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Old 31-07-2011, 18:45   #1
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Battery Monitor vs Voltage

I installed a battery monitor the other day and charged the battery till the monitor said it was 100% State of Charge (SOC). For reference I have 4 new 6V batteries wired in series/parallel for a total of 460 AH. I charged them the other day till the charge amps dropped to 8 amps and considered that 100%.

At end of my sail today the monitor said I had used 33 AH and that the battery was at 95% charge, which even though doesn't work by math I can accept this.

But the voltage at the time was 12.25V on the monitor and I was drawing 4.7 amps. This voltage is a less than what I would expect for the battery if they really were at 95% SOC.

Should I really believe the SOC on the monitor even though the voltage doesn't seem to match? Is that 4.7 amps enough to effect the voltage vs charge this much?

Has my voltmater been misleading me this much the past couple of years?
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Old 31-07-2011, 18:57   #2
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Re: Batery Monitor vs Voltage

Quote:
Has my voltmeter been misleading me this much the past couple of years?
Volt meters need calibration and the monitor assume when you start out you are 100%. If you do all that you'll find the monitor will be the better judge. The volt meters are pretty good at the high and low ends but in the middle they don't really tell you what you need to know. Once you can see the load in amps and live with it a while you get to know how many amps things really take instead of what you thin k they should take.

A high quality multimeter can be used to adjust the volt reading. There is a set screw to tweak the reading on the meter. When we got our boat the readings were bogus. If they are not close to accurate they are mostly worthless. It's also a good time to set the exponent on the monitor and double check the setting on the charger too. Once you get those right you can sleep knowing it's all pretty close. You'll treat your batteries better and maybe get a few more years out of them.
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Old 31-07-2011, 18:58   #3
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Re: Batery Monitor vs Voltage

Try turning stuff off, let the voltage stabilize then check it?
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Old 31-07-2011, 19:33   #4
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Re: Batery Monitor vs Voltage

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Originally Posted by Don Lucas View Post
I charged them the other day till the charge amps dropped to 8 amps and considered that 100%.
If the battery was still taking 8 amps the battery was not fully charged.
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Old 31-07-2011, 20:18   #5
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Re: Batery Monitor vs Voltage

Don,

When your batts were taking 8 amps was any other draw on? Ideally you want to check this with no loads and just the alternator or charger feeding the bank. Under 2% acceptance can be considered full when cruising but when really full you can be below .5% acceptance. You were at about 1.7% acceptance. For coastal cruising I would wait till your acceptance bottoms out before re-setting the monitor. Most monitors are factory set to re-set at about 2% but this can be changed. I moved ours down to 1% of bank size..

How did you calibrate the monitor?

As for open circuit voltage checks they can not be effectively done without allowing the batteries to rest with no loads and no charging for many hours...

For example I am sitting here typing this drawing about 8.2A with two fans running, stereo, 5 LED cabin lights a pure sine inverter powering a discharged 15" laptop, anchor light and some phone chargers going and my voltage tells me I am at 12.56 volts. If I went by this it tells me I am at 80% SOC when I am in fact at about 95.4% SOC (monitor was manually re-set Friday when the bank was accepting about .4%). If I turn off all the loads my battery voltage goes back up to reflect my actual SOC after about 12 hours of rest on the batts.

In fact I just turned off all loads except for -1A worth and my voltage went back up to 12.63 and it is still slowly been climbing over the last few minutes and is now sitting at 12.74 volts with a -1A draw...... This tells me that despite running at over 8 amps for a little while my batts still have some surface charge on them because 12.74 volts should be more than 100% SOC for my bank and I am down 17.9Ah's. Oops it just hit 12.75v still with the -1A load...

Inverter and fans back on and the voltage is dropping back again to where it was before...... Trust the monitor not the voltage unless you get very used to your bank and loads and what to expect....
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Old 31-07-2011, 20:25   #6
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Re: Batery Monitor vs Voltage

Don,

I agree with most of the above. If your batteries were accepting 8 amps with no other load, they weren't 100% charged.

You have to charge them to a full 100% and THEN calibrate the monitor.

RE: the voltage, with a draw under 5 amps on that battery bank, you should be seeing not less than 12.6 or 12.5 at the very least from a fully charged flooded cell battery bank.

One other thing: your new batteries won't show their full potential until they have been cycled quite a few times, i.e., discharged to 50% or so and fully charged. In other words, when new they aren't giving you 460AH of capacity (at the 20-hour rate), and won't for a few weeks or months....provided that you treat them right!

Bill
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Old 31-07-2011, 20:30   #7
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Re: Batery Monitor vs Voltage

Quote:
Originally Posted by btrayfors View Post

One other thing: your new batteries won't show their full potential until they have been cycled quite a few times, i.e., discharged to 50% or so and fully charged. In other words, when new they aren't giving you 460AH of capacity (at the 20-hour rate), and won't for a few weeks or months....provided that you treat them right!

