STC estimate sounds about right. Battleborn says there heater draws 1.8A when active, so that would be 3.6A for the typical STC which has two 100Ah batteries. Internal battery heaters are much more efficient per watt because they are within the battery housing. With the Victron batteries, the heating pads are much larger and external so they have to be much higher wattage to heat up the cells within the battery housing. When I saw the Victron setup for 400Ah (two 200Ah victron batteries), heating pads covered the entire front and back of the batteries, so 9" x 24" heating pads times two. I can see that drawing a fair bit considering the surface area compared to the small internal heaters.
It does seem odd that they are triggering when your lows are only 45F. Not sure where the thermostat is located on these heating pads. If external, it would have to deal with a lot of variables including the toughest, which is a below freezing cabin/battery that has recently been heated up by Truma heater. The battery itself would still be below 40F. The battery protect would not allow charging due to the internal cell temp sensor, but if the pads were controlled by an external sensor, it wouldn't know so functionally it would make sense to turn it on a warmer temps. Now you have me curious if the external heating pads are triggered by external sensor or somehow communicate with the Victron BMS. I've reached out to STC and will report back.
@roamie STC provided some more info on the heat pads used with the Victron battery solution.
As previously discussed, since Victron batteries don't have internal heater so external heat pads are used. STC shared that the heat pads watt/amp draw is 78W/5.8A for a 400Ah Victron battery bank (2 x 200Ah batteries). When the Battery Heater switch is on, the battery heat pad thermostat turns on when temp dips to 44F and turns off at 68F. The heat pad thermostat is measuring ambient temperature within the battery compartment and is not communicating directly with the Victron BMS nor internal Victron battery temp sensors. In order for the battery pads to communicate with the Victron internal sensors, a good deal more equipment would be needed including the Victron Servo system. Per the Victron rep, they have never had a customer experience issues using heat pads set up this way and they shared that the batteries are compact enough that the heating pads heat them up quickly, thus avoiding a large temperature delta between compartment temps and internal battery temps. And, STC has the Victron batteries programmed to not accept charge below 33F, so even if there was a delta, the batteries themselves would not accept charge until they were above that set temp.
A perk of external battery heater pads, vs internal battery heaters, is the draw is known to the shunt so you can see when the heat pads are drawing power. And, the Battery Monitor knows about the draw so your SOC will remain accurate. When batteries have internal heat pads, the power draw is not known to the shunt, so the battery monitor SOC does not decrease even though power is being consumed by the internal heat pads and cell SOC is decreasing.
I like external battery heater setups better as you are more aware of what is happening, SOC stays accurate, etc. And, you can always turn off the battery heater switch if you know batteries are above freezing (you've had cabin heat above freezing for a good period of time) and you are charging them. Or, when they are below freezing but you will not be charging them. No reason to have Battery Heater switch turned on in those situations.
Now, to digress and discuss batteries with internal heat. Different batteries use different methods to power internal heat pads. Last I checked, Battleborn uses the following method; the heat pads are powered only by battery cells. That allows the battery to be heated even if there is no incoming charge current. That is good if you want to preheat the batteries before a charge current in applied. Such as, before sun comes up for solar charging, to pre-heat before starting truck for DC charging, or before shore power charging is connected. The con is if you turn on battery heat and forget about it, it will run down your battery fairly low before it disconnects. Then you have a discharged battery bank. And, your battery monitor doesn't know about the draw, so it reports the same SOC from when you turned on the battery heat. This can be a unpleasant surprise if you trust the SOC when using internal battery heaters.
Some other brand batteries, with internal heaters, can only power internal heaters if there is charge input and it uses only that charge input to power the internal heat pads. Usually there is a minimum charge input, like 10A per 12v battery. The battery monitor will also be inaccurate with this method because the shunt thinks the inbound charge power is charging the battery, but all the inbound power is going to the heat pads. Note that no matter how much current is available, only the current that the heating pads use passes through the shunt and into the battery. Meaning, if you have a 50amp charge source, only what the internal heat pads use will be used. Such as 10A of the 50A available. No current is charging the battery until the BMS determines the cells are warmed up enough. Then the battery can start to charge, but heat pads remain on. So it will start using all 50A if the charger source, but only 40A is going to charge battery because 10A is still going to internal heater. Once heater turns off, all 50A can go to charge battery. However much current the heat pad used, that is how far off the Battery Monitor SOC will be.
Example: A 100Ah battery uses 10A for internal heater. It's at 50% SOC. You turn on heat pad and turn on a 20Amp charge source. If it takes exactly 1 hour to heat battery cells to adequate charging temp, the charge will have output 10A, the shunt knows that 10A went across it, but does not know that it went to internal heater rather than battery cells. Battery monitor will report that the battery SOC is now 60% SOC, but it's still only 50% SOC because all 10Amps when to the heat pad. At the end of the first hour, the cells are warm enough to take charge, the full 20amp charge goes through the shunt, but 10A is still being burned by internal heat pads, and the other 10A is charging cells. If this situation lasts exactly one hour, the battery monitor will now report the battery is at 80% SOC (reading goes from an incorrect 60% and increases by another 20% when the battery only went up 10A, so the real SOC has now moved from the real 50% to a real 60%. If the battery heater now turns off and 20amp charge rater another hour, both the battery monitors SOC and the real SOC goes up 20A (20% in this case). Battery monitor will inaccurately report 100%, but real SOC is now 80%. If you think it's at 100% and remove charge, you'll only be at 80%. The point is, when using internal battery heaters, the battery monitors SOC reading will not be accurate again until you charge to true 100% SOC. Not when the battery monitor reaches 100% SOC, because that will be inaccurate. But charged until the charger shuts off because voltage set point (typically 14.4v - 14.6v) is reached and current tapers off. Then SOC will be accurate again (until the next time internal battery heat pads are used). External heat pads have the advantage if an accurate SOC is important to you.
While I'm off on internal battery heaters, some batteries allow you to choose heat pad power source within their app and some have a third method of powering internal heaters; hybrid source. I've used batteries where you can set them to hybrid mode so, if the inbound current is not adequate to run the internal heaters, the remaining needed current will draw from the battery cells. Meaning, if they need 10A but inbound charge current is only 4A, the remaining 6A will come from battery cells to provide the needed 10A. If there is 25A of available inbound charger current, the internal heats pads will get 100% of the needed 10A current from the available inbound current. Only the needed heat pad current will be used and the remaining available current will go unused until the battery cells are warmed up and can start using all the inbound current to charge the battery cells. These same batteries allowed you to choose a standard method of internal heat that will use only current from the battery, but I always left them on the hybrid because if there was any inbound charge current available, I wanted the heaters to prioritize using that source and then fill in the remainder with battery power.
