Solar Panel charging Nora

Hi, i want to charge the Nora with a solar panel.
I have been looking at the TI BQ25504 energy harvester charge circuit, and it looks pretty neet!

I have some doubts however:
Currently the nora runs on 2xAA batteries, I know you can get 1.5V AA lithium batteries, so this could work.
I was also considering changing batteries to a normal 3.7V lithium battery, but I am unsure what happens then when the boost converter (XC9128B45) on the nora is not enabled. If then 3.7V will be passed directly through to the imp and fry something?

If you’re using a 3.7v battery, you should use a buck supply (that drops down to 3.0v, to give more headroom).

The Li-Ion design guide here: has good suggestions for suitable bucks.

The suggested TPS62233 has an Iq of 22uA. While this is not alot, i am only waking my device up to measure a few times a day, so as far as my calculations go, 22uA Iq will actually have a significant impact on battery life.

Design guide C suggests a cheap LDO (CMP1700) with 1.6uA Iq. But somehow in my mind, LDO’s are worse than DC/DC’s. Would there be drawbacks to using this LDO?

Another option could be the TPS62740 from TI ? It’s a buck capable of 300mA (400mA for the 62742 version), with 360nA Iq. Its a bit more costly thou.

Any thoughts on this?

I have ordered all three PSU’s and the BQ25504 charge circuit to play around with :slight_smile:

It depends on your usage model; if you’re asleep most of the time, then the lower efficiency of the LDO won’t hurt you much (as typical voltage will be around 3.7, you’re running at 3.0/3.7=81% efficiency anyway; you may get to 85-95% with a switcher, but if that’s at the cost of increasing your sleep power significantly the extra savings when on may not be worth it).

As with anything in life, if you want the nicest things (high efficiency and low Iq) it costs more :slight_smile:

Yeah ok makes sense :-). I will prototype one with the LDO and one with the TPS62740, see how the impact is in real life, with my small indoor solar panel.

Since i am using the BQ25504 i do not need the S-8241 charge protector.
I am however a bit confused as to which voltage level i should configure the VBAT_overvoltage.

The S-8241ABKMC-GBKT2G has:
Overcharge detection voltage 4.325V
Overcharge release voltage 4.075V

Should set my VBAT_OV to ~4.3V or ~4.0V? using single cell 3.7V battery?