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Old 03-21-2014, 06:45 AM   #1
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Understanding my Battery Monitor

I installed my Victron battery monitor the other day and am out now at anchor. The readings I'm getting are not consistant with the battery state of charge from the battery manufacturer.
Here is the data after 13 hrs on the hook.
Voltage 11.8
SOC 86.6%
CE (energy consumed) 73.6 Amps
TTG (time until depletion) 11.87 hrs

The battery manufacturer, Decca, provides the following data
% charge Voltage
100 12.8
75 12.6
50 12.3
25 12.0
0 11.8

According to the Decca table and my battery monitor voltage reading the batteries are DEAD.
Batteries are 4 paralleled 105 AH AGM's about 4 years old.
The voltage was confirmed by a separate v meter.
I calibrated the battery monitor to 420 AH.

The SOC and TTG can't be right.

Idea's?
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Old 03-21-2014, 08:18 AM   #2
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When you took the voltage reading was everything turned off and had been off for a while? Refrigeration, inverter, lights, etc everything off. All DC loads. Voltage readings of the batteries, that are used to indicate a state of charge, are only accurate after the batteries have been at rest with no loads.
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Old 03-21-2014, 08:36 AM   #3
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He's quite right. Battery voltage readings are valid only on a fully rested battery (usually 12 hrs w/o charge or discharge of any kind)
You can get close to rested voltage by cranking your starter for 15 sec. but if the engine starts and the alt. puts out, all bets are off.
Why would we even need SOC monitors if we could look at battery voltage at any time for the same info.
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Old 03-21-2014, 08:53 AM   #4
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I noticed it took awhile, a number of days, for my Victron to collect enough data, it knew what was going on.

Having said that, after almost a year, the SOC and TTG are frequently "off"

So I pretty much just use the CE, which I actually monitor and record each morning when not on shore power. I also record voltage, but as others have said, it's off and even when I rest the batts, it's still not the most accurate reading, depending upon how much rest the batteries actually got!
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Old 03-21-2014, 08:56 AM   #5
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Looking at your numbers, your SOC is only a few % off.

Just don't let your CE get below -200ah
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Old 03-21-2014, 09:19 AM   #6
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Quote:
Originally Posted by Brooksie View Post
Battery voltage readings are valid only on a fully rested battery...
What he said. Ideally the batteries should be rested for 24 hours. A fairly good reading can be gotten with 12 hours of rest. At the very least they need to be rested for a couple of hours, or the reading you are getting is pretty useless.
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Old 03-21-2014, 10:11 AM   #7
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The replies above about not relying on voltage are spot on unless done right.

If the Victron works similar to the Magnum, it takes a few full charges for it to accurately calculate the charge efficiency of the batteries, which eventually yields you a quite accurate SOC (which will still vary to volts, per above). Unfortunately, with AGMs you can't take a hydrometer reading. As I was breaking in my new bank of Trojan L16's, it seemed to me the voltage readings were noticeably lower than normal after the same AH out, than the old bank. When I tested it with a hydrometer, after about 8 full charge cycles, the SOC was spot on to the battery monitor. In discussing this with Trojan, they said SOC was the best measurement to use.
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Old 03-21-2014, 10:18 AM   #8
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I have spent A LOT of time reading and re-reading the manual to my Link 2000. It has been a steep learning curve for me. Early on in the manual it states you cannot rely on just a single parameter to understand the state of your battery (page 5). Also, you may need to resynchronize your monitor when the batteries are fully charged. "If the Link 2000 should ever get out of sync with the battery state of charge, it must be resynchronized. The best way to do this is to be sure the battery is discharged at least 10% of declared battery capacity and then recharge until the charged parameters are met. At this point the meter should automatically reset to zero."

I have found that when you sit on float charge for an extended periods, the meter kind of looses it's way a bit. Reasons for this are discussed on page 20 on the manual. The monitor can be reset at this point. This manual explains a lot of stuff about charging systems, more so than what I found described in the manual for my Victron monitor. Perhaps some of the Link manual is not relevant for more modern monitors, but much is probably still relevant.

http://www.xantrex.com/documents/Dis...-1(Vendor).pdf

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Old 03-21-2014, 10:24 AM   #9
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Quote:
Originally Posted by Wxx3 View Post
Looking at your numbers, your SOC is only a few % off.

Just don't let your CE get below -200ah
I think that would only be true (50% discharge, 200ah) if the batteries were new, which mine are not.

I did check the batteries after 2 hrs with very little draw, and the voltage rose to 11.97.

