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Remote/bluetooth voltage measurement?


james2
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I 'work from home' on my boat all day and sit at the bow end, and batteries and all the electrics gubbins are in the stern. There's a voltmeter in the stern that I really want to keep an eye on, but I end up walking up and down the boat all the time to check it. 

 

There is 12V all the way up the boat (cigarette lighter style sockets), but I'm assuming that this won't give an accurate reading due to the resistance of the wire being an unknown? (Although thinking about it now I suppose this resistance should be constant and could be factored in to give accurate reading of voltage at the battery?)

 

What I'd really like is a bluetooth voltmeter that'd work the same as bluetooth thermometers - stick it where you want it and install an app on your phone to get wireless readings. Annoyingly this doesn't seem to exist though, at least not in price range I want to pay for this sort of thing which would be £10-20

 

Longer term I'm going to get a Raspberry Pi or something and start hooking up loads of sensors and do some comprehensive data collection, but short term I need to be able to easily view battery voltage of my stern batteries in the bow.

 

Any suggestions?

 

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Victron smart battery sense £25 to £35 depending where you buy it, I know it is above you price range. Cheap seller here. I have not used one and the one thing I would mention is the the battery sense may be in the engine compartment with a steel bulkhead in the way which may weaken the signal. It should be possible to extend / route the wires into the electrical area inside the cabin to improve things. I have some Victron components with Bluetooth in the rear electrical cupboard and can pick the signal up on my phone throughout the cabin.

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It's worth investing in a decent battery meter which can measure current going in and out anyway - voltage only gives you half the story, as it will sag under load and recover. Likewise, if you charge a heavily depleted bank for an hour, the resting voltage after a few minutes will appear like it's fully charged. However, you're just seeing the surface charge on the plates and the batteries in reality aren't charged.

 

Victron's BMV series and Smart Shunt (same thing but minus a screen) have data ports which can be connected to the Pi via a USB converter, and the ve.direct protocol is largely open source. The BMV pushes almost every parameter over ve.direct once per second, so you can harvest and log all the data you want on the Pi...current, voltage, state of charge, amp-hours consumed, starter battery voltage.

 

The Smart versions have Bluetooth which is remarkably good, on a 45' boat it'll reach all the way to the front deck if mounted in a cupboard in the back.

 

https://www.victronenergy.com/battery-monitors/smart-battery-shunt

https://www.victronenergy.com/battery-monitors/bmv-712-smart

Edited by cheesegas
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Even digital voltmeters demand all but zero current so a cheap plug in 12V socket voltmeter should be as accurate as you need as long as the wiring is not undersized for any other loads you are putting on the circuit - KIS is king on boats. I would not suggest it would be 100% accurate but more because it is cheap far eastern stuff rather than because of cable resistance. It should be fine to tell you when you rally need to go and check the voltmeter at the back. In any case, blue tooth, Pi, or not no voltmeter will allow you to accurately infer state of charge either while charging or discharging.

  • Greenie 2
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Any old plug in voltmeter will do to give you an idea.

You just need to note the difference between the two so if the back meter reads 12.6v and the front reads 12.5v you know to add roughly 0.1v onto any reading on the front meter.

It only needs to be  comparative not absolute.

 

  • Greenie 1
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29 minutes ago, cheesegas said:

It's worth investing in a decent battery meter which can measure current going in and out anyway - voltage only gives you half the story, as it will sag under load and recover. Likewise, if you charge a heavily depleted bank for an hour, the resting voltage after a few minutes will appear like it's fully charged. However, you're just seeing the surface charge on the plates and the batteries in reality aren't charged.

 

Victron's BMV series and Smart Shunt (same thing but minus a screen) have data ports which can be connected to the Pi via a USB converter, and the ve.direct protocol is largely open source. The BMV pushes almost every parameter over ve.direct once per second, so you can harvest and log all the data you want on the Pi...current, voltage, state of charge, amp-hours consumed, starter battery voltage.

 

The Smart versions have Bluetooth which is remarkably good, on a 45' boat it'll reach all the way to the front deck if mounted in a cupboard in the back.

