Battery heater sketch. Please give feed back.

noenegdod

Solar Addict
Joined
Mar 21, 2020
Messages
949
I live in Canada and the truck will sit for weeks at a time in the winter not being used so heating the battery constantly doesn't make sense. As such there is a lot more power required to recover than would normally be required to simply maintain because the battery will have to be pulled up from well below freezing when I want to use the truck and it would be nice it it could happen in half a day.

I dont know how much I need to explain so Im just going to stick my cartoon below and please feel free to pick it apart and ask questions. Any criticisms are greatly appreciated.

Major points that may not be obvious are:

  1. Shore power is the default when plugged in
  2. Battery and alternator power via the DC-DC is the second option when not plugged in
  3. When it is below 5degC the BMS turns everything off so a manual switch allows power from the chassis system but only when engine running (Relay with coil power from fuel pump)
  4. The thermostat on the right controls the primary heat source. It controls two 120W heating pads in series for 60 watts total. Its thermocouple is on the top of the battery and the thermostat will be set to around 10degC.
  5. If Im in a hurry and/or have a long way to heat them up I can press the momentary switch coming off the output of the first thermostat which will energize the second thermostat. This controls a 120 watt heating pad and if I close the switch, it will also control a second 120watt heating pad. This thermostats thermocouple is in the 1" airspace between the aluminum heat transfer plate and the bottom of the battery. This thermostat will be set somewhere around 30deg
  6. When the first thermostat ( The one set to 10 degrees) reaches set point and opens the circuit, the relay controlling the second thermostat opens and deactivates the two 120 watt heating pads and does not reset. Heating from that point is done only by the 60 watt pads unless the momentary switch is pressed again.
  7. The battery is 16 cells and there is airspace between each cell so heat to the top of the battery is not by conduction through the battery alone. There will be a convection current moving heat as well.
I appreciate any suggestions!
 

Attachments

  • Scan0015.pdf
    141.3 KB · Views: 41

kenryan

Solar Addict
Joined
Jun 29, 2021
Messages
181
I would just decide on a number for watts of heating and run it through an on/off switch with an adjustable high temp cutoff, and then experiment in the real world.
 

HRTKD

Boondocker
Joined
Apr 24, 2020
Messages
7,481
Location
Somewhere South of Denver
I run 24 watts of heating pads on each 4s battery. That's two 12 watt pads, one on either of the long sides of the battery. Your watts with the first stage aren't far off from what I'm using.

I've never let mine go below 32° F, so I don't know how long it would take to bring them up to a chargeable temperature. My memory is a bit fuzzy, but I recall that it takes 45 minutes to go from 35° F to 45° F.

Why do you have space between your cells?
 

noenegdod

Solar Addict
Joined
Mar 21, 2020
Messages
949
I run 24 watts of heating pads on each 4s battery. That's two 12 watt pads, one on either of the long sides of the battery. Your watts with the first stage aren't far off from what I'm using.

I've never let mine go below 32° F, so I don't know how long it would take to bring them up to a chargeable temperature. My memory is a bit fuzzy, but I recall that it takes 45 minutes to go from 35° F to 45° F.

Why do you have space between your cells?
I used 2 kJ/kg/degC which is probably a bit on the high side and I used -40C - 10C as the worst case. I do believe it is recommended that -20 is the coldest they should be though.

2000J/kgx100kgx30degC=6,000,000J

6,000,000J/300W=20,000seconds = 5.5 hours to recover to 10C from -20 (it was around 9-10 hours from -40)

This is what the space is from:

20210209_072652.jpg


20210311_203444.jpg

The busbars go over the holes that come up from the bottom but there is still about 1/2" gap under them.

20211009_162015.jpg
 

noenegdod

Solar Addict
Joined
Mar 21, 2020
Messages
949
I just did a quick google search and found this:

"Storing batteries below freezing is fine, even at very low temperatures such as -40 Centigrade (that is the same in Fahrenheit), or even less! The electrolyte in LiFePO4 cells does not contain any water, so even when it freezes (which happens around -40 Centigrade, depending on the particular formulation) it does not expand, and does not damage the cells."

From:
 

HRTKD

Boondocker
Joined
Apr 24, 2020
Messages
7,481
Location
Somewhere South of Denver
It seems to me that with those holes, some of your heat is going to come up them. If the temperature probe has access to the heat coming up the holes then it could shut down the heating pads prematurely. If you can isolate the probe from the ambient air then that is less of a concern.

The foam on the back of my heating pads is surprisingly good at keeping the heat on the battery and not letting it radiate out away from the battery. Since I have rigid foam around my battery, I did try to isolate the probe from the ambient air, just in case.
 

noenegdod

Solar Addict
Joined
Mar 21, 2020
Messages
949
It seems to me that with those holes, some of your heat is going to come up them. If the temperature probe has access to the heat coming up the holes then it could shut down the heating pads prematurely. If you can isolate the probe from the ambient air then that is less of a concern.

The foam on the back of my heating pads is surprisingly good at keeping the heat on the battery and not letting it radiate out away from the battery. Since I have rigid foam around my battery, I did try to isolate the probe from the ambient air, just in case.
Heat coming up the holes is the goal. This is all theory right now but the temperature under the batteries being set to 30 will drive the heat up the holes. The additional surface area up the sides of the cells will increase heat transfer into the cells and cool the air traveling up. Once the cells are not able to cool the air enough to prevent the 10C setting for the top thermocouple from being reached, most of the work should be done by that point and the high power pads will be disabled. Then as the cells continue to absorb heat the top thermocouple will drop below 10 and the 60 watt elements will continue the remainder of the journey. The BMS temp probe is sandwiched between two of the cells and should not prematurely reach operating temperature until the cells are actually at temp (5C).

This battery will have EPS foam completely surrounding it as well.
 
Top