Connecting a Li-Ion Battery to PCB - Is it safe to solder?

[MixedSignalSystems]

I'm working on a small form-factor wearable device that uses a 120mAh single-cell Li-Ion battery for power.

As the form factor is so small, I'd like to connect the battery directly to pads on the PCB without the use of a connector.

The battery is supplied with an integrated PCB for temperature protection. The battery cell tabs are connected to the integrated PCB. There are two wires from that PCB for external connections to the battery.

My local assembly house have said that they do not have the facility to spot weld or ultrasonic weld the wires from the battery to my PCB.

My question is whether or not it is safe to solder the wires from the integrated PCB on the battery to my PCB.

I've seen a variety of opinions on this. There seems to be two basic schools of thought:
1) Simply do not do it under any circumstances.
2) If you do it, keep the soldering iron temperature to a minumum and be ligthning quick when you solder the connections.

Different Li-Ion battery datasheets seem to come down on different sides of this argument. Given the overall low quality of Li-Ion battery datasheets (compared to ICs or connectors, for example), I'm hesitant to accept either suggestion at face value.

I'd appreciate any input on this. Is it safe to do this? For the person soldering the wires and indeed the battery itself? If so, how best to go about this?

Many thanks,
Kieran

[anishkgt]

Could pls share the cell pic and the pcb you wanna solder it too.

Spot welding is the best way to go but soldering is also ok (not recommended). While soldering the cell make sure the solder iron is not in contact with the cell for a long time as heat in any for can kill Li-ion cells.


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[ar__systems]

Yep, you should use spot welding for production. for prototyping - who cares.

[djQUAN]

.......The battery cell tabs are connected to the integrated PCB. There are two wires from that PCB for external connections to the battery. ..... My question is whether or not it is safe to solder the wires from the integrated PCB on the battery to my PCB.....

if I understand correctly, cell -> bms -> your pcb. from BMS to your PCB are just normal wires.

I don't see any problem why not solder it. there are countless products out there with wires from the pack to the product PCB without problems.

[ChristopherN]

Hi,

you should avoid high temperature rise on the flat metal tabs that are the direct battery tabs.

If your battery looks like this it has internal protection:

You can solder those wires just like normal wires. However, I would suggest putting a special process in place for assembly. Your process should make sure that:
-The wires never short out, possibly by only stripping one of the wires.
-The polarity is correct for each battery / product, this can be achieved by using different length wires for GND and V+ with matching pads on the PCB.
-The batteries are not crushed in any way, a special tray for manufacturing is often a good choice.
-That dropped or in any way damaged batteries are thrown away, maybe by putting a corresponding work order in place.

For the factory test (if you want to be very safe), I would implement the following steps:
-Discharge the battery and measure discharge time (and rate) and the battery voltage until the BMS cuts it out.
-Charge the battery full while monitoring state of charge and battery temperature.
-Discharge the battery to 70% (or what ever level your shipping company wants to have for air transport).

This obviously takes a lot of time, but it will catch a lot of assembly errors and bad batteries as well as bad BMS systems. A special rig that takes multiple units is needed to do this in a timely manner.

Another thing to consider is the housing, don't underestimate that Li* cells swell and deflate quite a bit. I would add a millimeter on top and bottom of the battery (on the large faces of the battery) to allow for that. The gap can be filled with foam tape, this also adds a nice level of shock protection in case the unit is dropped.

Best regards

Christopher

[KL27x]

if I understand correctly, cell -> bms -> your pcb. from BMS to your PCB are just normal wires.
This is where I am losing the question, too. Soldering pcb to pcb, you use wires. And you solder them. No problem.

I have used the 120 mAh lipo. The tabs that come directly off the pouch? If they can be soldered, I have failed to discover how. I think they need to be spot welded. I have salvaged before using mechanical pressure connection.

[digsys]

Our Solar car club have soldered many 1000,s pouch cells over 20+ years. I can't recall if there have been ANY failures due to soldering.
We have had many different members doing the soldering, some taught their first time. I personally use a HD JBC and set it to damn high power, with a huge tip !!
The job is done in seconds, and very little heat transfers down the tab. In the high density format we do, there's no room for a heatsink clamp, and it's pretty
dangerous. We rarely have a failure (if any?). We leave a little length to the top pcb, with an insulation layer / padding layer underneath. ymmv
EV Pack  www.pbase.com/digsys/image/164509549
Bloatware  www.pbase.com/digsys/image/164509551
nb: The bloat was due to pushing a particular pack way past spec, to finish a race :-)