Martin Gregorie  wrote:
> On Mon, 09 Mar 2020 17:28:10 +0000, Theo wrote:
>
> > You don't want to power it directly from the lithium cell - you need a
> > cutoff circuit to prevent overcharge and overdischarge.  The cutoff is
> > often integrated into a 5V stepup chip, so there's no point using the
> > bare cell even if it would work.
> >
> Sure - using a BEC commonly used on electric models sorts that out. I'm
> aware of that problem, and intend to deal with it, but ATM am more
> concerned to find out of I should use a 1S or a 2S battery pack - 1S is a
> lot easier to chanrge since no cell-balancing is required.

I don't know why you'd want to use 2S if all you're powering is 5V stuff -
it's easy to charge 1S (4.2V) from USB, and then a boost converter to
generate 5V is easy.

Do BECs actually provide cutoffs?  The one linked to in this thread doesn't
seem to.  Many battery packs already come with their own BMS which handles
the cutoff - you don't get that if you're using bare cells.

> > However you can buy the power board from a power bank without the
> > batteries.  If you search ebay or better Aliexpress for 'power bank pcb'
> > there's lots of them of varying form factors.  For example, a selection:
> > https://www.ebay.co.uk/itm/5V-1A-2A-2-1A-Power-Bank-Charger-Module-
> > Battery-USB-LED-Boost-Step-Up-PCB-Board/283407817785
>
> That's a GOOD thought - thanks.

A colleague also suggested looking at Poundland power banks - if you take
them apart the board in the 2 pound ones seems to do the right thing.

There are some 'disposable' 1 pound one-time-use power banks that don't have
a charging port, but it turns out they're actually fully equipped with a
charging chip but they just save the cost of the connector - e-waste, sigh
:(.  If you solder on some wires they're a decent small pouch-cell
500mAh-ish power bank.
https://www.youtube.com/watch?v=6lflk6iY56w

As always, Poundland is pot luck as to what stock you'll find today.

Theo

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