On Mon, 18 Jan 2021 20:33:21 +0000, in
, Joe
 wrote:

>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.

This type of charger will work PROVIDED the battery doesn't run so low
that it's reluctant to take a recharge. In that case you need a REAL
LiIon charger (what you describe above doesn't even come close) that
detects this condition and charges at a very low rate until the
voltage rises to the point that the battery will accept charge.

Simple chargers like you describe above work for cell phones because
the logic that protects the battery is in the cell phone.

--
Jim H

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