Off-grid system review

[Mike90250]

........Well that is what you want out of your controller. You want to install 300 watts of panels, and only use 200 watts. Why?

I put up a 1200w array and limited to 30A @ 24V, because that's all the batteries can take in the summer. Winter, in the clouds. fog and rain, I need every inch of that array and more. wife does not want a generator there.

[sensij]

I feel that is a trick question, but I'll bite the bait because getting hooked is the only way to find out what is at the other end of the line. I put up 300W because I want to use the 200 watts to provide the correct charging currents to the battery, and the other 100 watts to run the daytime load. Or I want the other 100 watts facing east. Or I want the other 100 watts because I usually have a lot of clouds. But that is part of the 'Dumbest Question Ever' thread I'm fixing to ask when I have given up answering it myself. Go ahead, reel me in.

So, there is some stuff to deal with here.

Most entry level charge controller do not have a facility for limiting amps, beyond the hardcoded limit at the charge controller's rating. Yes, if you are using controllers at that end of the market, you need to replace the controller when you want to change the charge current limit.

More advanced controllers have the ability to set a current limit less than that controller's rating. However, that feature does you no good for operating daytime loads, since the available current will not be increased even if some of it is being routed off away form the battery. It does give more design flexibility and better future-proofing for the different battery configurations you might consider using.

Some controllers have relays or control signals available to connect with other equipment that may control opportunity loads. The more typical use case is for those loads to kick in when the battery is fully charged. Trying to run those loads in parallel with charging, on the assumption that there is still enough sunlight in the day to achieve both, is more thinking than the charge controller will offer.

I fully agree with the idea that the charge controller amp limit (whether it is the rating limit, or a lower one set by the user) is an effective tool to lengthen the charging day and safely over-panel the system, and builds more margin into a design than strictly limiting the PV rating to the controllers rating would allow.

[AzRoute66]

I feel that is a trick question, but I'll bite the bait because getting hooked is the only way to find out what is at the other end of the line. I put up 300W because I want to use the 200 watts to provide the correct charging currents to the battery, and the other 100 watts to run the daytime load. Or I want the other 100 watts facing east. Or I want the other 100 watts because I usually have a lot of clouds. But that is part of the 'Dumbest Question Ever' thread I'm fixing to ask when I have given up answering it myself. Go ahead, reel me in.

[Sunking]

Sure, and for my [next] 125 Ah battery I'll need a 13 amp controller, and when I want to try going from C/10 to C/8 I'll go get another controller, and if I want certain features I'm limited to 20 or 40 amp selections on that brand. Now, to the issue that I do not know the design process; I'll grant you that MAY be, but it seems that your process is not as elegant as just setting the max current to the C/x value that you want. If it turns out that such MPPT controllers are few and far between I'll apologize for that, but I get the sneaking suspicion that if we inventory them we'll find that is a pretty standard feature (it just makes too much sense, and I bet if it is on 'some' then it is on 'most'). It would make YOUR design process the one you are FORCED to use if you don't shop well. In your words, "With me so far?" I asked why the charge controller wants to know Ah during setup, could you address that part?

Non issue. No reason to ever limit charge current. I can take a 80 amp controller and make it a 10 amp controller real damn easy. Instead of installing 1000 watts of panels, I just install a 130 watts.

Now answer me this simple little question. If you buy a Pizza, do you expect to have a whole Pizza, or a slice or two missing? Well that is what you want out of your controller. You want to install 300 watts of panels, and only use 200 watts. Why?

[AzRoute66]

Mike, the last one I did, and thus the one I'm probably thinking of was a Renogy Charge Commander (?) 40 amp.

[Mike90250]

...... I asked why the charge controller wants to know Ah during setup, could you address that part?

Nope. YOU have to tell us what controller asks you for Ah in setup. The XW ComBox asks, but it's the main interface, not the controller. It does use the Genset size, and the battery size to set some limits, but that's all I know of.

Midnight initial setup has evolved and when I did my last configuration, I don't recall it asking battery size. Neither did my TS-MPPT-60. I think the PT-100 may ask, I've only set one up, and I recall it was pretty easy, if one follows the manual.

[AzRoute66]

Sure, and for my [next] 125 Ah battery I'll need a 13 amp controller, and when I want to try going from C/10 to C/8 I'll go get another controller, and if I want certain features I'm limited to 20 or 40 amp selections on that brand. Now, to the issue that I do not know the design process; I'll grant you that MAY be, but it seems that your process is not as elegant as just setting the max current to the C/x value that you want. If it turns out that such MPPT controllers are few and far between I'll apologize for that, but I get the sneaking suspicion that if we inventory them we'll find that is a pretty standard feature (it just makes too much sense, and I bet if it is on 'some' then it is on 'most'). It would make YOUR design process the one you are FORCED to use if you don't shop well. In your words, "With me so far?" I asked why the charge controller wants to know Ah during setup, could you address that part?

