Lead-Acid Battery charge question

[Hughman]

I'm using a 12v. deep cycle battery to run the bilge pump, that I remove from the boat to recharge using a 110v. multicharger.

I have been testing the battery with a specific gravity tester* (eyedropper with 4 floating balls ), and a voltmeter.

The voltmeter reads 16 Volts, but the specific gravity tester indicates 25% charge: The battery reaches full charge as indicated by the specific gravity tester in 12-16 hours more at a charge rate of 10 amps.

Here's the question: My backup battery has sealed cells- How can I determine the charge level without testing specific gravity?

*Edit: this gadget is called a hydrometer. smile.gif

[ 08-11-2004, 01:40 PM: Message edited by: Hughman ]

[nedL]

Interesting question. I don't know how you can. I have one of those "four floating balls" type of hydrometer. It took me years of searching to find a real hydrometer (glass vial w/paper scale inside so you can read the actual specific gravity - something like 1.150 dead, up to 1.300 fully charged). I finally found one at a tag sale. smile.gif I'll be curious what sort of answers come up.

[Fitz]

Home brew stores sell hydrometers.

[Tom M.]

Remove the battery from the charger, let it sit for a few hours, then read voltage. Your 25% charged battery will probably read something like 12 volts or a little under. Fully charged, and at rest (meaning not charged or discharged for a few hours), your battery should read about 12.6 volts.

You can do the same with a sealed lead acid battery.

Lead acid battery voltage makes wild swings when charging or discharging. So you can't trust the voltage reading during those times.

In my opinion, the best way to charge any lead acid battery is with automatic charger, which basically means it will stop putting amps in the battery when its full. Its a little more complicated than that, but not much smile.gif

[Lone Star]

Take a look at this...

State of Charge (http://www.homepower.com/files/battvoltandsoc.pdf)

[Klaus]

Just in case you did not know this, use a digital voltmeter for the suggested measurement above. The old pointer type multimeters are useless for detecting these small differences.
Digital multimeters are cheap nowadays and the cheapest will do the trick.
Klaus

[tidmarsh]

Originally posted by Fitz:
Home brew stores sell hydrometers.Most homebrew hydrometers only read up to about 1.100--too low the even register a dead battery, much less tell when you've reached a full charge @ 1.300.

Most beers start out at 1.040 to 1.060 and ferment down to 1.005 to 1.010--that's a very different range of SG than battery electrolyte, according to nedL's range of 1.150 to 1.300.

[nedL]

This had me curious about how a battery hydrometer matched up to a home brew one. Thank you.
The battery hydrometers, in addition to indicating the specific gravity also indicate levels of charge on the scale. Then there are also the antifreeze hydrometers, they still seem to be available though.

[nedL]

I just did a bit of looking & it appears that battery hydrometers are still available (at least on ebay). http://i7.ebayimg.com/01/i/02/0c/54/42_1_b.JPG

[Alan Peck]

Hughman: I doubt if you would have to go to EBAY to find a good hydrometer with a proper calibrated float.

Most decent auto parts stores, such as Auto Zone carry them.

[Chris Coose]

Thanks for all your replies. This forum is amazing!

Hydrometers are avaliable, I think I bought 3 the last time for about a buck. They break easily.

Now I got to buy a digital voltmeter.....