Electronic – Charging circuit for LiFePO4 cell

1) pretty standard. Get a 4S LiFePO balancing-protection board with over/under charge protection. About $8 on eBay. This will prevent the 4S to exceed the limit voltage, and will cut off the current if undervoltage is detected. This board simply goes in parallel with the CV power supply and the laptop. The supply CV voltage must be between 14.2 and 14.4 V. You can get a 5S or 7S board and rewire it to act as a 4S board.

2) I think the best solution is a 5S 18650 LCO Li-ion (i.e. non-LiFePO). These have more charge for the same weight and same cost. This will work well with your 19V laptop charger. You would need a 5S balancer-protection. 5S is 21 V, so the charger will never reach that. If the charger goes bad, the balancer-protector (BP) board shall protect your cells. You will not need a CV charger.

3) Yes, do not use self-protecting cells.

4) The theory is that you need a precise CCCV charger AND a balancer-protection board. I think with a 5S BP board, you can do away with the CCCV charger and use a standard 19 V laptop charger.

When working on the fringes of specifications, it is always best to speak with an applications engineer at the manufacturer. They often have access to data that is not part of their published specifications.

With that being said, you should give careful consideration to thermal management within your design. For example, a large insulated thermal mass in contact with the cells will retain heat generated during the exothermic charging cycle. This will help prop up the ambient temperature of the cells during non-charging periods - such as at night.

The choice of enclosure color can also play a role in raising the average ambient temperature of the cells.

You may also go so far as to use your solar power to warm the cells until they reach an acceptable charging temperature. While this is not the most energy efficient approach, it is a pragmatic one that may enable the application.