# Understanding battery hydrometer readings



## Graystone (May 1, 2008)

I have 2 marine starting batteries in my boat. Both assumed to be the same age at about 3 years. I keep them connected to a maintainer charger when not in use which is 99% of the time.

With a hydrometer I measure the specific gravity at each of the 6 cells as:

battery #1
1.275 - 1.285 - 1.285 - 1.285 - 1.285 - 1.275

battery #2
1.265 - 1.275 - 1.285 - 1.285 - 1.280 - 1.265

Does this look o.k.?

The instructions provided with the hydrometer are written for a brewer.

Thanks


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## Bluecometk (Jun 20, 2009)

Let me see if I can remember this. 1.265 is a fully charged battery at 70 degrees Fahrenheit. As temp goes up from that point the specific gravity will increase. I canât remember the ratio to temp but if your batteries are at 70 degrees or above and showing those numbers you are ok. The only thing to worry about is if those numbers deviate from each cell to the next. That in turn would show a bad cell.

State of Charge Specific Gravity voltage 

Battery volts 12V 6V 
100% 1.265 12.7 6.3 
*75% 1.225 12.4 6.2 
50% 1.190 12.2 6.1 
25% 1.155 12.0 6.0 
Discharged 1.120 11.9 6.0


I hope this helps.
These are just my opinions

Bluecometk


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## TnAndy (Sep 15, 2005)

Blue has it nailed......nothing more to say.


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## Graystone (May 1, 2008)

Thanks for the replies Bluecometk & TnAndy.

Graystone


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## wy_white_wolf (Oct 14, 2004)

How long had those batteries be off the charger when those readings were taken?

Batteries should sit at rest (no charge or load) for at least 2 before taking readings. If they had you might want to run a equalizing charge on battery 2 and recheck (just for reference).

edited to add:

The end cells have most likely lost a little capacity. That is not usual as they are exposed to the temperature changes more than the middle cells. But still nothing to worry about if they do the job you want. An equalizing charge might help to restore them.


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## Graystone (May 1, 2008)

wy_white_wolf -both batteries where off the charger for 24 hours when I tested them. Both are back on charge and I plan on retesting this Saturday.


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## adamtheha (Mar 14, 2007)

I have 8 large deep cycle Surrettes that I ran hard for about 6 months, weren't always charged or equalized properly. I figured that they were heavily sulfated, and I followed the manufacturers instructions to get them back into health. 
Has anyone had experience reviving a sulfated deep cycle? What was the result?
No matter what I do, I can't seem to get the SGrav up to the green mark (notwithstanding that the batteries were quite warm after equalizing for almost 14 hours in a stretch.)
I now equalize every two weeks, and the batteries stay over the 80% full mark almost all the time. Should I equalize the crap out of them a few more times, or are they permanently damaged? They were brand new, still less than 1 year old.


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