Hey SUNKING.. can you explain this please?

[Murby]

I've been doing research and reading the forums heavily... Really enjoy your posts.... but this one is confusing..

In this thread:

https://www.solarpaneltalk.com/forum/off-grid-solar/off-grid-solar-panel-systems/18060-off-grid-system-review

You make the following statements:

The difference is replacing the battery every two to three years instead of every 4 to 5 years assuming it was sized correctly to the load which you have not done.

We can tell from what little information you have provided your whole system is completely out of acceptable tolerances. The most important peice of information missing is you do not mention how many watt hours you intend to use in a day. As it stands right now your batteries are sized to only support 9.2 Kwh per day, and your panels are sized to support 26 Kwh per day. Do you see any problem with that statement? It means you get to replace them in less than a year. You are going to fry your batteries with 160 amps of charge current. That is I assume you know a 8400 watt panel system requires two very expensive 80 amp MPPT charge controllers. The minimum size battery to support those panels are 1280 Amp Hours or Rolls S-1725 a 2 volt 1300 AH battery so you would need 24 of them.

Having said all that is a moot point because you failed to do the very first requires step of determining your daily watt hour usage. Everything is determined by daily Watt Hour use and Location. At this point all anyone knows is location and a huge all electric home.

The way I'm reading this is you're suggesting that one must use up all the available power from their solar panels to charger their batteries.. That "you are going to fry your batteries with 160 amps".. Couldn't one just dial down the output of the charge controllers to match the battery bank size?

Or is your statement directed at a situation where someone is trying to harvest every watt of power from every component and assume that all things will be run at their maximum levels?

I don't see a problem with having an oversized PV array, and oversized inverter attached to an undersized bank of batteries so long as the loads (and any generator support) is managed correctly.. As a newbie to this, I read your statement as saying "you must charge your batteries with all the power your pv array or generator creates"

Did I get that right?

Thanks,

[SunEagle]

The only way to "dial down" the output of the CC is to reduce the amount of pv wattage or aim the panels toward a less optimum compass point so they becomes a lower % of their nameplate.

Remember with a MPPT CC it is Watts in = Watts out so the amount of charging amps is directly proportional to the Watts in / battery charging voltage. Too many watts in can exceed the speed or C/amp rating for a specific battery chemistry.

Sure you can find ways to siphon some of those charging amps to go to a load but that takes some type of controller and something to measure the amount of amps going into the batteries.

It can be done but it could be difficult doing it manually.

[bcroe]

There is no technical reason a charge controller can't control the output power, just as
a grid tie inverter limits its output power to a specified amount, but it might be more
expensive to add that feature. A home grown way to do it might be to just short out
some panels one at a time, each using a power MOSFET and an optical isolator driven
by a limit circuit. Bruce Roe

[SunEagle]

There is no technical reason a charge controller can't control the output power, just as
a grid tie inverter limits its output power to a specified amount, but it might be more
expensive to add that feature. A home grown way to do it might be to just short out
some panels one at a time, each using a power MOSFET and an optical isolator driven
by a limit circuit. Bruce Roe

Nice idea. That could be done with a logic operated current meter with multiple outputs. Solid state relays to short the panels would be a better choice then standard type due to the contacts experiencing arching when they are opened again.

[sensij]

The only way to "dial down" the output of the CC is to reduce the amount of pv wattage or aim the panels toward a less optimum compass point so they becomes a lower % of their nameplate.
.

No. Most of the better charge controllers allow the max output current to be user set, within the rating of the controller.

[SunEagle]

No. Most of the better charge controllers allow the max output current to be user set, within the rating of the controller.

I stand corrected. I guess I only have hands on experience with the cheaper CCs. Nice to know the quality CC's have more flexibility and control of the operation besides just voltage settings.

[max2k]

No. Most of the better charge controllers allow the max output current to be user set, within the rating of the controller.

that would mean they give up on MPPT when dialed down which makes sense otherwise where would they put extra energy?

[sensij]

that would mean they give up on MPPT when dialed down which makes sense otherwise where would they put extra energy?

Right... They don't operate at the maximum power point when limiting current. They still perform power conversion, so the benefit of efficient conversion from array voltage to battery voltage is intact. It is the same behavior as clipping, in grid tie vernacular.

[Murby]

No. Most of the better charge controllers allow the max output current to be user set, within the rating of the controller.

Thank you! Looking at the Flexmax programing instructions, the option to limit amps is right at the top of the list... His statement about frying batteries with 160 amps makes no sense at all to me if I'm understanding the information I've been researching.

Says you can limit current and voltage right here:

[max2k]

... His statement about frying batteries with 160 amps makes no sense at all to me if I'm understanding the information I've been researching...

I have a suggestion to admins- let's create new forum called 'SunKing is wrong!!!' and let everyone interested spend their efforts there so the rest of the forum sections can remain more relevant .

on the topic: I think SK was referring to simpler MPPT controllers which can't 'forget' MPPT duty: if they can convert full PV power available they would frying the batteries in the process. Now there are 2 ways to prevent this:
- reduce PV power available one way or the other
- utilize smarter controller which could honor max charging current at the expense of MPPT function as it is not really needed during output current limitation phase.

[sensij]

on the topic: I think SK was referring to simpler MPPT controllers which can't 'forget' MPPT duty: if they can convert full PV power available they would frying the batteries in the process. .

Umm.. could you suggest a model number for the "very expensive 80 A MPPT charge controller" that you think he was referring to? I'm unaware of any that fail to include a current limiting function.

[max2k]

Umm.. could you suggest a model number for the "very expensive 80 A MPPT charge controller" that you think he was referring to? I'm unaware of any that fail to include a current limiting function.

I can't- have no experience with any but that's the only way to make sense of the warning he was trying to communicate IMO: CC without charge current limit function have no choice but to dump all the available PV power to the battery up to their fixed current rating. If all CCs nowadays include charge current limit then all is well as long as end users don't forget to set the limit correctly.

[Murby]

I have a suggestion to admins- let's create new forum called 'SunKing is wrong!!!' and let everyone interested spend their efforts there so the rest of the forum sections can remain more relevant .

I'm not sure how to take this statement but it sounds sarcastic in an undesirable way.

I'm trying to learn all I can and when I see a statement from someone who seems to know a lot about this stuff that doesn't make sense, I feel the need to question it in order to make sure I understand things correctly.

Not trying to pound on SunKing, only trying to figure out what he's saying.. I don't want to be one of these folks who slap up an off grid system and then go "oh, I wish I would have known about that before I made that very expensive mistake"

[max2k]

I'm not sure how to take this statement but it sounds sarcastic in an undesirable way.

I'm trying to learn all I can and when I see a statement from someone who seems to know a lot about this stuff that doesn't make sense, I feel the need to question it in order to make sure I understand things correctly.

Not trying to pound on SunKing, only trying to figure out what he's saying.. I don't want to be one of these folks who slap up an off grid system and then go "oh, I wish I would have known about that before I made that very expensive mistake"

ok, my bad- got confused you with DK for a moment. Hope the explanations in this thread provide enough clarification for your learning exercise.

If CC has output current limiting feature and it is set correctly then the battery won't be damaged by excessive charging current even if your PV produces x10 of the required power. CC will simply 'ignore' the excess.