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Old 06-05-2016, 08:06   #1
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Combining Solar Panels of Different Output

What is the best way to combine a pair of 100W, 12VDC solar panels with a 280W, 20VDC 60cell panel? One method is to run the pair of 12VC in series and wire that 24V array in parallel with the 20V panel, then use one MPPT controller. I know there will be some inefficiency in running two arrays of slightly different power in parallel and I understand that this also may make it hard for the MPPT controller to find an optimum. But I don’t know how to quantify the inefficiency. A second method is to use a PWM controller for the 12V panels (2 in parallel) and an MPPT controller for the 280W/20V panel. My concern is that I’ve read on CF that if your engine alternator is charging, then the MPPT goes to float (does not charge) b/c it sees a high battery voltage. Wouldn’t the PWM charge look the same as the alternator charge and the MPPT would stop… In other words, I’m concerned the 2 separate controllers will confuse each other when measuring the state of charge… So What is the best way to combine a pair of 100W, 12VDC solar panels with a 280W, 20VDC 60cell panel?

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Zach
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Old 06-05-2016, 09:28   #2
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Re: Combining Solar Panels of Different Output

If I can glom on to this question, if I have an MPPT controller for a 100W panel, could I add on 60w panel on a cheap non-mppt controller?
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Old 06-05-2016, 09:47   #3
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Re: Combining Solar Panels of Different Output

This has been discussed around here to death, and I'm sure you'll get differing opinions, but you can readily have multiple controllers connected to the same charging system.

Voltage sensing is pretty simple, if the voltage is higher than X, change the charging state, or stop charging. Your MPPT should drop into float if the alternator is raising the voltage above the MPPT's absorption settings. Your batteries are full. Your alternator regulator should also drop back about the same time.

Same for multiple solar controllers. If you want to control how all this works together (not saying you have to/need to) set your controllers to slightly different voltages (i.e. have you PWM switch to float at 14.3, your MPPT at 14.4). Your PWM will stop charging near the end of the absorption phase, and your MPPT will finish it off (and if you are getting close then the MPPT probably has enough power to do that).

And generally it is all moot - if you aren't on the boat and are using the solar as a maintainer then the batteries will get fully charged each day. If you are on the boat then it is likely that usage while charging will be pulling down the voltages.
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Old 06-05-2016, 10:50   #4
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Re: Combining Solar Panels of Different Output

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Voltage sensing is pretty simple, if the voltage is higher than X, change the charging state, or stop charging. Your MPPT should drop into float if the alternator is raising the voltage above the MPPT's absorption settings. Your batteries are full. Your alternator regulator should also drop back about the same time.
This is not what I have read here on CF. If the batteries are at say 50%, ~12.0V and the PWM controller is charging at 14.7 V, the MPPT controller will not sense the 12.0V battery state, but something much higher due to the 14.7V charger. Just like when you put a volt meter on a dead battery that is being charged. The volt meter reads close to the charger's output voltage, not the disconnected battery voltage. So the sensing of battery state when you have other devices charging the battery is not straight forward. I think the controller would have to know the voltage, discharge/charge rate and battery bank size to determine state of charge. I'm pretty sure these chargers don't know that, since you don't get a shunt with them. At least this is how I understand it...am I wrong?

I intend to live aboard starting in 12mo and will be running a refrigerator and RO watermaker. so a full capacity install is important. 480W is marginal to keep up with my loads, but panel space is limited.
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Old 06-05-2016, 11:17   #5
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Re: Combining Solar Panels of Different Output

If the batteries are at 12.0V then the PWM won't be putting in 14.7. It may be set for 14.7, but it will only put in the voltage at the battery. It will raise the voltage as the battery charges. Your battery is a sink and controls the voltage on the circuit. There is no way to have 12V and 14.7 on the same set of wires. The voltage is controlled by the battery and its state of charge. The PWM, MPPT, alternator regulator, etc. all react to the voltage measured at the battery.
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Old 06-05-2016, 11:34   #6
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Re: Combining Solar Panels of Different Output

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Originally Posted by Dsanduril View Post
If the batteries are at 12.0V then the PWM won't be putting in 14.7. It may be set for 14.7, but it will only put in the voltage at the battery. It will raise the voltage as the battery charges. Your battery is a sink and controls the voltage on the circuit. There is no way to have 12V and 14.7 on the same set of wires. The voltage is controlled by the battery and its state of charge. The PWM, MPPT, alternator regulator, etc. all react to the voltage measured at the battery.
THIS ^^^^+1

What MPPT does is decouple the input voltage from the output voltage. It then takes input voltage and amps and then converts it to the output voltage of the battery at a somewhat higher amperage. Output voltage is the same for PWM and MPPT.

