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40amp mppt, battery max charge current 30amp - A problem?

Mike Si

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Jun 17, 2022
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I was about to hook up 2x 200w(10.1A) solar panels to a 40amp MPPT with 40amp charge current to charge a 12v 120ah AGM battery but just noticed the battery has a maximum charge current of 30amps. Does it matter that the controller charge current exceeds the battery maximum? I'm thinking the controller would manage to suit the battery and the max panel output is 33.3A which is unlikely in any case... Or should I be more cautious and only connect one 200w panel? Or something else, it's my first attempt at this stuff.
 
400W of panels on a 12V system can be up to about 33A as you say. With a charge voltage closer to 14V you'll only get 28A. So you will never get the full 40A out of the SCC unless you add more panels. But 33A is more than the battery maximum of 30A. I wouldn't exceed the maximum unless you want to shorten the battery life.

Most (many?) charge controllers can be configured to a user set maximum charge current. See if your SCC lets you set the max charge current to 30A. Then in the rare case your 400W of panels actually generate enough power to possibly get you over 30A, the SCC will limit it to 30A to keep the batteries happy.
 
Great, thanks for the response. If the controller output is not adjustable I'm tempted to try a 30A DC circuit breaker between SCC and the battery. A bad idea?
 
The AGM will not be over worried by an ocasional charge current over 30 amps. In reality the panel output will never reach 400 watts unless you are in cold locations.
The actual charge voltage will be 13.5 volts and higher, the panel loss with heating, cable losses, controller losses, and less than optimum mounting angle, all reduce the power into the battery.
My guess is the typical maximum will be in the low 20s amps.

Mike
 
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Most (many?) charge controllers can be configured to a user set maximum charge current. See if your SCC lets you set the max charge current to 30A.
I don’t think that most of the mid-shelf and bottom shelf SCCs allow setting output max. Although the 1012LV aio’s do. Straight up SCC components on the lower shelves don’t seem to offer that.
 
If you have the panels in series, you'll have 40v 10amps. I'm still a learning this stuff but I don't think you will have any issues at all.
 
If you have the panels in series, you'll have 40v 10amps. I'm still a learning this stuff but I don't think you will have any issues at all.
The OP will have up to 33A of battery charge current. The input voltage and current are not the same thing as the output battery charge voltage and current that the MPPT SCC produces. The max possible battery charge current is total panel wattage divided by battery charge voltage.
 
If you have the panels in series, you'll have 40v 10amps. I'm still a learning this stuff but I don't think you will have any issues at all.
Amps output by the solar panels and amps output by the solar charge controller to the battery are two very different things.
So, you are not correct. The solar charge controller converts the higher voltage and lower amps output by the panels to a lower voltage and higher amps required to charge the battery.
 
I don’t think that most of the mid-shelf and bottom shelf SCCs allow setting output max. Although the 1012LV aio’s do. Straight up SCC components on the lower shelves don’t seem to offer that.
My SRNE MC Series falls into this I think. I haven't powered it up to set parameters yet but there's no indication in the manual or website that suggests it is adjustable
 
The AGM will not be over worried by an ocasional charge current over 30 amps. In reality the panel output will never reach 400 watts unless you are in cold locations.
The actual charge voltage will be 13.5 volts and higher, the panel loss with heating, cable losses, controller losses, and less than optimum mounting angle, all reduce the power into the battery.
My guess is the typical maximum will be in the low 20s amps.

Mike
Thanks, this makes me feel more comfortable. Is a 30amp breaker between the SCC and the battery a good idea or a dumb one?
The AGM will not be over worried by an ocasional charge current over 30 amps. In reality the panel output will never reach 400 watts unless you are in cold locations.
The actual charge voltage will be 13.5 volts and higher, the panel loss with heating, cable losses, controller losses, and less than optimum mounting angle, all reduce the power into the battery.
My guess is the typical maximum will be in the low 20s amps.

Mike
 
Is a 30amp breaker between the SCC and the battery a good idea
As mentioned above:
1) fuse is to protect the cable. 30A is for 10ga. I’d use a minimum of 8ga but probably 6ga, and fuse that situation at 40A
2) with the potential of 33A it would not be a good idea.
 
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