[Sunking]

So, I am paying hundreds of dollars for a 'state of the art' charge controller that turns out to be no more sophisticated than the average $20 Walmart dumb charger. There is no setting for max current. Seems the first thing set up in a charge controller besides Battery Type is 'Battery Ah', what the hell is the controller doing with that information? Really, why should it ask? It seems to me I better start downloading and reading some charge controller manuals. Thanks for this extremely disturbing answer. I think I am almost ready to start my "Dumbest Question Ever" thread. For now I have to get to my blackboard: "I should never assume, I should never assume. I should never assume. I should neve...."

Not sure what your issue is?

As Mike said some very high end controllers will allow you to limit current. But why would you pay more to do that, when you can have that for free? It is just part of the Design Process. If you have say a 100 AH battery my friend and you had better be able to provide at least 10 amps charge current at 13 volts. That is easy to figure out I need a 10 amp controller and a 10 amp x 13 volts = 130 watt panel minimum. Now if that Controller is 10 amps, I can use a larger panel, but its output will be clipped at 130 watts. Example say use a 175 watt panel. It will produce 10 amps longer around noon hours. The Controller itself will limit the current to 10 Amps

So really there is no issue here. It all takes care of itself in the design. The issue is you do not know the design process and why it is done the way it is.

[max2k]

So, I am paying hundreds of dollars for a 'state of the art' charge controller that turns out to be no more sophisticated than the average $20 Walmart dumb charger. There is no setting for max current. Seems the first thing set up in a charge controller besides Battery Type is 'Battery Ah', what the hell is the controller doing with that information? Really, why should it ask? It seems to me I better start downloading and reading some charge controller manuals. Thanks for this extremely disturbing answer. I think I am almost ready to start my "Dumbest Question Ever" thread. For now I have to get to my blackboard: "I should never assume, I should never assume. I should never assume. I should neve...."

to the same tune:
Assumption is the mother of all f..ups
Assumption is the mother of ...

I'd agree it is rather dumb to call device 'charge controller' and not to implement the charge part of the name. IMO that is more important and primary requirement than correct MPPT- if the battery can't take it there's no reason to efficiently track PV max power point.

[Mike90250]

... There is no setting for max current.....

In the top end controllers, there is, both my Mornigstar and Classic. have deeply buried settings to limit max output current to less than full power.

[AzRoute66]

[...] For solar MPPT Charge Controllers Output Current = Panel Wattage / Battery Voltage. Secondary controlled by the MPPT CHARGE Controller Max Current rating. If you buy a 20 amp controller, it can limit the output current to 20 amps so it does not burn itself up. [...]

So, I am paying hundreds of dollars for a 'state of the art' charge controller that turns out to be no more sophisticated than the average $20 Walmart dumb charger. There is no setting for max current. Seems the first thing set up in a charge controller besides Battery Type is 'Battery Ah', what the hell is the controller doing with that information? Really, why should it ask? It seems to me I better start downloading and reading some charge controller manuals. Thanks for this extremely disturbing answer. I think I am almost ready to start my "Dumbest Question Ever" thread. For now I have to get to my blackboard: "I should never assume, I should never assume. I should never assume. I should neve...."

[Sunking]

This problem is now addressed completely in all modern MPPT charge controllers I'm thinking. I mean now we have the concept of Bulk Charging which is constant current charging, a max current setpoint is established and the above scenario cannot happen, right?

Well yes and no.

Charge controllers are really pretty dumb. Charge current in a solar system is primarily controlled by the panel wattage. When it comes right down to it a charge controller is not really Constant Current, it is Constant Power.. For solar MPPT Charge Controllers Output Current = Panel Wattage / Battery Voltage. Secondary controlled by the MPPT CHARGE Controller Max Current rating. If you buy a 20 amp controller, it can limit the output current to 20 amps so it does not burn itself up.

So if you had say a 12 volt 100 AH battery on a 20 amp controller with a 260 watt panel initially if the battery were 12 volt, the highest current possible 260 watts / 12 volts = 20 amps or C/5, no real problem. It is part of the design. Note as the battery voltage rises, charge current goes down because it is Constant Power, not Constant Current. Say once the battery gets close to being charged up to 14.6 volts current has to be 260 watts / 14.6 volts = 17.8 amps.

