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BMV 702 Temp or Midpoint


viper_za
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Hi All

 

A quick question since I will be getting my BMV 702 soon.

What would you guys say is better for it to monitor the midpoint or the temps.

I was thinking of the temps and in combination with Jaco's software maybe adjust the charging according to the spec of your batteries temps.

Sure this should be possible.

 

Thanks

Thys

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Hi Thys

Temp is easy to monitor with an aquarium thermometer whereas midpoint is more difficult. Midpoint also changes more rapidly and I would find midpoint more useful than a temperature probe.

Chris

 

 

Good idea to modify charging voltages but I think this is more of a seasonal change rather than a setting one changes on a daily basis.

Thanks Chris will maybe try and figure out a way to do both :)

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Something to look into if you already have an arduino in the general area. The atmel chips have an onboard temp sensor, with the right code you can ask for the value. Apparently it's not very well calibrated, but it is accurate, so once you calibrate it yourself (hold an ice cube in a plastic bag to it) it can be used to measure temperature. Probably not useful in this application... but good to know anyway :-)

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  • 7 months later...

Another question on the BMV 702, does anyone here now of any reason not to shorten the cable that is supplied with the unit. In the Instrumentation field we often find instruments that was calibrated with a specific cable length and altering that length affects the accuracy of the instrument. Does someone know if shortening the cable might affect accuracy?

 

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Hi Jaco

I think the cable is just a data feed not actually used by the pc to monitor the device (like a net cable) ... I would shorten it if needed.  Don't know for sure though?

I left mine in case I have lots of money one day and can build a battery room with lovely blue machines inside... ;) (Yes Chris!)

Regards

Mark

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I cut mine and have not noticed any difference. 10m is just not practical.  Axpert and Victron report voltages that are within 10mV of each other. I don't rely on Axpert readings at all really as they are not accurate enough (TTT smiles broadly and says nuthin'). Remember you need to crimp on a RJ12 adapter not a RJ11. I discovered this after I had cut the cable. RJ12s are not readily available in Graaff-Reinet so a waiting game ensued.

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I ignore all and will only comment on one thing. :D

Instead of snipping the original cable, make a new shorter one.

Then you can confirm that all is still identical between the two cables, as I expect it will be, and if for whatever reason the device must go back, you still have the original to send back.

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26 minutes ago, Chris Hobson said:

Axpert and Victron report voltages that are within 10mV of each other

Chris the reason for this is the extra cable length between the inverter and the bank not that much the accuracy.
Even on the sites that run all blue the MPPT gives +- the same difference in voltage when the BMV is connected.
Jaco
I also think it's just a data cable, would have cut mine ages ago just keep on forgetting to get new connectors

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5 hours ago, Jaco de Jongh said:

Another question on the BMV 702, does anyone here now of any reason not to shorten the cable that is supplied with the unit. In the Instrumentation field we often find instruments that was calibrated with a specific cable length and altering that length affects the accuracy of the instrument. Does someone know if shortening the cable might affect accuracy?

I've made up a longer cable and used 4 pair cable instead of the original 3 pair cable and had to run certain wires in parallel, because the voltage drops on certain wires have an effect on mostly the voltage reading. If you are going to make it shorter it might have the opposite effect or you might not notice it at all depending on the final length of the cable. When I did my testing I noticed a voltage drop on the 20+m cable and then experimented the round-robin way to determine which wires I had to double up to reduce the cable's resistance and associated voltage drops.

3 hours ago, viper_za said:

Chris the reason for this is the extra cable length between the inverter and the bank not that much the accuracy.
Even on the sites that run all blue the MPPT gives +- the same difference in voltage when the BMV is connected.
Jaco
I also think it's just a data cable, would have cut mine ages ago just keep on forgetting to get new connectors

It is NOT "just a data cable", the shunt does not contain anything digital e.g. A-D converters etc. The cable between the shunt and the actual monitor / display unit carries analogue voltages which are converted inside the monitor.

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@superdiyJust for interest sake, witch 2 wires were use to power the display? And what was the function of the others. 1 pair would be for the nominal voltage, the other 2 maybe for the temp signal or second battery monitoring, And of course the Current.  Could you figure out what kind of signals are used for all off them or are they all mV signals.

Just out of curiosity?

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Ja, and don't ever do like a idiot once did, that if you do cut a cable, make sure it is not connected to anything, even it is UTP. :)

Lucky for the idiot, and I am not mentioning any names, the inverter the UTP cable was connected to, was a blue colour and the manufacturer anticipated idiots will use it and made the inverter idiot proof by adding idiot protection in for just such a clever idiotic act. 

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8 minutes ago, Jaco de Jongh said:

@superdiyJust for interest sake, witch 2 wires were use to power the display? And what was the function of the others. 1 pair would be for the nominal voltage, the other 2 maybe for the temp signal or second battery monitoring, Could you figure out what kind of signals are used for all off them or are they all mV signals.

Just out of curiosity?

No, I cannot remember. I did draw up a diagram of the shunt and the components on the shunt pc board and the associated wires to the monitor, but I'll have to really dig deep to find that - it is somewhere in my piling system in the garage.

Basically you have the power to supply the monitor, the voltage drop across the shunt and the midpoint or temperature probe reading.

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The cable is completely analog and the only issue is that the voltage measurement on the BMV might be incorrect on a long cable length, when the backlight is on. I believe it uses just 4 wires, two for the voltage across the shunt, and two for power/battery voltage.

That's why @superdiy doubled up on the wires, to counteract the voltage drop caused by the backlight :-)

The ideal solution would have been a 6-wire system, separate power and V-sense wires, but if I recall correctly, that's not how it's implemented.

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