Bill
Bingo! Deep cycle batts can take a number of cycles to fully break in... Our bank holds voltage far better now than it did for the first season we had them and they are now on their fifth season.
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Old 01-08-2011, 00:09   #8
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Re: Batery Monitor vs Voltage

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Originally Posted by Don Lucas View Post
I installed a battery monitor the other day and charged the battery till the monitor said it was 100% State of Charge (SOC)...
Is this monitor just a voltage meter, or does it measure the current going in and out of the batteries? As others have said, to use the battery voltage as a charge indicator, the batteries have to be completely rested for several hours.
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Old 01-08-2011, 00:27   #9
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One should really make some readings with a hydrometer before drawing any conclusions about a battery charging issue. It's easy and quick and very accurate regarding state of charge. Too bad charging systems can't easily use such a sensor.
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Old 01-08-2011, 03:13   #10
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Re: Batery Monitor vs Voltage

I've noticed that when my batts are topped up and there are no charges other than the engine room verntilation and the displays themselves... the alternator puts out almost no current. But you have to get everything coordinated... that is charge until you batts accept no more amps and then set you monitor at 100%. Make sure when you charge with your engine's alternator all alternate charging sources are off line.
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Old 01-08-2011, 04:35   #11
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Re: Batery Monitor vs Voltage

Quote:
Originally Posted by Maine Sail View Post
Don,

When your batts were taking 8 amps was any other draw on? Ideally you want to check this with no loads and just the alternator or charger feeding the bank. Under 2% acceptance can be considered full when cruising but when really full you can be below .5% acceptance. You were at about 1.7% acceptance. For coastal cruising I would wait till your acceptance bottoms out before re-setting the monitor. Most monitors are factory set to re-set at about 2% but this can be changed. I moved ours down to 1% of bank size..

How did you calibrate the monitor?
...
I have the Victron monitor and it came with default programming and also says in it's instructions that 4% acceptance was fully charged. It automatically did reset to 100% and I also syn'd it that day. If someone has more info on what it should be set to I'm open to it.

It doesn't really matter to me if it is 100% or only 98% SOC. The confusing thing to me was the monitor displayed voltage vs the SOC, with the voltage being way less than what I expected to see based on the SOC (on the monitor digital display).

The Ah out on the monitor did kind of agree with the SOC change and by far the biggest load had been the coffee pot running off the inverter (so a high short load).

Maybe it is something about the newest of the battery bank.

I'm going to boat in the next day or so to do some work and will check the readings again as nothing should be on.
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Old 01-08-2011, 04:45   #12
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Re: Batery Monitor vs Voltage

Yes, you're on the right track!

Be sure to check battery voltage DIRECTLY AT THE TERMINALS with a good digital multimeter, and check agreement with the Victron.

Also, with the multimeter measure the voltage of each of the four batteries DIRECTLY to be sure you don't have a bad or weak cell. Do this with everything turned OFF (charger and all loads).

Let us know what you find.

BTW, here's a chart which gives you some idea of HOW MUCH less capacity new batteries may have as compared to older/exercised/well-treated batteries. Note that the two brand new T-105 windlass batteries don't have as much capacity as the five-year old T-105 house batteries. Hopefully, they will as they get exercised and used.

Bill

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NB> The increase in capacity over time reflected in the house battery bank readings is the result of setting my Victron charger to higher float and repeat absorption voltages about 8 months ago. The Victron charges the house bank batteries only. All others are charged with Iota DLS-45/IQ4 and DLS-55/IQ4 chargers, and all chargers are on 24/7.
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Old 01-08-2011, 12:50   #13
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Re: Batery Monitor vs Voltage

"Breaking In" New Wet Cell Batteries

Link-series Charging Algorithms -- The "Gotcha" Factor!
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Old 01-08-2011, 13:58   #14
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Re: Battery Monitor vs Voltage

Well I was at the boat today to do something else and checked the system. This is after the boat had been sitting for 19 hours with everything off.

On the Monitor:
voltage - 12.55
amps - 0
CE - -8.9 Ah
SOC - 98%

voltage at the batteries with meter - 12.50V

So it looks that everything is right and my problem is just that I'm too used to looking at the voltage on the main panel and will have to become a believer in the SOC display.

Since I don't sit in 1 stop for days with the refrigator running the only way I'm going to get a few cycles run though the batteries is to turn everything on and run the mircowave a few times. Otherwise 30 minutes or so I motor to get in/out of anchorage/mooring field is just going to charge it right back up.

On the other hand I found last weekend that my alternator works a LOT better now that it has a new belt installed!
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Old 03-08-2011, 05:29   #15
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Re: Battery Monitor vs Voltage

Bill,

Check the voltage at the terminal? What is the difference between a multi meter and a link meter which is connected to the terminals?

Granted the link reports to hundredths. But how important would thousandths be?

Is there much loss in the wiring to the link via the harness?

Is the contact of touching a meter probe as good a connection of a screwed down crimped on fitting... the later seems to have more contact.
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