So now lets change the battery out to a 12 volt 10 AH Battery. Same parameters battery is discharged to 12 volts, 260 watt panel, 20 amp controller. Guess what happens, we get the same 20 amps of charged current. Or 20 amps into a 10 AH battery and that thing is cooking and in 10 minutes or so is getting hot hot hot.

So like I said the answer yes and no. The charge current is designed in by the idiot buying the panels and controller. It is part of the deign, not an accident. I should say it is part of the design if you know what you are doing.

FWIW lets say we have a AC powered, 12 volt 20 amp charger.. We set the charger to 14.8 volts and put back on 12 volt battery. Whatever size, does not matter. For grins say a 12 volt 100 AH like above. Initially we put the battery on charge note the voltage and current. Voltage = 12.2 volts @ 20 amps = 244 watts, Some 4 hours later we see 14.6 volts @ 20 amps = 292 watts. Power went up with voltage, current was constant. Now once we hit 14.601 volts the charger enters constant voltage, current will start to taper. By the time the battery reaches 14.8 volts, charge current = 0 amps.

[AzRoute66]

[...] Allow me to elaborate. If you took say a 12 volt 100 AH battery @ 60% SOC the Open Circuit Voltage is roughly 12.2 volts with an Ri of .01 Ohms. Now lets say I have a 12 volt 200 Amp charger set to the recommended 14.8 volt setting. The instant you connect the charger you will have 200 amps of charge current, and the battery voltage will go to 12.2 + (100 amps x .01) = 14.2 volts. With me so far.

Well magic happens at 14.2 volts or 2.4 vpc called Gassing and Corrosion. You are still at 60% SOc and will now be at and above Gassing Voltage for a very extended period of time. The electrolyte will be boiling profusely, and unless you have special caps to collect the spewing electrolyte and return it to the battery, otherwise is spew out the vent caps and you cannot replace electrolyte. [...]

Hope that helps.

I know this is an old Topic but I am having a really hard time wading through even just the 'Off Grid' section to see if my questions have already been addressed. I found this part of this one disturbing for what Sunking said above.

This problem is now addressed completely in all modern MPPT charge controllers I'm thinking. I mean now we have the concept of Bulk Charging which is constant current charging, a max current setpoint is established and the above scenario cannot happen, right? If your answer is 'yes', do you have a recommendation on how to go about digesting older Topics versus just going ahead and asking my dumbest question in a new thread?

[bcroe]

I'm also thinking about building a box with about six 0.1F 80V electrolytics ($40/each from United Chemi-Con via DigiKey) to put in parallel with the battery (switched and fused, of course). I'd like to reduce the amount of micro-cycling that goes on when there are significant loads and PV power at the same time, and reduce the peak current out of the battery from the 120 Hz inverter load.

Before buying any super caps, CALCULATE how much their voltage would DROP to supply the
actual surges you are worried about. This isn't difficult. The problem with caps is, the only way
to get much of their stored energy out, is to DRASTICALLY drop their voltage. Before that
happens the batteries (that give up energy with little voltage drop) will be picking up the load
anyway. Bruce Roe

[Sunking]

You really are an invaluable asset to this forum, man.

Thank you.

Glad to know I don't really need the BMS. But spending $600 on an Orion Jr. for some peace of mind and a more accurate SOC info about an $8000 battery, and to avoid having to take everything apart to manually balance every year, seems reasonable enough to me.

Don't let me give you the wrong impression. There are 3-Schools of thought with Lithium Batteries and how to Balance them.

1. Top Balance is the commercial solution, where you charge the cells to 100% using Vampire Boards. Downside of that is it cuts cycle life in half, and puts you at a high risk of destoying your batteries from over discharge and fire.

2. Middle Balance is what commercial EV's use and cannot be done DIY. It requires using anything but LiFeP04. EV Manufactures used matched cells within 1% capacity. They never allow the customer to fully charge or discharge any cell. It is the only way they can offer the warranties..

3. Bottom Balance which is what I use and most DIY EV builders. Makes it impossible to over discharge a cell which is instant death and cause for most fires. We also never ever fully charge our cells which doubles cycle life. We mimic what commercial EV manufactures do, and commercial Solar Battery manufacturers do.

You have to pick 1 of the two methods, and the Orion can be used. You just disable the Balance feature. I had one for I sold.

I'm also thinking about building a box with about six 0.1F 80V electrolytics ($40/each from United Chemi-Con via DigiKey) to put in parallel with the battery (switched and fused, of course). I'd like to reduce the amount of micro-cycling that goes on when there are significant loads and PV power at the same time, and reduce the peak current out of the battery from the 120 Hz inverter load.

Don't waste your money on that. Trust me. I can save you a lot of money if you are willing to listen and do homework.