Using Voltmeter Readings as indication of Battery Capacity left.......

Have just recently purchashed a couple of plug in type voltmeters to keep eye on my deep cycle battery. In the state that I checked it's workability, about all that would have been draining it at the time would have been an LED indicator on external 12v socket. Consequently this is the lightest load battery will ever have. Turned on the radio and noticed a significant voltage drop whilst it was on, and minor voltage drops when the larger of my LED lights were on(voltage restored to "normal" after items turned off). I am ex Telstra tech of some 39 yrs., so should know better, but can someone tell me at which would be the best time to check voltage to indicate capacity left? & also tell me what said voltages should roughly be to indicate Full, 75%, down to 50% for instance.( will be double checking accuracy of said voltmeters with a multimeter as soon as have the time). Does anyone think leaving voltmeter plugged in would drag much? Also is there a rough way of working out current drag for certain items, using the difference in voltage drop between max voltage and and that indicated when items turned on?

Thx in anticipation
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Reply By: Robin Miller - Monday, Aug 12, 2013 at 20:23

Monday, Aug 12, 2013 at 20:23
Hi Kev

The best time is when the battery has been at rest for a while after either charging or discharge. (Like 1 hour)

However you can get a good indication within 5 minutes of charging if you put you headlights on for a minute then rest for 5

Your battery type will affect the actual readings at full/ half etc and so best do with help of manufacturers data.

For My cal/cal exide I use 12.7 as full and never discharge below 12

What sort of voltmeter ? - if its the plug in cig lighter Jaycar type then they are remarkably accurate even with temp change - they draw under 10ma and can be left in permanently (I have 2 and do this).

I use a clamp type current meter for measuring which means no circuit break needs to be made but also carry a 0.47ohm 10 watt resistor to put in series to measure smaller things.

Robin Miller

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AnswerID: 516316

Follow Up By: Robin Miller - Monday, Aug 12, 2013 at 20:45

Monday, Aug 12, 2013 at 20:45
P.S. One thing you can do to improve the accuracy of using a plug in voltmeter is to put in a socket and wire it permanently straight back to your battery , this prevents every accesory you switch on, or that starts up (e.g. fridge) from causing a voltage drop and giving you false readings.
Robin Miller

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Reply By: Ross M - Monday, Aug 12, 2013 at 20:36

Monday, Aug 12, 2013 at 20:36
I don't think it is an accurate way of telling the capacity level because batteries are all different in their charge capacity, ability deliver and type of battery. You will know hte voltage level but may not know the SOC.

Unless you test ie 2 amp draw for 50hrs on a 100ah battery you won't get much idea of the particular batteries you use.
Also a small load for many hours is quite different to a far heavier load for less but equivalent hours. Both may equate to be the same by maths but the batteries ability and characteristics will dictate what is real life and the resultant state of charge level.

Modern battery level indicators, microprocessor controlled, which read the charge amount input and then the output so it can be compared is Highly "testical" stuff, but even they aren't a real accurate/good indicator.

Once you get to know the readings for your particular circumstances from a known full charge down to 12.1v or thereabouts and the usage which caused it to drop you will be as accurate as the smart metering stuff. You will record typical use won't you?

Why don't you test the volt meter to know how much it draws, then you will know what to expect it to use and if to leave it constantly on or not.

Ross M
AnswerID: 516317

Reply By: Member - Alan H (QLD) - Tuesday, Aug 13, 2013 at 07:59

Tuesday, Aug 13, 2013 at 07:59
Elite monitors

I hear that the Elite monitors have a great reputation and work well to show state of battery. I don't have one but others who swear by them.

Expensive but they work

AnswerID: 516328

Follow Up By: Nomadic Navara - Tuesday, Aug 13, 2013 at 09:06

Tuesday, Aug 13, 2013 at 09:06
Kev, there are a few of that type of monitor. Here is a couple of others. Sample 1 Sample 2

Here is a bit more on Alan's example. What ever model like these suggested ones you may get, make sure it features Peukert's constant in its calculation.

The theory books generally state you should leave a battery rest for 6 - 8 hours before taking voltage readings for best accuracy. We do the following. The cook arises before the sun. She takes a voltage reading by torch light before anything is switched on and the solar commences charging. If someone in your camp is prepared to do that then you should be able to get by without one or the SOC (state of charge) monitors listed above.
Retired radio and electronics technician

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FollowupID: 795657

Reply By: olcoolone - Tuesday, Aug 13, 2013 at 08:33

Tuesday, Aug 13, 2013 at 08:33
This is going to be a BIG thread!

Hope you have your calculator ready...LOL
AnswerID: 516330

Reply By: kev.h - Tuesday, Aug 13, 2013 at 18:19

Tuesday, Aug 13, 2013 at 18:19
Check this site I think this is what you are looking for also google "battery state of charge"
heaps of info there
AnswerID: 516350

Reply By: SuperGrover - Wednesday, Aug 14, 2013 at 10:18

Wednesday, Aug 14, 2013 at 10:18
A member has previously posted a "Temperature Compensated Battery State of Charge (SOC) Table" which I found very useful in determining my battery charge and was easy to use. All readings were taken after four hours without charge/discharge. You may find it with the search function. If I remember it was Sidewinder who posted it but I cannot be sure. Hope this helps.
AnswerID: 516378

Reply By: greybeard - Wednesday, Aug 14, 2013 at 10:46

Wednesday, Aug 14, 2013 at 10:46
Just remember that using battery voltage to determine state of charge is like trying to work out someone's weight by using a tape measure ;)
You're using the wrong measuring instrument and measuring the wrong value. That said, you can use it, under certain conditions, to get an approximation of the state of charge. But that's all it will ever be.

Keep that in mind with the 'advise given' and you'll be ok.
AnswerID: 516379

Reply By: kevmac....(WA) - Thursday, Aug 15, 2013 at 20:44

Thursday, Aug 15, 2013 at 20:44
Would like to thank all the respondents for their input and will soak them all up. At least I now have a better understanding
AnswerID: 516457

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