Solar controller amps vs. what goes in battery

[SKMT]

Originally Posted by sailingchiro

A good clamp on amp meter with voltage and ohm meter is one of the best tools on a boat.

It's been on my boat toy list. But with the battery monitor i always could just flip on off whatever i wanted to measure, and compare the difference

[mitiempo]

Originally Posted by SKMT

.......But with the battery monitor I always could just flip on off whatever i wanted to measure, and compare the difference

Assuming the monitor is correct. A clamp meter of good quality will allow you to check for accuracy.

[BMD21]

One might think of Kirchhoffs laws, which I got in basic physics..


1. Kirchhoffs current law: The sum of all the currents into a junction is equal to the sum of the currents leaving the junction. This means if 75 amps is goiing into the junction, with no processing to change the voltage, the same current is coming out.


2. Kirchhoffs voltage law. The sum of voltages around a circuit is equal to zero, without a generator of course.


So if the is less current going in than going out, check the voltage in both input and output. Since the controller is not a generator, EI in must equal EI out. We cannot create energy except by a generator/battery.


And you will have a voltage drop through the battery wire, since it takes energy to push the electrons through. Since the current through the wire is fixed, the voltage has to drop. No generator in that wire.


Check the circuits with volt and current meters, of course, but first think about what you would expect to find.



Boris

[BillKny]

Originally Posted by flee27

All asked/stated as a question... Amps are amps but the acceptance rate of the batteries does change with charge. Like donradcliffe is asking. If the solar is producing the stated 105 amps and the lead acid bank is small or past the bulk stage of charging the internal resistance of the batteries will limit the current they will accept. So in the case you would see less amps going into the batteries the the solar has the potential to deliver.

No, your question is a bit hazy, but when the controller reports "105 Amps" that is not a capability, it is an ACTUAL. If the controller is outputting 105 Amps, and the battery is accepting 75, the other 30 amps have to be going somewhere, or there is a reporting error.

That's a lot of amps to lose(30 * 12= 360 Watts). A LOT. On my 52 foot boat with a lot of power consumers, my normal power draw is 200 Watts.

Something is very wrong.

[noelex 77]

As others have pointed out, current does not diminish along a wire like voltage does so both devices should give the same reading. The first step is to determine which reading is correct. A clamp on multimeter has sufficient accuracy to provide the answer with such a large discrepancy.

[Nord Sal]

If you disconnect all your solar panels, your solar charge controller(s) should show 0 amps output.


What does your battery monitor read in this case?


On the Vitron 700 series BMs (and probably others) you can disconnect your negative terminal from your battery so that no current can pass through the shunt while keeping your BM connected. Your BM should show zero amps, if it doesn't it's out of calibration.


I'd check this first if you can.

[Perdiem]

"If the controller is outputting 105 Amps, and the battery is accepting 75, the other 30 amps have to be going somewhere, or there is a reporting error."

I thought I understood this - but maybe I don't!

We have 3 x 100 watt panels going through a Victron 50 amp smart controller. In full sunshine I get ~16 amps on the battery side of the controller and if the batteries are down it all shows up on the battery monitor.

If the batteries are charged or nearly charged I might only get 3 or 4 amps on the battery monitor. Then if my 8 amp freezer cycles on, I still get the 3 or 4 amps going into the batteries but the 8 amps appears to be picked up by the solar panels.

I thought of it this way. If I go down to the boat and unplug the shore power (batteries have been on float for days) and then leave the boathouse and go out in the sun the panels will be outputting 16 amps but there's no way the batteries will accept it.

[noelex 77]

Originally Posted by Perdiem

"If the controller is outputting 105 Amps, and the battery is accepting 75, the other 30 amps have to be going somewhere, or there is a reporting error."

Correct.

Originally Posted by Perdiem

iIf the batteries are charged or nearly charged I might only get 3 or 4 amps on the battery monitor. Then if my 8 amp freezer cycles on, I still get the 3 or 4 amps going into the batteries but the 8 amps appears to be picked up by the solar panels.

Correct.

Originally Posted by Perdiem

I thought of it this way. If I go down to the boat and unplug the shore power (batteries have been on float for days) and then leave the boathouse and go out in the sun the panels will be outputting 16 amps but there's no way the batteries will accept it.

In the situation you describe, the solar panels will not be producing 16 amps. The solar controller modifies the panel output so the current they produce is less. The simple way of doing this is for the solar controller to rapidly connect and disconnect the solar panels. The solar panels cannot produce any power when disconnected so by modifying the time on and off, the current produced by the solar panels is also modified. This is PWM.

