Battery voltage conversion to % charged while lightly loaded at say 5a

[GordMay]

Originally Posted by btrayfors

While I generally agree with that, Gord, I believe that there is nevertheless value in such a table IF the user knows his batteries, his measurement devices, and his boat ...

... Bottom line: if you know your boat, your voltage measurement equipment, etc., then measured voltage can be a good indicator of SOC.

Bill

Even so, his chart won't represent (or even approximate) any other application.
A rested open-circuit voltage chart, on the other hand, approximates all batteries, of a type.

[sailorboy1]

Originally Posted by btrayfors

If it did, it means one of three things:

1. your voltage measuring device is way off;
2. you didn't measure voltage directly at the batteries; or
3. those batteries are shot.

FWIW,

Bill

measured directly on the battery, all measurements taken from the battery monitor, batteries less than I year old and working great

while there was less than 1 amp outgoing at the time, the frig had been cycling on/off all day (takes about 3 amps)

now I turn off the frig for a couple of hours and only use the stereo at that 1 amp I would get a different voltage reading and a chart would be closer

the point is if you are on your boat using stuff, unless you turn all your loads off charts of charge verse voltage is useless

[btrayfors]

Originally Posted by Don Lucas

measured directly on the battery, all measurements taken from the battery monitor, batteries less than I year old and working great

while there was less than 1 amp outgoing at the time, the frig had been cycling on/off all day (takes about 3 amps)

now I turn off the frig for a couple of hours and only use the stereo at that 1 amp I would get a different voltage reading and a chart would be closer

the point is if you are on your boat using stuff, unless you turn all your loads off charts of charge verse voltage is useless

Taking voltage readings off the monitor MAY be OK, so long as it is connected properly. Much better to take them DIRECTLY from the batteries, though, meaning you put the meter probes directly onto the battery terminals.

If you really measured 12.1 volts after that light load regime, and if the batteries are in good condition (age doesn't matter....it's perfectly possible to kill good batteries in less than one year--ask me how I know:-), then....
the batteries weren't fully charged to begin with and they were at about 50% SOC (irrespective of what the battery monitor said).

I've seen so many cases where these wonderful little devices tell erroneous stories, because they're either not connected properly or not calibrated and re-calibrated as the batteries age and lose capacity.

Bill

[btrayfors]

Originally Posted by hellosailor

Bill-
"especially since many battery monitors are uncalibrated " You mean, a Xantrex or Victron (or worse yet, an MPPT controller with integrated display and monitor) can't be trusted any more than the rest? (I haven't had a chance to take the calibration source out exploring.)
And I couldn't help noticing your test chart on the echo-charge seems to indicate that it reduces voltage (to the starting battery) about .4-.5v compared to the house battery voltage?? Shouldn't that be a Very Bad Thing?

Assume that remark re: battery monitors was tongue-in-cheek...yes, they often are misleading.

Won't get into the EchoCharge thing here...too much thread drift already...but the EchoCharge is fine. The start battery was fully charged.

Bill

[Maine Sail]

Originally Posted by hellosailor

Gord, there's always a value in q&d ballpark calculations. That's all the voltage is supposed to be.

As to temperature compensation, sure, that's important too when you're not sailing at 20C/68F. But when was the last time you saw a temperature compensated hydrometer, than automatically took the temperature into account? Right then, same shortcoming as voltmeters. You still have to know how to use them, still have to make the same compensation. Still have to wait for the electrolyte to stir itself up and unstratify.

There's more than one way to turn a cat into furry mittens.

Bill-
"especially since many battery monitors are uncalibrated " You mean, a Xantrex or Victron (or worse yet, an MPPT controller with integrated display and monitor) can't be trusted any more than the rest? (I haven't had a chance to take the calibration source out exploring.)
And I couldn't help noticing your test chart on the echo-charge seems to indicate that it reduces voltage (to the starting battery) about .4-.5v compared to the house battery voltage?? Shouldn't that be a Very Bad Thing?

The Echo's do have a volt drop in them. This is actually not necessarily a bad thing because a start battery is normally at 98-99% state of charge and has no real need to be pegged at the 14.4V limit of the Echo while the house is in absorption..... As you can see you realy need to push a house bank voltage to get the Echo to its max of 14.4V..

[Maine Sail]

Originally Posted by btrayfors

I've seen so many cases where these wonderful little devices tell erroneous stories, because they're either not connected properly or not calibrated and re-calibrated as the batteries age and lose capacity.

Bill

Bingo.. It's rarely the monitor and usually wiring, lack of proper programming or lack of manual re-sets when known full. The factory defaults are usually very dumbed down..

[sailorboy1]

since I never gave a voltage reading I don't know how you all are getting to all these "facts", I didn't even record voltage when I was recording data over 24 hours the other day

nothing wrong with my installation or monitor programing, the SOC and CE (amps out) match pretty much, amps into the batteries when charging match what would be expected based on the SOC, there is no indication of anything problem in my system and the monitor is tracking well based on everything I have seen

just because there was only 1 amp out at the time it doesn't mean there weren't a lot more 10-20 minutes prior, I was using my boat normally and probably had charged my lap top that hour

don't really know how this became that there was something wrong with me or my system when all I said was that voltage isn't a reliable indication of charge when you are on the boat using your systems

[btrayfors]

Don,

Don't be so sensitive....much of the discussion above is not about you! It's about the issue of whether or not accurate voltage measurements can be a reliable indicator of SOC. I maintain that they certainly can, given a knowledgeable owner, accurate measuring instrument and technique, and someone familiar with their own boat, equipment, batteries, etc.

