Battery Monitor, Solar, Shunts

[Yellow Ballad]

One for the electric techs possibly, and to get my head around it.

I don't want a battery monitor as such but with my recent seatalk upgrade I think it would be a good way of seeing what the current voltage and current draw on my I70 and to make a note on long passages in the logbook.

I can't find anything reasonable (compact) and I don't want a second display like Victron BM range but I have found this Yacht Devices News: New product: NMEA 2000 Battery Monitor

Although not out yet and no instructions available I think it'll suit my needs but a couple of questions if I may. I assume it outputs NMEA2K PGN127508 which is voltage, current and temperature but it does mention state of charge in the link.

Firstly wiring, it measures on the positive side. 3 wires, battery side of shunt, load side of shunt and negative.

I have at the moment

Leisure Battery to Master Switch post A

Master Switch post B to Switch panel

I was thinking of putting the shunt between the post B and the switch panel so the whole caboodle gets shut off. I'm guess it takes it power from the seatalk network so wouldn't draw any power once this was turned off anyway. Does this sound reasonable?

Secondly
Solar onto Post A so itself feeding the batteries with the master off so before the shunt. Will this matter/confuse the current draw whether it was before or after the shunt? If I'm drawing 10amps through the shunt I assume say the solar is pumping in 1amp before the shunt the actual battery is dropping 9AH. If after the shunt will the BM will show 9amp draw?

Lastly, shunts, they say in the limited information to use a 75mV shunt. I imagine with everything on and being used I would use less than 100amps, probably closer to 50 as I doubt I'll be using the fridge, heater, bilge pump, all the instruments, autopilot, lights but I would hope the usual power draw will be less than 10AH at any one time. Is there a disadvantage having a 100amp shunt rather than a 50amp shunt, thinking it may be possible to draw more than the 50amp limit but hopefully only using about 10% of it's rating? Or does it not really matter?

Obviously once it's out and I've read the instructions it may all become clear but for now I'm just trying to plan how to plan my rewire/upgrade.

Many thanks in advance

Tom

 

[pvb]

If you want a battery monitor, you'll put the shunt next to the battery, with only the battery connected to one end of it. That way, it will measure all the current in and out of the battery.

Shunts will cope with excess current, within reasonable limits.

Does the system you're considering use Peukert algorithms to achieve state of charge? If not, it might not be too accurate.

 

[Yellow Ballad]

I have no idea but I'm not really fussed about state of charge, just interested in the battery voltage and current draw when I look.

There will be nothing before the shunt apart from the master switch and the solar input connection hence asking if the solar is best to go before the load side or after and if after will it reduce the "gross" load figure? (so showing "net" so to speak).

What would you suggest for a shunt rating then? I have a choice of 100, 50, 40, 30, 20, 15, 10.

 

[William_H]

For a completely different approach to OP problem there is an old fashioned solution re current in the form of analogue moving coil meter. The meters built say for 100 microamp full scale deflection can use a section of supply cable as the shunt. They do not then drop a lot of voltage in the shunt and require no power to supply them. The down side is that you will not get an absolute current value in amps unless you can calibrate the meter in some way. Calibration can be done with a digital multimeter having a 10 or 20 amp range. Insert this temporarily into the battery circuit and fit the microamp meter across from one end to the other of the feed cable. The analogue meter will measure volt drop of the cable. If there is not enough analogue meter indication for the current you have in mind then you need a longer cable or the idea is a bad one. To reduce the analogue meter movement you can fit a resistor in series or even a variable resistor to get the numbers on the analogue dial to match the current you have. You may be able to find a centre zero meter measuring charge and discharge or use a toggle switch to reverse the meter connections.
Here are some analogue meters on Ebay Analogue microamp meter | eBay
Of course you could buy a shunt and use it with calculation as to meter reading versus actual current. Don't forget fuses in the wires to the meter at the main wire if you are measuring in the positive line. ol'will

 

[BabaYaga]

I have no idea but I'm not really fussed about state of charge, just interested in the battery voltage and current draw when I look.

There will be nothing before the shunt apart from the master switch and the solar input connection hence asking if the solar is best to go before the load side or after and if after will it reduce the "gross" load figure? (so showing "net" so to speak).

