Voltage Charts / Hydrometer

[Joshua]

I want to measure my battery banks SOC as accurately as possible from Voltage readings.

 

I have a couple of Voltage / SOC charts but there is quite a discrepancy between them.

 

Is there a well-respected chart out there that has proven itself?

 

I also want to buy a good hydrometer (one that gives accurate and actual specific gravity readings rather than just traffic light indicators) can anyone recommend one?

 

Many Thanks.

 

Joshua

Edited August 29, 2012 by Joshua

[Keeping Up]

I've got at least three with wildly different figures, so I plotted them all on the same chart and each time now I just select the one that gives me the answer I was secretly hoping for

[RLWP]

I've got at least three with wildly different figures, so I plotted them all on the same chart and each time now I just select the one that gives me the answer I was secretly hoping for

 

= it doesn't work?

 

Richard

[Gibbo]

Can you post the ones you have?

 

The reasons for the differences are many. If you get one for the African market it will be different than one for the Canadian market (different temperature ranges so different acid strength). Different battery types (wet cell antimony, calcium, AGM, gel etc) have different voltages. Some graphs are shown under load, some are shown open circuit. For northern european wet cells. open circuit, it is:-

 

12.7 = 100%

12.6 = 90%

12.5 = 80%

12.4 = 70%

12.3 = 60%

12.2 = 50%

12.1 = 40%

12.0 = 30%

11.9 = 20%

11.8 = 10%

11.7 = 0%

 

The bottom end of this table (below about 30%) can vary quite a lot between different batteries.

 

You need a rest period (no charge/discharge at all) of at least 4 hours and preferably much longer for these readings to be even remotely usefull. Some people will tell you a shorter period will suffice. They are wrong. I have a normal leisure battery here and after a full charge it takes over 3 days for the voltage to settle to the true voltage.

 

PS. It takes skill and practice to use a bulb type hydrometer. No one seems to believe that either. It's easy to prove. Take 10 readings, one after the other. No two will be the same.

Edited August 29, 2012 by Gibbo

[Keeping Up]

= it doesn't work?

 

Richard

 

I should have made it clear, I meant 3 charts not hydrometers (I have one of those but I'm having some trouble using it on my sealed batteries)

[RLWP]

I should have made it clear, I meant 3 charts not hydrometers (I have one of those but I'm having some trouble using it on my sealed batteries)

 

I got that part. It was the 'what answer would you like' bit that suggested to me that it doesn't really work

 

Richard

[Keeping Up]

I got that part. It was the 'what answer would you like' bit that suggested to me that it doesn't really work

 

Richard

 

On the contrary, I regard 'giving me the answer I was hoping for' as working perfectly

[Grace and Favour]

On the contrary, I regard 'giving me the answer I was hoping for' as working perfectly

 

I love your logic - - - I keep looking for weather forecast services that service my requirements in a similar manner

 

I'm currently using the forecast for the Sahara - -

[larkshall]

I love your logic - - - I keep looking for weather forecast services that service my requirements in a similar manner

 

I'm currently using the forecast for the Sahara - -

 

Its raining militants there

[colin stone]

I want to measure my battery banks SOC as accurately as possible from Voltage readings.

 

I have a couple of Voltage / SOC charts but there is quite a discrepancy between them.

 

Is there a well-respected chart out there that has proven itself?

 

I also want to buy a good hydrometer (one that gives accurate and actual specific gravity readings rather than just traffic light indicators) can anyone recommend one?

 

Many Thanks.

 

Joshua

 

This is quite a good article with charts - www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf

 

I found an Optilux hydrometer on Ebay which is temp compensated. Also called Hella Hydrovolt - http://www.autocomp.ru/catalogue/accessories/400-73/areometry/areometr-hella-hydro-volt.html

 

Where one would find one now, I do not know.

Edited August 29, 2012 by colin stone

[luctor et emergo]

Can you post the ones you have?

 

The reasons for the differences are many. If you get one for the African market it will be different than one for the Canadian market (different temperature ranges so different acid strength). Different battery types (wet cell antimony, calcium, AGM, gel etc) have different voltages. Some graphs are shown under load, some are shown open circuit. For northern european wet cells. open circuit, it is:-

 

12.7 = 100%

12.6 = 90%

12.5 = 80%

12.4 = 70%

12.3 = 60%

12.2 = 50%

12.1 = 40%

12.0 = 30%

11.9 = 20%

11.8 = 10%

11.7 = 0%

 

The bottom end of this table (below about 30%) can vary quite a lot between different batteries.

 

You need a rest period (no charge/discharge at all) of at least 4 hours and preferably much longer for these readings to be even remotely usefull. Some people will tell you a shorter period will suffice. They are wrong. I have a normal leisure battery here and after a full charge it takes over 3 days for the voltage to settle to the true voltage.

