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Old 31-08-2015, 23:46   #1
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Mastervolt BTM-iii not accurate

My Mastervolt BMT-iii monitor is showing some very innacurate readings. It's tleling me I have 60% battery power left when it's charing on float. With a multimeter it's showing 13.55v but the Mastervolt baterry monitor is showing 12.55. Has this battery monitor gone bad or are there any tips/tricks that I should consider before replacing it since they're quite expensive...

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Old 01-09-2015, 02:23   #2
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Re: Mastervolt BTM-iii not accurate

The main problem with amp-counting battery monitors is zeroing them out -- resetting them to 100% when the batts are really fully charged. If you don't do that regularly, then what they read doesn't even come close to reality.

I would bet that this is your problem, as these monitors, as far as I know, don't "go bad" by giving wrong readings -- they just go dead if they're bad. I suppose there is some slight possibility that you have a bad connection somewhere which is throwing off the reading, but again, I wouldn't expect that the reading would be further off than that, and erratic to boot. The deviation of voltage you talk about might be a sign of this, however, so maybe it would make sense to remake this connection, and be sure that the connections to the shunt are tight and sound. Did you measure voltage at the shunt?

The other thing which has to be corrected on these monitors is the real capacity of the batteries. As they age, they lose their real capacity in amp/hours, and the battery monitor needs to know this. This begs the question of how do you measure the real capacity -- and the answer is that there is no easy way to do it; you have to guess. Even the initial setting of battery capacity is a guess -- since the manufacturer rating often varies greatly from reality.

For all these reasons, I don't like this type of battery monitor and I have thrown away my Victron one. I bought a SmartGauge which I think is a superior approach -- it analyzes voltage. That Balmar has recently started marketing these with their brand (at nearly double cost) is also a pretty good vote for this approach.

I have not installed my SmartGauge yet (this week, I hope), but after tossing my Victron battery monitor, I started monitoring my batteries by simply observing voltage under various load conditions. This approach has worked very well for me -- I think better than what I was getting from the Victron (was a BM-602 IIRC). If your bank is fairly large, then you can get an excellent picture of the state of charge by observing voltage under light loads when no heavy loads have been on for a while. This does not fulfill the requirements of the ideal measurement of state of charge by open circuit voltage, which requires that the battery has rested for a certain time with no load at all, but any error in this method will understate battery charge, so for purposes of practical everyday management of your batteries, it is excellent, in my opinion far better than amp-counting battery monitors with zero chance of overestimating the state of charge. I have checked my assessment of state of charge with specific gravity of the batts (the most accurate way to know SOC), and the deviation is very small, just a few percent, and always on the conservative side.

The SmartGauge actually works on the same principle -- it observes voltage, including long and short term changes in voltage in response to loads. It doesn't know amps going in our out, but since your boat has a certain number of devices, it can recognize the signature of different loads and can recognize how the voltage change in response to them changes in response to changing state of charge. By all accounts, it works far better than amp counting, and the device is inexpensive (if you buy it directly from the UK manufacturer rather than through Balmar) and easy to install (no shunt).
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Old 01-09-2015, 02:44   #3
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Re: Mastervolt BTM-iii not accurate

if you can't even read the correct voltage something is wrong. can you measure the voltage at the back of the gauge? if it's wrong there your wiring is bad. if it's correct I'd say the meter is shot.
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Old 01-09-2015, 02:46   #4
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Re: Mastervolt BTM-iii not accurate

No battery monitor will be accurate over time due to Peukert's law and temperature variations during charge / discharge, There is not a battery monitor that takes these into calculations,
They are best used as a display and re-calibrated to real battery condition/SOC every few months, Use SG's on FLA, Sealed, let them rest for min 12h & test voltage,
Also most will default to 100% SOC reading on initial connection, You MUST make sure your batteries are 100% at this time or else everything on is false,

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Old 01-09-2015, 17:23   #5
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Re: Mastervolt BTM-iii not accurate

For an excellent discussion and description of battery monitors, including Victron and SmartGauge, see Compass Marine's site:

Compass Marine How To Articles Photo Gallery by Compass Marine How To at pbase.com
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Old 09-09-2015, 21:47   #6
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Re: Mastervolt BTM-iii not accurate

I have the same problem with my Mastervolt.

