Are you tired of buying expensive heating mats that don’t do the job very well? Or maybe you need a heating pad for a specific purpose, but can’t find the right one? Why not try making your own heating pad? It’s easier than you think and can save you a lot of money in the long run.
Here’s how one of our customers made his own heating pad using a low voltage heating cable. He mounted it on the base of an aluminum battery box. This is a very clever way to use the friendly heat transfer capability of the aluminum battery box to heat the entire battery pack.
Here is a step-by-step guide to making your own heating pad.
Step 1: Gather the materials
To make your own heating pad, you’ll need a few materials.
1, Low voltage heating cable(15 ohm/m)
3, DPST switch
You can find most of these materials at your local hardware store or online.
For 15 ohm/meter heating cable here’s a link provided by our customer: https://a.co/d/2dTh3zK
Step 2: Milling slots in the aluminum battery box
Using a milling machine or drill press, mill a 1/4-inch slot in the 1/2-inch base of the aluminum battery box. This is where you will mount the heating cable.
*In this case our customers use aluminum battery box. Depending on your battery box, you can decide whether to install the heating cable on the base of the battery box or add an additional pad to the bottom to install the heating cable.
Step 3: Install the heating cable
Place three parallel strands of low-voltage heating cable into the milled slots and secure it with silicone.
Step 4: Connect the heating cables
Connect one end of the heating cable to the JK BMS(JK-B2A8S20P) heat connector and the other end to the positive terminal of the battery. This will allow the heating function to be turned on and off directly by the JK BMS without the need for an isolation relay.
Step 5: Add a DPST Switch
Insert a DPST switch into the heater circuit to bypass the BMS’s control and manually force the heater on. This is useful if you need to turn the heater on without waiting for the BMS to turn it on.
Step 6: Test the heating pad
Test the heating pad to make sure it is working properly. A 50W heater should work well and not get too hot. You can also adjust the temperature by increasing or decreasing the number of strands of heating cable.
Below are installation photos shared by our customer.
1, Base with heater
2, Battery cells installed
3, BMS installed
4, Completed battery pack
How to Choose the Right Heating Wire and Calculate Heating Power
If you open the Amazon listing for low voltage heating cables mentioned earlier, you will find several models to choose from. They mainly differ in their resistance, with options of 0.3, 1, 1.4, 3, 5, 10, 15, 20, 28, 40 ohm/m.
So, how do you choose the appropriate model?
First, you need to determine the layout and length of the heating cable based on the battery pack’s size. Then, taking into account the maximum current and heating power that the BMS heating port can provide, you can plan the model (ohm/m) of heating cable within the safe operating range of the BMS.
For safety reasons, the heating cable should be as long as possible and in multiple strands connected in parallel, to reduce the power per meter.
Let’s take the case of our customer as an example.
Based on the battery pack’s geometry, the heating cable layout here requires three strands of 2.7m(106 inches), which will be connected in parallel.
The BMS JK-B2A8S20P‘s heating port allows a maximum current of 3A.
The battery pack consists of eight LF280K cells with a nominal voltage of 25.6V.
So, the heating cable’s resistance should not be lower than:
R=Voltage/Current = 25.6V/3A = 8.5 ohm
In practice, we should not operate the heating cable at the BMS’s maximum current limit. Otherwise, a simple current fluctuation could cause the BMS to shut down the heating function. It’s best to leave at least 30% margin.
So, the heating cable’s current should not exceed 3A * 70% = 2.1A
The minimum resistance of the heating cable should be recalculated as:
R=Voltage/Current = 25.6V/2.1A = 12.2 ohm
As this is a heating cable formed by connecting three strands of 2.7m (106 inches) in parallel, the resistance of each heating cable is:
R= 12.2 ohm * 3 = 36.6 ohm
Hence, the resistance per meter of the heating cable should be:
36.6 ohm/2.7m = 13.5 ohm/m
That’s why our customer chose a 15 ohm/m heating cable for this system.
Let’s now see what the heating cable’s resistance would be if we use the 15 ohm/m heating cable.
Resistance = (15 ohm/m * 2.7m) / 3 = 13.5 ohm
Current = Voltage/Resistance = 25.6V / 13.5 ohm = 1.9A
Next, we need to confirm that the heating cable’s power does not exceed the maximum power it can withstand.
First, let’s look at the power of the heating wire:
P=Voltage * Current = 25.6 V * 1.9 A = 48.6 W
So, the power of each heating cable will be:
P= 48.6 W/3 = 16.2 W
The power per meter will be:
P= 16.2 W/2.7 m = 6 W/m
We need to ensure that the power per meter does not exceed the heating cable’s specifications.
According to some listings on Amazon, the specifications of the heating cable are as follows: the surface temperature for 5 W/m is 29.4°C – 40.5°C (80°F – 105°F), and the surface temperature for 10 W/m is 40.5°C – 51.6°C (105°F – 125°F).
The power of the 6 W/m wire does not exceed the specifications of the heating cable.
Thus, the selection of the heating cable is complete.
In practical operation, the actual temperature reaching the battery will be much lower than the surface temperature of the cable due to the influence of the thermal medium.
Therefore, temperature testing is also required after installation to see if it meets the expected heating requirements for the battery pack.
In the customer’s case, after running the heater for 3 hours, the temperature of the silicon was about 43°C (110°F). After running the heater for 4 hours, the temperature of the fully assembled battery increased by 11.1°C (20°F).
For the customer’s case, this is perfect because he used an aluminum box and installed the heating cable inside the bottom of box, which has excellent heat conduction ability.
In addition, the following points should be noted:
1. Different media have different conduction capabilities, which have a direct impact on the final heating results.
2. Avoid using short cables and do not let the power produced per meter exceed the rated power of the heating cable.
3. Cable evenly and handle heat dissipation well, avoiding direct contact between the heating cable and the battery cell, which can cause a large amount of heat to accumulate in a small area.
4. Verify repeatedly and carefully to ensure safe heating.
Making your own heating pad is a great way to save money and create a custom solution for your needs. With a few simple materials and some basic tools, you can create a heating pad that works just as well as a store-bought one. So why not give it a try?
*All of the content in this article about DIY heating pad comes from what one of our customers has shared. We salute this customer for selflessly sharing his experiences.
Energiepanda can currently deliver JK BMS quickly to customers in North America, Europe, Australia, Asia and South Africa
Charging LiFePO4 battery at freezing temperatures can be challenging as the low temperature can affe…