Again from annother forum------
-2009
Maine Sail
Swab
Join Date: Feb 2009
Posts: 189
Measuring Battery State of Charge / Volts
I have seen the discussion many times here where folks say they monitor their batteries state of charge via a volt meter. While this can be fairly accurate when the battery has been allowed to come to a resting voltage, the reality is that when you are on the boat and cruising, attaining a 12+ hour resting voltage can be very difficult.
I wanted to do a photo/time/voltage experiment to illustrate why using a simple voltage test on a boat may not be the most accurate way of checking the resting voltage of a battery especially with short resting times.
In order to check the state of charge (SOC) on a lead acid battery the battery should have rested, meaning no input voltage/amps or output/loads, not even the stereo memory, for a considerable amount of time. If the battery is not allowed to rest, and you have recently run the engine, or had a load turned on, the SOC will not be accurate for many hours.
To conduct this illustration I used a bench charger, stop watch and a DVM. I set my camera up on the tripod and photographed the battery over 24 hours (actually I still have 12 hours to go as of this posting).
The battery, a Trojan group 24 deep cycle, had been on my bench and drained to about a 60% SOC. It then sat for a week before I took my baseline SOC reading. The resting voltage was 12.24 volts or about 60% of charge according to Trojan Battery. I then plugged in the 20 amp charger for about 130 seconds and then unplugged it.
This was only 130 seconds of charging and it affected the resting SOC for at least 11 hours and was off by about 10% even after sitting for 11+ hours.
Battery SOC at Resting Voltage and 80f (Source Trojan Battery)
100% = 12.73
90% = 12.62
80% = 12.50
70% = 12.37
60% = 12.24
50% = 12.10
40% = 11.96
30% = 11.81
20% = 11.66
10% = 11.51
Resting voltage Fri 10:04 P.M. / 12.24 Volts:
Charge Applied Fri 10:05:42 P.M. Voltage rises quickly to 13.97 volts:
By 10:05:56, or within 14 seconds, the voltage was already 14.3 volts:
I had unplugged the charge at roughly 130 seconds and the voltage was already dropping back to 13.45 volts at 10:07:15 which was 1:57 seconds from where I turned on the charger:
I took the next photo on Saturday at 2:13 A.M. a little over four hours after turning off the 130 second charge. The voltage still read 12.44 volts or roughly 76% SOC:
Unfortunately I fell asleep and did not take the next photo until Saturday morning at 8:42 A.M. when the voltage still read 12.38 volts or roughly 70% SOC. Based on my calculations the battery should still be at about 12.24-12.25 resting based on 130 seconds of charging and what went into it:
Here's the photo from the 12+ hour mark, 10:26 A.M. Saturday, still reading approx 70% SOC:
Here's the photo after 24 hours rest. The battery is back to just about where I had calculated it would be. As of this morning at roughly 34 hours it was still at 12.26 volts and seems to have finally attained a resting voltage:
If you were to run your engine before bed at say 10:00 P.M. then wake up and take an SOC reading at 8:45 A.M. you could be off by as much as 10% in the wrong direction thinking you are at a higher SOC when in reality you are as much as 10% lower. If that lower range is 60% you only have 10% bank capacity not 20% before you hit the 50% re-charge threshold.
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-Maine Sail
Casco Bay, ME
1979 CS-36T
My Sailing Photo Galleries (LINK)
04-06-2009
Camaraderie
Founder Emeritus!
Join Date: Feb 2009
Location: East Coast US
Posts: 3,174
Why is Maine's Post Important?
I received a question asking about why Mainesails post was important.
It is a great piece of work and let me try to explain the implications for cruising sailors that don't have a lot of battery knowledge.
It is really for cruising sailors (and that includes weekenders!) who need to KNOW what charge their batteries have, and when they need to re-charge, and how to measure the % of charge.
To begin with, ANY 12V battery will have a fully charged voltage reading of right about 12.7V.
You can't get "fuller" than that as it is 100% full.
