So a mobile device needs power, and to make a mobile device more useful, more power is always better right?

Initially I had considered just popping a JST socket on the board and calling it a day. That would make the case design much simpler, and could even save 2mm on the thickness of the device, making it even easier to slip in a pocket. Job done right??

Well if I ever want to sell these (and I would like to!! ) then an enclosed battery becomes a major issue! Anyone following along for a while might have seen my past fun with Royal Mail and batteries . Being able to ship the device without a battery makes life much easier, but it needs to be a battery that the end user can easily get locally, for a reasonable price.

18650

Any local high street these days will have a choice of vape shops, and those shops will generally be a source of the ever useful 18650 lithium battery. They have become the AA battery of the modern world. The biggest issue with these batteries is there cylindrical nature. The battery being 18mm in diameter makes any device you use them in harder to make thin. Take that 18mm, add 2mm each side for the case, and 1.6mm for a pcb, and your up to a chunky inch without trying. Pockets are not too happy about that....

That Nokia battery that everyone uses

If you buy an electronic device that has a removable battery, the chances are it will be rocking a "BL-5C". This means it will super easy for people all over the world to get their hands on the battery. The use of these types of cells also has added advantages. They have hardware based reverse polarity protection (as long as you design the battery bay right, you cant connect the battery the wrong way) and they are supposed to have protection circuitry built in too (although i wouldnt want to trust that on the cheaper cells). The battery is only 5mm thick too, so even with some case and pcb, its still going to be thinner than a 18650 cell on its own!! All is not perfect though, when you look at the battery, its contacts are little gold squares on the battery, you need to find the right kind of connector to mate with this efficiently. a quick search and you can find hundreds of people selling you a 'nokia battery connector' which will do the job

The problem comes back to 'if I want to sell these'. its easy to find random connectors in small quantities, but if your thinking of getting a fab house to make your boards, you need part numbers and someone who can reliable offer larger quantities of the part. This is where I fell short

The Other (one of many) Nokia Batteries

While i was hunting for battery connectors, a friend mentioned a Light he had acquired from AliExpress, and it too had a removable battery, but not the magical standard cell. This one was a BP-4L from the more modern range of Nokia phones (E61, E63, etc) and the first advantage I came across was a reduction of thickness!! 4mm rather then 5mm (a win for pocketability!!) the next bonus was a 50% increase in capacity! 1500mah rather than 1000mah. Then the final decider was that i could find a battery connector!! This cell has a spring based socket connector on the battery, and 3 simple blades attached to the pcb

We Have A Winner...

So here we are, BP-4L is the way to go!! Still got hardware based reverse protection, slim to make it easy on the pockets, and a session busting 1500mah battery!! That means that even if you have an average 100mah running load, and you average an hour a day usage, that battery could be lasting over 2 weeks!! I call that a win :)