schroederjd
Solar Enthusiast
- Joined
- Nov 12, 2020
- Messages
- 175
Battery Case Update: I completed a 24-hour test of the battery heating 'system' (ie, heating pads + insulated box + controller). For the first 6 hrs, the heating pads were cycling at a 33.3% duty cycle (1 sec ON, 2 sec OFF, ~33watts). The heating pads, which are in direct contact with the bottom compression plates, were getting a bit higher than I wanted to see it (trying to keep the compression plates below 40C), so after 6 hrs, I dropped the cycle down to 16.7% (1 sec ON, 5 sec OFF, ~17 watts). The box remained undisturbed with the heater on that duty cycle for the remainder of the 24 hour test.
Results are pasted below. Again, I had three thermistors in the box (one between the pad and the bottom plate (T1), one center pack (T2), and one top of box (T3)), and one thermistor outside the box measuring ambient (T4). Overall, I was quite surprised by just how little heat was required to increase the pack temp. Based on my average duty cycle over the 24 hrs, I applied an average of about 20 watts of heat. That level of heat was able to raise the pack temp to 12C (22F) above ambient over the 24 hr test, and temps were still rising steadily. I did take some additional readings at the end of the test, and found that the bottom cells (those in direct contact with the compression plates) were around 32C. The temperature gradient from bottom cells to top cells (32C to 24C) does concern me a bit, and make me think that it's important to maintain temp, rather than to raise it with long periods at higher duty cycle.
I'm thinking that I should be able to keep the heat control fairly simple for the final configuration. Maybe have 5 different duty cycles, which are triggered based on pack temp. Maybe something like this.
Let me know if you think this will work, or if you think there might be a better approach to consider. I'm also interested in thoughts on the 'optimal' relay switching frequency for a setup like this might be. Not really sure what the pros/cons are to longer vs. shorter cycle time.
Results are pasted below. Again, I had three thermistors in the box (one between the pad and the bottom plate (T1), one center pack (T2), and one top of box (T3)), and one thermistor outside the box measuring ambient (T4). Overall, I was quite surprised by just how little heat was required to increase the pack temp. Based on my average duty cycle over the 24 hrs, I applied an average of about 20 watts of heat. That level of heat was able to raise the pack temp to 12C (22F) above ambient over the 24 hr test, and temps were still rising steadily. I did take some additional readings at the end of the test, and found that the bottom cells (those in direct contact with the compression plates) were around 32C. The temperature gradient from bottom cells to top cells (32C to 24C) does concern me a bit, and make me think that it's important to maintain temp, rather than to raise it with long periods at higher duty cycle.
I'm thinking that I should be able to keep the heat control fairly simple for the final configuration. Maybe have 5 different duty cycles, which are triggered based on pack temp. Maybe something like this.
Pack Temp Duty Cycle
>10C 0%
7.5 - 10C 5%
5 - 7.5C 10%
2.5 - 5C 15%
<2.5C 20%
Let me know if you think this will work, or if you think there might be a better approach to consider. I'm also interested in thoughts on the 'optimal' relay switching frequency for a setup like this might be. Not really sure what the pros/cons are to longer vs. shorter cycle time.