Now...when you use a charger to charge a battery up to 100% it will start out charging at 14.5V or so and gradually taper down to 13.3V when the battery is full. The funny thing is that when you remove the charger from the battery and take a voltage reading on the battery an hour later...the battery alone will read 13.3V...even though 12.7 if FULL. This is a FALSE reading and not REAL battery capacity...just a surface charge left on the battery plates from the charger. You can get rid of this surface charge by placing a heavy load on the battery for a few minutes...and THEN take a reading after the surface charge has been removed.
OK so far? Hope so.
Now cruisers KNOW that once their batteries reach 12.2V they are 50% discharged and need to be re-charged or their battery lifetime will be reduced. So...it is very important to know WHEN you've hit 12.2V AND that your voltmeter is giving a TRUE reading of the actual battery voltage and NOT some voltage mixed with a surface charge.
MaineSails little experiment has proven that even a very brief charge to a 12V battery will leave a VERY long lasting (up to 24 hours) false reading on a voltmeter. And this could cause you to slowly destroy your batteries by letting them go too far discharged without knowing it. Example: If you had a battery that was REALLY 50% discharged...but got a little "sun" from a small solar panel...that battery would still be showing it was nearly full at 10PM at night while you burned the lights and ran the TV or stereo and drove the battery into deep discharge!
You see...his experiment proved that even with only 1/2 of an amp/hour being put back into a 100 amp/hour 50% discharged battery by a charger, an alternator or a solar panel or wind vane...the batery voltages readings will indicate that the battery does NOT need to be charged until a full 24 hours have passed and a true reading is once again obtained.
Of course...anyone cruising cannot wait 24 hours to use their batteries. They need to KNOW when the batteries need charging AND they need to run the stuff on their boat like knotmeters and reading lamps!
MaineSails point is that it is highly unlikely that the voltage reading you are taking from your batery indicate anything of value (unless the battery is dead!
)
Cruisers had better get a real amp/hour meter and not rely on a voltmeter if they don't want to murder their batteries.
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04-06-2009
Camaraderie
Founder Emeritus!
Join Date: Feb 2009
Location: East Coast US
Posts: 3,174
The next question in the original thread that arose was...so how do I monitor battery state of charge without a voltmeter?
The traditional answer is with a Xantrex link monitor...since West Marine sells them. I've used them with great success but the cheapest one today is $349 bucks!
Mainesail has found another one that actually does more than the Links do...and for only $199....the Victron BMV-600. Great find!!
This product will measure AMPS both in and out of the battery as well as AMPhours remaining in the battery and the TIME you have remaining on the battery at the present amp usage! It will also sound an alarm at low battery and give you voltage readings.
It is quite easy to hook up. It comes packed with a shunt that looks like this:
You simply move ALL cables off the negative side of your battery and attach them to the bolt on one side of the shunt. Then you make up a new battery cable that goes from the other bolt on the shunt to the negative terminal on the battery. A couple of other cables are included with the monitor and they just plug in as directed. Simple and easy and no more worries about how much juice you have left!
Read more about it or read the installation manual here:
http://www.victronenergy.com/battery...and%20bmv-602/
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04-06-2009
Izzy1414
Mast Man
Join Date: Mar 2009
Location: Grants Pass, OR
Posts: 548
My post was more or less asking to take the experiment one more step to see how puting a load on the battery after charging to see how that affected the time it took to get to the rested voltage state. My thinking was that for those of us who only have volt meters, maybe the reading might be more accurate with the surface charge removed. That's important in the real world situation (at least mine) because I usually run electrical systems right up until we leave the boat and would like to know the general state of the batteries then instead next weekend when I get back to the boat. BTW, Maine Sail, I forgot in my original post, THANK YOU for your excellent work and post.
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04-06-2009
Giulietta
PARTY HOST!!!
Join Date: Feb 2009
Location: Portugal
Posts: 5,505
Quote:
Originally Posted by
SCOTTY
what size is your alternator?
BLUE!!
and you dont want to mix old and new batteries. what i would do would be to take one of the old ones and use that as a starter battery. that is if they are a combo type or a starting battery. if they are deep cycle only then starting using them can be rough on them.