What would you suggest for a shunt rating then? I have a choice of 100, 50, 40, 30, 20, 15, 10.

Is there no alternator charging of the leisure battery? If there is, would you not be interested in measuring it? In that case this current would likely be the highest passing through the shunt, so would have to be taken into account when deciding the rating. (Given that the monitor/A-meter can read both positive and negative values).

Regarding where to put the solar (and other charging) input in relation to the shunt: If input goes between shunt and battery, the charging current will not be measured, only the load current will. If the shunt is placed closest to the battery, it will measure net flow into or out of the battery.

 

[halcyon]

One for the electric techs possibly, and to get my head around it.

I don't want a battery monitor as such but with my recent seatalk upgrade I think it would be a good way of seeing what the current voltage and current draw on my I70 and to make a note on long passages in the logbook.



Tom

What sort of current load are you looking at measuring / see ?

Brian

 

[Yellow Ballad]

I'm unsure, I've never measure it before but from my list of "things" and what they draw...I imagine in usual sailing conditions 10AH would be about right but sticking the fridge on and the autopilot on would double this.

I imagine with everything on and being used I would use less than 100amps, probably closer to 50 as I doubt I'll be using the fridge, heater, bilge pump, all the instruments, autopilot, lights but I would hope the usual power draw will be less than 10AH at any one time. Is there a disadvantage having a 100amp shunt rather than a 50amp shunt, thinking it may be possible to draw more than the 50amp limit but hopefully only using about 10% of it's rating? Or does it not really matter?

BabyYada thanks, I though that'll be the case. Although charging currrent would be a good to know I'm quite happy to just know what's being used by things I have on and not the net discharge. Although I may change my mind, I've not fully decided yet.

Regarding where to put the solar (and other charging) input in relation to the shunt: If input goes between shunt and battery, the charging current will not be measured, only the load current will. If the shunt is placed closest to the battery, it will measure net flow into or out of the battery.

 

[Richard10002]

You say you don't want a monitor, and only want instantaneous amps and volts....

Why reinvent the wheel? If you get a NASA BM2 you get a 200A shunt, volts and amps, and can ignore everything else it does.

I say this because, in 2011, I was of the same mind as you, and bought a cheap volt meter and ammeter on ebay. It didnt take long for me to want to know more, and I now have a NASA BM2 and Smartguage on my Lead Acid bank, and a BMV 712 on my lithium bank.

For not too much money, you future proof yourself

 

[Poey50]

I broadly agree with the above. Amps and voltage are useful in many ways but the single most valuable figure - if accurate - is state of charge. Smartguage is very good for this, Victron BMV7** is pretty good as long as it is regularly synchronised at 100% charge (I have both of these) but NASA, by reputation, is a bit carp for SOC.

 

[Yellow Ballad]

I do agree SOC would be good to know but as it's been said not always reliable/needs syncronising and if I'm honest I don't want another screen with a big hole somewhere/wires being run over the boat. I've done without a BM since owning the boat it was just the fact I've got a spare seatalk plug and it would be "nice" to see what's being used. I can put the shunt and wiring near the backbone and it wouldn't impact on the limited space I have in my electrics cupboard.

The fact the Nasa comes with an 100 amp shunt makes me think I'll be able to use one with no ill affects then?

BabyYada has made me have a good think about it and putting the charging methods through the shunt to get a net flow does sound like a good idea so you'

 

[Poey50]

You say you don't want a monitor, and only want instantaneous amps and volts....

Why reinvent the wheel? If you get a NASA BM2 you get a 200A shunt, volts and amps, and can ignore everything else it does.

I say this because, in 2011, I was of the same mind as you, and bought a cheap volt meter and ammeter on ebay. It didnt take long for me to want to know more, and I now have a NASA BM2 and Smartguage on my Lead Acid bank, and a BMV 712 on my lithium bank.

For not too much money, you future proof yourself

Richard, I've just PM'd you on a related side-issue.

 

[Richard10002]

I do agree SOC would be good to know but as it's been said not always reliable/needs syncronising and if I'm honest I don't want another screen with a big hole somewhere/wires being run over the boat. I've done without a BM since owning the boat it was just the fact I've got a spare seatalk plug and it would be "nice" to see what's being used. I can put the shunt and wiring near the backbone and it wouldn't impact on the limited space I have in my electrics cupboard.

