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Can this switch do what I want?

svqueenjane

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Apr 24, 2022
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On a boat, with a 12v DC system, I want to wire a switch between a solar panel and the MPPT controller - I want to use a small, low profile LED switch which claims to be rated at 12v/25amp. But my 175watt solar panel has an open circuit voltage of 21.6 (operating voltage 18v) which means I am pushing about 9amps (max) at 18v - is that safe for a switch rated to handle 25amps @ 12v?

low-profile-led-switch.png
 
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No. It probably would work, but could be damaged or life shortened if you operate it at 20+v. You should be able to find a 24v or 32v switch that will work.

Being an LED switch is also problematic. Regardless of voltage rating of the switch, the LED will operate "oddly" as the voltage will vary greatly depending on sun and battery charge. You should opt for a non lighted switch.
 
I always thought the rule was higher voltage, lower amperage, lower amperage, higher gauge wire (thinner). So if it can handle 25amps at 12v, shouldn't it be able to handle 12amps at 24volts, or in my case 8amps @19v? I don't get where I went wrong with this logic.

But, I ordered a different switch. It's rated for 10amps @24vdc - it looks identical, probably same factory in China. But, this one claims it is rated for 20a @ 12v or 10a @ 24v. Assuming they are not lying, that should be okay.

 
I always thought the rule was higher voltage, lower amperage, lower amperage, higher gauge wire (thinner). So if it can handle 25amps at 12v, shouldn't it be able to handle 12amps at 24volts, or in my case 8amps @19v? I don't get where I went wrong with this logic.

But, I ordered a different switch. It's rated for 10amps @24vdc - it looks identical, probably same factory in China. But, this one claims it is rated for 20a @ 12v or 10a @ 24v. Assuming they are not lying, that should be okay.

This is correct, but not complete. In addition to the voltage/current ratio, there is insulator breakdown voltage. That is, the voltage at which current will cross through a wire's insulation or bridge a gap of air. In this case as a switch opens or closes, there will be a small arc as it just starts to open, before the gap is large enough to stop it. The contacts need to be large enough and robust enough to handle that at higher voltages. If they aren't they will over time burn and pit.
 
Also, realize the LED is not going to work as you probably expect. In the morning as your batteries are charging at a high rate, the current from the panels will be high, and the voltage low. The LED will be off or dim. At the end of the day when the batteries are fully charged, the current will be low, and the voltage high, and the LED will be lit. Then, as the sun goes down, the LED will go off.
 
Also, realize the LED is not going to work as you probably expect. In the morning as your batteries are charging at a high rate, the current from the panels will be high, and the voltage low. The LED will be off or dim. At the end of the day when the batteries are fully charged, the current will be low, and the voltage high, and the LED will be lit. Then, as the sun goes down, the LED will go off.
I am aware of this and I view this as a feature not a bug. I think of this as a solar panel activity light.

Also, with a very large LifePo bank, high daily power usage, and very small panels (relative to bank size), the solar controllers are always in bulk. Always. "when the batteries are fully charged" - this never happens. If there was a risk of going anywhere near fully charged I would intercede and disable charging sources.
 
This is correct, but not complete. In addition to the voltage/current ratio, there is insulator breakdown voltage. That is, the voltage at which current will cross through a wire's insulation or bridge a gap of air. In this case as a switch opens or closes, there will be a small arc as it just starts to open, before the gap is large enough to stop it. The contacts need to be large enough and robust enough to handle that at higher voltages. If they aren't they will over time burn and pit.
Thank you, this is very clear!
 
On a boat, with a 12v DC system, I want to wire a switch between a solar panel and the MPPT controller - I want to use a small, low profile LED switch which claims to be rated at 12v/25amp. But my 175watt solar panel has an open circuit voltage of 21.6 (operating voltage 18v) which means I am pushing about 9amps (max) at 18v - is that safe for a switch rated to handle 25amps @ 12v?

View attachment 119346
Perhaps use the switch to control a relay that's got the correct rating.
 
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