Car batteries from the local supermarket 50 Euros each..but I went a mile further...got the ones that are sealed...
if you start from scratch i would get a battery for a 4 cylinder car ( i dont know if american sizes are the same as over there ) and hook that to the 1 position. then get 2 80 or even better 110 amp batteries for the house load on position 2.
Our cars have same size batteries, they just go faster and longer than yours and look better too...but if I have already 2, I just get a new 80 for the engine, no?
as for the genny, for 10 days on the hook charging your battery 4 hours a day on a honda 1000 I genny you might burn 1.5 gallons of gas in 10 days. if you get a much bigger charger, ie a 80 amp one, you could probably only run it 1.5 to 2 hours a day. if you get a honda 2000 i you might be able to run the air conditioning if you have it
even 1 hour on the diesel is too much..$8.00 a gallon, plus the inconveniece of wearing the engine and all...
Thankst.
04-07-2009
eMKay
Big Freakin' Lake Sailor
Join Date: Feb 2009
Location: Buffalo, NY
Posts: 684
04-07-2009
Maine Sail
Swab
Join Date: Feb 2009
Posts: 189
Did that
Quote:
Originally Posted by
eMKay 
Put a load on that battery for about 30 seconds after your charge experiment, I would be willing to bet you 5 bucks it goes back down to the real state of charge. Try it out.
Did that still gave a false reading when compared to a 24 hour rest. I don't know what the magic time/amp draw would be to get you there.
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-Maine Sail
Casco Bay, ME
1979 CS-36T
04-07-2009
Camaraderie
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Join Date: Feb 2009
Location: East Coast US
Posts: 3,174
Conventional wisdom seems to be a load of 1/3 the battery capacity for 5 minutes removes any surface charge. I have no idea whether this is true. But for a 100amp hour battery this would mean a 33 amp load and at the end of 5 minutes you would have depleted the actual battery by 3% more (3 amp/hours) while removing the surface charge. Would a 16.5 amp load for 10 minutes do the same...or an 8 amp load for 20 minutes? Dunno.
I guess this could be tested with a known state of charge...a brief charging event like in Maines experiment and a Link type monitor hooked up to do the measurements.
Perhaps Maine can tackle this one NEXT winter!
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04-07-2009
eMKay
Big Freakin' Lake Sailor
Join Date: Feb 2009
Location: Buffalo, NY
Posts: 684
Quote:
Originally Posted by
Maine Sail
Did that still gave a false reading when compared to a 24 hour rest. I don't know what the magic time/amp draw would be to get you there.
Cool. How much off was it? That's my way of testing batteries at work, but I don't have to get it perfect. It does explain why our automatic tester is usually wrong. We have to use that tester to generate warranty claims, and a lot of time it shows a battery as good when I know it ain't. On cars that don't need a warranty claim I just use my voltmeter.
Last edited by eMKay; 04-07-2009 at 10:14 PM.
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04-07-2009
sailaway21
Tactician
Join Date: Feb 2009
Location: SW Michigan
Posts: 3,307
04-07-2009
Camaraderie
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Sway...there is a lag time on SG rise and fall so you do have to wait a bit after charging or after discharge loads to allow the diffusion of the H2SO4 to fully take place. I have heard to wait from 20 minutes to an hour. (Obviously you cant do it if you just added water either...do a charge cycle)
For the benefit of the thread...here's a specific gravity chartlet:
1.265 100%
1.225 75%
1.190 50%
1.155 25%
1.120 0%
This is at 80 degrees a 4 for every 10 degrees over that and subtract 4 for every 10 degrees under that.
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07-21-2009
tachyon46
Fluctuat nec mergitur
Join Date: Jul 2009
Location: Long Island
Posts: 44
Quote:
Originally Posted by
Camaraderie
The next question in the original thread that arose was...so how do I monitor battery state of charge without a voltmeter?
The traditional answer is with a Xantrex link monitor...since West Marine sells them. I've used them with great success but the cheapest one today is $349 bucks! Mainesail has found another one that actually does more than the Links do...and for only $199....the Victron BMV-600. Great find!!
