Electronic – How to store/use energy that could be generated from a solar cell duing the off stage of a buck converter

I found a mppt solar charger ... but again I don't want the bulky battery.

If you don't want a battery, why did you look for a charger?!

Can you suggest a way to power a solar charger with a very small battery and not have it discharge when clouds block the sun?

First, when a battery powers something it discharges. That's how it works. Second, most likely there is a way to hack charger schematics to keep it "operational", i.e. powered up without the sun. But what would be the point of that if there is no sun to get power from and no battery to charge?

Is there a mppt out there that doesn't require a battery?

I believe they called "Direct-coupling photovoltaic systems". You have to research those yourself. The MPPT controllers (not MPPT chargers!) are commonly used for loads that do not have strict power requirements, for example water pumps, water heaters etc.

However the fact is, most loads cannot operate in the wild output power range of the solar panels. Using them without battery basically negates the efficiency gains of the MPPT, because they will shut down in low light when just a little extra juice from the battery could have kept them working. At the same time in very bright light the extra available power is wasted when it could have been used to top up the battery.

So if your device (you haven't given us enough details) is the kind that can operate intermittently and you want it to draw as much power as available but don't mind shutting down in low light, then MPPT controller is what you need.

If you want your device to have stable operation in variable light conditions then what you really need is a bigger battery and MPPT charger. Then you can limit power available to your device at say, stable 100W 10V and use the remaining 20W (when they available) to charge the battery.

This is a very specific case of the more usual "can I charge and operate a load simultaneously"

In this case the answer is clear: If you value correct absorption phase charging then

1. The solar system must be able to supply at least enough current at the desired voltage to allow this. You need to be able to specify what this requirement is.

2. An additional load can only be operated if the PV output is able to provide it without preventing 1.

The above is "almost obvious" but introduces some extra questions.

• If the available PV energy falls below that needed to perform absorption mode charging in the middle of an absorption cycle, what should you do? You can continue at a lower "float" rate, or stop charging until enough energy is available to continue absorption charging, or ... . If you stop and then recommence absorption charging are the periods simply additive, or is there a more complex algorithm involved? If so, does it matter where in the absorption cycle the pause(s) are or how many there are or how long between them? What fun!

• If you can 'spare' surplus energy for another load, is it able to handle the variation from eg clouds, or complete cut off if the charging demands all the energy. eg a water heater would usually happily consume excess energy as available. A television (or its viewer) would not be so happy. An AC compressor is intermediate.

• Unlikely - food for thought: You MAY be able to split the battery into two (or more) portions to allow largely uninterrupted boost charging in two stages - with the risk of having an unbalanced battery if the solar output fades sooner than expected on a given day.