Stop trying to charge standard 18650 batteries with solar. This is just a bad combination.

It can be done, but when you combine multiple cells it gets complicated by charge balancing and the risks are too great.

Dual cell charge packs have become pretty common, the problem probably arises more when you do a lot of charge and discharge cycles in larger arrays.

The fact is that most large arrays are not the standard Lithium Polymer but a more robust and versatile Lithium Iron battery.

There are, however, a prepackaged solution that is cheaper and fun to play with. Look for the small, prepackaged solutions.

Walmart

Walmart for $12.99 has the iMeshbean 50000mAh Solar Charger, Solar Power Bank 10000mAh External Backup Battery Pack Dual USB Solar Panel Charger(Black)

Ebay for $15.00 has the TG Waterproof 900000mAh Portable Solar Charger Dual USB Battery Power Bank F Phone

Amazon for $18.00 has the Solar Charger 20000mAh Power Bank, Portable Charger Solar Phone Charger with 2 USB Port 2 LED Light External Battery Pack for Emergency Travelling Camping, iPhone Android Cellphone Charging (Orange)

Search these guys for Solar Power USB Charger and save yourself time and money.

For larger storage projects stick to the battery technologies that are tried and true for use with the charging controllers that are sold with the solar panels.