This product will measure AMPS both in and out of the battery as well as AMPhours remaining in the battery and the TIME you have remaining on the battery at the present amp usage! It will also sound an alarm at low battery and give you voltage readings.
07-21-2009
Camaraderie
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Posts: 3,174
Tachyon... I actually like the approach of the Battery Bug for some things and I would think it would be of particular interest to those with small boats and relatively small battery banks. Note that despite the single link you provided...there is a different Bug for different capacity batteries UP to 300ah's and prices range from $79 to $169.
They are to be installed on each battery which makes them affordable only for small systems and they cannot be used over 300ah's.
In terms of function...the one "plus" they bring is to measure the batteries ACTUAL decrease in capacity over time...so you can tell that your 100ah battery when fully charged now only holds 70ah's. A very nice feature.
On the other hand...it does not tell you how many amps you are using NOW or how many amps you have put back in with solar panels or alternator, or how many hours you have left at the current rate of usage.
Both meters provide voltage readings and a good indication of remaining amp hours available.
The BB MUST be attached to the battery terminals directly...which may make monitoring it a bit of a pain.
For someone wanting to take care of their battery and monitor capacity and not on a big budget...the BB sounds like a good value.
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07-22-2009
tachyon46
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Join Date: Jul 2009
Location: Long Island
Posts: 44
Quote:
Originally Posted by
Camaraderie
Tachyon... I actually like the approach of the Battery Bug for some things and I would think it would be of particular interest to those with small boats and relatively small battery banks.
<--snip-->
The BB MUST be attached to the battery terminals directly...which may make monitoring it a bit of a pain.
FWIW -- I sent off an email query to Argusanalalyzers.com (manufacturer of the Bug) regarding the direct mounting constraint. I was told this is due to the calibrated resistance in the leads, but that they are working on a model with leads long enough to allow it to be mounted in a more convenient location.
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Well that was what I was doing until Walt suggested to me that my two batteries might/would? charge faster if I had the switch on "both".
Now I have a charging system using the car alternator that has a potential of putting out 60 amps if the batteries would accept that much whereas the outboard alternator can only put out 6 amps max. Is that the reason you suggested that Walt?
I use to charge the batteries one at a time with the alternator watching their voltage with a ....................
http://purplesagetradingpost.com/sumner/macgregor/inside-12.html
........................digital meter that gives voltage to a tenth of a volt. When I would get one up to 14.1 to 14.3 with the alternator I would switch to the other. So on the last trip of 18 days I would charge them both at the same time and it seemed to be faster and I can see the logic as the alternator had the ability to supply all of the current either one could take at any moment of charge. One factor that stopped this from being a fair test with what I did before is that I had added the 40 watt solar panel, so the batteries weren't run down as much as before. Sometimes though I wouldn't charge them every day and then they were lower. With the solar panel and charging like this my charging times running the generator went down from about 40 minutes a day to about 15 to 20 on the average.
Now I use a lot of electricity with the CPAP machine I run at night, about 36 to 45 amp/hours a day. Our other main user is the depth finder that we run all the time we are underway. I don't know at this point what it uses, but will once I add my amp meter to the system.
Since I now charge both batteries at the same time I use them usually at the same time. I figure it is better on them to not draw them down as far as using them by themselves. If I check them separately with the volt meter they still are never exactly the same voltage, but only vary by .1 to .2 volts. They are different ages and different manufactures and sizes.
Now that I use the system like this maybe I'll try two 6 volt batteries like Walt in the future.
Rob I don't have a generator that runs a battery charger, the system charges the batteries directly with the alternator. This is more efficient than running a generator that then powers a battery charger.
I'm about ready to order a second solar panel. It will be at least another 40 watt and I think I might be able to add an 80 watt. An 80 watt is only a little over $200 now. We are going to add a frig/freeze that runs with a real compressor to the boat and do away with the ice problem on our long trips. That is one reason to increase the solar panels.
c ya,
Sum
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