I would like to build a IoT project which needs need 5V. I have some 6V 1w solar panels which I would like to connect in parallel to charge some LiPo batteries 3.7V


I have some TP4056 modules which has a max. input voltage of 5.5V https://www.ebay.de/itm/5pcs-5V-MICRO-USB-1A-Lithium-Battery-Charging-Protection-in-one-Board-Module/351493712096?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

1: What is the best way to bring the 6V from the solar panels down to 5V. A buck converter or a diode 1N4007?

2: Can I charge 2 or more batteries from one TP4056 module or do I need one for each battery?

3: Can I use different size mAh batteries in the circuit?

Thank you in advance

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    \$\begingroup\$ Useful app note link added \$\endgroup\$
    – Russell McMahon
    Jul 18, 2019 at 2:40

1 Answer 1


The TP4056 has a Vinmax of 8V and can be safely operated, if desired, at 6V input.
TP4056 datasheet here

PV panel voltage regulation:

A reason to limit Vin is that the IC will charge at a decreasing maximum current as Vin increases due to thermal dissipation issues. Charging specifications are given at Vcc = 5V (datasheet page 1) so limiting Vin to 5VDC makes sense. Due to the non-critical voltage requirement, a zener diode of suitable wattage could be used (option A below). If tighter Vin control is wanted, a simple shunt regulator will suffice - eg a TL431 driving a shunt transistor (P Channel MOSFET or PNP BJT) (option B below).

Components are sized for regulation at 5V, 2 watt.


simulate this circuit – Schematic created using CircuitLab

TP4056 modules are relatively low cost (your price is OK but they can be substantially lower than your example if you buy 10 from some sites). It is better to use one module per battery although more than one battery can be connected in parallel with a variable degree of success. If connecting more than one battery, ENSURE that they are balanced first - connect them together via say a 10 Ohm resistor if voltages are about equal and maybe charge them individually first and then parallel them.

Paralleling batteries of different mAh capacity is potentially doable but is inadvisable unless there is some good reason to do so. As above, equalise voltages first.

There are two main styles of TP4056 modules available - those with a low voltage load cutout (which you have cited) and those without the cutout circuitry. Those with the cutout are much preferred in simple circuits where the user does not manage the load and battery, as they prevent cell overdischarge and battery damage or destruction.

Battery protection circuitry:

The diagram below shows a TP4056 charger plus a DW01A battery management IC and FS8205A dual MOSFET. The DW01A disables the path to B- when various conditions are not met. The TP4056 already provides charging and overvoltage control, so in normal use the main specification is battery undervoltage protection, preventing a load from discharging the battery below a safe level. On TP4056 modules with Out+ B+ B- and Out- terminals, the two extra ICs are the DW01A and the dual MOSFET.

Very useful TP4056 & DW01A related application note here

enter image description here


The PV panels you cited are encapsulated in epoxy resin. This is potentially acceptable for occasional outdoor exposure. If they are to be used outdoors on a semi-permanent basis, you can expect a lifetime of as little as a summer (say 3 months), typically about a year, and in exceptional circumstances, a few years. A far better choice is "PET" encapsulated panels (or, less liable to be found, a fluorocarbon plastic encapsulation). Depending of manufacturing quality, a PET panel may give in excess of 10 years of full time outdoor use.

eg PET PV panel 6V 1.5W $US3.06 free shipping - quality unknown but LOOKS reasonable.

PET (Polyethylene Terephthalate) is the same plastic used in softdrink bottles. It is laminated to the PV material and PCB backing using EVA laminating plastic (as also used in standard glass-fronted PV panels.) PET chemical bonds resist breakdown by UV light, whereas epoxy resin carbon-carbon bonds are susceptible to UV degradation leading to frosting and crazing of the surface over relatively short time periods.

Sunshine hours and related statistics for Esbjerg, Denmark from www.gaisma.com

  • 1
    \$\begingroup\$ Thank you very much for your detailed and brilliant explanations. I was planning to use polycrystalline solar panels but will now buy some PET laminated instead. Thanks to you. May I ask you for a link to the TP4056 with cutout circuitry? Thanks again \$\endgroup\$ Jul 17, 2019 at 12:05
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    \$\begingroup\$ @HenrikLauridsen - See added diagram for battery protection functionality. More to come. || Poly or Mono crystalline refers to the silicon PV material and either are OK. Mono usually provides slightly more output than Poly so is slightly smaller pe Watt Wmp but this is usually unimportant in this sort of panel. | PET and Epoxy are encapsulation materials and independent of the poly or mono crystalline choice. \$\endgroup\$
    – Russell McMahon
    Jul 18, 2019 at 2:32
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    \$\begingroup\$ Thank you very much again. I checked my TP4056 modules and they are all equipped with DW01 chip. I have plans to run a Raspberry Pi Zero with camera from batteries / solar at some time. I have read that the Pi with camera consumes about 230 mA, so it should be doable. About the solar panels I have ordered the 6V 3,5W \$\endgroup\$ Jul 18, 2019 at 7:48
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    \$\begingroup\$ @HenrikLauridsen If you use a buck regulator then an 18V panel with voltage converted to 5V is "OK". A linear regulator or clamp regulator from 18V would waste most of the energy. A 6V 2W panel will deliver most of the energy to the charger when needed. See TP4056 datasheet for maximum available charge current. Note that the device tends to be thermally limited if Vin is high and/or cooling is inadequate. \$\endgroup\$
    – Russell McMahon
    Jul 18, 2019 at 11:16
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    \$\begingroup\$ @HenrikLauridsen Note that the available energy per day will be limited by available sunlight, PV panel wattage and battery capacity. Sunlight equivalent hours may be lower than expected. See Gaisma page for Esbjerg Sunshine hours/day average by month are January to December: 0.57 1.24 2.32 3.82 5.14 5.25 5.17 4.28 2.78 1.55 0.78 0.50 \$\endgroup\$
    – Russell McMahon
    Jul 18, 2019 at 11:25

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