Battery Monitors & Shunt size? (1 Viewer)

[Langtoftlad]

In non technical lingo & short words please;

Battery Monitor "Shunts" - what are they & why does size matter?

I'm thinking of getting WildAx to install one when they fit my solar panel.

Apparently my best choices are either the Victron BMV-700 or a Nasa BM2 or Nasa BM1
500a, 200a & 100a "shunts" respectively.

I can appreciate bigger is better but its also more expensive.

As I don't want bells & whistles, just an ability to see what the solar is putting in, the state of the batteries, and what is going out.

So 120w panel, feeding two 100ah leisure batteries via a PWM controller (which also tops up starter battery)
No inverter, no extra panels envisioned (no room) & don't imagine a third leisure battery)

So is the cheaper Nasa BM1 adequate ??

 

[Lenny HB]

A shunt is basically a very low value high power resistor that is wired in series with the negative battery terminal. By reading the volt drop across the shunt you can calculate the current passing through it, the battery monitor does the calculation and displays the result.

Unless running a large invertor a 100 amp one will be more than adequate.

I think there were some problems with the BM1 giving accurate results in Motorhomes I don't know if this has been fixed.

 

[tonyidle]

Shunts (to be very non-technical) output a tiny voltage that varies in value as the current in the circuit varies. To do this they introduce a very low value resistance into the cable connected to them. Because of how they work they produce heat. If they are sized correctly the amount of heat is insignificant. So you need a shunt designed for the maximum current your circuit is likely to handle which is, with a 120W panel, less than 10A. The 100A shunt is not only adequate - it is overkill. You do not need anything larger. If you change your mind and fit more panels bear in mind that 100A shunt can only manage 1200W solar. The number of leisure batteries has no effect on the shunt required.

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[peterc10]

Usually the limiting factor for a shunt will be the size of the inverter you fit if, that is, you fit one. We have the Victron BMV 700 and it works well for us. But we have a 2000W inverter (4000W peak). .

 

[Langtoftlad]

To be honest I didn't understand the technical explanations but thanks @Lenny HB and @tonyidle

However, I did understand that, for my limited needs, shunt size is not a factor.

From reading another thread I think the BM1 might need to be reset after being plugged into EHU otherwise could read batteries charged in excess of 100%.
No biggie as far as I'm concerned.

But @Lenny HB , in a couple of batt monitor threads you mentioned the value of monitoring (Greece?) and that you have a "cheapie" Chinese one, which does the job - can you tell me which one (if it is still available) ???
As I said, no bells or whistles needed.

• Just to see what solar is doing - interest only.
• What's going out - valuable to ensure not drawing too much juice.
• Charge state of batteries - important for their health.

Cheers

 

[TheCaller]

Just to be clear - the size of the shunt isn't governed by the size of the charging current - it's governed by the maximum current flowing either into or out of the batteries. For anyone who has an inverter, this will be the defining load - which of course is added to whatever else you are running at the time.

So you need to consider what you will be running from the batteries as much as how it will be charged.

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[eddie]

The NASA has issues with solar, so if your going to use NASA use the bm2

the victron is neater and much better quality, also the choice of fascia to suit the fitting requirements is better

 

[Gromett]

I agree with @eddievanbitz I have had both a nasa and a victron. The victron was much better. The Nasa dicks around and gets confused by solar. You do learn how to read around it's strange readings but for a few quid more the Victron is better.

As for shunt size. Add your battery chargers max current to your solar panels max current to give you your input current. If you have an invertor take this as your output current. Which ever is the larger, spec your shunt for this plus 10% at least.

If you have a B2B that is larger than your battery charger use this value instead of your charger.