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Requesting Review Of PCB Design For A BQ25570 Based Solar Charger

mcfaang

New Member
Hello all,

I am trying to make a solar charger such that solar cells will trickle charge a 18650 battery while the battery powers an arduino pro mini. This may be a long post but I want to explain my reasoning for the schematic.

When it comes to the parts: the main part is ofc the BQ25570 IC, there is also kind of an array of resistors used to set voltage divisions to set up V_OUT, overcharge voltage and MPPT tracking. I chose the values of the resistors by putting in desired values in an Excel sheet gotten from TI instruments here. The idea is using a set of jumper caps to set a specific MPPT ratio and also using the jumper caps on setting V_OUT and overcharge voltage.

For the capacitors were from reading the datasheet and using mostly typical values and the ones used in the solar application example.

Inductors were picked on the same principle. As a note, I am hoping to get a 3.6V output and around 12mA output.

Wasn't a long post after all but I would appreciate feedback on this and also if I need to clarify anything else.

Here is the schematic (do pls tell if quality is bad, I uploaded png):

r/PrintedCircuitBoard - [Review Request] BQ25570 Based Solar Charger
I set it up as a 4-layer board. The Back copper layer is GND. The third layer is VRDIV and the second layer is VOC_SAMP. Here is the PCB routing on the 3D view (was unsure how to best show the 2D routing)

r/PrintedCircuitBoard - [Review Request] BQ25570 Based Solar Charger
This is the image I mostly took inspo from for the schematic (it's in the datasheet as well):

r/PrintedCircuitBoard - [Review Request] BQ25570 Based Solar Charger
Thank you for your time.
 

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You don't trickle charge Li-Ion batteries, there are specific charging techniques required, which are VERY, VERY important for Li-Ion to avoid the risk of fire.

Also, you suggest charging at 3.6V, at 3.6V a L-Ion cell is flat (if you go much lower the battery can be permanently damaged) - Li-Ion cells are 4.2V when fully charged.
 
Totally agree — the BQ25570 isn’t a full Li-Ion charger. It can reach 4.2V, but doesn’t handle proper CC/CV charging or termination. If you’re using it with Li-Ion, you really need a protection IC or dedicated charger in the path. 3.6V is near empty, not a safe charge level.
 
You don't trickle charge Li-Ion batteries, there are specific charging techniques required, which are VERY, VERY important for Li-Ion to avoid the risk of fire.

Also, you suggest charging at 3.6V, at 3.6V a L-Ion cell is flat (if you go much lower the battery can be permanently damaged) - Li-Ion cells are 4.2V when fully charged.
Hello, I am not charging at 3.6V, overcharge is set at 4.2V and it is the output that I can use for the arduino that is 3.6V. I basically want something similar to an AEM10941 (which I have) but the output is mainly from the solar cells instead of the battery. Solar cells charge battery and also try to power load. I have contacted e-peas about how I can change this configuration but no sufficient response.
 
Totally agree — the BQ25570 isn’t a full Li-Ion charger. It can reach 4.2V, but doesn’t handle proper CC/CV charging or termination. If you’re using it with Li-Ion, you really need a protection IC or dedicated charger in the path. 3.6V is near empty, not a safe charge level.
Hello, I am not charging at 3.6V, overcharge is set at 4.2V and it is the output that I can use for the arduino that is 3.6V. I basically want something similar to an AEM10941 (which I have) but the output is mainly from the solar cells instead of the battery. Solar cells charge battery and also try to power load. I have contacted e-peas about how I can change this configuration but no sufficient response.
I also know the batteries I have are protected from overcharge or discharge, I am guessing that is not enough?
 
Hello, I am not charging at 3.6V, overcharge is set at 4.2V and it is the output that I can use for the arduino that is 3.6V. I basically want something similar to an AEM10941 (which I have) but the output is mainly from the solar cells instead of the battery. Solar cells charge battery and also try to power load. I have contacted e-peas about how I can change this configuration but no sufficient response.
I also know the batteries I have are protected from overcharge or discharge, I am guessing that is not enough?

No it's not, you need a charging circuit - if you look on AliExpress they sell various specific solar Li-Ion charger modules.
 
