I have Li-poly battery that is 4.2V 2000mAh with a protection circuit.

I have solar panel that is 5V 250mA.

The aim is to charge the battery to maintain a fully charged battery, so it really preforms as a top up charger.So if the battery is completely low the first step would be to use a Li-poly charger to charge the battery and then use the solar panel.

I would like to know if there is a disadvantage of my setup as my tests have found it to be the most efficient way.

  • Setup I have not used a li-poly charge IC, as i have found they require a minimum voltage and when the sun is low this would mean no charge at all as the current draw from battery would completely drop the input voltage, as my panel size is very small.I feel as my current charge is very low, as it is below 300mA, i can get away with this setup, but if i had a much larger solar panel i would need a dedicated charger IC.

But what i have done is the following:

1)I have connected my solar panel via a diode and load switch to the battery. 2) Using a ADC and voltage divider circuit(via 2nd load switch) i am continuously monitoring the voltage so that once it reaches 4V i would switch off the charging process. This is done using a micro controller.

I have noticed even when the sun is low and therefore the solar panel is producing only few mA, the battery still get some charge. The current draw on battery never effects input voltage by dropping it to low as , as the voltage drop on solar cell never drop below the battery voltage.Where as with a Li-poly IC i have noticed, that there would be no charge in these conditions.

Ay disadvantage of this setup? As it seems to be working well.

  • \$\begingroup\$ Sounds reasonably conservative. A commercial charge controller would probably continue charging at 4.1V (constant voltage mode) until either the battery current reduced to a percentage of nominal charge rate (say 50 or 100mA) or a time cutoff (say an hour). So yau're giving up a few percent capacity for simplicity and safety - sounds OK to me. \$\endgroup\$
    – user16324
    Commented Dec 18, 2017 at 12:30
  • \$\begingroup\$ I second what Brian says, but in the batteries I have seen between 4 V and 4.1 V you have something like 20% of the total charge. It is worth checking the datasheet in my opinion. \$\endgroup\$ Commented Dec 18, 2017 at 12:51
  • \$\begingroup\$ I say 4V, as i am giving a little bit of a clearance for safty purpose, instead of charging all the way to 4.2V. If i was to aim for 4.2V, so with 5V solar cell there is a chance of over charging. This just reduce the chances of any issues. \$\endgroup\$ Commented Dec 18, 2017 at 20:34
  • \$\begingroup\$ By the way i frequently use cheap and cheerful LTC4054, with 5V source, and programmed to output 500mA \$\endgroup\$ Commented Dec 18, 2017 at 20:34

2 Answers 2


I do not see any disadvantages so far. For clarification, can you explain what Li-poly IC you mean. As far as I understand you always use the same battery (Li-Poly) and it does get charged by the setup you provided.

Some improvement suggestions:

1) use Schottky diode to reduce voltage drop 2) since you battery has integrated protection circuit, you do not need to monitor the voltage while charging.

Notice: be careful not to esceed maximum charging current in case of too much sunlight. Typically maximum charging current is C/2 ,where C is capacity of you battery divided by hours (i.e. 10000mAH would give you C=10000mA, so C/2= 5000mA). But check your datasheet!

  • \$\begingroup\$ Depending on the battery used C/2 is very conservative. 1C is the standard slow charge rate for li-poly batteries meant for RC aircraft. In fact, manufacturers sometimes claim up to 5C charging capability! \$\endgroup\$ Commented Dec 18, 2017 at 15:47
  • \$\begingroup\$ There is Schotty Diode, this insures voltage does not flow from battery to solar panel, when solar panel output is low. I need to monitor the voltage to insure over voltage does not occur. \$\endgroup\$ Commented Dec 18, 2017 at 20:36

I have only one objection and that is low amp charging in low light. I have seen discussions on this subject and people have referred to mfg datasheets mentioning "charging be finished within 5 hours". Allegedly the chemistry isn't "mixed" properly during weak charging, "lithium clogging the electrodes", creating problems similar to "undercharging" lead-acid batteries, i.e. deterioration. (And I have indeed ruined a 12V AGM by undercharging it with a weak panel.)
Maybe this is a reason for the charge IC to shut off at low voltage. So, pls do a 1-year test and share your results! :)


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