I am designing a backup system for a gate (240V, 3/4 HP motor) in an extreme climate of -10°F (-23°C). I will use a MultiPlus-II 48/3000 and two Victron LiFePO4 25.6V/200Ah Smart batteries.
Since the manual prohibits charging below 5°C, I plan to install an 80W heater controlled by the Cerbo GX relay inside an insulated cabinet.
Technical question: Does Victron consider 80W to be an adequate and safe load to maintain the thermal mass of these batteries in that temperature range without compromising the integrity of the battery casing?
We build our battery heaters using small silicon pads encapsulated in sheet metal. There’s a little more to it, but that’s the gist. This helps to distribute the heat from the pads across the bottom of the battery. A 20W silicone heating pad can deform or melt the plastic case if you just stick the pads directly to the battery. Make yourself a heat spreader and you should be ok.
I solved the heating of the LFP this way;
This has nothing to do with the battery’s specific heat capacity (“thermal mass”), but solely with the insulation around the battery. You need to heat it exactly as much as the battery loses through the insulation.
The insulation has a physical parameter for this: it’s thermal transmittance, or U-value.
Let’s assume the U-value is 1 W/m²K (this says: heat loss is 1 Watt per square meter and per temperature difference (in Kelvin)), and your insulated battery box has an outer surface area of 2 m², and the temperature difference is 30 K (+6°C inside, -24°C outside). Then your battery loses heat with a power output of 60 W. To maintain the internal temperature at +6°C, you therefore need to reheat it with exactly this 60 W.
You can find many U-value calculators online, which you can use to calculate your heat loss for a desired temperature difference.
2 Likes
I used an AI to size the heating system.
One of the crucial considerations is the heatflux W/dm2 through the battery casing.
I think the lower, the better. It avoids thermal stresses in the casing but also in the interior of the LFP
I would never trust an AI for that. Calculating the heat loss itself is a trivial matter that requires only elementary arithmetic. Don’t give control of your life to an AI.
The lower the better in cold conditions. But you should definitely make sure that your battery doesn’t overheat in warm environments.
To me, the main advantage of an AI is that it searches the web for components much faster than I ever could.
Always use more than one.
I keep on monitoring the coretemperatur of the LFP and the temperature measured at the +.
How the difference between the two develops is a measure of how quick the heat flows through the battery.
Of course charging and discharging have an influence of the temperature.
I stay on the vary safe side switching between 7C en 8C measured on the +.
Victron stipulates a maximum operating temperature of +50C.
The heating system remains for from this, it could not even achieve it.
The purpose of the system is to keep the LFP (BMS) from switching off due to a core-temperature <5C.
It has no other function.
The insulation is the easiest part to be altered. I can alway add more or remove some.
Bad insulation shows itself by long periods of being switched on.
What matters is the % switched on time.
Based on the same environmental conditions;
If this is low the heater has too much power.
If it is close to 100% a bid more Watts or better insulation help.
Current AI does nothing more than search the web. And because there’s so much utter rubbish on the web, AI contributes to the further spread and amplification of this rubbish.
Yes, AI is currently good for searching. But it’s certainly not good for determining reliable technical solutions.
But you can easily do it right from the very beginning. This is not a lottery. You can calculate exactly the thickness of your insulation for your indiviual conditions. No need for try and error …
I designed a battery room for someone in Sweden, en they do not need any heating. The U factor of the shell is 0.1385 and that of the batteries another 0.45 and because of that the discharge en charging takes care of the heating.
Yes. It can get pretty hot. Got word the other day that they ran into a VRM alarm (35°C) twice. Something which is easily solved. In the summer the door is opened.
Anyway. Good luck 
1 Like
I do not understand the focus on the insulation?
This is not the key topic.
Key is not to put too much heat into the sidewalls which will damage the LFP but also not too little in order to have enough to warm up the interior.
Heatloss is secundary.
Heatloss can be compensated for by keeping the heating switched on for a longer period.
The most important roll for the insulation is to avoid cold spots.
The warmth from the heating elements should be destributed evenly over de exterior of the LFP.
In my opinion, insulation is a matter of suck and see.
In an existing installation there are not always all liberties’ to insulate because of other components, wires, pipes, structural elements etc around the LFP.
Think about a cramped camper or boat.
If have the luxury of designing a new system everything can be done optima forma.
Oh yes, this is the key topic:
If 80 W is OK is ONLY a question of insulation. Nothing else. I have explained the physics.
1 Like
I use a 100w ceramic heater for my pylontech as I wanted it too look more professional so they are installed in a server rack now.
Previously I used insulation PRI board as a box for them and one of the light bulbs that you would have in like the reptile enclosure think it was 20watts because the heat loss was really low charge was enough to increase the heat so it didn’t use the bulb much to be honest, but if you can access the battery temp use a Shelly temp device to automate the light/heater.
Summer is the issue with such a construction as it locks the heat in. Now I’m using server racks I’ve put a portal AC in the garage and core drilled the pipe outside and automated the fans in the rack and the AC so that it would have to see temp in the garage and temp in the battery over a certain point before it turns on the AC and once it goes down which is determined with the battery temp AC will turn off. I use Home Assistant for all of this
This, this is the answer. You want all the cells in the battery to be at as uniform a temperature as possible so how you apply (spread) the heat is important. One question is “how big does the heater need to be?” and this, as pointed out, is largely driven by the insulation/heat loss and is easily ballpark estimated. You want it big enough, but bigger is not necessarily better. The second question is HOW to apply the heat and Ed’s answer is spot on. As a point of reference, keep in mind that most soldering irons consume just 20-60W so the devil is in the details when you have 80W.
1 Like