 

https://www.victronenergy.com/battery-monitors/smart-battery-shunt

https://www.victronenergy.com/battery-monitors/bmv-712-smart

 

Thanks I have the BMV 501 right now which is basic but seems good and accurate and shows net current in which is very useful.

 

Do you happen to know what I need to connect my BMV 501 to PC to be able to get at the data? Assuming PC is close to the meter for now, I'll deal with the distance issue on the data wire separately as that's much more in my wheel house. BMV501 seems older so it's a bit harder to google for, but I found the manual which says it does have 'PC Link'

 

Getting the accurate reading out of the BMV501 onto my PC would be a great solution.

 

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9 minutes ago, james2 said:

 

Thanks I have the BMV 501 right now which is basic but seems good and accurate and shows net current in which is very useful.

 

Do you happen to know what I need to connect my BMV 501 to PC to be able to get at the data? Assuming PC is close to the meter for now, I'll deal with the distance issue on the data wire separately as that's much more in my wheel house. BMV501 seems older so it's a bit harder to google for, but I found the manual which says it does have 'PC Link'

 

Getting the accurate reading out of the BMV501 onto my PC would be a great solution.

 

The 501 is very old, and I don't think it speaks ve.direct as it has an RJ-11 port, so the ve.direct to USB kits won't work. I think the original PC interface kit converted to RS232 only, to give you some idea! There's no documentation available on the protocol so unless you happen to find the interface kit on ebay, you'd need to do some serious reverse engineering to find out what data it's spitting out. Chances are it's RS232/RS485 at a non standard voltage level...

 

You'd be better off getting a Smart Shunt and a ve.direct to USB cable. I use a 712 with a ve.direct to USB with a RPi 3+ and it works very well.

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7 hours ago, Tony Brooks said:

Even digital voltmeters demand all but zero current so a cheap plug in 12V socket voltmeter should be as accurate as you need as long as the wiring is not undersized for any other loads you are putting on the circuit - KIS is king on boats. I would not suggest it would be 100% accurate but more because it is cheap far eastern stuff rather than because of cable resistance.

You could avoid the cable resistance issue completely by running a couple of thin wires the length of the cabin, connected to the voltmeter at the bow end, and at the stern connecting one wire to the negative bus bar (or negative battery terminal) and connecting the other wire to the non-battery end of the fuse/circuit breaker on any of the 12V circuits.

 

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On 12/10/2021 at 11:59, Tony Brooks said:

Even digital voltmeters demand all but zero current so a cheap plug in 12V socket voltmeter should be as accurate as you need as long as the wiring is not undersized for any other loads you are putting on the circuit - KIS is king on boats. I would not suggest it would be 100% accurate but more because it is cheap far eastern stuff rather than because of cable resistance. It should be fine to tell you when you rally need to go and check the voltmeter at the back. In any case, blue tooth, Pi, or not no voltmeter will allow you to accurately infer state of charge either while charging or discharging.

Just to amplify that a bit.  You need the terminal voltage of the batteries.  Buy a cheap digital as Tony suggest but run a pair of thin wires all the way from the battery bank to where you sit.  Battery bank connected to one end, voltmeter connected to other end.  Virtually zero current drawn therefore no voltage drop.  Simples?

 

N

  • Greenie 1
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9 minutes ago, Theo said:

Just to amplify that a bit.  You need the terminal voltage of the batteries.  Buy a cheap digital as Tony suggest but run a pair of thin wires all the way from the battery bank to where you sit.  Battery bank connected to one end, voltmeter connected to other end.  Virtually zero current drawn therefore no voltage drop.  Simples?

 

N

 

All true except the OP really needs the rested voltage, so he will need to turn all the loads off. This probably means that he wants just to keep an eye on the voltage so he knows when it's time to stop using electricity and start recharging. To know that and start charging he needs to go to the back of the boat where his accurate meter is, so in my view no need to worry about extra wires. He will soon get to know by how much the front voltmeter differs from the accurate one.

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