# MPPT charging a li-ion battery

I just designed this MPPT solar charge controller which outputs 4.2 V regulated via MPPT, it uses the SPV1040 IC. This will be used on a satellite with 4 side panels, each with 8 solar cells in parallel outputting 2.6 V at approximately 122 Ma. Each panel will have this exact circuit and all the outputs will be connected in parallel to a 3.7 V lithium ion camera battery. When active only 1 solar panel will be active and the other MPPT circuits will be offline or providing a very low amount of current. How should I connect the 4.2v to the battery, just directly or using some sort of controller to stop charging at 4.2v? Keep in mind the battery will always have a load and it will charge for around 50m in a 90m cycle due to being eclipsed. I would obviously like the battery to last as long as possible so I am not sure what are the implications of constantly having a battery connected to 4.2 V during 50m. Thanks.

The schematic was made based off a simulation made using the ST Edesignsuite directly from the provider.

This IC has an integrated MPPT controller that uses a perturb and observe algorithm so if the IC is not in CV or CC mode is in MPPT mode. The voltage on the output depend on the state of charge of the battery but you can define both a maximum CV charge and a maximum CC charge. For the CV you use R6 and R5 in order to get 1.25V from the voltage divider, that is, CV_set=1.25*((R5+R6)/R6) if you want to start charging with constant voltage at 4V just change the resistors accordingly. For the CC you use R4, the voltage drop shall be higher than 50mV to start CC so CC_set= 0.05/R4, so in the case of your figure the CC will be 50mA (quite low), so if you want to increase it just decrease the value of R4.

You can connect the battery to the output of this IC, you don't need anything else but the voltage is not and cannot be regulated, as I stated before it depends on the state of charge of the battery. It is clamped to the battery voltage.

I recommend you to install a protection IC for the Li-Ion cell so you can define a secondary protection for both overvoltage and overcurrent, this ICs normally include protection against short circuit and undervoltage too, this is very important otherwise you can keep discharging your battery to dangerous levels.

You can keep lithium ion batteries at their rated float voltage indefinitely with no penalty, no charge termination is required. So assuming SPV1040's output is a maximum 4.2V then you are good.

• Please delete this answer before it gets dowvoted into oblivion. It is completely and totally wrong. Jul 9, 2018 at 2:35
• I would like to maximize the life of the battery so im not sure how well suited this option is. Thanks though Jul 9, 2018 at 7:20
• From another answer "However, it is safe to "float" a lithium polymer cell at a lower voltage -- typically somewhere between 3.9V and 4.05V, depending on the manufacturer and cell specifics. Thus, it is totally safe to design a cell "charge/float" circuit that provides a float voltage that won't go above the safe float voltage. This circuit will basically provide no current into the battery when there is no load. When there is load applied, the power will come from the battery and from the "float" voltage in relation to their respective source impedances (internal resistances.)" Jul 10, 2018 at 16:01
• There is nothing wrong about this answer. The trade off to be made is maximum SOC vs lifespan. The lower you set the float voltage the longer the lifespan of the cells. "In terms of longevity, the optimal charge voltage is 3.92V/cell. Battery experts believe that this threshold eliminates all voltage-related stresses; going lower may not gain further benefits but induce other symptoms" Jul 10, 2018 at 16:03
• There is a big difference between floating at 4.05 and floating at 4.2. Floating at 4.2 is completely contrary to all established safety rules for Lithium ion battery charging. It is completely unacceptable to do it, and completely unacceptable to recommend it in an answer. Jul 11, 2018 at 5:30