 

PS. It takes skill and practice to use a bulb type hydrometer. No one seems to believe that either. It's easy to prove. Take 10 readings, one after the other. No two will be the same.

 

I believe that. I have found widely varying readings with my hydrometer, some indicating that a batt is utterly death, when in actual fact the battery charges, and holds charge for several days. Likewise two batts indicated in the green on all cells, but when isolated, would not even take a charge, let alone hold it...

 

I now have two working batteries, and have trown my multi-meter and hydro-meter in the canal, and simply burn some incence at an image of Gibbo, which I have positioned over my battery bank. It appears to work...

[smileypete]

I also want to buy a good hydrometer (one that gives accurate and actual specific gravity readings rather than just traffic light indicators) can anyone recommend one?

One from with a scaled glass float from one of the established brands should do, maybe best avoid the el-cheapo tool brands though.

 

They're best for comparative readings though, between cells of a batt, or batts in a bank, or to when the batt was new.

 

They can help to show up batt problems, bank wiring problems, declining batt condition, or need for better charging and equalisation.

 

As long as they're all reasonably inline after a full charge, and not way out of line after a discharge to around 50%, things should be fine.

 

Edit:

Some have a rudimentary form of temp compensation in that the float is part glass and part plastic. If the float is a different temp to the batt acid, this may explain why the readings come up different, but they should stabilise as the temp of the float does.

 

cheers, Pete.

~smpt~

Edited August 29, 2012 by smileypete

[Pretty Funked Up]

[jonathanA]

here's a SG chart i use, I'm sure I lifted it off this site, so apologies if someone recognises it as their own work...

 

 

State of Charge Specific Gravity

100% charged 1.265

75% charged 1.239

50% charged 1.2

25% charged 1.17

Fully discharged 1.11

subtract 0.004 for each 5°C above or add 0.004 for each 5°C below 25°C. Do this for each of the cells of the battery.

 

 

One tip when using the standard bulb type hydrometers that my old physics teacher Mr Limb taught me, is to flick the thing with your finger to shake off any bubbles and make sure the float isn't sticking to the side...

 

I used the same technique for checking the two banks of washing machine sized 2v cells used by the GPO in my youth every month....

[Peter-Bullfinch]

Gibbo, thanks for posting your list....very useful. We have copied it down to have it handy. I had been previously misled by my motorhoming brother who told me that I should never let my battery bank get below 12.5. My battery bank gets down to 12.3 after a couple or so days and the fridge still operates. We rarely stop that long however. Our fridge is the heaviest consumer nowadays. We recently had the ten rectangular old hungry fluorescent type light fittings modified to have lovely frugal LED strips (3 to each unit). This seems to help in keeping up our battery charge obviously. The only time we ever use the inverter these days is for my wife's hair straighteners!

[bottle]

You were not misled, if you do not let your batteries go below 12.5 that is better than not letting them go below 12.3 but not better than, not letting them go below 12.7.

 

Battery SOC and usage is a compromise and only you can decide the levels you wish to maintain.

 

The 50% SOC/DOD is only a guide, for probably the best compromise.

[Pretty Funked Up]

One tip when using the standard bulb type hydrometers..., is to flick the thing with your finger to shake off any bubbles and make sure the float isn't sticking to the side.

 

why didnt I think of that.

 

 

you get an idea of capacity from sg vs voltage too

Edited August 29, 2012 by Pretty Funked Up

[Gibbo]

The normal bulb ones are actually really difficult to use properly.

 

As JonathanA pointed out bubbles make a bit of a mess. Bubbles will make the SG reading higher than reality. Conversely, acid drops on the top of the float (above the liquid level) will make the SG reading lower than reality. The amount of the error is much greater than one would expect.

 

Also, they tend to stick on the sides of the tube.

 

You also really need to stir the electrolyte up in the battery first, which is a bit difficult.

 

It really does take quite a bit of practice to get consistent readings.

 

Worse, the SG reading won't tell you the state of charge unless you know the state of health. It won't tell you the state of health unless you know the state of charge. As PFU alluded to.

[ditchcrawler]

I use to like the ones that worked on light refracting ( hope that's the right word) through the liquid which you look through like a telescope. Not going to google them as I have a slow connection tonight at Nassington on the Nene

[Pretty Funked Up]

The normal bulb ones are actually really difficult to use properly.

 

so true. you need to take the reading at least 3 times and average that.

 

Those opitical ones seem like a much better idea but i've not tried one.

[Gibbo]

I use to like the ones that worked on light refracting ( hope that's the right word) through the liquid which you look through like a telescope. Not going to google them as I have a slow connection tonight at Nassington on the Nene

 

I've heard good reports of those but never tried one. Aren't they more a bit "lab equipment" than "summat fo' mi boat"?

 

so true. you need to take the reading at least 3 times and average that.

