Solar Charge Controller Sizing

chrisblessing

Solar Enthusiast
Joined
Sep 20, 2019
Messages
115
Greetings. I recently purchase 6 250 watt panels to affix to my garage roof where I have no utility power. The panels specifications are:

Rated Power: 250W
Open circuit voltage (VOC): 37.6 V
Max power voltage (VMP): 30.3 V
Short circuit current (ISC): 8.85 A
Max power current: 8.27 A

The panels will have good exposure and will be used to charge a 280Ah LiFePo4 12 volt battery. I find specifications such as "12V battery system:eek:pen circuit voltage ≤48V(Voc) maximum. PV input power:1040W" a bit confusing, so I would be appreciative of recommendations for sizing the solar charge controller.

I am open to connecting the panels in a series-parallel combination.

Thanks much for the input.
 

iamrich

Solar Addict
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Dec 17, 2020
Messages
832
Location
Elgin, Texas
What are you trying to power? That will be the deciding factor. If you just want to power lights and low amperage items (120v AC), a good all in one will likely work perfect, but if you are trying to fire up the 12 gigawatt band saw or chop saw you will need a big inverter to handle the surge power draw.
 

chrisblessing

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Joined
Sep 20, 2019
Messages
115
What are you trying to power? That will be the deciding factor. If you just want to power lights and low amperage items (120v AC), a good all in one will likely work perfect, but if you are trying to fire up the 12 gigawatt band saw or chop saw you will need a big inverter to handle the surge power draw.
Currently I only anticipate running 5-8 LED trac lights and a 12v fan. Additionally, 4 Battery Tenders for our motorbikes when in winter storage. I don't anticipate any large demands and will likely only have a 1500w inverter attached.

To clarify my situation further: the crate battery will accompany us in our little camper when we boon dock. That sizing has all been sorted out. Its use for the garage is just a matter of cost saving in that the battery will be used continuously.

I suspect that the number of panels is simply too many and exceeds the battery's requirements. I may pull 4 of the panels down and leave just 2 250 watt panels @ 500w total, which I suspect is more than enough for my needs.

Thank you for your response.
 

Hedges

I See Electromagnetic Fields!
Joined
Mar 28, 2020
Messages
9,235
1500W of PV isn't necessarily too much charge current for 3360W PV, a bit under 0.5C
If your SCC supports voltage of 2 or 3 panels in series, you might wire PV as 2s3p or 3s2p
The multiple strings can be oriented toward sun at different times of day. If area presented to sun at any given time is 0.7x total panel area, they would produce 950W peak or about 0.28C

What charge rates would your battery support?

Will the batteries get below 32F? If so, need low-temperature cutout, might be implemented by BMS.
If charged at low temperature like 40F, allowed charge current would be reduced. If battery specs don't show that, assume it is true and refer to other brands.

Downsizing PV array to perhaps 0.1C or 0.15C, and tilting to optimize for winter sun, might provide the needed watt-hours while keeping charge current low enough.
Overcast days means less power, so better is to have SCC sized for that low targeted charge current, but oversize PV so SCC maxed out under most weather conditions.
 

chrisblessing

Solar Enthusiast
Joined
Sep 20, 2019
Messages
115
1500W of PV isn't necessarily too much charge current for 3360W PV, a bit under 0.5C
If your SCC supports voltage of 2 or 3 panels in series, you might wire PV as 2s3p or 3s2p
The multiple strings can be oriented toward sun at different times of day. If area presented to sun at any given time is 0.7x total panel area, they would produce 950W peak or about 0.28C

What charge rates would your battery support?

Will the batteries get below 32F? If so, need low-temperature cutout, might be implemented by BMS.
If charged at low temperature like 40F, allowed charge current would be reduced. If battery specs don't show that, assume it is true and refer to other brands.

Downsizing PV array to perhaps 0.1C or 0.15C, and tilting to optimize for winter sun, might provide the needed watt-hours while keeping charge current low enough.
Overcast days means less power, so better is to have SCC sized for that low targeted charge current, but oversize PV so SCC maxed out under most weather conditions.
Thanks Hedges. I do know that the charge controller currently installed in the crate, alongside the battery, will not be able to handle the voltage output of even two of the panels, so my thought was to purchase a separate SCC for the garage alone.

The panels are well positioned flat on the roof which is itself pitched just about perfectly. We average 360 days of full sunshine, and because of our location in a canyon I know pretty precisely, depending on the season, when the sun abruptly rises and when it abruptly sets.

I think your suggestion to do a series/parallel combination suits me; I may remove two of the panels and do a 2s2p configuration which should allow me to keep the cost of the charge controller within reason.

Thank you for your detailed reply.
 

Hedges

I See Electromagnetic Fields!
Joined
Mar 28, 2020
Messages
9,235
You're welcome.

Sounds like you might have a PWM SCC (although a few MPPT are low voltage).
Most MPPT can be over-paneled because they regulate output current, but PWM can't.

Here's an assortment of SCC from one retailer:


A 30A SCC for instance would charge the battery at 0.1C, which I think together with BMS cut-out below 32F would be good for battery life pretty much regardless of temperature.

The Epever model I'm looking at would be good for the voltage of 2s PV panels.
But it says, "Max. PV input power:390W(12V Battery)"
I don't know if that is true, that it can't handle overpaneling (fails to limit current and gets damaged) or if they just mean that's all the power it can process.

The Victron 100/30 similarly says "Max. PV input power: 440W(12V Battery)"

I'd want to be able to over-panel in order to collect 2kW or more in almost any conditions.
 

fafrd

Photon Sorcerer
Joined
Aug 11, 2020
Messages
2,453
You're welcome.

Sounds like you might have a PWM SCC (although a few MPPT are low voltage).
Most MPPT can be over-paneled because they regulate output current, but PWM can't.

Here's an assortment of SCC from one retailer:


A 30A SCC for instance would charge the battery at 0.1C, which I think together with BMS cut-out below 32F would be good for battery life pretty much regardless of temperature.

The Epever model I'm looking at would be good for the voltage of 2s PV panels.
But it says, "Max. PV input power:390W(12V Battery)"
I don't know if that is true, that it can't handle overpaneling (fails to limit current and gets damaged) or if they just mean that's all the power it can process.

The Victron 100/30 similarly says "Max. PV input power: 440W(12V Battery)"

I'd want to be able to over-panel in order to collect 2kW or more in almost any conditions.
In general, the max power these MPPTs specify is the maximum power they can fully-utilize. 390W for charging a 12V battery = 30A @ 13V, so I’m guessing that’s a 30A Epever MPPT you’re looking at.

The Epever’s (at least the A-Series) specify a maximum overpanelling of 50% in the section on PV array. That should mean a maximum of ~585W for your 30A 12V model.

In terms of what that translates to if it get’s exceeded, my best guess is that these cheapo inverters are rated for maximum power but not for continuous use (ie: they’d overheat running at peak power for 24h straight).

Since the ‘pattern’ of solar power output is very well-established (peak power lasting 2-3 hours, dropping to half-of-peak power after an hour at either side of that), I’m guessing they might want to limit peak charging rate to no more than ~5 hours continuous, which would result from specifying a maximum array rating.

If there were no upper limit, you could reach peak power soon after sunrise and stay there until just before sunset (over 10 hours continuous in the summer months).

It’s the only explanation I can come up with for maximum overpanelling ratings (unless they are trying to prevent you from wasting too much of your money on solar power you will never be able to extract ;)).
 
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