Low current lithium battery recharge and hold (float charge)

[AloyseTech]

Hi,

I have a lithium-ion powered device which can be sometimes connected to an external communication bus. A little bit of energy can be "harvested" from this bus, around 4mA at 5V. I'd like to use this available energy to maintain the lithium ion battery at roughly 50% of its charge. The battery is a protected (typical DW01-like circuit + PTC fuse) 700mAh cell. The device itself uses around 200uA on average with a few activity phases at around 200mA.

I though about using a current limited voltage source (based on TL431), set to 4.1V 3mA, with a schottky diode in series (so around 3.8V) to the battery like this :

https://tinyurl.com/25tarcvq


I'm not sure about the behavior of the battery but I assume it could sink current even when it is charged in CV mode. And in this case current would flow continuously which is bad and could cause damage to the battery (not overcharging though, as it should not charge above the set voltage of 3.8V).

Is it necessary to implement duty-cycled charging or some sort of very low current threshold detection to stop charging ?

I understand that it's a complicated topic and and I could not find really factual evidence wether or not the idea is safe in general. So if anyone has some literatur about the topic that would also be awesome.

Thanks

Related threads :
https://www.eevblog.com/forum/projects/float-charge-voltage-for-liion-cells/
https://www.eevblog.com/forum/chat/lipo-battery-on-holdingfloat-charge/
https://www.eevblog.com/forum/chat/is-big-clive-right-about-lithium-battery-charging/
https://www.eevblog.com/forum/projects/float-charging-lithium-ion-batteries-at-lower-voltages/
https://www.eevblog.com/forum/projects/float-charging-li-battery-for-fluke-845ab-null-meter/
https://www.eevblog.com/forum/projects/lithium-charging-ic-with-low-ground-pin-current/msg1185739/#msg1185739

[AloyseTech]

I found that Analog Device sells a shunt based battery charger for energy harvesting application : LTC4070 and LTC4071. According to the datasheet (https://www.analog.com/media/en/technical-documentation/data-sheets/LTC4070.pdf):

Extensive measurements on Li-Ion cells show that the cell charge current drops to nanoamps with the shunt charge control circuit effectively terminating the charge.

Unfortunately they don't provide much more information.