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A few 75/15 observations and dislikes

I just towed my solar electric boat home after leaving it in the water for the last month during which time it has been completely self sufficient. During use I noticed a few things about my 75/15 charge controllers.

First and foremost is the issue about parallel chargers. I don’t know where things stand with the Firmware update to include this feature, but at the moment my two chargers are wired in parallel to a 24V bank, and when in full sun with each fed 60V+ of solar, and a load on the system with the chargers in Bulk, the most I can get is 15A total out of them, split mostly equally between the two, with each providing 7.5A or so. Is this the issue that the Firmware update will address? Shouldn’t each charger be putting out 15A? My motors are pulling far more than that from the system, so I would think each charger should be operating at Max output.

Second, is there a way to avoid having to do Firmware updates? I understand the update is in the App and that WiFi is not needed, however I don’t like the fact that I can not use the App to monitor my system without first updating the hardware. What if something goes wrong during the update? I would rather do it at the end of an outing rather than just before. Is there a way to avoid this? The App seems to require an update before you can be allowed to fully pair with a device.

Lastly, one of my Blue LED’s had stopped working completely yet the unit continued to work otherwise normally. For almost a month it was like that, then suddenly (without any updates or changes) it started working again. I guess it just needed a vacation? Kinda disconcerting, but at least it is once again blinking in offbeat harmony with the other.

Ohh and one more, does the battery bar on a BMV712 indicate the current level of 50% of the bank capacity while the SOC percentage relates to 100% of the banks capacity?

MPPT Controllers
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solgato avatar image solgato commented ·

System is 3 x 100w panels in series feeding 60V to a MPPT 75/15 charger X 2 paralleled to a 24V bank.

Each charger is only doing half the work, using only half the available power, etc.. when both arrays are seeing equal sun which they typically do.

My motors can pull 80+ Amps at times, so when the chargers go into Bulk mode in full sun with full panel voltage, I would expect to see 30A going back into the system to offset some of the motor load.

I am only seeing half that as if each charger is being confused by the other. The same behavior can be seen in my history chart. Each day both chargers are pretty much splitting up the work load to recharge the bank unless one of the arrays is shaded.

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David avatar image David solgato commented ·

So if the batteries are charging at say 28V typically (seems reasonable?), a 300V PV array at FULL harvesting capability put in around 10.5A. You will never get 15A from a 300W array into a 24V bank. Simple Maths ... P=IV, or P/V=I.
P = 300W (absolute max) / V = 24V (realistic minimum) = 12.5A at the very very (very unlikely) most per Array/Controller combo.

And the amount of time you will getting a full 300W from a 300W array is pretty small as well. You are getting 75% of the 300W it sounds like? Sounds about right to be to be honest.

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solgato avatar image solgato David commented ·

Thanks. Yes, I understand the math. My panels are brand new with Sunpower cells and I am on the water, often with full sun overhead in Hawaii, so I should see close to max.

Point is, I’m not even seeing what you think should be possible as I have two independent 300W arrays each feeding their own 75/15 which are paralleled to the 24V bank.

If the batteries are fully charged with 100% SOC and I put a heavy amp load on the system in ideal solar conditions, and the chargers switch to Bulk mode, I should be seeing at least 20A based on your math, and I’m only seeing 15A combined as if they are hitting a wall, with each providing half the amperage.

Of course I can see how much wattage is being converted into power by monitoring each MPPT. Each charger appears to be using only using half the available wattage with plenty of voltage to spare over 28V since the panels are in series often supplying each charger 60V+ In ideal conditions.

My wire runs are very short with my motors powered directly off a double pull double throw master power switch to which the solar chargers are also wired, and there is very little discrepancy between the V reading from the shunt of my BMV-712 and what the chargers are measuring at their inputs, so there is negligible loss in the system.


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David avatar image David solgato commented ·

Well, I would say based on my maths you would be very fortunate to see 20A in total. You seem to be assuming that the panels will be getting the full wattage harvested which really is unrealistic. Also remember some losses in the controller. it is not massive but it is still a factor to include.

My numbers didn't even take conversion and cable losses into account!

Do you still think you should get 30A into a 24V Bank from a total of 600W of PV panels? you say "half the available wattage" - how do you know what the avaikable wattage actually is? have you got an irradiation meter that is telling you what should be harvested, or are you assuming the available wattage is 300W always?

You keep mentioning the voltage ("plenty of voltage", "over 60V", etc). Voltage is only part of the picture. I could connect 3 of my Victron 30W (THIRTY) watt panels together in series and I would have a 90W array that is presenting 60V to the controller. It is still only a 90W array.

Anyway, hopefully you will get the answers you will like from someone as you don't like my thoughts and explanations. I wish you luck and look forward to seeing a sucessful conclusion to your quest :)


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David answered ·

I think to get a better answer, you need to provide more information.

You say "Shouldn’t each charger be putting out 15A? My motors are pulling far more than that from the system, so I would think each charger should be operating at Max output." But what is the size of your PV Array?

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