Multiple Battery Monitors on shunt

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SeamanStaines

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I am pretty sure I am correct on this but I just want to check.

I currently have a NASA BM1 battery monitor using their shunt. I want to add on a Studer monitor to get logging from my inverter. The NASA shunt is 100A 50mv and the Studer is 500A also 50mv. From my recollection of ohms law and the way shunts work I should be able to connect both of them to the new shunt.

Am I forgetting anything?

Ta
 
SeamanStaines said:
I am pretty sure I am correct on this but I just want to check.

I currently have a NASA BM1 battery monitor using their shunt. I want to add on a Studer monitor to get logging from my inverter. The NASA shunt is 100A 50mv and the Studer is 500A also 50mv. From my recollection of ohms law and the way shunts work I should be able to connect both of them to the new shunt.
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No, you're not correct, you can't connect both monitors to the new shunt. However, what you can do is add the new shunt in series with the old one; the old one will connect to the Nasa monitor, the new one connects to the Studer monitor.
 
SeamanStaines said:
OK OK. I will repost on PBO, but everyone here usually likes me :-)
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Dont do that. Multiple posting is against the forum rules . The mods will move the post if they see fit.


pvb said:
No, you're not correct, you can't connect both monitors to the new shunt. However, what you can do is add the new shunt in series with the old one; the old one will connect to the Nasa monitor, the new one connects to the Studer monitor.
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Well ???

one instrument is expecting to see 50mV at 100amps the other 50mV at 500 amps. If the BM1 is connected to the Studer's shunt it will only see 10mV at 100 amps and its current readings will therefore be only 1/5th of what they should be.

But you wont want 500 amps through the BM1s shunt. It will take a good bit morethan 100amps but maybe not 500. The BM1 itself may not respond correctly to such large currents.
 
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pvb said:
No, you're not correct, you can't connect both monitors to the new shunt. However, what you can do is add the new shunt in series with the old one; the old one will connect to the Nasa monitor, the new one connects to the Studer monitor.
Click to expand...
Correct!! Beat me to it......
 
VicS said:
But you wont want 500 amps through the BM1s shunt. It will take a good bit morethan 100amps but maybe not 500. The BM1 itself may not respond correctly to such large currents.
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Good point, I think we need to know what size the inverter is before going further. If the inverter is much more than 1500W, the OP will probably have to replace the BM1 with a proper battery monitor.
 
pvb said:
Good point, I think we need to know what size the inverter is before going further. If the inverter is much more than 1500W, the OP will probably have to replace the BM1 with a proper battery monitor.
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Yes also need clarification of what the OP wants to achieve.

If he wants to properly monitor the battery be needs a single battery monitor that will handle the max current in or out. with everything going through its shunt, inverter included.

If he also wants to separately monitor the inverter use ( but why ??) then he needs an additional monitor on the inverter circuit
 
Ok,

Here is the thinking.

I have a BM1 that works well. I do occasionally draw more than 100Amps as its a 2kva inverter so assuming batteries at 12v (normally higher) then by my calculations that is 166amps which the 100 Amp shunt should handle for a short period if not permanently. I would think its never more that a few minutes.

Its been like that for six years and I have never had a problem, the BM1 simply shows 99A when it goes over but I am sure it probably screw the calculations a bit.

The studer has much better monitoring and also data logging so I can see what is going on over a month period.

I can set the studer to match the shunt size so the real question was can I connect both monitors to the same shunt in parallel. Since they are both effectively volt meters with a high resistance (for example if you had two multimeters connected to the same supply in parallel you would expect the same voltage reading on them as if you just had one?

I guess I ought to replace the BM1 to a BM2 though which has a 200A shunt and is then designed for 200A
 
SeamanStaines said:
Ok,

Here is the thinking.

I have a BM1 that works well. I do occasionally draw more than 100Amps as its a 2kva inverter so assuming batteries at 12v (normally higher) then by my calculations that is 166amps which the 100 Amp shunt should handle for a short period if not permanently. I would think its never more that a few minutes.

Its been like that for six years and I have never had a problem, the BM1 simply shows 99A when it goes over but I am sure it probably screw the calculations a bit.

The studer has much better monitoring and also data logging so I can see what is going on over a month period.

I can set the studer to match the shunt size so the real question was can I connect both monitors to the same shunt in parallel. Since they are both effectively volt meters with a high resistance (for example if you had two multimeters connected to the same supply in parallel you would expect the same voltage reading on them as if you just had one?

I guess I ought to replace the BM1 to a BM2 though which has a 200A shunt and is then designed for 200A
Click to expand...

If your inverter is 2kW, and has worked well through the BM1 shunt for years, I guess it's OK to keep the BM1 shunt. It's likely that the BM1 won't track the current accurately though, so a BM2 might be better if you want an accurate reading.

If you can set the Studer monitor to match different shunts, simply set it to the parameters of your BM1 shunt (50mV at 100A). Then you don't need a separate shunt.
 
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