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My team and I are working on developing a wearable device (like a heart rate monitor with IP68 dust/waterproofing) and we're utilising wireless charging for our 3.7V 350mAh non-removable battery.

We'll be using TI's BQ51050B IC and whilst it performs the roles of wireless receiver and battery charger in one IC, I noted that it does not perform the full suite of voltage, current and temperature protections that a good battery management system (like the BQ77905) does.

I have reviewed a few different safety standards including IEC, UL and CE and they all discuss the potential hazards however don't give a definitive verdict as to whether the BMS would be advisable for small wearables specifically.

Given the experience on this forum, would these extra protections be necessary/advisable in an internationally distributed small wearable product or would they be unnecessary? In other words, is a BMS necessary with a 3.7V battery or are they are primarily designed and aimed at things like cordless drills which are usually 18V+?

Kind Regards, Andy

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closed as primarily opinion-based by Neil_UK, Enric Blanco, PeterJ, laptop2d, Dave Tweed Jun 26 '17 at 16:46

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ Would you buy a product and wear it, where the safety standards were debatable? Are you asking us for permission to put your clients at risk? Are you asking for us to tell you to design it according to your conscience? Are you asking us whether if your conscience and your financial models don't align, which one you should follow? Voting to close as opinion based. \$\endgroup\$ – Neil_UK Jun 25 '17 at 5:21
  • \$\begingroup\$ Finance is not a factor here. Safety is the aim of the game and that is what I'm trying to learn. The purpose of the question is to find out if a BMS is necessary with a 3.7V battery or if they are primarily designed and aimed at things like cordless drills which are usually 18V+. \$\endgroup\$ – astanton Jun 25 '17 at 5:27
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    \$\begingroup\$ Usually single cell consumer devices use a battery protection circuit, not usually called a BMS. It is partly a terminology thing, I guess. Although with a single cell, there is no balancing, for example. So a BMS may have to deal with balancing, but a protection circuit doesn't. Usually the protection circuit is integral with the battery itself. Seiko seems to be the industry leader in battery protection circuits. sii-ic.com/en/semicon/products/power-management-ic/… \$\endgroup\$ – mkeith Jun 25 '17 at 5:34
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    \$\begingroup\$ You need a charger IC (like the BQ51050b) on your device. And the battery pack needs to have (or typically has) a protection IC, such as one of the seiko ones I mentioned earlier. The two IC's perform different functions. The charger IC charges the battery. If it should fail in such a way that the battery is charged too fast or to too high a voltage, the protection circuit will open up and prevent further charging. The protection circuit is typically a very small PCB built into the battery. \$\endgroup\$ – mkeith Jun 25 '17 at 6:18
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    \$\begingroup\$ 90 degree C wash cycle should kill most batteries. \$\endgroup\$ – winny Jun 26 '17 at 13:50
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There are too many documented cases of batteries causing fires. You need to make sure that protection is included, especialy as the person wearing it may not be able to remove it quickly unlike a phone which can be thrown away.

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What exactly is needed will depend on how your device is marketed: in which countries you will sell it, will it be a certified medical device or not, etc. Specific requirements will apply in each case and it's too broad for an answer.

Also, most standards don't ask you to implement a particular feature such as BMS but to satisfy specific requirements. A good start would be the norm IEC 62133, which defines specific tests your battery should pass (mechanical damage, over-current, overcharge etc.). AFAIK this norm is used a lot for wearable batteries, but it might be insufficient or overly strict depending on what device you have exactly, as explained above.

AFAIK a simple wearable gadget with no medical functionality will be OK with a simple fuse for over-current protection. Some medical devices also get away with very simple chargers if the user is unable to charge the battery while wearing/using the device (e.g. electrical toothbrushes).

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