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I have what seems to be a rather common problem, a circuit in which i wish to be able to simultaneously charge and discharge (use) a LiPo battery. The "charge" going "into" the battery will be coming from a solar panel, and therefore may have varying voltage, the discharge from the battery will be going through a powerboost 500 and to an iphone to charge it.

Would this be a functioning circuit, where if the iphone is not plugged, the solar panel will charge the lipo, and when it is plugged in, the solar panel and lipo will be regulated to 5V and charge the phone?

Circuit Diagram

Thanks in advance, all answers are accepted

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You need to have a charge controller circuit between the solar panel and the battery. The voltage and current from the solar panel will vary depending on the amount of light landing on it and so it needs to be regulated. The battery has requirements for charging so that it is looked after and not over heated.

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Presumably the battery is a single LiPo cell.

You cannot simultaneously charge and discharge a battery.
I_battery = +Icharge - Iload.
If I battery above is +ve the battery is charging.
If Ibattery above is negative the battery is discharging.
This is not a problem, just needs to be understood.

A LiPo cell MUST NOT EVER be charged above 4.2V and MUST NOT EVER be "floated" at 4.2V. There are two choices

  • Charge with a purpose designed charger controller.
    There are effective low cost modules available using a TP4056 IC.
    Buy ONLY the versions with separate Vout and battery terminals - these handle charge control, over and under voltage protection and more.

Like this:

enter image description here

Examples only - NOT a recommendation HERE AND HERE

OR

  • Clamp the battery at 4.1V or less so that it NEVER reaches 4.2V. Voltages in the range 4.0V - 4.1V are OK.
    Higher than 4.1V risks early death of the battery.
    Lower than 4.0 will result in a low battery capacity.

If you check operation in full sun with a fully charged battery a zener diode (perhaps 3.9V) may do the job of battery clamp well enough.
A TL431 clamp regulator will allow a precision clamp voltage of say 4.0V to be set.
Depending on the battery mAh and PV panel rating you may need a clamp circuit with a external pass transistor as well.


Q1: What is the battery mAh?
Q2: What are the PV panel specifications.
Any datasheet links?

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  • \$\begingroup\$ 1. Very low battery capacity, just what I had lying around (I believe it is 400mAh) 2. The solar panel is rated at 3v at a maximum of 240mA \$\endgroup\$ – 3NiGMa Mar 30 at 22:20
  • \$\begingroup\$ @3NiGMa 3V is too low to charge a LiPo. Minimum for full charge is over 4V. 4.2V plus headroom for full formal charge and maybe slightly less for an OK charge. Probably 4.5V for the 4056 (see spec sheet). A PV panel rated at 3V loaded will make about 3.75V open circuit in full sun - too low. \$\endgroup\$ – Russell McMahon Mar 31 at 3:02
  • \$\begingroup\$ Ok, I have ordered a new 5V solar panel \$\endgroup\$ – 3NiGMa Mar 31 at 21:56
  • \$\begingroup\$ @3NiGMa Measure Vo/c of your existing panel in bright sunlight. If it is 5V or above it will be useful. \$\endgroup\$ – Russell McMahon Apr 1 at 0:59

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