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I was looking for some simple and cost effective solar lipo charger for my DIY projects...I've seen the GreatScott video about solar lipo charger and i've realized one of it using the same schematic reported in the original video and easyeda project page: here

For my own needs, I've added a MCP1700T 3.3V regulator at the same point of connection of BAT+, as you can see in the attached image, the rest of circuit is the same as reported on EasyEda.

sch

The (when i attach the external Solar Panel) circuit charges without any issue the Lipo battery, the MPTT works fine (with 21V panel) and the 5V boost converter and the 3.3V step down regulator (MCP) works flawlessly!

...but there is a strange issue that I want to solve about...basically, when I attach only the battery (without the external Solar Panel), the 3.3V and 5V step up converter does not work unless I connect manually the GND of the circuit and the BAT- pin.

Every time i need to "start-up the board" i have to do this kind of magic trick...is this an issue? Why is the circuit acting like that?

I need to wire the Gnd of the circuit to the BAT- pin?

If I do this, the battery protection will still be operative?

I'm a little bit confused...it's my first approach to solar and lipo charging world, so, please.. Be patient!

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  • \$\begingroup\$ If I recall regulation is done on the gnd path , so check the chip schematic \$\endgroup\$
    – Tony Stewart EE75
    Mar 17, 2021 at 21:48
  • \$\begingroup\$ The solar charger schematic is identical to the recommended schematic of the manufacturer (Consonance - CN3791) \$\endgroup\$
    – VirtApp
    Mar 17, 2021 at 22:06
  • \$\begingroup\$ " when I attach only the battery (without the external Solar Panel), the 3.3V and 5V step up converter does not work unless I connect manually the GND of the circuit and the BAT- pin." - What happens if you connect the battery and then use the solar panel to charge it? \$\endgroup\$
    – Bruce Abbott
    Mar 17, 2021 at 22:10
  • \$\begingroup\$ How is U7 LDO gnd connected to BAT-? \$\endgroup\$
    – Tony Stewart EE75
    Mar 17, 2021 at 22:19
  • \$\begingroup\$ Your whole design is flawed. The charger is designed to charge a lithium battery, nothing more. Connecting a load to the battery at the same time will confuse it and possibly stop it working or damage the battery. If you want to power a load at the same time as charging the battery you need to find a way of doing it directly from the panel or some other source. \$\endgroup\$
    – Finbarr
    Mar 17, 2021 at 22:59

1 Answer 1

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If your Buck converter does not manage a soft start, it is possible that to ODC turns off the OD gate voltage = G2.
Meaning the overdischarge current is tripped.

For Discharge battery connection to Buck grounds both external gnd FETs need a >=3.2V high gate voltage satisfied by the protection IC Logic so bypassing them will also block the ODC protection.

Check your CSI R1 value for its threshold to be above your surge start, unless there is a cap fix.(?)

Check for OD and OC to be high to start, and what condition might turn off the OD gate. Both gates need = 3.7V to drive the FETs to enable the battery to Gnd. While charging should disable the load so that CC cutoff is achieved (I think) . OD= “0”, OC=“1”

(Someone correct my answer if I am wrong) Add PIN numbers that are missing on your schematic and verify they are correct.

Measure that start surge current and voltage.

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I have never used this circuit before.

Try a small then large cap from CSI to Vss and use the smallest size that works.

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  • \$\begingroup\$ Thanks Tony, I've tested yesterday and today this modification: i've connected CS to BAT- pin, with this, i'm able to get the Boost converter and the LDO working without the solar panel. I'm currently testing the overdischarge to see what will happen, anyway, i've not yet measured the Gates voltage...what you think about the direct connection to CS? \$\endgroup\$
    – VirtApp
    Mar 19, 2021 at 12:41
  • \$\begingroup\$ CSI is. For current sense input. NG \$\endgroup\$
    – Tony Stewart EE75
    Mar 19, 2021 at 12:45
  • \$\begingroup\$ Nevermind...i suppose that we need to find a solution to solve what is reported in the datasheet of the FS312F: When a battery is connected to FS312F-G for the first time, it may not enter the normal condition (dischargeable may not be enabled). In this case, short the CS and VSS pins or connect to a charger to restore to the normal condition...so, to do so (short momentarly the CS and VSS pins) what kind of circuit i need to put in place? \$\endgroup\$
    – VirtApp
    Mar 19, 2021 at 14:05
  • \$\begingroup\$ A capacitor of suitable size? \$\endgroup\$
    – Tony Stewart EE75
    Mar 19, 2021 at 14:43
  • \$\begingroup\$ connected between the two pins? which size i can use? i mean i think that 500ms/1 second it will be enough... \$\endgroup\$
    – VirtApp
    Mar 19, 2021 at 14:46

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