Charging a battery from a solar panel?

[Flyback]

We have an SPP30-12 solar panel (datasheet below) whose output is boosted up to 30V and then this is fed to a buck battery charger. (battery is 24V)
Will it be OK for us to simply use an input-current-regulated boost converter (in constant off time mode) and set its input current to 1.72Amps, and then just let it transfer energy to its output, where the buck battery charger will use the energy to charge the battery?
(1.72A is the “max power current” level of the solar panel)
Victron SPP30-12 Solar panel datasheet:-
https://www.es-store.co.uk/documents/product/Polycrystalline-Panels-EN.pdf

 

[MikeMl]

You need to know about the charging algorithm as implemented by the charger. The current into the charger is a function of the state-of-charge of the battery itself, and if you are trying to minimize the charging time by playing the MPPT game you cannot do that without knowing about what the charger input requires.

 

[Flyback]

thanks, we will be making the buck charger ourselves, and so can suit that to the battery as required. We're just wondering whether we sould always limit the solar panel current to the "max power current" level of 1.72A?.....ie whether its bright or dim daylight to the panel.

 

[MikeMl]

I think that the goal would be to maximize the current into the battery, regardless of what is happening at the panel...

 

[Flyback]

but I believe according to the datasheet of the panel, we mustn't draw more than 1.72 amps out of the panel?

 

[kubeek]

No that the max power current is referring to the max power point under standard test condition.

 

[MikeMl]

Does the panel have a short circuit current rating? I would guess that the Data Sheet is "suggesting" that if you draw more than 1.72A, you are past the MPPT point for the panel; it is not an hard and fast requirement.

The battery voltage during charge is between 24V and ~28V, and the goal is to maximize current during Likely, the maximum available current from the panel is less than the battery accepts as a maximum allowed charging current.

 

[Flyback]

s/c current for panel is 1.85A, and max power current is 1.72A..... I would have thought it was pointless to ever try and draw more than 1.72A out of the panel?

 

[MikeMl]

So you want a step-up converter that keeps the panel current near 1.72A while the battery determines the output voltage between 24 and 29V (stage 1).

After the battery reaches ~29V, the converter keeps the battery voltage constant at ~29V until the battery current drops below a threshold. (stage 2). We don't care what the panel current is during this stage.

Then the converter keeps the battery voltage at ~27.4V for the rest of the day (stage 3). We don't care what the panel current is during this stage.

 

[mab2]

The current out of the panel is (very approximately) proportional to the light falling on it - so at STC (noon, clear blue sky, panel square to the sun) it will produce max power at ~18v and 1.72A; if it's dull, grey and overcast (say 10% light intensity) the max power point (MPP) may be more like 16.5V, 0.17A. if it's a bright noon and there are clouds around (but not blocking) the sun you can get 'cloud lensing' effects which might give you a MPP at, say, 18.5v and 1.9A.

A MPPTracker uses a uprocessor to measure power output and constantly adjusts the voltage to optimise power for the light conditions. If you want to try and approximate the max power point without calculations you would be better off regulating to a voltage setpoint rather than current, as voltage doesn't vary as much with light level.

Unless you can make exceptionally efficient dc-dc converters, your would be better of using a single stage to go from 18v to battery volts - or better yet, get a 60 cell panel (MPP ~30v) and directly connect to the 24v battery without MPPT's of dc-dc convertors IMHO.

 

[Flyback]

thanks, there will actually be multiple solar panels, and we therefore need to have each one with its own boost converter, boosting the panel voltage up to ~35V ready for the buck charger to to use. So all these boost converters will be supplying into the input capacitor bank of the buck charger.

 

[mab2]

If that's the case, could you not put the panels in series pairs which would have a Vmp ~36v straight into the buck charger? Or is the Voc * 2 too high (in which case use a simple dump load controller to limit volts)?

 

[moty22]

The setup that you have is the best and the easiest to transfer most of the energy out of the solar panel.
The step-up converter works on the idea of charging a coil with energy and dumping it on the battery. This way whatever voltage or current the panel gives you put into the battery.
If you design your converter to work down to 1V then when there is little light you can still step up the voltage to 30V and charge the battery with it.
The way to make such converter is to drive the drive circuitry of the mosfet by the 24V from the battery and to connect the coil to take its current from the output of the panel.
You can limit the current to 1.72A or 1.85A, it isn't important. You need a current limit for protecting your electronics. The panel is already protected by it's own high resistance.
You don't have to worry about MPPT because that applies only when you charge
directly a battery which has lower voltage than the panel.

 

[Flyback]

You don't have to worry about MPPT because that applies only when you charge
directly a battery which has lower voltage than the panel.

Thankyou for this quote, this is the missing piece of the jigsaw puzzle which I have been seeking.

 

[moty22]

You can google MPPT and get much better knowledge than I can give.
Existing MPPT systems charge the battery directly when the sun is out. On cloudy days when the output voltage drops below the battery voltage they switch on a step up inverter that uses the low anergy of cloudy day. You of course have the inverter on all the time.
MPPT are complex systems because their job is to find what is the ideal voltage current combination to take from the panel. This task is complicated because solar panels don't respond in a linear way to sunlight.