So do I disregard the voltage and rely on SOC and CE and give the monitor several charge discharge cycles to calibrate itself to my batteries?

A voltage reading below 12 v seems bad, very bad.
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Old 03-21-2014, 10:32 AM   #10
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You can always pull one or two out and take them to be tested if you are paranoid (and there is nothing wrong with that in this case). A high end battery shop that does a lot of deep cycle batteries should have the proper equipment and know how to do the test right. The process is described in Calder's "Boat Owner's Mechanical and Electrical Manual" which has all the answers to the posts you have made on batteries and chargers, explained in detail and English. Absolutely an essential book on any cruising boat.
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Old 03-21-2014, 10:40 AM   #11
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Timjet...perhaps "bad, very bad" but perhaps not as bad as you think it is. I went through similar angst with my batteries and replaced the lot. I think I was right to do this, but in hindsight, I don't think that things were quite as bad as I thought they were. These deep cycle batteries take considerable time for the voltage to equalize across the battery plates (reference, Calder). I would follow what others have said about letting the batteries rest before measuring the voltage. Also, I would consider taking steps to reset the monitor and possible the monitor parameters, if at all possible, before I replace the batteries.

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Old 03-21-2014, 11:08 AM   #12
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As others have said measuring the voltage on a house bank is not an accurate method to determine SOC, since by nature house banks are always under load.

How battery SOC monitors work is simple.

They measure amps, and apply a time base to them to calculate amp hours.

If you tell them the amp hour capacity of your house bank
and
If youi tell them the current state of charge, they can figure the State of Charge from there.

The Xantrex link2000 is programable to decide just what constitutes a "full charge". I think I set mine to declare the batteries full if the charge current was less than 5 amps for 1/2 hour. This is important as it keeps your SOC meter in sync.

When reading your SOC meter, although it is digital, it is not 100% accurate, and you really do not care anyway. Your batteries cpapacty will vary somewhat due to temperature, current draw down rate, and age.

The general goal of the SOC meter is to determine the approximate state of charge of the batteries, and just as important, actually more important, measure charging and discharging current and voltage.

This information is critical to determine the best time to recharge to optimize batterylife , and to determine the state of your boats charging systems.
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Old 03-21-2014, 11:11 AM   #13
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When you are at the dock and the batteries are fully charged, shut everything off, any DC draw, inverter and the battery charger. Watch the battery voltage and see where it goes. Does it slowly drop to around 12.8 and stay there or does it go lower. If it goes lower, test your batteries individually At 4 years old, batteries usually aren't 100%. You may also have a bad cell (s) in one.
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Old 03-21-2014, 04:00 PM   #14
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Quote:
Originally Posted by ksanders View Post
As others have said measuring the voltage on a house bank is not an accurate method to determine SOC, since by nature house banks are always under load.

How battery SOC monitors work is simple.

They measure amps, and apply a time base to them to calculate amp hours.

If you tell them the amp hour capacity of your house bank
and
If youi tell them the current state of charge, they can figure the State of Charge from there.

The Xantrex link2000 is programable to decide just what constitutes a "full charge". I think I set mine to declare the batteries full if the charge current was less than 5 amps for 1/2 hour. This is important as it keeps your SOC meter in sync.

When reading your SOC meter, although it is digital, it is not 100% accurate, and you really do not care anyway. Your batteries cpapacty will vary somewhat due to temperature, current draw down rate, and age.

The general goal of the SOC meter is to determine the approximate state of charge of the batteries, and just as important, actually more important, measure charging and discharging current and voltage.

This information is critical to determine the best time to recharge to optimize batterylife , and to determine the state of your boats charging systems.
Actually, the newer more sophisticated monitors have an algorithm that takes variables into account and arrives at a charge efficiency factor. . It is almost always the case for the number of amps in to exceed those that were taken out before you are at 100% SOC. My Magnum gives you a meter for both (total AH out, net AH in/out) so it was easy to see. It does drop off percipitously as batteries reach end of life, they just can't accept a full charge.

It may be that Tim's batteries are near or at end of life. We don't know how much they have been used and discharged over time. With his new Victron, he will have a much better handle on that with the next set he buys.
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Old 03-21-2014, 04:32 PM   #15
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Actually, the newer more sophisticated monitors have an algorithm that takes variables into account and arrives at a charge efficiency factor. . It is almost always the case for the number of amps in to exceed those that were taken out before you are at 100% SOC. My Magnum gives you a meter for both (total AH out, net AH in/out) so it was easy to see. It does drop off percipitously as batteries reach end of life, they just can't accept a full charge.

