BMS with heater function. 30w too low?

[nebulight]

I'll be installing a 8sp2 system into an EEL server rack battery box for a 24 system with the new JK BMS inverter BMS. EEL sells heating pads (designed for 48v systems) that has two pads in series. I figured the lower voltage should just halve the power. I measured the resistance and it's around 27.5 ohms which should output around 60w on a 48v systems and around 30w on a 24 system. These are designed to go on the bottom of the case and heat the batteries from below.

My question is 30w too low? Since these are in series, should I cut the series connection and put them in parallel which will half the resistance to get back to around 60w? I typically don't recommend heating batteries since it just wastes battery storage, but the way these new bms' work, they just take the charge current and divert to the heaters until the battery warms up, then charges so really there is no wasted energy. Previous setups would just heat all the time until they came to temp (battery or solar charge, it didn't matter)

 

[Partimewages]

I can't answer your specific question.
1. Did you ever try to hold a 60 watt incandescent bulb? Lots of heat.
2. Is the pack going to be in a cold climate all the time?
3. Will the pack have to be charged when it's below 32F or 0C?

 

[nebulight]

I can't answer your specific question.
1. Did you ever try to hold a 60 watt incandescent bulb? Lots of heat.
2. Is the pack going to be in a cold climate all the time?
3. Will the pack have to be charged when it's below 32F or 0C?

1. I was thinking the same thing, however this 30 or 60w will be distributed over a larger surface area so I figured that it would probably heat much slower and less direct so it wouldn't have the same impact as a lightbulb with the resistor over less than an inch.
2. No, not all the time. It's going to be installed in my camper van. We do take winter trips.
3. Maybe? We are running a combo AC/Heat pump so if we are running the heat pump in the winter off the batteries, I'd like to be able to recharge. The battery will be in a heated space but if we did a long hike away from the van, we probably wouldn't leave the heater on while away.

 

[12vDC]

For reference I have 6 small pads attached to an aluminum plate for 50W @13V.

In my testing in open air I found it acceptable with heat being absorbed into the battery cases surprisingly quickly.

 

[nebulight]

I’ve had the series pad connected for about 30 min and it’s hardly warm. I cut the series connection and put them in parallel it’s much better. It’s noticeable warmer but still able to leave my hand on the pad as it’s distributed over a wider area. I think this should be good enough as I probably won’t be camping in single digit temps.

 

[12vDC]

I’ve had the series pad connected for about 30 min and it’s hardly warm. I cut the series connection and put them in parallel it’s much better. It’s noticeable warmer but still able to leave my hand on the pad as it’s distributed over a wider area. I think this should be good enough as I probably won’t be camping in single digit temps.

"Probably" famous last words...

Yeah I also tested with each two-pad string in series and it took too long to achieve any warmth so I decided to stick with the 7W 12V rating of each pad. Maybe in a sealed container a battery box exposed to harsh conditions even for long durations the lower wattage would be okay. My tray is enclosed but not sealed and sits under the van, I will need the power.

 

[Partimewages]

Just remember 1 watt = 3.412 btu's.
It takes 1 btu to raise 1 pound of water 1 degree. 100 pounds of batteries I personally would use the 30 watts. It's going to take quite some time to make that bank below freezing.
You could always put a switch in to go back and forth.

 

[OTRwSolar]

We did some informal testing with a few 200-300ah 4s cell packs placed outside our home during the winter and inside our home freezer. We found ~1.5w of heat per kg (or ~1w per 1.5 lb.) of cell weight was generally enough to keep the cells heated to 55 - 60f (13c - 16c) when exposed to ambient temps of approx. 0f (-18c). The heating pads were applied directly (and evenly distributed) to all the cells. Used no formal insulation---just placed the cells inside a plastic battery box with a cover.

 

[12vDC]

also

the CORE temp is what we are aiming for vice a dropdead ambient temp of 0°C

once the cell core becomes “frozen” it will take much more external thermal compensation to overcome. conversely when the core is warm or hot it takes a while for that temperature to recede.

 

[OTRwSolar]

the CORE temp is what we are aiming for vice a dropdead ambient temp of 0°C

once the cell core becomes “frozen” it will take much more external thermal compensation to overcome . . .

To my knowledge there's no way for the average Joe to accurately determine core temperature. We always assume worst case----the core is at ambient (or the lowest known ambient).

Also, to clarify, I believe it's heating "time" that will increase when heating a "frozen" (vs. unfrozen) core. This, of course, assumes the same ambient, same heating source, and the same cell/core temperature (as your final heating goal) for both the frozen core vs. the warmer, unfrozen core. Increased heat will result in reduced heating times, but could also cause uneven heating (thermal lag) and ultimately higher cell temperatures (if a thermostat is not used). Have to be careful with this approach. As they say, low and slow.

In any event, core temperature during the winter is more or less a moot point for us. During the winter we maintain our batteries/cells at a constant 60-65f with thermostatically controlled heating pads. Very little thermal lag issues (core vs. outer case temperatures) when the cells are maintained at a constant temperature. We maintain 60-65f because we do .4c charges and .6c discharges on a regular basis. We were surprised just how few ah's it takes to maintain cells at 60-65f, 24/7---especially with good insulation. Ah's well-spent vs. potential cell damage.

 

[12vDC]

I didnt infer time which is correct, I was attempting to be succinct. My heating system is temp controlled, no worry for over heating.

We maintain 60-65f because we do .4c charges and .6c discharges on a regular basis.

Please clarify the above statement. The heating you provide maintains the heat or the discharging does?

You already agreed maintaining a CORE temperature is the goal but the average Joe would also not know the precise discharge calculation over time to have any appreciable effect. You are charting these results?