What is tje most efficient solution to power a 3,2w 4,2v solar panel? Addind a diode and using lifepo 4000mah battery? Or using a contoller and 4000mah lipo battery? In this case which contoller available on the market should I use? I add few of them I've found but feel free to suggest anyone else Thanks in advance Andrea




You should not use a solar panel directly connected to a lithium battery. This will destroy your battery soon. You should instead find a solar panel that has it's Vmpp voltage around 3.5V and use a li-po specific battery charger connected to it.

Li-po batteries use constant current charging up to 60% charge and then finish with constant voltage at aprox. 4.2V. For the constant current, up to 1C is recommended, so at 4000mAh you can charge with up to 4A. Your panel is 3.2W so charge current limiting won´t be an issue.

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I can´t tell if the charger from your first link will work, as it is rated at 4.5-5.5V. It might work but you will lose some power for not working in maximum power point (MPP).

You might get a better result by simply using a constant current circuit, tweaking the charge current so that your solar panel voltage is close to Vmpp at most of the time, followed by a voltage limiter at 4.2V. That will require only a few components and would be more efficient.

  • \$\begingroup\$ To give you an update I'm using a panel 3v 3w and I have found the step-down input 3-6v below and set it up at 4,2v as you recomanded, I have notice bigger smartphone has problem charging from the usb port, but they are do fine if I set output at 5v. Do you believe it will damage the battery at this voltage? m.aliexpress.com/item/32747199929.html \$\endgroup\$ – Andrea Mar 31 '17 at 12:38
  • \$\begingroup\$ If you are charging a smartphone, then you need 5v and not 4.2. Smartphones have their own internal battery charger so they need a higher voltage than the battery itself will use. But this way you will not be running your panel in MPP. If you wish to charge a smartphone directly, get a 5v solar panel and make a circuit with a voltage limiter and accumulator (capacitor) that only gives charge to the phone when a certain voltage has been achieved in the capacitor. \$\endgroup\$ – cyberponk Apr 1 '17 at 18:47
  • \$\begingroup\$ I believe the stepup converter I'm trying to use they are working following the same principle you explain, they have little cilinder looking like small capacitor to achieve 5v. The only problem I'm facing is stability, using the one working from 3v I suppose sometimes my panel is not always achieving that voltage, so semetimes is charging the phone and sometimes even if the ameter is says it charge the percentage doesn't rise. I will try with one starting at 2,5v and another one at 1v. Even if the latter could manage only 1A. Unfortunately I cannot change anymore the panel. \$\endgroup\$ – Andrea Apr 3 '17 at 15:12
  • \$\begingroup\$ Your problem is that the panel output varies a lot, so is not always giving 3W. Most of the time the output energy will not be enough to power the smartphone itself, so there will be no energy left to charge the battery, that´s why battery level isn´t rising. Your smartphone is also smart, so if usb voltage is not at a fixed 5V it will probably cut the power input to avoid damage. Solutions for this: leave phone turned off while powering, or leave it in low power mode (airplane mode, gps and bluetooth off, screen off), and definetely get yourself a bigger solar panel, I would say at least 10W. \$\endgroup\$ – cyberponk Apr 3 '17 at 17:12
  • \$\begingroup\$ What about if I connect a battery? 3v 3w Solar panel with step up converter might be able to charge the 3,7v 4000mah battery and then the battery will steady charge the smartphone via usb port. Battery arrive today do you think I can connect using the same input connection poles I used for connecting the panel? I will try several usb converters and some have dedicated battery port but the only one (almost) worked so far has only single input port. By the way, thanks so much for following me in this dangerous path :) \$\endgroup\$ – Andrea Apr 4 '17 at 20:49

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