Voltage is voltage, amps is amps. When dealing with an incorrect voltage reading this has nothing to do with Peukert values or amps.

The % use may be off because of the P settings, as been mentioned, but at least the voltage should be reading correctly, which apparently Mastervolt has a problem with.

It reads voltage too low... but it is not the panel that is reading the voltage, its the shunt. The shunt in Mastervolt is not just a shunt... its an intelligent box that stores all the readings and can pass them between devices.

I have never found an answer to the voltage problem.

I ended up adding a SmartGuage, which I have found to read voltage much more accurately.

FWIW - if you have lead acid batteries I would check to make sure there is water in them.
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Old 10-09-2015, 02:49   #7
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Re: Mastervolt BTM-iii not accurate

Quote:
Originally Posted by Dockhead View Post
... bought a SmartGauge which I think is a superior approach -- it analyzes voltage.....
SmartGauge actually does more than analyse voltage over time which is why the installation manual says use 14 gauge wires which is far bigger than needed just to measure voltage. See this posting from the designer himself from about 10 years ago - yes SmartGauge is that old - I have had mine for 5 years.

Chris Gibson posting:

Obviously I am not prepared to disclose how SmartGauge works. Anyone who asks me to do so is simply being totally unreasonable.

But to say that only voltage can be measured via 2 wires is *completely* incorrect.....

Pull a brief current pulse from the battery and measure the voltage drop, this will give an indication of internal resistance.

Present an AC voltage across the battery and measure the phase angle and amplitude of the resultant current. This will show the AC impedance of the battery.

Do the same thing with a wide variety of frequencies and analyse the results. This is know as AC impedance spectrography.
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Old 10-09-2015, 05:33   #8
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Re: Mastervolt BTM-iii not accurate

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Originally Posted by wegman View Post
For an excellent discussion and description of battery monitors, including Victron and SmartGauge, see Compass Marine's site:

Compass Marine How To Articles Photo Gallery by Compass Marine How To at pbase.com

That is the best advice you can get!


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Old 10-09-2015, 06:56   #9
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Re: Mastervolt BTM-iii not accurate

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Originally Posted by Snore View Post
That is the best advice you can get!....
The best advice on how and why SmartGauge works comes from the inventor Chris Gibson who I quoted in part above. Here is the link to his FULL post from 2005 on Battery Monitors and SmartGauge. MainSail's post is a very good independent test of the product.

Smartbank battery management - Page 2
post number #42
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Old 10-09-2015, 10:01   #10
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Re: Mastervolt BTM-iii not accurate

Lots of talk here about SOC etc but the fundamental problem is that the monitor is not giving the correct voltage reading. Voltage readings are never (at least on any of the numerous brands/models I have seen) taken from a lead to a shunt. The voltage measurement leads are supposed to go direct to the battery positive (or a buss or switch tied directly to the battery) along with a fuse to protect the wire.

Shunt leads only measure the miniscule voltage difference across a shunt, on the negative side of the circuit, and should never carry full battery voltage.

Lots of reasons why the SOC/battery capacity readings can be off. Very few as to the battery voltage. Battery voltage can be measured by a very skinny wire often 16 gauge but doesn't even have to be that big to do the job. A bad connection - corrosion being the most common problem - could be the issue. Or it could be the wire terminations. Or the meter itself. Try taking the same wire (with a jumper if needed) to another battery and see what the meter reads then. If it reads correctly then the meter is most likely okay and it is something else. But check all the connections. I think I would actually recommend taking the sensing lead off of the battery and putting it back on and see what you get. Make sure you measure the battery voltage at the same place with your handheld meter too. The problem may be between that and the battery itself if it does not go direct to the battery post. Check the fuse holder as they sometimes get corroded inside.

