Sourcing 3-lead 3.7v LiPo batteries

I'm attempting to source replacement batteries for some medical devices (digital goniometer, inclinometer, etc.) The markings are:

GMB063048 3.7V 800mAh 2.0WH 2010.12.08 R

I've found batteries of suitable size and capacity (6mm x 30mm x 48mm, 850mAh). My issue is that the OEM batteries have a 3rd blue lead attached to a thermistor that is connected to a charging circuit on the main PCB.

All of the batteries I've been able to locate have only 2 leads. I find myself ignorant and frustrated here. Will I need to order a 2-lead battery and then simply connect the thermistor to the negative terminal in order to make this work, or is there a search term that I'm missing (being ignorant of battery configurations and features) that will help me locate what I'm looking for?

Thanks in advance for your advice and knowledge. I was raised by an electrical engineer, and consider myself an advanced hobbyist. I know very little about batteries, though.

My plan is to keep the existing thermistor and reuse it on a replacement LiPo cell. If it's too much trouble to explain why that's a terrible idea (if indeed it is), then a pointer to a resource I can use to teach myself the pertinent details would be fantastic.

• The markings are for the cell only. – Ignacio Vazquez-Abrams Apr 20 '18 at 16:08
• The thermistor attachment is probably there for an internal temperature check as Li+ / LiPo cells can only be charged across a relatively narrow range safely. 0 - 40C to start, 0 - 45 C to continue. – Peter Smith Apr 20 '18 at 16:17
• Thanks for the replies. I've got a pretty good handle on why the thermistor is there and what it does. This question is more about sourcing compatible replacement components. If any old cell will work and I can just add the thermistor, that makes it easy. – Jeff H. Apr 20 '18 at 16:21
• Well, you could add your own thermistor, but to do that, you need to fully understand the charging circuit. – Peter Smith Apr 20 '18 at 16:23
• My plan, if I go that route, is simply to reuse the thermistor on the existing circuit and thereby avoid changing any of the parameters of the circuit. I wouldn't be adding a thermistor to a system without one, I would be simply replacing the lipo cell and keeping the thermistor in place. My amateur understanding doesn't lead me to believe that this will cause any problems... but... you know.... – Jeff H. Apr 20 '18 at 16:27

2 Answers

If you are looking for replacement of a 3-pin 800 mAh Li-Po battery which is 8 years old, you need to look for something in the 1200 mAh range today. Because the Li-Po technology progresses. Here is an example from e-Bay:

Most of two-wire batteries are made cheap for RC (Radio-Controlled) toy applications. They have super-high discharge rates (15C to 90C) and can be harmful.

For individual replacement and for your private use you can do whatever it takes, but if you aim at industrial scale, consider all warnings in Olin's answer.

Actually, I have seen the opposite, an original China-made battery would came with three-wire connection form its BMS protecting circuit, but a European-based medical supplier re-packaged it with the yellow thermistor wire cut-off, making it a two-wire connector. So go figure the safety and reliability of this re-branding.

Your device will likely need the thermistor connection (usually PTC102, aka 10k to ground) to operate, otherwise the charger chip will sense a faulty battery (like most of Texas Instruments chargers would do, for example). You can re-use the old thermistor, or just fake it with corresponding chip resistor with a nominal value (10k at 25 deg.C, or whatever it is).

NOTE: there is an informal convention on pouched batteries dimensions. This one is labelled as "6043450", which means that it is 6.0 mm thick, 34 mm wide, and 50 mm long. Just FYI, you can use this as a search parameter to find a better fit.

• Thank you. Apparently they didn’t follow the convention with this battery. But it was 8 years ago. – Jeff H. Apr 21 '18 at 19:02
• And that’s exactly the configuration on the device currently. Thanks again. – Jeff H. Apr 21 '18 at 19:08

You say these things are medical devices. The general answer is therefore that you shouldn't be in there messing with them. You should only replace parts if they are intended to be user-replaceable. In that case, the details would be in the manual.

Medical devices are governed by a large set of regulations. The manufacturer had to perform lots of specific tests, keep very specific records of the design process, and get certification. Here in the US, it is the FDA (Food and Drug Administration) that handles such certification.

If you do anything to the device that is not sanctioned by the manufacturer, then the device will no longer be certified. You would be committing a federal offense if you then use the device on patients.

Most likely, nobody would ever notice. However, if something happens and somebody gets hurt, even if ultimately not due to your mod of the device, you would be in very serious trouble. The jury will hear all about how you were so irresponsible and had so little regard for patients that you performed your own highly technical jargon-laden unauthorized modification of a very sensitive medical device. The final nail in the coffin will be when the plantiff's lawyer enters this Stack Exchange question into evidence, showing that you even knew you had no idea what you were doing. Even if by some miracle you escape a serious judgement and somehow manage to keep your medical license, the FDA may decide to file criminal charges.

Or, you could avoid all that and have the manufacturer perform a authorized repair. Sure, they might charge $150 for what will probably be just at$3 battery, but you get to keep your house, the privilidge to practice medicine, and you won't be public declared a dirtbag by a jury.

• I appreciate your concern. These devices are not used in any circumstances where their failure can have any potential negative consequences beyond having to reschedule an appointment. I had thoroughly considered the risks before deciding to take on this project. (I’d never try to replace the batteries in an AED, for example.) Upvoted. This answer should be read and understood by anyone deciding to tinker with anything used in the healthcare field. – Jeff H. Apr 21 '18 at 19:07