It may be that Tim's batteries are near or at end of life. We don't know how much they have been used and discharged over time. With his new Victron, he will have a much better handle on that with the next set he buys.
Any net difference between amps in vs amps out is why you have to be careful when you set these meters up. Thats also why you need to get your bank to a full charge occasionally to re-sync it with the battery bank.

What is a "full charge", and how do we define it electrically?

I told my system I beleive that less than 5 amps in for 1/2 hour is a full charge. That way it keeps things straight.

I do not know what opther variables come into play, and would be very interested from a theoritical standpoint to see just what the more advanced meters are doing.

A good thing about these meters is that you can see the batteries age. Look at the bank voltage under load when the batteries are at say 50% when new, and the voltage will be lower when the batteries are near end of life, indicating less amp hour capacity.
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Old 03-21-2014, 04:38 PM   #16
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Old 03-21-2014, 05:14 PM   #17
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Is the shunt you installed for the monitor inline with ALL loads on the battery? Some times people connect other loads directly to the battery.

Have the batteries been maintaine well; watering and smart charger? If not my gut tells me 5-6 years is it for thattype of battery.

Can you check all the cell specific gravities and get another data point for battery health?

You said in a previous post that you had an old inefficient fridge. 73 amp-hrs for almost 12 hours seems low for an in efficient fridge. What did the monitor read for amps when the fridge compressor was running?

Please ignore if I have asked stupid questions.
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Old 03-21-2014, 06:06 PM   #18
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I agree with the above replies about battery voltage. It's tough seeing 11.9V and thinking my batts are OK, but I'm getting used to it. Are you running an inverter to power your 120V systems? If so, this constant draw will keep the batteries from reaching that 'rested' state. Cycling refrigerators will do the same.

I agree that the key component to watch is the AH consumed. Your boat sounds like it has similar loads to mine...I burn, give or take, about 150 AH per day. Once my 660 AH lead-acid house bank reaches about 81-82% charge, the charge mode shifts to absorption mode and the acceptance rate diminishes to about half of my charger or alternator output.

I just returned from 3 days on the hook chasing sturgeon around Suisun Bay. After day one, I ran between 70% and 85% with daily recharge. The more I move the boat, the less I need to run the generator. For me, one day's battery use represents about 25% of the bank capacity...or 50% of the usable charge which ranges from full to 50% at the depleted state. This means I need to replenish 25% or 150AH per day. If I don't move the boat, that means running the gen for a couple hours in the morning and a couple hours in the evening. For my L-A batts, this recharge goes most quickly running between 60% and 80% capacity.

With 420AH, you must replenish after about 210AH has been consumed. If you typically run from 55-90% state of charge, you need to replace about 75AH per recharge (35% of 210AH). That represents only half of what you need per day, so two recharges, morning and evening, may be a good pattern for your boat.

I suppose your AGM's might accept the high charge rate well above the 82% that I see with my L-As.
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Old 03-21-2014, 07:15 PM   #19
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Quote:
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Is the shunt you installed for the monitor inline with ALL loads on the battery? Some times people connect other loads directly to the battery.

Have the batteries been maintaine well; watering and smart charger? If not my gut tells me 5-6 years is it for thattype of battery.

Can you check all the cell specific gravities and get another data point for battery health?

You said in a previous post that you had an old inefficient fridge. 73 amp-hrs for almost 12 hours seems low for an in efficient fridge. What did the monitor read for amps when the fridge compressor was running?

Please ignore if I have asked stupid questions.
Shunt should be on negative lead coming out of battery.

He has AGMs, no watering, no taking specific gravity measurements.
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Old 03-22-2014, 08:38 AM   #20
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Ok thanks guys.
I charged the batteries last night via the genny for 2 hrs. Not near enough for my 420 ah bank and 30 amp charger. The charger was putting out over 20 ams when I shut down the genny so the monitor never synchronize. Syn happens when v>
13.2 and charge current < 16.8 amps.

Based on the monitors historical data like Al, I'm using roughly 150 amps per day. That's 5 hrs genny run time and why I ordered a 100 amp charger.

When I got up this morning the v is 10.8 verified by a second v meter. A florescent light in the V berth would not come on. SOC read 80% and TTG is 20 hrs. The house bank is dead. Monitor is not calibrated correctly or something else is wrong.

We will move the boat today and should top off the batteries with the far more powerful alternators. I'll check before I shut down the engines to see if the monitor re-synched.
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