Solar Circuit Design Help - Page 1
Forum section: electronics / general

author: Dicko
author_rank: Contributor
timestamp: July 21, 2023, 09:58:32 pm
content: Hello there kind people,I’m reaching out for some help with regards to a circuit I’m trying to develop.I have a 150 mA load (max), a 3.7 V (3700 mAh) LiPo battery, and a 0.5 W, 100 mA, 5.5V Solar Panel that I would like to use to recharge my battery (not drive my load in any way).The battery connects to an Arduino WAN 1310 via a jst connector 3.7V input.I have used the DFRobot solar charger in the past and has worked ok but I would like to incorporate a solar charger within a single PCB with my main PCB rather than having separate PCBs connected by wires.One thing to note is that at present, when I connect my battery to the load, I get some inrush current which can sometimes cause me some trouble, 1.8 A in fact, lasting about 0.4 ms.Also, my load needs to come on for 30 seconds and then go off for an hour in deep sleep, and I’d like an ultra-low sleep current.To this end, I believe I could use an ‘Ultra Low Power Boost Converter’ it’s the BQ25504 from Texas instruments to be exact.Then I plan to use a nano power timer, the TPL5110 to switch a load switch (and possibly the enable to the BQ25504 unless the polarity is different and I can use an output from the microcontroller to switch off the BQ25504), then I’d use the SiP32431 Load Switch, the nice thing about this switch is that is has a low Rdson and has some softstart capabilities which I hope would help with my inrush situation.My difficulty lies with the BQ25504, I would like some help in how to configure it so that it works for my purpose, to recharge the lipo battery.In my head I think I should have an input for the solar panel, input for the LiPo battery and an output voltage for my Load Switch and then the Load Switch to my load.I plan to take a parallel connection from the battery terminals, one pair will go to the BQ25504 input so that it can be recharged etc and the other pair will go to the TPL5110 timer circuit.But yes, I would like some help please on how to configure the BQ25504 for my application. That would be greatly appreciated.Links for the components I have mentioned are as follows.Solar Panel:https://wiki.seeedstudio.com/0.5w_Solar_Panel_55x70/BQ25504:https://www.ti.com/lit/gpn/bq25504TPL5110:https://www.ti.com/lit/gpn/tpl5110SiP32431 (Load Switch):https://www.farnell.com/datasheets/2049671.pdfThank you in advance for your valuable time and kindness.

author: Peabody
author_rank: Super Contributor
timestamp: July 21, 2023, 11:32:53 pm
content: I believe the 1310 has a battery charger (BQ24195L) onboard, along with a 3.3V linear regulator. So you probably don't need a load switch or a boost converter. The question is whether it will work with a solar panel.

author: Dicko
author_rank: Contributor
timestamp: July 22, 2023, 06:23:18 am
content: Thank you for your reply. Yes I believe it has a charger inboard too but no option to charge with a panel. I would like to get the circuit I mentioned above working, the Arduino is just part of the load which required 3.7V

author: Peabody
author_rank: Super Contributor
timestamp: July 22, 2023, 02:16:02 pm
content: I have no experience with the BQ25504, so can't really help with that. But there is an example solar circuit in the datasheet. Looks pretty complicated.It appears the On/Off input of the load switch is On-high. And the enable of the BQ is also active high. So you might want to look at the TPL5111, which has an active-high output. The TPL5110 is active low.Edit: I think you'll have to leave the BQ on all the time so the battery can be charged even when the load is shut off. Also, what is causing the current inrush? 1.8A seems like a lot.Edit2: The word "lithium" does not appear anywhere in the BQ datasheet. Do you have any examples of this part being used with a rechargeable lithium battery? There are recommended algorithms for charging lithium batteries, including charge termination, that you may want to follow.