# Solar charger for Lipo battery

Im trying to build a little solar charger for a Li-po battery using

• 2 solar panels rated at 6V/120mA
• a linear charger based on the TP4056 chip

The idea is to connect the two panels in paralel so it can get the most current at 5V at best (since its a variable source), the charger module has an overcharge/overdischarge protection (the DWO1A), so in theory,

• Its enough to hook the lipo battery to the charger?
• Its needed a diode at the Vin pin?
• Supposing a constant value of light (ie. in the case the cells are providing the 120mA) how much time it does take to charge a 2000mA?
• Its worth to attach at the output of the charger a boost converter (based on AOD4184,SD1040CS and LTC1871 ), so it can be charged at least two battery on parallel?

Is it enough to hook the lipo battery to the charger?

ANS.: What do you mean? According to me, it is quiet enough, only if the setup is for charging the battery only.

Its needed a diode at Vin pin?

ANS.: I don't think so. Why would the pin need a diode? It is of no use as the solar panels would never provide reverse current. Moreover, these diodes have at least a resistance of 5 to 10 ohms. This will reduce your input current. Do not add a diode.

Supposing a constant value of light (ie. in the case the cells are providing the 120mA) how much time it does take to charge a 2000mA?

ANS.: I guess you mean "2000mAh". Supposing the cells provide 120mA current, and due to internal resistance of the charger, the output current is 100mA, then the time taken to charge the battery would be 2000mAh/100mA = it would take 20 hours to completely charge the battery. (Or about 2000/120 ~= 17 hours at 120 mA). HOWEVER, tat's "equivalent full sun hours, which is usually far less than daylight hours. Typical 'full sunshine hours equivalent' are 2 to 6 hours winter to summer in many places. A few are somewhat better (eg Kabuhl, Afghanistan) and a few are much worse in winter (ge Moscow, Russia). Realistically, in Summer it would take about 3 to 4 days of good cloud free sun to fully charge a 2000 mAh cell from a 120 mA panel.

Its worth to attach at the output of the charger a boost converter (based on AOD4184,SD1040CS and LTC1871 ), so it can be charged at least two battery on parallel?

ANS.: Yes you could do that.

• Can you explain how to calculate the time, Im doing the cocient and it takes 20 hours. Commented Feb 26, 2020 at 19:08
• The battery's capacity is in mAh (milli-Ampere hours) usually. The charging current should be in milli-Ampere too. Just divide the mAh value by the mA value and you get the time required in hours. Commented Feb 28, 2020 at 17:23
• 2000 mAh/ 100mA = 20 hours, not 10. I'll edit the answer. However, the charger internal resistance probably has little or no effect on charge current. If the cells provide 120 mA at maximum power point there will be "some voltage to spare" as maximum power point of the 6V panel he specified is either 6V (usually MPP is specified) or at least 4.8V if Voc = 6V (6 x 80%) and probably more. even at full charge (4.2V) that gives you 1.2V to 1.7V "headroom" so Icharge in full sun will be 120 mA. Commented Feb 17 at 6:14
• Also note that equivalent sunshine hours is not equal to hours of daylight. (Good 1st approximation is 2 to 6 hours winter to summer. Commented Feb 17 at 6:14

Its worth to attach at the output of the charger a boost converter (based on AOD4184,SD1040CS and LTC1871 ), so it can be charged at least two battery on parallel?

Two batteries in parallel require the same voltage as a single battery. Connecting two in parallel takes twice as long to charge, but needs no additional converter. Adding a converter adds inefficiency but serves no useful purpose.

HOWEVER - if you connect two batteries in series you would need to double the output voltage. This is probably not trivial as the TP4056 module "looks at" the voltage and current draw of its load to function. Without special care to maintain the voltage doubler below 4.2V it would shut down.
Also, the end of charge sensing may not work as intended.
So, it may be doable, but is probably far more complicated that just adding a boost converter.

There are IC's and modules available that are designed to charge two LiIon cells in series frfom a 5V input