The cell to buy
One lithium cell, 3.7 V, with a protection circuit on it and a JST PH 2.0 plug. Size matters as well: the board charges at up to 500 mA, and a cell much smaller than 500 mAh is being charged faster than it was made for.
What the label says
A lithium cell says 3.7 V on its label. That is its middle, not its only voltage: it is 4.2 V when full, sits near 3.7 V for most of its charge, and should be treated as empty at 3.0 V. The board's charger takes it up to 4.2 V and stops.
The other number is mAh, the capacity. 1000 mAh supplies 1000 mA for an hour, or 100 mA for ten.
The protection circuit
Almost every pouch cell sold with a plug on it has a tiny circuit board under the tape at the end where the wires come out. That board disconnects the cell if it is run too flat, overcharged or shorted.
The board's charger looks after the top of the cell: it stops at 4.2 V. Nothing on the ESP32 board looks after the bottom. Run a cell without protection down past 3 V and it is damaged rather than flat, and short one and it will deliver far more current than the wires can take. Only use a cell with its own protection circuit. For 18650 cells the phrase to look for is "protected".
How fast is too fast
A charger's current is measured against the cell's capacity. 500 mA into a 500 mAh cell is 1C — the whole capacity, per hour — and that is as fast as most small cells allow. Many say 0.5C.
The board's charger delivers up to 500 mA whatever you plug in. So the small 100–200 mAh cells sold for tiny projects are charged at 2.5C to 5C here, which is how cells get hot. A 1000 mAh pouch charges at 0.5C and fills in about two and a half hours; a protected 18650 is gentler still and lasts the longest.
One cell, not two
A pack labelled 7.4 V is two cells in series. It needs a two-cell charger and a regulator that takes 8.4 V, and this board has neither. The same goes for a pack of AA batteries: the socket is a charger's output, not a power input.
The plug
JST PH, 2.0 mm between the pins, two pins. Which pin is plus is not standardised — the same red and black lead comes wired both ways round, and this kit has two sockets that are wired opposite ways. Which pin is plus is the page to read before you plug anything in.
When it does not work
Unplug USB and the cell at once, and put the cell somewhere it cannot touch anything that burns. A swollen lithium cell is finished and should go to battery recycling, not back on the board. A small cell on this 500 mA charger is the usual cause.
Measure the gap between the two pins. The board wants JST PH, which is 2.0 mm; JST XH is 2.5 mm and will not seat, and the tiny 1.0 mm plugs on some drone cells are JST SH. Do not force one into the other. Buy a cell with a PH lead, or an adapter lead.
A protected 18650 has a small circuit board and a metal strip under the wrapper at one end, and is usually a few millimetres longer than 65 mm. The seller will say protected or PCB. If it does not say, assume it is not.
No. The socket is wired to a lithium charger, and the charger will try to charge whatever you plug in. Alkaline cells are not made to be charged and NiMH cells need a different charger. Only a single lithium cell goes in.
What is in the box, and which shield to start with.
Three boards, three shields →Edit this page — content/books/esplipo/the-cell-to-buy.mdx
Questions about this product
See what other owners have asked, and read their solutions.
ESP32 LiPo Dev Kit: 3 Boards with LiPo Charger + 3 Shields
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