 

I once got someone who swore by them to take 6 consecutive readings. Every one was completely different.

[Pretty Funked Up]

I've heard good reports of those but never tried one. Aren't they more a bit "lab equipment" than "summat fo' mi boat"?

 

I've seen em in use (modern heating engineers 'aka plumbers' carry em these days).

 

Battery acid up to the eye doesnt sound very bright but if used propery i cant see it being a problem.

Edited August 29, 2012 by Pretty Funked Up

[Joshua]

Can you post the ones you have?

 

The reasons for the differences are many. If you get one for the African market it will be different than one for the Canadian market (different temperature ranges so different acid strength). Different battery types (wet cell antimony, calcium, AGM, gel etc) have different voltages. Some graphs are shown under load, some are shown open circuit. For northern european wet cells. open circuit, it is:-

 

12.7 = 100%

12.6 = 90%

12.5 = 80%

12.4 = 70%

12.3 = 60%

12.2 = 50%

12.1 = 40%

12.0 = 30%

11.9 = 20%

11.8 = 10%

11.7 = 0%

 

The bottom end of this table (below about 30%) can vary quite a lot between different batteries.

 

You need a rest period (no charge/discharge at all) of at least 4 hours and preferably much longer for these readings to be even remotely usefull. Some people will tell you a shorter period will suffice. They are wrong. I have a normal leisure battery here and after a full charge it takes over 3 days for the voltage to settle to the true voltage.

 

PS. It takes skill and practice to use a bulb type hydrometer. No one seems to believe that either. It's easy to prove. Take 10 readings, one after the other. No two will be the same.

 

Many thanks for that chart.

 

 

The two I have are as follows:

 

 

Chart 1 Chart 2

 

 

100% 12.70 / 12.60+

 

95% 12.64

 

90% 12.58 / 12.50

 

85% 12.52

 

80% 12.46 / 12.42

 

75% 12.40

 

70% 12.36 / 12.32

 

65% 12.32

 

60% 12.28 / 12.20

 

55% 12.24

 

50% 12.20 / 12.06

 

40% 12.12 / 11.90

 

30% 12.04 / 11.75

 

20% 11.98 / 11.58

 

10% 11.94 / 11.31

 

0% 10.50

 

As you can see, at 12.20 one reads this as 50% the other 60% SOC.

 

This morning, I completed the little test recommended by grahame r in an earlier thread ([1] fully charge [2] discharge to reasonable indicated SOC [3] note indicated amp use [4] read open circuit Voltage [5] back calculate to get true battery bank capacity) from this I calculate my actual capacity is about 525ah rather than the 660ah I had assumed (6 x 110 ah 4 week old batteries).

 

I have now programmed this into my Bat. Mon. and have begun a new discharge test.

 

I have a feeling this will solve the problem of the discrepancy between my indicated SOC and actual voltage readings, time will tell.

 

You have, however, raised another doubt in my mind, namely the accuracy of my voltage readings, I only let the bank rest for about 2 hours, I was actually amazed how much they recovered in that time from the reading I had taken with just the inverter on and nothing else.

 

With inverter on and drawing an indicated 1.9 a and an indicated SOC of 81% my voltage read 12.14, with everything off for 2 hours this increased to 12.28!

 

 

 

One question, is it normal for a labelled battery bank of 660ah (6 x 110 ah open wet cell batteries) to provide only 525ah or so?

 

 

Joshua

Edited August 29, 2012 by Joshua

[Gibbo]

One question, is it normal for a labelled battery bank of 660ah (6 x 110 ah open wet cell batteries) to provide only 525ah or so?

 

After a few months (or even weeks or days if you've been really abusive) use, in a word, yes.

 

Last year, whilst running some tests, I bought a brand new 90Ahr battery on tuesday. By wednesday it was a 60Ahr battery. By thursday is it didn't work at all.

 

PS. Your first figures for voltage look much closer to reality than the second figures which appear, to me at least, to have come from someone's imagination.

Edited August 29, 2012 by Gibbo

[dmr]

It sounds like you are getting serious about batteries and so next time you get new ones it might be worth getting something like Trojans.

Trojan publish figures for their voltage and specific gravity against state of charge which is a huge help.

100% = 1.277 sg

 

With practice it is possible to get some fairly accurate specific gravity measurements with a hydrometer, though you will have to measure the temperature and apply the correction. I have found it very interesting to discover just how quickly the cells get out of balance, and specific gravity starts to fall (sulphation), during the winter when we struggle to keep the batteries fully charged.

 

I can't recommend any particularity hydrometer. I got a cheap one that was rubbish, and then an even cheaper one that was spot on.

One possible problem is that the paper scale can sometimes move within the float if the hydrometer is handled roughly during use or shipping.

 

Messing with acid is a risky business, its very easy to put holes in your clothes!

 

...............Dave