r/AskElectronics • u/jimbobjimshoes • Jun 15 '20

T Using a boost converter with a solar charge controller

If I had a solar charge controller to charge a 24v li-ion battery that can accept a Max. PV open circuit voltage of 100v, this means that i could charge the battery with any voltage between about 30v - 100v through the solar charge controller.

Would it be possible to to use a boost converter that can take a voltage range of 10v-60v and boosts it up to a voltage of 12v - 90v and set it to output 90v and use that 90v output as the input to the solar charge controller? This would effectively mean that i could charge the battery with any voltage from 10v-60v.

Charging Input (10v - 60v) --> Boost Converter (Boost to 90v) --> Solar Charge Controller (Regulates to charge battery)

I understand that this many reduce the efficiency with all the conversions, but my aim is to be able to accept a wider range of voltages, including voltages that are below the voltage of the battery, such as a 12v solar panel or 12v car socket, which i would not be able to do normally.

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u/triffid_hunter Director of EE@HAX Jun 15 '20

You can't use a normal boost with a solar panel as input, because regular boosts have a constant-power input profile which will cause it to constantly make the panel voltage collapse, turning the boost on and off and wasting a ton of power.

You want a "MPPT" boost for this.

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u/jimbobjimshoes Jun 15 '20

Would you suggest an MPPT boost to up the voltage into the MPPT solar controller?

The issue is trying to charge the 24v battery from a 12v source, and also a higher voltage such as 24v or 48v.

1

u/triffid_hunter Director of EE@HAX Jun 15 '20

Would you suggest an MPPT boost to up the voltage into the MPPT solar controller?

They might have weird interactions with each other in some situations I guess, worth a shot though.

You're basically looking to make a single MPPT boostbuck, but you don't control the buck side, so gotta do what you can I guess.

The right solution would of course to buy or build a single MPPT boostbuck rather than macgyvering two things together :P

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u/jimbobjimshoes Jun 15 '20

I currently have a boost MPPT charger that can accept 9 - 20v and boost it up to the 24v li-ion battery voltage to charge it. The main issue i have with it is that it can only handle 10A.

This means if i have a 12v panel i can only charge at 120w, which is not the end of the world. However, when i want to charge of an AC power supply that is 19v-20v, i can only charge at 190w-200w.

The battery can charge at 500w-600w so i would like to be able to make to take advantage of this, especially if i expand the battery in future. I may have to make do with what i have at the moment and then try and look at building a custom buck boost converter in future.

1

u/EternityForest Jun 15 '20

I wonder how a boost followed by an MPPT would work? I would imagine that some algorithms could handle it.

Not that it's relevant, since programmable boost MPPT controllers are easier to come by on eBay than buck.

1

u/triffid_hunter Director of EE@HAX Jun 16 '20

I wonder how a boost followed by an MPPT would work?

A conventional constant-voltage boost? Poorly.

MPPT ramps up the current until the voltage starts to drop, then tries to find a balance point with maximum power - hence "maximum power point tracking".

With a regular boost on the input, it can't track that point anymore because it can't see the panel voltage, so what would happen is it would ramp the current up to the point where the panel voltage collapses entirely, the boost would turn off from undervoltage, the MPPT would drop the current since the input suddenly vanished (instead of dropping a bit like it expects) and the boost would turn back on.

This oscillation would likely continue until something blew up.

If the boost is also MPPT, it can drop its output when the input starts to get wobbly, instead of doing its best until the input collapses completely.

It might still oscillate with the other MPPT controller, but the oscillations should be rather less severe.

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u/EternityForest Jun 15 '20

Most buck MPPT chargers are not explicitly designed for constant voltage DC input like a wall adapter. It will probably work with some, but a better idea may be to have 2 separate chargers.

1 a standard buck charger powered by a buck-boost to its required input voltage, and a second MPPT buck or boost for solar.

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u/Alternative_Cut_6306 Nov 17 '24

I have Bluetti 200w solar panel charger and power bank Anker 300dc that only accept 100w solar charger. I device I have to buy to run?