Minimal electronics to interface a solar cell to a battery powered system

Question

I've got a bunch of solar cells whose maximum power point is at around 18V and a battery-powered system that uses 3S LiPo batteries (9-12.6V). I would like to interface these solar cells to the system in order to increase the endurance (i.e. operation time from battery full to battery empty) compared to the battery-only setup.

What are the minimal electronics required to achieve this?

I'm aware that the proper way to go is to use a MPPT, including possibly one that support recharging LiPo batteries. What I'm after is the minimal effort required to a working prototype where part of the system's power requirement is provided the solar cell. In particular, I don't require battery charging capability and it would be ok if the solar cells don't operate the maximum power point (i.e. it would be ok if it operated at battery voltage instead, which would yield about half the efficiency). Also, since the solar cell can provide only ~30% (~8W) of the total average power (~25W), the battery cannot be completely replaced and the power supply cannot be simply switched from either battery or solar cell depending on battery charge or lighting condition.

Answer 1

The minimum requirement is a diode from the PV (solar) panel to the battery.
This will clamp the PV panel voltage to a diode drop above the battery voltage.
Panel output will be > Imp and < Isc. It will effectively be close to being a constant current source. This arrangement will give you about Vbat/Vmp of the panel maximum wattage or about 11/18 or or around 60%.

You can get much closer to full panel output wattage with a simple buck converter. This could be a discrete design using a few transistors and an inductor - but it's easier and almost as cheap tp use something like a MC34063. These are very old, lower max frequency than most modern SMPS IC's, but very flexible, available and low cost.
A MC34063 buck converter can be built with the IC, an inductor, a Schottky diode and a few resistors and capacitors. Efficiency can probably approach 90%. Using an external MOSFET will help efficiency.

You can get close to MPPT performance by using the panel loaded voltage as your controlled voltage - the converter functions to try and keep Vpanel at Vmp. This can get within a few percent of MPPT power out over much of the range of solar input.