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post #1 of 11 Old 09-05-2012 Thread Starter
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Stupid state of charge question

Up until now, I have only been day sailing. I have a starter battery and a house battery. They both get plenty of charge motoring out and back into the harbor every time I sail. So, I have never worried much about the state of charge. Now that I am planning overnights, and weekends, what would be a simple way to monitor my batteries? I understand resting voltage would be too unreliable (don't want to rest battery for 24 hours). I may get a real battery monitor, but, don't want to buy one yet.

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post #2 of 11 Old 09-05-2012
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Re: Stupid state of charge question

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Up until now, I have only been day sailing. I have a starter battery and a house battery. They both get plenty of charge motoring out and back into the harbor every time I sail. So, I have never worried much about the state of charge. Now that I am planning overnights, and weekends, what would be a simple way to monitor my batteries? I understand resting voltage would be too unreliable (don't want to rest battery for 24 hours). I may get a real battery monitor, but, don't want to buy one yet.
Lead acid use a hydrometer- available at local auto store for less than $5
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post #3 of 11 Old 09-05-2012
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Re: Stupid state of charge question

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Lead acid use a hydrometer- available at local auto store for less than $5
Please don't do this unless you want lots of holes in your clothes... A hydrometer needs "rest" for the electrolyte just as a resting OCV reading does. The vast majority of people taking SG readings are doing it incorrectly and with a tool akin to a pitchfork being used for brain surgery. The best use for an SG reading is to determine cell balance but again a quality tool should be used to get anything even close to accurate. I've yet to see any $5.00 hydrometer be consistent or accurate. Keeping the batteries closed and free of external contamination will generally serve you better than taking SG readings any more than once per year to determine cell balance and the need for equalization.

An OCV reading will tell you what you need to know but ideally it should be "resting" and done with an accurate volt meter for the best accuracy. How long to rest will depend upon temp and the battery type. I have had some batteries that don't hit resting voltage for over 40 hours and some hit it in 8 hours in warmer temps..

There are a number of ways to determine state of charge without a battery monitor and using voltage but they require a good sense of your bank and you'll need to do some homework with your bank for it to be close to accurate. With each 0.1V equaling 10% of the banks capacity the accuracy of the voltage readings is key.

Also keep in mind that no amount of SG, OCV readings or battery monitor can tell you the "capacity" the bank has left in it. You can have a perfect SG or OCV reading for 100% charged 100AH battery but the battery may only have a capacity of 25 Ah's due to age and condition. Battery monitors are also not foolproof and they require an educated user and programmer to make them give you the most accurate data they can. They are anything but "plug & play"...

If you set yourself up with a "cut off" for charging that keeps you above 50% SOC and gets you back to 80-85% when off cruising that is all that really matters.
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Last edited by Maine Sail; 09-05-2012 at 04:15 PM.
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post #4 of 11 Old 09-05-2012 Thread Starter
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Re: Stupid state of charge question

So, in the real world, what did cruisers do b/f fancy battery monitoring systems? Just run your stuff, and if the lights start getting dim, run the motor more the next day? (I have found that most solutions for being fugal are found by looking back at what people did 30-40 years ago)

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post #5 of 11 Old 09-05-2012
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Re: Stupid state of charge question

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Please don't do this unless you want lots of holes in your clothes... A hydrometer needs "rest" for the electrolyte just as a resting OCV reading does. The vast majority of people taking SG readings are doing it incorrectly and with a tool akin to a pitchfork being used for brain surgery. The best use for an SG reading is to determine cell balance but again a quality tool should be used to get anything even close to accurate. I've yet to see any $5.00 hydrometer be consistent or accurate. Keeping the batteries closed and free of external contamination will generally serve you better than taking SG readings any more than once per year to determine cell balance and the need for equalization.

An OCV reading will tell you what you need to know but ideally it should be "resting" and done with an accurate volt meter for the best accuracy. How long to rest will depend upon temp and the battery type. I have had some batteries that don't hit resting voltage for over 40 hours and some hit it in 8 hours in warmer temps..

There are a number of ways to determine state of charge without a battery monitor and using voltage but they require a good sense of your bank and you'll need to do some homework with your bank for it to be close to accurate. With each 0.1V equaling 10% of the banks capacity the accuracy of the voltage readings is key.

Also keep in mind that no amount of SG, OCV readings or battery monitor can tell you the "capacity" the bank has left in it. You can have a perfect SG or OCV reading for 100% charged 100AH battery but the battery may only have a capacity of 25 Ah's due to age and condition. Battery monitors are also not foolproof and they require an educated user and programmer to make them give you the most accurate data they can. They are anything but "plug & play"...

If you set yourself up with a "cut off" for charging that keeps you above 50% SOC and gets you back to 80-85% when off cruising that is all that really matters.
Been using hydrometer for 35 years and yet to get a hole in me clothes. They are actually pretty acurate (even a $5 unit, as all they are based on is the weight of an equal sized ball) and used on the most high tech marine vessels to check battery charge and cell equalization. It is all in technique I think.
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Re: Stupid state of charge question

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So, in the real world, what did cruisers do b/f fancy battery monitoring systems? Just run your stuff, and if the lights start getting dim, run the motor more the next day? (I have found that most solutions for being fugal are found by looking back at what people did 30-40 years ago)
Good point. With some of the responses you wonder how we ever survived the trip from cave man to the susposed modern human race we now are.
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Re: Stupid state of charge question

"Please don't do this unless you want lots of holes in your clothes..."
So it ain't just me!

And considering that you need to let batteries stand so the electrolyte can equalize, which is the exact same process and reason that you let them stand before using a voltmeter, what a pointless invitation to ruin. Especially if that one drop that splashes off the hyrometer gets in your EYE because, of course, you do wear eye protection whenever you're screwing around with acid, right?

Barquito, you can get a battery deep-cycle tester under $50 on Amazon, it will tell you roughly how many AH in capacity are left in the batteries. Or just get a CALIBRATED multimeter, and read the voltage. If you've just been charging, throw a load on 'em for two minutes before you read the voltage. Will that be perfect? No, but it will be perfectly adequate for your needs.
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post #8 of 11 Old 09-05-2012
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Re: Stupid state of charge question

Use a digital voltmeter like this CLICKY

A switch like this clicky

Wire it to both batts. Power the voltmeter throught the switch so it is only energised when reading the batts. Center off push one way to read one batt other way for the other batt.

All you really need.

NB an expanded scale analogue meter will do as well.
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Re: Stupid state of charge question

"Use a digital voltmeter like this CLICKY"
Caveat emptor. I ordered a couple like that last year for a project, with similar high accuracy, and one says 14.3 when the other says 14.4, so I know there's something not right. Haven't had a chance to check yet if it is sloppy calibration or simply not accurate as quoted, but I've also seen them read high enough to mean there's an outright bad regulator if they're right. Oneathesedays, I'll get a chance to throw the good meter in and see where the problem is.
Caveat emptor.
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post #10 of 11 Old 09-05-2012 Thread Starter
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Re: Stupid state of charge question

Quote:
Use a digital voltmeter like this CLICKY

A switch like this clicky

Wire it to both batts. Power the voltmeter throught the switch so it is only energised when reading the batts. Center off push one way to read one batt other way for the other batt.

All you really need.
That is exactly what I was thinking I would do. However, on looking into this, I'm not sure I could get a meaningful reading. Would it be good enough information to shut the solar panel off, put a load on for a couple minutes, then check the voltage? Or, would that still be too variable? I really just need a ball-park reading to see if I am closer to 50% SoC or 90% SoC.

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