Totally agree — the BQ25570 isn’t a full Li-Ion charger. It can reach 4.2V, but doesn’t handle proper CC/CV charging or termination. If you’re using it with Li-Ion, you really need a protection IC or dedicated charger in the path. 3.6V is near empty, not a safe charge level.
Hello, I am not charging at 3.6V, overcharge is set at 4.2V and it is the output that I can use for the arduino that is 3.6V. I basically want something similar to an AEM10941 (which I have) but the output is mainly from the solar cells instead of the battery. Solar cells charge battery and also try to power load. I have contacted e-peas about how I can change this configuration but no sufficient response.
I also know the batteries I have are protected from overcharge or discharge, I am guessing that is not enough?
No it's not, you need a charging circuit - if you look on AliExpress they sell various specific solar Li-Ion charger modules.
Most of the modules I've seen are best suited for solar panels left in sun. The panels I am dealing with are for indoor harvesting from indoor light
 
A safety cutout, as included in some cells, is not charge control.

For the applications you describe, you may possibly be better off using a supercapacitor type system?

Is the battery supposed to power the Arduino when there is no power from the solar cells?
 
Hello, I am not charging at 3.6V, overcharge is set at 4.2V and it is the output that I can use for the arduino that is 3.6V. I basically want something similar to an AEM10941 (which I have) but the output is mainly from the solar cells instead of the battery. Solar cells charge battery and also try to power load. I have contacted e-peas about how I can change this configuration but no sufficient response.
I also know the batteries I have are protected from overcharge or discharge, I am guessing that is not enough?

Most of the modules I've seen are best suited for solar panels left in sun. The panels I am dealing with are for indoor harvesting from indoor light

Which is what solar panels are designed for, using them indoors only produces a tiny fraction of their potential output, as light levels are so much lower. Have you measured the kind of power levels you will get from the panel(s) you're thinking of using, and how many panels you might actually need?.

A good few years back there was a TV show, something like "Robot Olympics", where teams from schools competed in a series of robotic tests. One of them was a solar powered sprint. As the show was made indoors (in a huge hanger or whatever) there was a vast array of high power lighting directly above the race track - it was staggering how much lighting it took to try and simulate a sunny day.

Have you considered using NiMh batteries, instead of Li-Ion?, something you can trickle charge, and that with indoor charging there's little that will be needed (a blocking diode from the solar panel?) as it won't be able to produce enough power to over charge them.
 
Which is what solar panels are designed for, using them indoors only produces a tiny fraction of their potential output, as light levels are so much lower. Have you measured the kind of power levels you will get from the panel(s) you're thinking of using, and how many panels you might actually need?.

A good few years back there was a TV show, something like "Robot Olympics", where teams from schools competed in a series of robotic tests. One of them was a solar powered sprint. As the show was made indoors (in a huge hanger or whatever) there was a vast array of high power lighting directly above the race track - it was staggering how much lighting it took to try and simulate a sunny day.

Have you considered using NiMh batteries, instead of Li-Ion?, something you can trickle charge, and that with indoor charging there's little that will be needed (a blocking diode from the solar panel?) as it won't be able to produce enough power to over charge them.
Hello. Yes I have measured from the solar cells and they collapse under they arduino load since they cant sustain the current needed, its expeccted. Here are some readings:
one solar cell:
V = 0.94V
I = 106uA
V_oc = 1.53V
I_sc = 142uA

four in series:
V = 1.423V
I = 147uA
R = 9.41k
V_oc = 7.33V
I_sc = 133uA


I know I can likely use two in series and two in parallel since the BQ25570 has max input voltage of 5.5V. The solar cells I am using are more optimized for indoor lighting, though ofc, they still wont produce the same amount of power.

To answer about NiMh, no I hadnt really thought about them.
 
A safety cutout, as included in some cells, is not charge control.

For the applications you describe, you may possibly be better off using a supercapacitor type system?

Is the battery supposed to power the Arduino when there is no power from the solar cells?
Yes, the battery is supposed to continue to power the arduino when there is no power from the solar cells. The project is meant to see how much, if any, the battery life can be extended through this charging. I know the battery will likely not be able to be kept charged by the cells.
As for a supercapacitor, I was unsure it would be able to produce / sustain the current needed?
 
As for a supercapacitor, I was unsure it would be able to produce / sustain the current needed?
High current types are readily available.
These are rated for up to 1A normal discharge current, 6A peak:

As an example of more extreme ones, my spot welder uses a couple of massive screw terminal ones, something like the Chinese equivalent of these - rated at over 2000A pulse discharge!
 
High current types are readily available.
These are rated for up to 1A normal discharge current, 6A peak:

As an example of more extreme ones, my spot welder uses a couple of massive screw terminal ones, something like the Chinese equivalent of these - rated at over 2000A pulse discharge!
Thank you for bringing this to my attention, I did not know that.
 

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