Choosing components: DIY surgical headlamp

[Phlogiston]

@RadioCured

Charger:

Here I need some help again. I would really like to include the ability to charge the batteries via micro USB instead of having to remove them every time. This way I could charge on any cell phone charger or from a computer USB if necessary. It would also make my enclosure design less critical - I can use screws instead of having to design a battery door or something for easy access.

I found this really nice little charger circuit, but it’s obviously built to charge to 4.2v. Since I’m planning 3 batteries in series to run my driver, is there any way to include this or something like it?

As you know, you won’t be able to use that to charge 3 cells. Don’t be tempted to try and wire 3 of them with one per cell - even if you use 3 separate USB cables, many USB power supplies are wired in a way which will create a short circuit in that configuration.

For 3 cells in series, you’ll need a BMS (“battery management system”) module (a.k.a. “balancing charger”) with balancing connections to each cell. As you already know, you’ll need a higher voltage power supply for that.

@RadioCured

Phlogiston mentioned sterilization - this will not be a concern as the light does not have to be sterilized in any way. During surgery the only the region from the hips to the chest are considered sterile. We wear clean caps to keep hair out of the operating field and masks to prevent spreading bacteria from nose/mouth, but glasses, loupes, head lights, shoes, etc are not actually sterile. For this reason any buttons or control mounted to the light would not be helpful, so controls at the battery pack that can be adjusted through the sterile surgical gown are necessary.

That’s interesting, I didn’t know that. Learned something new today

@Yokiamy

You need the cells in parallel, in series won’t give the required runtime.

If the driver is a switch-mode buck driver with reasonable efficiency, then the runtime will be unaffected by whether the cells are in series or in parallel. Part of the prototyping process would obviously include runtime tests to make certain of that.