Short question: can I connect some load to a solar panel while it is charging a battery or the current across this load can cause a confusion in the battery load controllers?

Explanation: Maybe too smart, I'm thinking on reuse a garden light (a 10$ one that has a solar panel, battery, motion sensor and leds) also as motion alarm. The circuit I'm thinking about is:

enter image description here

Where left part is the power one:

  • a solar panel, nominal 5 V, 300 mA, 1.5 W, that, across a schottky diode (0.2 ... 0.3 V of Vf) is used to charge the battery and also to feed the load
  • a 18650 battery (3.7 V, 2200 mAh, charged during solar hours) and its charge controller (the box labelled "1" in the circuit). This part is based on a HY2111 IC, following manufacturer hint:

enter image description here

(where PB+ and PB- are the connections to the solar panel and schottky diode. WARNING: the ground reference on this schematic is not the same than in previous one).

The right part is the load:

  • a PIR sensor and related components (box "PIR" in the circuit). It contains an internal 3.3 V DC regulator, a PIR motion sensor and its related components. The output is 0 V when no motion, when motion is detected the output is a pulse of 3.3 V and 60 seconds duration.
  • the output of the PIR sensor goes to two elements. First one is used to wakeup a FireBeetle ESP board (box "FB" in the circuit) that sends a message using WiFi network
  • the PIR also activates a MOSFET that controls the leds. In order that leds doesn't activates during day, the box "2" contains another MOSFET that cuts the LED power when the solar panel provides enough voltage, that is, during day (warning: the first schematic contained an error on this part. fixed).

During day, the solar panel is powering the battery, the PIR and the FireBeetle. The PIR and the FireBeetle needs less than 1 mA when idle, 150 mA during 5 s (average time) when motion is detected and the FireBeetle is transmitting by WiFi. Is this current load (1 mA in idle mode, 150 mA when a trigger) a problem for the correct function of the battery controller?

ing around 300mA.

  • 2
    \$\begingroup\$ Having designed commercial battery charger (and associated algorithms), I can say a definitely maybe. Some don’t care and charge forever, which is great in your case but will boil out all the water in healthy cells (lead acid) if one is shorted. Others will be very upset by small parallel loads. The manual is unlikely to give you specific values. Email the manufacturer? Single cell should greatly simplify the situation. \$\endgroup\$
    – winny
    Commented Aug 12, 2021 at 18:50
  • 1
    \$\begingroup\$ The HY2111 isn't a charger IC, it's what is commonly referred to as a BMS, a device that keeps the battery safe from over/under voltage/current. That is all it cares about. Many of these garden light don't have a proper battery charger. \$\endgroup\$
    – ocrdu
    Commented Aug 12, 2021 at 18:56
  • \$\begingroup\$ @winny: but manufacturer you mean the one of the battery? I've added link to the battery datasheet. \$\endgroup\$ Commented Aug 12, 2021 at 19:01
  • \$\begingroup\$ @ocrdu: yes, the question is if this element will continue doing correctly its tasks of keep battery safe, when some additional load is used. \$\endgroup\$ Commented Aug 12, 2021 at 19:06
  • \$\begingroup\$ If properly connected to the battery it will, but it may disconnect the load if it thinks 150mA is too high a discharge current for the battery. You should be able to find out about that from the datasheet. \$\endgroup\$
    – ocrdu
    Commented Aug 12, 2021 at 19:10

1 Answer 1


can I connect some load to a solar panel while it is charging a battery or the current across this load can cause a confusion in the battery load controllers ?

Not if you have a solar charge controller (solar panels have a variable source impedance and to maximize the power output a special algorithm needs to be used, called MPPT to maximize the power output of the solar cell. Otherwise there could be power instability on the shared power rail)

Another problem is I see the battery connected nearly direct to the solar panel, there are two problems with this. As long as the HY2111 is preventing overvoltage and undervotlage then this will prevent the battery from failing, but the battery charging cycle will not be happening which could cause the battery lifetime to be shortened. A battery should have a charging controller and the HY2111 does not provide this function. If the design needs to be cheap, then using only the HY2111 will be ok at the cost of battery life and the design will not be robust.

Is this current load (1 mA in idle mode, 150 mA when a trigger) a problem for the correct function of the battery controller?

The overdischarge of the HY2111 varies by model, make sure you get the HY2111-HB (200mA overdischarge limit) or HY2111-KB (225mA overdischarge limit). the standard discharge for the battery you listed is 440mA so 150mA shouldn't be a problem for the battery.

  • \$\begingroup\$ In fact, all this part (solar panel, battery, usage of HY21111, ...) was already in the commercial garden light. My changes has been in the load side: addition of the FireBeetle and reconnect the PIR (in the original circuit, the PIR is active only at night, when system is powered by the battery, in the same way than the leds). That is, my change is the addition of the some load when battery is charging. \$\endgroup\$ Commented Aug 12, 2021 at 19:14
  • \$\begingroup\$ @pasabaporaqui See edit. \$\endgroup\$
    – Voltage Spike
    Commented Aug 12, 2021 at 19:18
  • \$\begingroup\$ One thing that confuses me is that, in day mode (battery charging) current across load doesn't goes across battery nor HY2111 (circuit is powered by the solar panel, load current should be mainly from it). How it can affect the HY2111 ? At night mode (battery is powering circuit) the HY211 should be "inactive" ? \$\endgroup\$ Commented Aug 12, 2021 at 19:21
  • \$\begingroup\$ At night the power for the HY2111 would be supplied by the battery itself \$\endgroup\$
    – Voltage Spike
    Commented Aug 12, 2021 at 19:23
  • \$\begingroup\$ you say "HY2111-KB (225mA overdischarge limit)". Could you clarify this value comes from ? Datasheet talks about V_DIP of 225 mV. \$\endgroup\$ Commented Aug 12, 2021 at 20:14

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