The fact the Nasa comes with an 100 amp shunt makes me think I'll be able to use one with no ill affects then?

BabyYada has made me have a good think about it and putting the charging methods through the shunt to get a net flow does sound like a good idea so you'

The SOC reading on the NASA is useless.

The Smartgauge is only useful and reliable below 80% SOC, so cannot be relied upon to tell you that your batteries are full... in fact, stopping charging when a Smartgauge first reaches 100% will almost certainly destroy batteries quickly.

I thing the Smartgauge developer has been quoted as saying, “why would anybody need to know the SOC of their batteries, if it is over 80%?”, or words to that effect.

Thus the Smartgauge is only really useful in the sense that it can tell you when to start charging your batteries.

 

[Poey50]

The SOC reading on the NASA is useless.

The Smartgauge is only useful and reliable below 80% SOC, so cannot be relied upon to tell you that your batteries are full... in fact, stopping charging when a Smartgauge first reaches 100% will almost certainly destroy batteries quickly.

I thing the Smartgauge developer has been quoted as saying, “why would anybody need to know the SOC of their batteries, if it is over 80%?”, or words to that effect.

Thus the Smartgauge is only really useful in the sense that it can tell you when to start charging your batteries.

As said above, I've got a Smartgauge and a Victron BMV700. The Victron is good (as you know) once you are sure it the batteries are fully charged (overnight on shore power for example). When accurate it can be used as a check on the Smartguage. On my setup the Smartguage is registering 100% charge when the Victron is showing 96-97%. So while I agree with you that you would be unwise to stop charging (if you can keep going) when Smartguage shows 100% I don't find it anywhere near your 80%. I've repeated this numerous times so I think it reliable for my setup.

I find it hard to believe that this is an accurate statement of the Smartgauge developer. It would be only the rankest newbie to batteries who would think it was unnecessary to know if the batteries were more than 80%. This is one of the main reasons for a reasonably accurate battery monitor in the continuous fight against sulphating.

I would have thought that Rod of the Marinehowto site would not be so enthusiastic if the 80% was usual. Balmar Smartgauge Battery Monitoring Unit

 

[GHA]

As said above, I've got a Smartgauge and a Victron BMV700. The Victron is good (as you know) once you are sure it the batteries are fully charged (overnight on shore power for example). When accurate it can be used as a check on the Smartguage. On my setup the Smartguage is registering 100% charge when the Victron is showing 96-97%. So while I agree with you that you would be unwise to stop charging (if you can keep going) when Smartguage shows 100% I don't find it anywhere near your 80%. I've repeated this numerous times so I think it reliable for my setup.

I find it hard to believe that this is an accurate statement of the Smartgauge developer. It would be only the rankest newbie to batteries who would think it was unnecessary to know if the batteries were more than 80%. This is one of the main reasons for a reasonably accurate battery monitor in the continuous fight against sulphating.

I would have thought that Rod of the Marinehowto site would not be so enthusiastic if the 80% was usual. Balmar Smartgauge Battery Monitoring Unit

The smartgauge does seem very accurate on discharge, as tested by compass marine, it's charging where it can go astray. Also, don't forget the smart gauge is measuring actual capacity, a battery monitor will display percentage of the figures programmed into it. Which can be well off the mark just with temperature differences even if the batteries are actually still really healthy.

Cold batteries are a fair bit smaller in terms of capacity than warm batteries.

I thought my smartgauge was reading wrong as the BEP monitor in the morning was showing a lower SOC than the SG in the heat but then as temperature dropped the SG was showing less capacity than the BEP.
But charging the SG gets a bit lost. ISTM tail current is the only reliable day to day method of determining if a battery is fully charged. My T105's level off at about 0.5% going in , in amps of capacity in Ah at 14.8v. Changes a little with temperature. And takes quite a while to finally level off........

