I have a pair of motorcycle heated gloves that use a small 7.4V 2200mAh Li-On battery. In the highest setting they claim to last 1.5hours so I figured that the maximum current draw is about 1.5Amps.
Unfortunately the gloves are not warm enough for very cold temperatures (40F and below). I would like to make a 3S1P battery pack using 18500 cells (18500s are shorter than 18650s and they can fit inside the battery pouch while not being too bulky when using the gloves). Each battery is 4.2V so that would give me 11.1V when fully charged and about 50% more heat. I would also need to get a charger for that pack but that is easy to find.
There are a couple of vendors on aliexpress and alibaba who wrote that they can make the pack for me but from what I have been reading in this forum it is difficult to trust that the batteries will have the rated capacity they claim.
The other option would be to get the trusted brand cells (from fasttech or other trusted vendors), glue them together, solder a PCM and shrink wrap them. Unfortunately I don’t have a spot welder so I can’t make the battery pack myself. Can anyone on this forum help me with this?
Problem is that 18500s, while being only a smidge shorter’n 18650s, likely have lots less capacity. Just comparing 18350s at 800mAH-900mAH vs 18650s at anywhere from 2600mAH to 3400mAH, 18500s are even less popular than 18350s so might also have abysmal capacity comparable to 18350s.
And running at higher voltage (1.5×) won’t burn off at 1.5× the current, but maybe 1.2×. Resistive metal, just like a lightbulb filament, so resistance also increases with increased temperature, limiting current somewhat.
And make sure that even at boosted heat if you still want to go down that road, that it won’t melt the material immediately around the heating element wires.
The rated capacity of the original 7.4V is 2200mAh. A good 18500 such as Panasonic should be around 2000mAh, so the capacity of the 18500 would be acceptable. The problem with 18650s for this particular application is that there is not enough room in the glove pouch for 3 18650s.
I already check that the increased heat is safe. The glove manufacturer sells a harness for connecting them to the motorcycle battery. I did that and at 14+V from the motorcycle battery they indeed produce a lot more heat and the material does not melt.
Aha. Any reason why the harness wouldn’t work? I was gonna suggest as an alternative a connection to, say, a bigger external battery pack like some headlamps use. Then you can make it whatever you want. 3S8P if you want, etc.
Soldering the cells together, unless they come with the welded-on solder-tabs, will be a nightmare, though. Hard to do, easy to fry the cell.
For the battery pack, it could be made yourself, or perhaps have a look at some of the USB power banks which can provide 9V/12V output, if you’d prefer not to handle the LiPo safety directly.
The P = V^2/R equation and various others would put the heat increase at 2.25X, for a purely resistive element.
I’m surprised that the gloves can handle this increase, but there will be a certain amount of PTC properties in the heating element to limit it.
You are correct about the power calculations. I forgot to mention that the gloves have a button where you can regulate the power/heat with four settings (100, 75, 50% and 25%). I believe this is done with PWM. When I connect them to the the motorcycle battery the highest setting is too hot and I have to use the intermediate setting. So there is no problem with too much heating.
Connecting to the motorcycle battery is a solution of last resort. Although the gloves are warmer I have to deal with wires tangling, connecting, disconnecting them from the battery (three connections, one for each glove and another one the motorcycle battery) and is not a practical solution for short commutes. I used to have wired gloves before and I didn’t like it.
The original battery pack is 7.4V, the maximum current draw is about 1.5A that gives a P=V*I=7.4*1.5=11.1Watts
The resistance is then calculated as R=V*V/P=7.4*7.4/11.1= 5 Ohms
Going from 7.4V to 11.1V the power at the maximum setting would be P=V*V/R=11.1*11.1/5=24.6 Watts, which is more than twice the heat. However since the gloves have four settings, if I use the 75% one then I would get about 18.5W or 66% more heat. That’s fine with me. The problem I have is how to get or make a battery pack?
Making 2S 18650 Li-on Battery Packs for RC FPV Models - YouTube pretty long video but he covers what you need. I think you can find a little plastic box with contacts in them to insert two 18650s that already have leads coming out if you don’t want to solder onto the batteries. Search for 2s 18650 battery holder. Or 3s for 12ish volts
It may be a better idea to assemble a pack out of prismatic cells. Prismatic cells are manufactured in multiple sizes, and so you must first determine how much area you can afford for the batteries. They are named with a figure in which its size is encoded: thickness by width by lenght. Examples:
These aren't high discharge cells, but for the required power should suffice. They come with built-in BMS circuitry, which may do fine or require some upgrade. Their casing isn't very though I guess, and so I would enclose them in some shockproof cover. They're fairly cheap, though, so testing them is budget harmless.
To search for cells like those one can go for polymer+battery or prismatic+cell. Remember to use “sort by” and remember to make specific searches for sizes you may find “fitting” (so you can find best prices).
I contacted Aliya aliexpress store, which I found on the list of trusted stores for genuine 18650 batteries and they said they could do one or two custom 3S1P battery packs with 3 NCR18500 in series. I have some doubts about the proper way of making these packs. A lot of 3S1P packs have two plugs, one with two wires, and another one with four wires, the latter I understand is for balance charging. But I have also seen some packs with only one plug with two wires. The latter would be preferred in my case but is it safe? Is it made differently, protected cells with a BMS?
3.7V is the nominal voltage; they charge up to 4.2V just like cylindricals do.
The packs with only 2 wires should have a BMS circuit board internally. That board should have provision to shutdown the output and charging FETs in the event of OverVoltage, UnderVoltage, and OverCurrent conditions. It would automatically control charging to a setpoint voltage, likely not adjustable by the user.
The packs with the multi pin terminal blocks allow monitoring of individual cells by the user/charger device. There may be a BMS board also if desired.
Thank for the explanation. So the pack with only 2 wires can not monitor the voltage of individual cells. Is it safe? Is there a situation where it is preferred over a pack with multi pin terminals?
14500 cells are another option, 2x 14500s can have the same capacity as a single 18500 while possibly making a better fit if packing them interleaved:
https://www.aliexpress.com/item/32377924520.html This is a 6x pack, inexpensive and also with USA local shipping. Check a few reviews of them here and there. I am waiting for a couple of these, they are going inside my hair clipper.
Thank you for the suggestion. I am leaning towards a 3S1P 18500 battery pack from Aliya’s aliexpress store, since I can trust the rated capacity of the cells.
I checked the BMS that you referenced
A BMS like this one will support balance charging, correct? Is it possible to have two wires sticking out with a single plug (a DC female plug 1.35x3.5mm in my case)?
It likely will but we need to see the other side of the board to know what chips it is using for the bms and balancing, etc.
Your 2 wires would connect to the + and - solder junctions.
A pack with only 2 wires may still be doing internal balancing, it’s just not putting the wires out where you can check it yourself. For example the packs used by power tools only have a connection for the tool or the charger, but inside is a circuit board that is doing the bms function.