There are other ways the solar controller can modify the output of the solar panels, but the above simple system is easy to understand and is usually the primary means of controlling the panel output for both PWM and MPPT controllers.

So the solar panels will not be producing 16A because they will be disconected for part of the time.

[SKMT]

The issue was monitor related. My engine negatives would ideally go to the monitor shunt, but it says 500a max, and I suspect my cummins starting amps are in that range. So engine negatives go to batteries. The issue was one of the engine alternators has a negative lead to the monitor shunt. So about 25% of the negative current was passing thru that resulting in the incorrect amperage showing on the monitor.

[Thumbs Up]

Congrats on solving the problem.
There are however some misconceptions about amperage being tossed around.

As others have pointed out, current does not diminish along a wire like voltage does

While it is true that the current does not diminish, it must be stated that as voltage drops, the current increases.

Amps is not amps because it is entirely dependent on the voltage. Some mppt controllers will show the solar in amperage reading based on the array voltage. If this voltage is say 17.5 and you are bulk charging at 13 volts then you will see a higher amperage going into the battery bank than into the controller.

Similarly you will always get a higher amperage reading near the battery than near the controller due to the voltage drop in the cabling. This voltage drop will be negligible at low amperages and greater at higher amps. It is not just from the wire voltage drop because each connection plus the shunt itself will have their own voltage drops.

If you have too much amperage for one shunt or it is impractical to run all the negatives through a single shunt, you can add a second shunt and parallel the sensor wires (stack the connectors) at the monitor. Just make sure that the milliamp rating is the same (usually 500 ma)

[noelex 77]

Originally Posted by Thumbs Up

Congrats on solving the problem.
There are however some misconceptions about amperage being tossed around.
While it is true that the current does not diminish, it must be stated that as voltage drops, the current increases.

Originally Posted by Thumbs Up

Similarly you will always get a higher amperage reading near the battery than near the controller due to the voltage drop in the cabling.

This is not true along a wire. The current in = The current out. The voltage on the other hand will drop along the wire. As the voltage decreases the net power is also decreasing. This "lost power" will heat up the wire.

The device that the wire is connected to may demand more current from the source as a consequence of the reduced voltage along the wire, or it may not and just run on this reduced power depending on the nature of the device, but the "The current in = The current out" for the wire still applies.

[SKMT]

Originally Posted by Thumbs Up

If you have too much amperage for one shunt or it is impractical to run all the negatives through a single shunt, you can add a second shunt and parallel the sensor wires (stack the connectors) at the monitor. Just make sure that the milliamp rating is the same (usually 500 ma)

Do you know how much of a concern it is to possibly exceed the shunt rating by about 25% for half a second on engine start up?

[Bill Seal]

"While it is true that the current does not diminish, it must be stated that as voltage drops, the current increases." ????
I=E/R. if R remains the same, (why wouldn't it?) as the voltage drops, the current also drops. Or is this some sort of "new math"

[Thumbs Up]

Originally Posted by SKMT

Do you know how much of a concern it is to possibly exceed the shunt rating by about 25% for half a second on engine start up?

Not much of a concern. In some situations though, due to cable routing problems, it makes sense to use a second shunt.

[Thumbs Up]

Originally Posted by noelex 77

This is not true along a wire. The current in = The current out. The voltage on the other hand will drop along the wire. As the voltage decreases the net power is also decreasing. This "lost power" will heat up the wire.

The device that the wire is connected to may demand more current from the source as a consequence of the reduced voltage along the wire, or it may not and just run on this reduced power depending on the nature of the device, but the "The current in = The current out" for the wire still applies.

Originally Posted by Bill Seal

"While it is true that the current does not diminish, it must be stated that as voltage drops, the current increases." ????
I=E/R. if R remains the same, (why wouldn't it?) as the voltage drops, the current also drops. Or is this some sort of "new math"

Ok , I stand corrected. I was not versed on Kirchhoff's Laws. You will get the same amperage reading along the same wire (in a circuit) no matter where you measure it.

As the wire heats up, the total resistance in the circuit increases, then voltage decreases requiring more amps to deliver the same amount of watts (work)
My statement, " higher amperage reading near the battery than near the controller due to the voltage drop in the cabling" was wrong. However, (in line with what I had said) as voltage decreases amperage will increase (to deliver the same output)

Generally speaking, stating that amps is amps is confusing because as voltage drops it does require more amps to deliver the same amount of work.