 

[davidmh]

If you are using a shunt the simplest way to measure the amps going through the shunt is to use a didital multimeter to read the voltage drop across the shunt and this will give you the amps flowing. My set up gives 1amp at 1 mV drop. I don't know the theory but I was told the information by an electrical engineer and it seems to work. I expect someone on this forum will explain how it works.
Regards
DAVID H

 

[pvb]

If you are using a shunt the simplest way to measure the amps going through the shunt is to use a didital multimeter to read the voltage drop across the shunt and this will give you the amps flowing. My set up gives 1amp at 1 mV drop. I don't know the theory but I was told the information by an electrical engineer and it seems to work. I expect someone on this forum will explain how it works.

That's the whole basis of how shunts work. The shunt has a very small resistance, which produces a tiny voltage directly proportional to the current flowing through it. Measure the voltage and you know the current.

 

[doug748]

....

The fact the Nasa comes with an 100 amp shunt makes me think I'll be able to use one with no ill affects then?

BabyYada has made me have a good think about it and putting the charging methods through the shunt to get a net flow does sound like a good idea so you'

100 amp should be fine. I often start my 20hp engine through the NASA shunt with no ill effects over 5 years or so. It's a great thing to have.

.

 

[Mistroma]

The smartgauge does seem very accurate on discharge, as tested by compass marine, it's charging where it can go astray. Also, don't forget the smart gauge is measuring actual capacity, a battery monitor will display percentage of the figures programmed into it. Which can be well off the mark just with temperature differences even if the batteries are actually still really healthy.

Cold batteries are a fair bit smaller in terms of capacity than warm batteries.

I thought my smartgauge was reading wrong as the BEP monitor in the morning was showing a lower SOC than the SG in the heat but then as temperature dropped the SG was showing less capacity than the BEP.
But charging the SG gets a bit lost. ISTM tail current is the only reliable day to day method of determining if a battery is fully charged. My T105's level off at about 0.5% going in , in amps of capacity in Ah at 14.8v. Changes a little with temperature. And takes quite a while to finally level off........

Yes, I find the same thing and check final current for daily indication of 100% charge as Smartgauge will usually read 100% early. Their tech. guys did say that it uses a different algorithm for charging and last part isn't that accurate. I'm certain you will have noticed that if it just reaches 100% there's a pretty rapid drop when the sun goes because it drifts down towards the actual charge pretty quickly. The next morning's reading is a decent indicator of actual % capacity remaining and usually agrees with temp. compensated SG.

I was lucky to find a "Bluetooth DC ammeter voltmeter power Timer battery Monitor tester" for about £18-£20 on eBay a couple of years ago. Current versions seem to lack Bluetooth and also require a shunt. I couldn't find a cheap current sensor at the time and the price was unbeatable. I didn't even try to mount the display in a visible location, just stuck it in a locker and slipped the sensor over domestic +ve connection. It's very convenient to get all the V, A, Ah, W, Wh readings form my phone. I think I'd probably connect the sensor to an ESP32 if the LCD/Bluetooth unit died.

 

[William_H]

If you are using a shunt the simplest way to measure the amps going through the shunt is to use a didital multimeter to read the voltage drop across the shunt and this will give you the amps flowing. My set up gives 1amp at 1 mV drop. I don't know the theory but I was told the information by an electrical engineer and it seems to work. I expect someone on this forum will explain how it works.
Regards
DAVID H

David is quite right however, we note that his digital multimeter would have a 200mv lowest volt range so a max of 200 amps measured. Which is fine however not much accuracy or definition at low currents. ie around 1 amp. It seems to be a fact that typical cheap digital amp meters are just 200mv meters inside so to measure low currents you need a shunt of greater resistance. So for a typical reading of amp meter at mid range you will have a drop of 100mv or 1/10 volt which is significant drop or waste. It is possible to fit an amplifier so that lower voltage can be measured but this does not seem common in cheap digital amp meters. Though necessary in complex battery monitors. Hence my advocating moving coil amp meter requiring a lot lower voltage drop in the shunt. ol'will

 

[GHA]

I think I'd probably connect the sensor to an ESP32 if the LCD/Bluetooth unit died.

If you have an existing shunt then an ESP32/8266 works well with an INA219 current sensor, just piggy backed across the existing shunt and use a clamp meter to calibrate it. The INA219 is a few quid on ebay or even better design a PCB from JLCPCB with everything already soldered on. Little project for someone
Adafruit INA219 Current Sensor Breakout
Seems to be accurate down to about 100/200mA.