With PWM, the input voltage is the same as the output voltage minus any internal controller voltage drops. As solar panels are constant amperage, output is voltage dependent.
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Old 06-05-2016, 12:33   #7
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Re: Combining Solar Panels of Different Output

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I think the controller would have to know the voltage, discharge/charge rate and battery bank size to determine state of charge. I'm pretty sure these chargers don't know that, since you don't get a shunt with them. At least this is how I understand it...am I wrong?
As far as I know, there is no charger known to man that knows the discharge/charge rate and battery bank size. If there were, it would be in the manufacturer's literature. Chargers work on voltage sensing. That all they got.
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Old 06-05-2016, 12:49   #8
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Re: Combining Solar Panels of Different Output

And to add to that, the controllers estimate SoC based on the voltage they read. When you get to the end of the absorption phase of the charge cycle (let's say 14.4V for convenience) the controller will start to dial back current to maintain the voltage at 14.4. All, of course, depending on the exact settings for the controller.

If you have two (or three, or six) controllers that all sense 14.4 they will all start to dial back current, with their goal being to maintain 14.4 for whatever time/current limits they have set. If you have one controller that is set to hold 14.4 for one hour, and another that is set for two hours (because those were the out-of-the-box settings and you never changed them) then at the end of one hour at 14.4 your first controller will drop to float and effectively stop charging. But your batteries are pretty full at this point, and the second controller will likely maintain the voltage on its own for the rest of the period.

So, yes, multiple controllers can conflict, either by reading the voltage differently (different sensing locations or different sensing accuracy) or by having differing settings (different voltage limits, or absorption times, or...). In general, however, these conflicts only come into play near the tail-end of the charging process. You should pay a little attention to this, if you largest controller is the one that drops out first then your smallest one may not be able to complete the cycle but still keep the voltage above the float setting of your larger controller, thus essentially disconnecting your larger source. If you start using electricity in this scenario your voltage will rapidly drop below the float level and your larger unit will come back online.

But now we're getting way deep in the weeds, for the most part if you configure them all for the same settings then you'll be fine.
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Old 06-05-2016, 14:01   #9
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Re: Combining Solar Panels of Different Output

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If the batteries are at 12.0V then the PWM won't be putting in 14.7. It may be set for 14.7, but it will only put in the voltage at the battery. It will raise the voltage as the battery charges. Your battery is a sink and controls the voltage on the circuit. There is no way to have 12V and 14.7 on the same set of wires. The voltage is controlled by the battery and its state of charge. The PWM, MPPT, alternator regulator, etc. all react to the voltage measured at the battery.
Yes, I agree that you can't have 2 voltages on the same wire, excluding voltage drop in the wire itself. I do not agree that the batteries are a huge sink and if they are at 12V disconnected that when you connect a charger, you will still see 12V. In fact batteries are quite resistant to being charged and you need to bring terminal voltage well above static state to induce a charge current. When I wake up in the morning and the batt's are down at 12V and then I start my engine, my volt meter immediately jumps up to 13.5V or so. And that 13.5V is the issue, because that is what the MPPT will see as the battery voltage and since it doesn't know that the voltage increase is coming from a separate charger, it will assume the state of charge is 'full'. I do not think this is just an end of absorption phase issue.

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... Chargers work on voltage sensing. That all they got.
Yup, and that's the issue. How does it know the true state of charge when you can have other chargers pulling the battery voltage up higher?
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Old 06-05-2016, 14:03   #10
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Re: Combining Solar Panels of Different Output

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As far as I know, there is no charger known to man that knows the discharge/charge rate and battery bank size. If there were, it would be in the manufacturer's literature. Chargers work on voltage sensing. That all they got.
A Midnight Solar Kid MPPT with the optional Wiz-Bang-Jr current sense module kinda does. (Who comes up with the names for this stuff?)

From bulk to absorb it'll switch based on voltage.
From absorb to float it'll switch on current (if you've got the wiz-bang)

It maintains a rudimentary amps out vs amps in battery status, which is really useful when a large load hits and drops the voltage. I've got this setup in my RV, with a domestic fridge (pwrd by 2Kw inverter) and around 350W of solar.

Scenario:
It's sunny,
The charger has already made it to float,
The fridge kicks on and drops the voltage at the batteries to around 12v for a half-second or so to start the fridge.

The nice thing about the Kid is that it knows that's a load - not a discharged battery and just happily supplies the 7A or so to run the fridge. it'll show the .5a or so going into the batts as float even in this instance. When it's not so sunny it's pretty cool to see that we're getting 5A from the solar and pulling 2A out of the batts.

(Not exactly related to boats) After using a domestic fridge in the RV, I have no idea why anybody would use propane unless its somewhere that there's never any sun.
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Old 06-05-2016, 15:18   #11
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Re: Combining Solar Panels of Different Output

When you start your engine and start charging the battery voltage is 13.5. That's the voltage that is required to overcome internal resistance during charging. The state of charge isn't reflected in that voltage because you are actively charging. Your solar charger should be set with the same battery voltages as your alternator regulator. Is there a chance that one or the other will become confused because when it starts up it already see another charging source? Yes, there is a chance. But, most of decent controllers will go to bulk and then absorb and try them out before switching to float, for exactly that reason (if your controller sees 13.5 it first tries full current, if it doesn't see the expected voltage rise then it knows it is in bulk, if it does see a quick voltage rise then it drops back to float). El cheapo controllers from e-bay that don't publish their algorithms, well, you get what you pay for.
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