In case anyone's wondering why I'm bothering with 10-20A of solar panels, I live on a boat and this is all I have

 

The "you" I was referring to was the OP, not RB.

Well that is very different. Never mind.

 

When you set up a MPPT power system you normally program what the max current will be to charge your batteries. If there is enough power being generated by the panels to support that charge rate then great. If the panels are not generating enough power then the MPPT will adjust/reduce the charge rate to maximize the power that is available.

When you read about an MPPT adjusting the load to maximize the transfer of power, the load that it is adjusting is the battery chargers output.

Mark

 

Hi Lestraveled, what you say is very clear, however I thought that the feedback information to the charger, which governs transition from bulk -> topping -> float depended on at least one fairly stable variable such as constant current or voltage, or otherwise there was the risk that the feedback information was not relevant, due to a substantial delay between a change in the charging power level and the feedback signal stabilizing at the new value.

 

I don't think it is too bad. During bulk charge current is limited - if there is not enough current it doesn't matter, it will just take longer to charge the battery. The rest of the time you limit the maximum voltage to about 14.2 or so. So you will need to wait until you have 14.2 from the solar panel and then measure the current to determine if it is time to shut it off. I think they usually use about 3% of the c/10 current value to say it is charged.

 

Hi The_RB, panel voltage will be 24v, battery voltage 12v, max current 20A (edit: I only have 10A of panels at the moment if I will be putting them in series for 24V but will probably add later).

So its 24v 10A panels into 12v battery at 20A? That requires a >20A buck converter. That is within the realm of do-able at hobby level but will not be "easy".

You are going to need a specialty inductor good for 25-30A without saturating, and a 24v 10A power supply, and a 12v >20A adjustable dummy load. I have all those things in arms reach (plus the obvious test gear) but you might need to build those first to allow you to test and fine tune the SMPS.

You might want to re-consider just buying a 20A 24v->12v MPPT from ebay...

Are the p mosfet losses due to relatively high Rds(on) compared to n mosfets? I've found some reasonably low ones (~0.020 Ohms). I'd be interested to know how a high side n-mos can be switched with a positive Vgs without having two separate power sources and not adding too much complexity and further losses.
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Yes. And, you can get a "high side mosfet driver IC" to drive a high side NFET. A google search will show you lots of options.

 

Billy
In the "bulk" mode, the charger remains in constant current mode until the battery voltage reaches 14.4 volts. (This voltage varies between manufacturers). If the sun goes down in mid charge, of course the charging stops. Then charging will begin again the next morning with the charger in bulk mode. (A smart charger will always begin in bulk mode.)

Another issue you need to address is your panel power verses your battery capacity verses your daily draw. If you do not produce enough power to be able to bring your batteries all the way through a charging cycle (bulk/absorption/float) at least once a day, your batteries will become sulfated quickly.

Mark

 

Hi Lestraveled, thanks for the tip, I have a small generator for that very purpose but would rather not use it if possible.

Apart from the fact that MPPT chargers are generally quite expensive, and it would be great to get more power from the panels, (especially in cloudy weather) of course I would not be doing this if it were not mostly for enjoyment. Also, as stated before, it is imperative that I understand how it works and I'm able to make repairs on it since I may not always be close to a place where I can get a new one.

Cheers everyone for your help

 

@The_RB I will hook up two of the batteries in series for the 24V test supply and limiting to 10A won't be a problem.

I might buy a couple of 200W TO-3s and plant them on the steering column (thick aluminium). Would it be reasonable to expect good dissipation through heat-transferring insulating pads?

I have a book on making inductors. I have salvaged a few big toroids and will just get some inductor wire to suit.

Also (I know real life is different ) I am using LTSpice to simulate things so hopefully by the time testing comes along things are fairly well configured. I also have a scope to test ripple etc.

Edit: Could you please suggest a thickness of wire for the inductor? It has been on my mind as I am always surprised to see how thin they are in speakers etc compared to the current/size ratios suggested online.
Edit2: I KNEW the book had a table somewhere! Never mind

Thanks for the advice.

 

Simply stated, MPPT works between the limits of 'constant current' and 'constant voltage'. This current is the maximum current that would be used for charging the battery safely, and the voltage is the float voltage of the battery.

 

It's also the size of the core that determines where the inductor saturates. Spice does not model this so you need to check it against the specs.

 

@The_RB I will hook up two of the batteries in series for the 24V test supply and limiting to 10A won't be a problem.
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Yeah it will still be a problem.

24v solar panel will reach its MPP at about 34v DC, so to simulate and test that you need a PSU that can make 34v DC 10A.

And it really helps that the PSU is voltage adjustable for when you are fine tuning the MPPT algorithm.

To properly test the MPPT feature you need the PSU to be >34v (like 40v or 45v) then use a high power series resistor between the PSU output and MPPT buck unit input. Then you will be able to see the MPPT adjusting its input current to find the peak power, ie the max power it can get after the series resistor.

If you post photos of your inductor cores (and size specs) we might be able to suggest if they are suitable or big enough.

 

Billy, Guys
Here is an app note on building a switching lead acid battery charger that has a simple MPPT function. The chip is a Unitrode/TI BQ2031. I have used this chip many times in battery charger designs, but as a controller only. (I integrated the PWM control output and turned it into a voltage that controlled a large (>25Amps) power supply.)

The examples in the app notes are all less then 10 amps output. I have never designed/built a 25 amp buck switcher, so I am asking you guys what you think about using this chip at that power level?

Attached is the general app note and the MPPT app note.

Mark

 

I've had a look and it looks pretty good, however atm I would like to at least design the circuit with as many discrete components as possible so I know how things work and what to watch out for. I might end up getting something like that if I find that it can bring much better efficiency. Thanks!