Now, about you: what I said was that if you are seeing 12.1VDC after only a light discharge, SOMETHING IS WRONG. It is.

If you were here I could tell you in 5 minutes what's wrong. But, maybe up there in NH the laws of chemistry and physics work differently :-)

Bill

[sailorboy1]

Originally Posted by btrayfors

.

Now, about you: what I said was that if you are seeing 12.1VDC after only a light discharge, SOMETHING IS WRONG. It is.


If you were here I could tell you in 5 minutes what's wrong. But, maybe up there in NH the laws of chemistry and physics work differently :-)

When did I say my voltage was 12.1? Didn't just say I didn't record voltage?

I like how you start making qualification statements while knowing nothing about my qualifications. Nothing is wrong with my system!

[Maine Sail]

Originally Posted by Don Lucas

That $120 would almost get you a monitor!

Far as using voltage as an indicator under small loads as an indication of charge: on the boat this weekend I was logging SOC, amp ahours out, voltage, amps; and with less than 1 amp the voltage would indicate the battery was less than 50% charge when it was in the 80s. So to me unless the load has been off the voltage is not worth much as 1 amp was enough to effect the voltage reading.

This is where it does not make much sense and where I suspect Bill was going with it.

50% SOC is roughly 12.1 V (resting) and I am sure Bill extrapolated the 12.1V from your 1A load and 50% statement above to the 12.1V.... A 1A load on a healthy bank, supposedly in the 80% SOC range, should not be at 12.1V with a 1A load unless the bank is teeeny tiny... I think that is why Bill is saying something is wrong and based on the above sentence you wrote....

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

[Jbaffoh]

Originally Posted by Don Lucas

When did I say my voltage was 12.1? Didn't just say I didn't record voltage?

I like how you start making qualification statements while knowing nothing about my qualifications. Nothing is wrong with my system!

Didn't you say the measured voltage indicated you had a SOC of less than 50%? I think most people would interpret that as a measurement of 12.1 volts, or less.

[Jbaffoh]

Sorry for cross-posting. Typing on an iPhone so it's difficult to see recent comments while typing.

[sailorboy1]

what I said was that using voltage and a chart one would believe I was at 50% SOC, my bateries have never been below 80% SOC in their lifetime

Mainesail didn't you post somewhere a whole thread about taking voltages and RE resting batteries

I'm sure if I only ran my radio for a couple of hours the voltage reading would be close to a chart SOC and match my monitor, in fact I know this for a fact as I check all this when I first get to the boat! After that I don't really believe most of what my volt neter says.

are you guys trying to get jobs as news reporters

other than that you guys "win"

[hellosailor]

Don, do you think the folks at Deka / East Penn are intentionally lying, or simply mislead, when they say:

How can you tell if an
VRLA battery is fully charged?
By using a voltmeter.

Open Circuit Voltage vs. State of Charge Comparison*
% Open Circuit Voltage Charge - Flooded Gel AGM
100 - 12.60 or higher 12.85 or higher 12.80 or higher
75 - 12.40 12.65 12.60
50 - 12.20 12.35 12.30
25 - 12.00 12.00 12.00
0 - 11.80 11.80 11.80
NOTE: Divide values in half for 6-volt batteries.
* The “true” O.C.V. of a battery can only be determined after the battery
has been removed from the load (charge or discharge) for 24 hours.

Lyon Station, PA 19536-0147 • Phone: 610-682-6361 • Fax: 610-682-4781
Order Department Hotline: 610-682-4231
EAST PENN manufacturing co., inc.: Lead-Acid Batteries; Battery Manufacturers; Thousands of Different Types of Batteries, Cable & Wire Product
E-mail: eastpenn@eastpenn-deka.com
E.P.M. Form No. 0139 Rev. 5/07
© 2007 by EPM Printed in U.S.A.

(Sorry about the columns, that's the web)

Your hydrometer will be equally inaccurate if the electrolyte hasn't been allowed to circulate and equalize. Or needs topping up.

[Dreaming Yachtsman]

All this battery theory is great and ideally a Xantrax or equivalent monitor would be convenient for keeping track of battery SOC. However, my boat came with a digital voltmeter (VM), digital ammeter (AM), and a selector switch to choose port or starboard battery and I have never seen a strong need to add more instrumentation. When I want to check the SOC of the house bank, I either wait until the refrigerator and freezer have both cycled off on their own or open their breakers on the DC panel immediately below the VM and AM, wait 30 seconds for the voltage to stabilize (usually rises ~ 0.1-0.2 VDC), then read the voltage. If less than 12.3 V, I start the generator.

In response to the OP, when I can't unload the battery (e.g. underway with navigation instruments energized), I adjust the meter reading by adding 0.2 VDC for each 5A of load to compensate for (I*I)*R voltage loss in the battery cables. This correction factor is based on empirical observations with fridge and freezer, each of which draws about 5 amps.

This is one of those cases where perfect is the enemy of good enough.