rule of thumb we in the HVAC industry use is that the insides of controls and equipment are normally about 10degC higher than the surrounding ambient air temperature.

If Sunsynk wanted batteries to the thermally insulated or lagged they would include that in their product. Externally applied insulation or enclosures around them will only be detrimental to the product, as it may cause it to overheat meaning the BMS has to intervene and protect the battery. I really don't think you need to worry about batteries in a garage or provide any form of heating or supplementary cooling. Garages are quite good locations. A built in frost protection heater that energises when the garage gets to, say, 5degC, could be installed, but it's probably not necessary.

In my day job I'm working on 200kVA Li-ion UPS battery racks, they like to be at 20degC and max 50% RH, but they are quite specialist items by Siemens. 

thanks @allyfish. In the winter, I would think my garage could close to 0C on a very cold night. I have not really checked in the past but I have now put a thermometer on top of one of the batteries to monitor when we get into the cold period in winter. 

Where you say "no de-rating between 0 and 50degC" - what do you exactly mean by that? What happens if the temp drops below 0C? 

@windsor-tg I’m looking for a solution to keep my batteries warm too. Also have a sunsynk system in a detached garage. Last winter the batteries went below 0°C on several occasions and this made them refuse to charge. I tried many things like getting the batteries to discharge so they warm themselves up enough to charge again and pointing a fan heater at them. Neither worked very well. Once the batteries reach 0°C they refuse to charge until the temperature goes above 2°C. Mine are also mounted straight on to a single brick wall. I’ve seen mention of mounting them on wood instead of straight on to the brick wall or a kitchen unit like you mentioned or celotex. I’m not sure which route to go down but definitely need to do something to prevent it happening again this winter.

@allyfish sorry wrong forum but picked up on HVAC. I've been thinking about getting some decentralised units for utility/ensuite areas am looking at Blauberg if you have any recommendations?

@lukeja - did you look any further on what best to do this coming winter to keep your Sunsynk batteries warm? 

I have a temperature sensor socket that I’m using to cool down the inverter in the summer. The sensor is attached to the casing of the inverter and the socket powers 4 x small fans when a set temperature is reached.

I was thinking about moving the temperature sensor and attaching this to one of the batteries and using the temperature sensor socket to attach a tubular heater to heat up the batteries when the temp dropped below a set temperature. 

is this an option?

@windsor-tg I put some thick celotex around the front and sides, it has made very little difference and the batteries are currently getting to 2°C and winter is just getting started so it’s not going to work. I think the problem must be that it’s mounted on the brick wall.