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Heating Base for 12v LiFePo4 Battery

chalupa_monster

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Feb 27, 2022
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After absorbing so much information from this forum when building my 12v LiFePo4 battery using 230Ah cells for my shed, I wanted to share the solution I came up with for heating the batteries during winter in Northern Illinois.

7b11PRq.jpg
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I'm using the JBD 200a Smart BMS with the heating lead. The BMS only engages this lead when the temperature is below the low charge temp cutoff and energy is being supplied by the solar charger. I wish it would start running at the low temperature release value, but maybe future iterations will improve on this.

I cut 1x3s to border around the edges of a 12x12in aluminum sheet and drilled holes through through the sheet to run wood screws into the 1x3's. I'm using these 12v silicon heating pads, with two sets of pads wired in parallel to divide the voltage and decrease the output. I used silicone glue from the local home improvement store to adhere them to 12x12in 1/8 thick aluminum sheet.

The performance is about what I expected. In -6C weather, it heats the battery by 3-4C per hour using ~30w. I don't need super fast heating as my panels face west and there is enough light in the AM to heat the batteries before direct sunlight reaches the panels. The temperature probes are mounted to the top of the battery, insulated with tape and foam strips. The slow heating helps reassure me I'm not cooking the battery and the temperature delta between the top and bottom isn't too great.

Anyhow, I'm open to input or suggestions. Personally I think came out great and works really well for a non-critical DIY battery.
 
The end product looks neat and tidy. You could use some edge banding if you want to hide the wood joints. But for a DIY garage battery heater, I wouldn't worry about it.

Good job.
 
Those heating pads, looks like two in series and two strings. 2S2P configuration.

That means each pad is only seeing ~6v each. That doesn’t increase heat output. Each pad should be run in parallel for max heating.

Are you using a relay from the BMS to control voltage for the pads? I forget what the max output capacity is for the BMS heating circuit is.
 
That's what I was thinking of doing. But just ONE of those pad should be enough don't you think? I mean, they get really hot...unless they're not the ones made for transmission and oil pans on trucks.
 
Those heating pads, looks like two in series and two strings. 2S2P configuration.

That means each pad is only seeing ~6v each. That doesn’t increase heat output. Each pad should be run in parallel for max heating.

Are you using a relay from the BMS to control voltage for the pads? I forget what the max output capacity is for the BMS heating circuit is.

Correct 2S2P with the heating pads, my goal was to decrease the output. I didn't want 100+ watts of heating necessarily. Voltage from the Victron when charging is 14.2v and since it's only heating when it's in low temp protection that's what the pads see.

That's what I was thinking of doing. But just ONE of those pad should be enough don't you think? I mean, they get really hot...unless they're not the ones made for transmission and oil pans on trucks.

One might enough, but I think they mostly come in 4 packs from Amazon - https://www.amazon.com/gp/product/B075WVPP5Y/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1.
 
Our 4S pack uses the same (physical) size cells, (4) 25w heating pads, and JBD 200a BMS (with the heating pad option) as the OP. We also wired the 4 pads in a 2S2P configuration so each pad only produces 6w (total 24w or 2a at 12v). When the pads are producing their rated 25w, they can potentially burn you. At 6w they're only very warm to the touch.

As for mounting the heating pads, we took a different approach than the OP. We simply taped a pair of heating pads, directly to the cells, on opposite sides of the cell pack (total 4 pads), using only heavy-duty HVAC tape. Took about 5-10 min. These pads have kept our cells heated to 60f with an overnight ambient of 0f several times (cells are mounted inside an uninsulated Group 24 plastic battery box). Used them all last winter with zero issues.

We use a 2-position toggle switch to select how the heating pads are controlled and powered. One switch position lets the JBD 200a BMS (with the heating pad option) divert charge current to the heating pads when charging is attempted below the charge low-temp cut-off. The other switch position powers the heating pads directly from the cells themselves via a separate thermostat. We usually find it less hassle to use the latter option. The occasional 2a draw to keep the cells heated to 60f is (for us) a small price to pay to have the option to safely and efficiently, charge or discharge, at a very high c rate at *any* time in very cold weather. YMMV.
 
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Forgot to mention in my previous post, turning on the heating pads with the cells at an ambient temp of, say 20f, the 4 pads (total 24w/2a at 12v) will heat the cells approx. 10 deg (f) per hour. Temp sensors are mounted on the top, middle of the cell pack. Verified the cell pack was being heated evenly (+/- 1f) on all 6 sides with an infrared temp gun. Although this is a decent heating rate, we'd still have to wait an hour or two to charge or discharge at a high c rate in cold weather. A big reason why we prefer using a thermostat to keep the cells at 60f the entire time we're camping in cold weather.

We did some testing with the cells (inside an uninsulated Group 24 battery box) sitting outside on our deck last winter. With ambient temps of ~20f, I'd guess-estimate the pads were operating at a ~50% duty cycle to maintain 60f. This went up to a 100% duty cycle at 0f (to maintain 60f).
 
Forgot to mention in my previous post, turning on the heating pads with the cells at an ambient temp of, say 20f, the 4 pads (total 24w/2a at 12v) will heat the cells approx. 10 deg (f) per hour. Temp sensors are mounted on the top, middle of the cell pack. Verified the cell pack was being heated evenly (+/- 1f) on all 6 sides with an infrared temp gun. Although this is a decent heating rate, we'd still have to wait an hour or two to charge or discharge at a high c rate in cold weather. A big reason why we prefer using a thermostat to keep the cells at 60f the entire time we're camping in cold weather.

We did some testing with the cells (inside an uninsulated Group 24 battery box) sitting outside on our deck last winter. With ambient temps of ~20f, I'd guess-estimate the pads were operating at a ~50% duty cycle to maintain 60f. This went up to a 100% duty cycle at 0f (to maintain 60f).
Would really like to see a pic if you have any. About to do the same for my camper.
 
Would really like to see a pic if you have any. About to do the same for my camper.
There are two more heating pads on the opposite side of the pack (4 pads total). Would only recommend applying the pads like this if the 4, 25w pads are used in a 2S2P configuration with each pad providing 6w of heat. Used in a 4P configuration (with each pad producing 25w) could potentially damage the cells IMO.

Lifepo4 - Heating Pads_sm.jpg
 
I was worried about heating the cells directly so I suspended mine and heat the air around them, one 25 watt pad between thin aluminum per battery with air flow.
 

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