Hydrometers, how do you use them

Submitted: Thursday, Jan 27, 2005 at 18:36
ThreadID: 19830 Views:3639 Replies:10 FollowUps:11
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On the advice of forumites I bought an hydrometer to test an old wet cell DC I have lying around. The markings range from 1100 to 1300. The garduations from 1100 to 1200 are red and read recharge, 1200 to 1250 read fair, and 1250 to 1300 read good.

The battery has had a solid charge and after settling for 24 hours is showing 12.42 volts on a voltmeter and the hydrometer readings on each cell are 1225, 1230, 1210, 1225, 1225 1220. These readings all fall into the "fair" category after a full charge suggesting to me it is not in real good nick, but not stuffed.

I have now attached a thermo cooler which draws 4 amps and will run it for 4 hours, let the batt settle again and take more readings. Then I'll add INOX and do it all again.

The guts of it is exactly what do these numbers mean? What does 1220 or 1275 represent. The bloody thing had no useful instructions.

Any help would be greatly appreciated.


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Reply By: Member - Mike H (VIC) - Thursday, Jan 27, 2005 at 19:02

Thursday, Jan 27, 2005 at 19:02
G'day Jim,
the numbers represent "Specifig Graphity"

Here are tow links that explain about Hydrometers,
Cheer, Mike
AnswerID: 95194

Follow Up By: Member - Mike H (VIC) - Thursday, Jan 27, 2005 at 19:06

Thursday, Jan 27, 2005 at 19:06
oops, i stuffed up :-)

link #1

link #2

FollowupID: 353993

Follow Up By: Member - Jimbo (VIC) - Thursday, Jan 27, 2005 at 21:34

Thursday, Jan 27, 2005 at 21:34
Thanks Mike.
FollowupID: 354034

Reply By: Bonz (Vic) - Thursday, Jan 27, 2005 at 19:11

Thursday, Jan 27, 2005 at 19:11
I would be looking for around 1280 from a fully charged in good condition battery. It refers to specific gravity and that about all I know. We still measure our batteries in Substations with hydrometers and they hold 1250-1280 after three years.
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AnswerID: 95196

Follow Up By: Member - Jimbo (VIC) - Thursday, Jan 27, 2005 at 21:36

Thursday, Jan 27, 2005 at 21:36
So Geoff,

Does this mean under thes figures the batt is on the way out?



PS See you at Easter?
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Follow Up By: Bonz (Vic) - Thursday, Jan 27, 2005 at 21:45

Thursday, Jan 27, 2005 at 21:45
Well if we got figures like that it meant we kept a closer eye on them, once it got to the lower "fair" figures the battes were replaced, but we were relying on them to switch off faults and things.

Theyre not on their way out but theyre sure making for the door in my opinion, how long till they go is anyones guess. My guess'd be 12 months? Depends on the figures you get, keep a log of each cell and readings and see how fast they deteriorate.

We head off to the Simpson at Easter, so we'll do Murray Sunset but not Corrying.
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Reply By: cokeaddict - Thursday, Jan 27, 2005 at 19:18

Thursday, Jan 27, 2005 at 19:18
Im not expert but i was surfing at the time i read your question and this is what i found....
Using a hydrometer to measure the specific gravity of the electrolyte. The more charge on the battery, the more sulfuric acid there is in the electrolyte and the higher the hydrometer float will be on the scale. A fully charged battery has a specific gravity or about 1.265

Specific gravity means exact weight. A "Hydrometer" or a "Refractometer" compares the exact weight of electrolyte with that of water. Strong electrolyte in a charged battery is heavier than weak electrolyte in a discharged battery. By weight, the electrolyte in a fully charged battery is about 36% acid and 64% water. The specific gravity of water is 1.000. The acid is 1.835 times heavier than water, so its specific gravity is 1.835. The electrolyte mixture of water and acid has a specific gravity of 1.270, usually stated as "twelve and seventy."

Variation in specific gravity among cells cannot vary more than 0.050. The variance is the difference between the lowest cell and the highest cell. A battery must be condemned for excessive cell variation if more that 0.050. In the example below, the highest SG reading is cell #1 (shown in green) while the lowest SG reading is cell #5 (shown in blue); the difference is 0.070 which requires battery replacement. Cell #5 if failing.

