On Mon, 18 Jan 2021 17:07:59 +0000
Jim H  wrote:

> On Thu, 14 Jan 2021 03:46:08 +0000, in ,
> The Natural Philosopher  wrote:
>

> >The way to run a lithium in this application is to use a 2 cell
> >lithium, a constant voltage constant current charger limited to 8.4V
> >and probably no more than the battery mAh capacity divided by one
> >hour... and a switched mode 5V regulator to feed the Pi and then
> >somehow monitor raw battery voltage and switch it all off when it
> >gets to say 6.6V and hope that the SMPS and monitoring circuit don't
> >then still draw enough to damage the battery, or even if it is take
> >it on the chin and replace the battery when that happens.
>
> Yes... with special care to observe that safe cutoff voltage so that
> when power returns the constant current, constant voltage charger
> doesn't charge the battery any faster than it can accept safely. But
> wait... the constant current needs to be all that is needed to run the
> Pi PLUS the additional safe amount to recharge the battery, but... how
> do you assure the Pi and the battery share the current appropriately
> so that the Pi runs and the battery recharges safely? Maybe a bit more
> control is needed to assure a battery voltage above a certain value
> before the Pi is rebooted.
>

The way to do it is to use a purpose-made lithium battery charger IC,
for a pound or two. A diode, an inductor, a capacitor, a resistor and a
transistor are all that is additionally needed, and some ICs
incorporate the transistor. You then need a power source that will
provide current for both charging and a reasonable amount to run the
device itself. The resistor sets the main charging current, typically
0.3-0.5 of the battery capacity, independently of the device
consumption. The diode, inductor, capacitor and transistor do the same
jobs as in any switch-mode power supply.

--
Joe

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