Nasa BM1 and Solar controller don't agree.

[truscott]

I have a single 120 watt Kyocera panel set up temporarily at the moment. Output goes via a 30Amp PWM type controller. With the sun shining at the moment, the display on the controller shows amps out of around 5 Amps but the BM1 shows half that.

The output from the controller is connected on the correct side of the shunt ie the positive wire goes to the positive post on the battery and the negative to the same side as the yellow wire on the shunt. There are no connections on the other side of the shunt other than the black and white BM1 cables and cable to negative post.

The controller is only a couple of feet from the battery bank and the wire is the same gauge for both in and out (sadly I''ve forgotten what gauge, but the outer diameter is 5mm and at a guess the actual core diameter is 4mm).

Curious to hear what folk think could be the cause of the mismatch. Voltage drop over such a short distance seems a stretch but am happy to be proved wrong.

Cheers PT.

 

[nigelmercier]

Optimistic controller? Test with a multimeter.

 

[ALB]

PWM creates ac like current. BM 1 designed to measure DC only?

 

[truscott]

PWM creates ac like current. BM 1 designed to measure DC only?

Can you explain that a bit more?

The voltage being displayed on the PWM controller and the BM1 are the same. The amps are different. The only thing I can think would influence this would be voltage drop across the wires from the PWM controller to the batteries. But then wouldn't they show different values?

Happy to be shown where I've gone wrong.

Cheers PT.

 

[ALB]

My guess is that the PWM (Pulse width modulator) is producing pulses of current that the ammeter on the controller is smoothing to get a DC value. The ammeter on the BM1 is not capable of dealing with the pulses and is therefore giving a lower reading. The volts are being measured from the battery terminals with the battery providing the effective smoothing. Not sure if a capacitor across the BM1 shunt would help much, but could be worth a try. Can't think of a suitable value for the capacitor off the top of my head. Any non-polarised capacitor would be worth trying.

 

[affinite]

On my Stecca controller there are two output currents that can be displayed.
One is the max current available from the panels at that instant and the other is the actual current being drawn by the battery or load.
They are frequently different and I assumed that if the battery was pretty much charged the current drawn would reduce, hence the two readings.
Current drawn on my Stecca does correspond with current shown on my BM2
Could you be displaying current available on your controller ?

 

[truscott]

On my Stecca controller there are two output currents that can be displayed.
One is the max current available from the panels at that instant and the other is the actual current being drawn by the battery or load.
They are frequently different and I assumed that if the battery was pretty much charged the current drawn would reduce, hence the two readings.
Current drawn on my Stecca does correspond with current shown on my BM2
Could you be displaying current available on your controller ?

Thanks for this Affinite! Looking at the little info that is on the controller, the amp value i am seeing is "PV A" which I think is as you say the current from the panel, NOT the current going to the batteries! So, the BM1 value is correct (and does go down as the batteries get charged).

Now I'm disappointed but not as confused.

Many thanks to all.

PT

 

[jol]

just doing a little reading before wiring my setup and came across this thread. why can't the panel via the controller be connected directly to the battery before the shunt; surely the Nasa monitor is just there to monitor battery state and discharge.

ps anyone got a spare Nasa bm1 shunt they're not using (i've got the unit i took off my old wrecked boat but forgot -- didn't know about the bus. many thx

 

[duncan99210]

just doing a little reading before wiring my setup and came across this thread. why can't the panel via the controller be connected directly to the battery before the shunt; surely the Nasa monitor is just there to monitor battery state and discharge.

ps anyone got a spare Nasa bm1 shunt they're not using (i've got the unit i took off my old wrecked boat but forgot -- didn't know about the bus. many thx

The BM1 shunt will only measure current into and out of the batteries if you direct the all loads and inputs via the shunt. If you connect your panels directly to the battery, bypassing the shunt, then the BM1 won't 'see' the current and won't factor it into the current state of the battery.

For a replacement shunt see here http://www.nasamarine.com/product/spare-shunt/ .

 

[MagnusS]

The most common cause when a solar controller shows a higher current than a NASA (or similar) shunt measurement, is that you have some device in the boat that consumes current. If all is correctly wired, this means that the current from the solar controller arrives at the shut, but then goes away to your electrical panel and this current consuming device without passing the shunt into the battery. Hence the difference.