Cell #1 Cell #2 Cell #3 Cell #4 Cell #5 Cell #6
1.260 1.230 1.240 1.220 1.190 1.250

Many factors contribute to cell variation; for example, if water was just added to that cell, the cell is then diluted with water resulting is a lower specific gravity reading. Recharging the battery would correct this false reading. In some cases if a battery that has cell variation slightly over the specification and is only about 50% charge, charging the battery at a slow rate of charge (5A) may reduce the cell variation, thus saving the battery.

By measuring the specific gravity of the electrolyte, you can tell if the battery is fully charged, requires charging, or must be replaced. It can tell you if the battery is sufficiently charged for a capacity (heavy-load) test. The battery must be at least 75% charged to perform a heavy load test. (The heavy load test will be discussed later). In other words, each cell must have a specific gravity of 1.230 or higher to proceed.

1.270 100%
1.230 75%
1.190 50%
1.145 25%
1.100 0%

If the battery is less than 75% charged, it must be fully recharged before proceeding. If the battery is 75% or higher proceed to a heavy load test. A battery not sufficiently charged will fail because it is discharged.

Fairdinkum mate ...it just goes on and on and on....hope that helps
AnswerID: 95200

Follow Up By: Member - Jimbo (VIC) - Thursday, Jan 27, 2005 at 21:31

Thursday, Jan 27, 2005 at 21:31
Thanks Ange for your detailed response and the time you have taken.

I now have a better understanding, due to your effort.


FollowupID: 354032

Reply By: tonysmc - Thursday, Jan 27, 2005 at 19:22

Thursday, Jan 27, 2005 at 19:22
You can also use your hydrometer to see how much alcohol is in your home brew! Tony
AnswerID: 95206

Follow Up By: Member - Jimbo (VIC) - Thursday, Jan 27, 2005 at 21:38

Thursday, Jan 27, 2005 at 21:38
That much I do know LOL.
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Reply By: Member - John C (QLD) - Thursday, Jan 27, 2005 at 19:22

Thursday, Jan 27, 2005 at 19:22
Just bought one also, says this on the back.

Charge scale:

1.265 = 100%
1.245 = 75%
1.190 = 50%
1.155 = 25%
Below 1.155 = 0%

AnswerID: 95207

Follow Up By: Member - Jimbo (VIC) - Thursday, Jan 27, 2005 at 21:33

Thursday, Jan 27, 2005 at 21:33
Thanks John,

That helps.


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Reply By: Member - 'Lucy' - Thursday, Jan 27, 2005 at 22:48

Thursday, Jan 27, 2005 at 22:48
Evening Jimbo.

Mate I respectfully suggest that you get your son-in-laws GMC gennie in the first instance.

Strap,clip,attach the 12V outlet to the subject battery and fire up the gennie in the second.

Take out the cell caps and watch the show begin.

It will start to bubble after about 5mins and if you test it you will find it is pumping in anything between 14 - 16 volts.

Maaaaaaaaaaaaaaaate the white build up crappola on the plates disappears and the battery starts thinking along the lines of a 'teenager' again.

Good idea to turn it off at this point and continue with a regulated battery charger, however this depends on the the individual battery being ' rejuvenated'.

The end result is just phenomenol.

I rejuvenated a Deep Cycle wet cell battery in this manner about two weeks ago..

It didn't matter if it failed as it was on the way out anyway.

Holds 12.8V - 13.00V now.


Ken Robinson
AnswerID: 95276

Follow Up By: Chucky - Friday, Jan 28, 2005 at 10:59

Friday, Jan 28, 2005 at 10:59
Sounds like a good way to create thermal runaway to me.

Nothing like a lead acid battery explosion to start the day. Have seen it happen on a boat when someone tried something similar, wasn't very pretty.
FollowupID: 354106

Follow Up By: Member - 'Lucy' - Friday, Jan 28, 2005 at 12:50

Friday, Jan 28, 2005 at 12:50
There is nothing quite like a little forced re-juvenating

However I wouldn't want a 'mini Chernobyl' in the garage either.

So I'll stick to the the 5min quick Zap.

When I was kid in the goldfields of Western Australia, there used to be a practise on the mine sites of quick chatging the batteries in some of the trucks with an arc welder.

That was exciting to say the least, especially when the sparks fired off.
FollowupID: 354124

Reply By: Member - Sand Man (SA) - Friday, Jan 28, 2005 at 00:33

Friday, Jan 28, 2005 at 00:33
As one of the guilty party that advised you to buy a hydrometer, here is another reference from the Windsun WEB site.