You have to get good voltage readings to get the battery capacity function to work. As far as charging the battery and then letting it rest, etc., you need to do that perhaps to reset the battery capacity calculation but a voltage reading can be taken with a handheld and compared to the meter at any time, whether the battery is charging, discharging, whatever.
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Old 10-09-2015, 11:31   #11
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Re: Mastervolt BTM-iii not accurate

Check the negative cablel to the battery monitor. Both the positive and the negative must go directly to the battery and not via a positive bus bar or an engine earth point.
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Old 10-09-2015, 12:12   #12
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Re: Mastervolt BTM-iii not accurate

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Originally Posted by sailinglegend View Post
Check the negative cablel to the battery monitor. Both the positive and the negative must go directly to the battery and not via a positive bus bar or an engine earth point.
Excellent point! Should check the meter negative as it is just as important as the battery sense lead to the battery.
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Old 10-09-2015, 12:12   #13
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Re: Mastervolt BTM-iii not accurate

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Originally Posted by exMaggieDrum View Post
Lots of talk here about SOC etc but the fundamental problem is that the monitor is not giving the correct voltage reading. Voltage readings are never (at least on any of the numerous brands/models I have seen) taken from a lead to a shunt. The voltage measurement leads are supposed to go direct to the battery positive (or a buss or switch tied directly to the battery) along with a fuse to protect the wire.

Shunt leads only measure the miniscule voltage difference across a shunt, on the negative side of the circuit, and should never carry full battery voltage.

Lots of reasons why the SOC/battery capacity readings can be off. Very few as to the battery voltage. Battery voltage can be measured by a very skinny wire often 16 gauge but doesn't even have to be that big to do the job. A bad connection - corrosion being the most common problem - could be the issue. Or it could be the wire terminations. Or the meter itself. Try taking the same wire (with a jumper if needed) to another battery and see what the meter reads then. If it reads correctly then the meter is most likely okay and it is something else. But check all the connections. I think I would actually recommend taking the sensing lead off of the battery and putting it back on and see what you get. Make sure you measure the battery voltage at the same place with your handheld meter too. The problem may be between that and the battery itself if it does not go direct to the battery post. Check the fuse holder as they sometimes get corroded inside.

You have to get good voltage readings to get the battery capacity function to work. As far as charging the battery and then letting it rest, etc., you need to do that perhaps to reset the battery capacity calculation but a voltage reading can be taken with a handheld and compared to the meter at any time, whether the battery is charging, discharging, whatever.
The mastervolt "shunt" is NOT a shunt as you think of it. It goes between the batteries and before the distribution centers.

Mastervolt - Innovative power systems<br>for autonomous use
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Old 10-09-2015, 12:29   #14
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Re: Mastervolt BTM-iii not accurate

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The mastervolt "shunt" is NOT a shunt as you think of it. It goes between the batteries and before the distribution centers.

Mastervolt - Innovative power systems<br>for autonomous use
The Mastervolt BTMiii uses a regular negative-side shunt and not the "MasterShunt" you link to. So my comments still apply. The MasterShunt is, indeed, a special animal and is wired in between the battery bank, on both the positive and negative sides. It uses the "MasterView" series of monitors all connected with the MasterBus. All part of the MasterRace line of equipment.
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Old 10-09-2015, 13:54   #15
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Re: Mastervolt BTM-iii not accurate

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Originally Posted by exMaggieDrum View Post
The Mastervolt BTMiii uses a regular negative-side shunt and not the "MasterShunt" you link to. So my comments still apply. The MasterShunt is, indeed, a special animal and is wired in between the battery bank, on both the positive and negative sides. It uses the "MasterView" series of monitors all connected with the MasterBus. All part of the MasterRace line of equipment.
That's true but I think its logical to conclude that they use the same brains. Usually a company uses a common component and/or common code base... the difference is just in form factor.
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