State of Charge
State of charge, or conversely, the depth of discharge (DOD) can be determined by measuring the voltage and/or the specific gravity of the acid with a hydrometer. This will NOT tell you how good (capacity in AH) the battery condition is - only a sustained load test can do that. Voltage on a fully charged battery will read 2.12 to 2.15 volts per cell, or 12.7 volts for a 12 volt battery. At 50% the reading will be 2.03 VPC (Volts Per Cell), and at 0% will be 1.75 VPC or less. Specific gravity will be about 1.265 for a fully charged cell, and 1.13 or less for a totally discharged cell. This can vary with battery types and brands somewhat - when you buy new batteries you should charge them up and let them sit for a while, then take a reference measurement. Many batteries are sealed, and hydrometer reading cannot be taken, so you must rely on voltage. Hydrometer readings may not tell the whole story, as it takes a while for the acid to get mixed up in wet cells. If measured right after charging, you might see 1.27 at the top of the cell, even though it is much less at the bottom. This does not apply to gelled or AGM batteries.


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AnswerID: 95294

Reply By: Wok - Friday, Jan 28, 2005 at 06:51

Friday, Jan 28, 2005 at 06:51

Becareful about using 'general values' for specific-gravity readings

eg: Exide Extreme : full charge = 1.275 = 12.80V [o/c]
..............................50% = 1.230 = 12.35V [o/c]

These values are from the Exide chemist, as you can see they don't match values quoted elsewhere in this thread. Maybe an idea to contact your battery manufacturer for specific values?

Another poster has outlined Equalisation [I do it once a year..BUT I LEAVE THE CAPS ON] You need to keep a record of the SG from each cell to track the performance of your battery.

AnswerID: 95306

Follow Up By: Stew53 - Friday, Jan 28, 2005 at 07:32

Friday, Jan 28, 2005 at 07:32
At work my Auto electrician told me that he was never able to charge battery’s up so that the hydrometer was in the green (not sure of the SG), over the years we had changed from Caterpillar battery’s to just about every brand name under the sun and had not found a battery that was better than the rest, although we had never changed our acid supplier or type. I investigated further and found that the acid we were being supplied was still being supplied by Caterpillar and what Caterpillar called tropical strength and was 30% Sulphuric acid, the then supplier of battery’s was Yuasa, so I spoke to their technical people and they suggested that since the days when we first started using the cat battery’s, battery’s had come a long way and they were stronger and less prone to exploding in the tropics and it was suggested that we use 37% Sulphuric acid although we only went to the 35% as it is the common blend and more readily available, ( as an example the tech rep told me that in some motor bikes to keep the battery’s small Sulphuric acid up to 70% is used and there were all types of combinations in between). The battery’s now charge up into the green and we get better performance from the battery’s and they last longer. We also changed acid suppliers and we now get a carton of 4 by 5 liter bottle of acid for the same price as 1, 5 liter bottle from Cat. I suppose I told the group this so that you are aware that not all acid is the same concentration and different combinations give different results.
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Follow Up By: Wok - Friday, Jan 28, 2005 at 08:25

Friday, Jan 28, 2005 at 08:25

Thanks for bringing this point up, explains the variation in SG values quoted all round the world!

I ignore the colour bands, just use the the SG values given by the manufacturer. Do you charge[ie acid fill] the Cats when you need them? Thats a good way...guarantees a fresh battery everytime.

This 35% blend is used in the tropics? What performance would you expect if this battery were to used in say Tassie?

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Reply By: Stew53 - Friday, Jan 28, 2005 at 17:57

Friday, Jan 28, 2005 at 17:57
AnswerID: 95411

Reply By: Stew53 - Friday, Jan 28, 2005 at 18:21

Friday, Jan 28, 2005 at 18:21
Yes we get the battery's and acid from the warehouse on site (if their not nil stock, that’s an insiders joke for all the miners out there) fill them up with acid then install them, I've had the Auto lecy install some solar panel on the roof of their shed and run some wires to a positive and negative bus bar through a BP regulator, if we get the chance we will make some battery's up of all sizes and have them sitting on the solar power until they are required, usually no longer than a couple of days, works well.
As for the acid type why don’t you ring Yuasa / Century head office and ask for some technical help, they were very helpful for me, years back.
AnswerID: 95415

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