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Old 12-26-2010, 11:18 AM   #1
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Voltage of a good battery

OK since I decided to read everything I could find on batteries I am hoping I ma not overthinking it, but here is the situation in regard to my house marine batteries, after charging both the volt meter read between 12.4 to 12.5 and the data I have been reading says that a newly charge battery should be showing no less than 13 volts, and if it read even 12.5 it is a bad battery. Beforehand I would have thought if it read 12 volt or more than the battery was still in great shape.

I did a hydrometer [new bulb type] test and it too read in the red after charging, so I am good now to assume that I shod not put these battery back into my rig after rewiring.

I have had luck with Optima gel cell battery so if everyone agree I guess I will buy new new blue tops to replace these wet cell ones.
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Old 12-26-2010, 11:30 AM   #2
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Quote:
Originally Posted by kartvines View Post
OK since I decided to read everything I could find on batteries I am hoping I ma not overthinking it, but here is the situation in regard to my house marine batteries, after charging both the volt meter read between 12.4 to 12.5 and the data I have been reading says that a newly charge battery should be showing no less than 13 volts, and if it read even 12.5 it is a bad battery. Beforehand I would have thought if it read 12 volt or more than the battery was still in great shape.

I did a hydrometer [new bulb type] test and it too read in the red after charging, so I am good now to assume that I shod not put these battery back into my rig after rewiring.

I have had luck with Optima gel cell battery so if everyone agree I guess I will buy new new blue tops to replace these wet cell ones.
Here's a pretty good article : Battery Tutorial

As far as Optima batteries, they can be a great battery, but they have to stay charged. They can be 'brought back from the dead' occasionally, but have to be in otherwise good condition. I have one in a street rod that is about 5 years old and still tests out perfectly, but I do leave a Battery Tender charger on it if the car is not being driven - there are a couple of 'memory' devices that will after a couple of weeks do a pretty good discharge
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Old 12-26-2010, 11:44 AM   #3
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The battery voltage should read about 13.5 volts right after charging unless there is some current being drawn that dissipates the surface charge. After the surface charge dissipates a fully charged battery will read about 12.6 volts.

If the hydrometer reads in the red after temperature compensation then I would replace the battery.
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Old 12-26-2010, 11:53 AM   #4
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I use Optima batteries in my boat and car, but am not sure I would in my RV. The reason being my coach is not always plugged in. I don't think Optimas are known for repeated charging from being well drained. They also cost a lot!
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Old 12-26-2010, 12:19 PM   #5
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Voltage and specific gravity do not tell you how 'good' a battery is - they only tell you the state of charge. Both measures need to be properly interpreted in regards to temperature and recent battery history.

There are two ways to determine the condition of a battery. The traditional method is a load test. Put a decent load (say 60 to 100 watts for a typical RV battery) and watch how the voltage drops over a period of an hour or two. SmartGauge Electronics - battery metering uses this sort of technique. The classic technique puts a bit bigger load on the battery and drains it to a low voltage to determine capacity but this is hard on the batteries.

The other method is called conductance testing. It showed up in the nineties and is less harmful to batteries. The batteryuniversity.com guy's business is this sort of tester.

To get good results, you first need to charge the battery with an intelligent multiple stage battery charger for a day or two (or longer) as it can take 8 to 12 hours to fully charge a lead acid battery. The battery will then have what is called a surface charge and show a voltage more than 13 volts. You have to put a load on it to remove the surface charge and then wait at least a half hour before you test voltage or specific gravity to get a usable measure.

If the battery has a bad cell, its voltage will rapidly drop to below 12 volts under load after charging. Old batteries will also drop in voltage but not so fast or far. New batteries will generally take quite a bit to get them to drop voltage much and they recover voltage faster after resting.

AGM's, like the Optima, are nice batteries but they are still lead acid batteries and pretty much follow the same rules.
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Old 12-26-2010, 02:39 PM   #6
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Originally Posted by Clay L View Post
The battery voltage should read about 13.5 volts right after charging unless there is some current being drawn that dissipates the surface charge. After the surface charge dissipates a fully charged battery will read about 12.6 volts.

If the hydrometer reads in the red after temperature compensation then I would replace the battery.
Right on!

Only related comment I'd add, is that many sub-standard chargers actually aren't capable of fully charging a battery to it's maximum charge capability, as pointed out above, about 13.5 volts or so. The surface charge will rather quickly drop to about 12.6 or so, and SHOULD stay at that level for quite a while, depending on loads applied - when voltage drops below 12 volts, a recharge is needed. And, if the voltage quickly drops to 12 volts or below, even with minimal loads, time for a new battery.

IF a battery is usually only charged to somewhere well below 13.5, it encourages plate sulfation - and unless a higher charge is regularly applied, eventually the battery will become damaged by sulfation, and may well not be able to be fully charged any more.

And yes - a frequent readout with a hydrometer is the best way to determine battery condition when it has been charged as fully as it is capable of.
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Old 12-26-2010, 03:09 PM   #7
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The only thing I might add is that the voltages that are related to the state of charge of a battery are usually "no load" values for a single battery. "No load" meaning nothing connected to the battery terminals.

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Old 12-26-2010, 04:13 PM   #8
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re: "a frequent readout with a hydrometer is the best way"

I disagree with this. Voltage and specific gravity are closely related. Hydrometry is a hazardous business and has a risk of electrolyte contamination. A modern DVM will tell you all you need to know regarding battery state of charge for the typical RV situation and the DVM is much safer to use to get an effective measure.

With either specific gravity or voltage, you need to interpret the measure properly. You cannot just assume the number you get means what you think it does.

And yes, I know there are some folks out there that still use their fingers to tell if a light bulb socket is live or not - that doesn't mean it is a method that should be recommended.
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Old 12-26-2010, 09:22 PM   #9
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Difference in measurements

A Hydrometer is going to tell the densitiy of thefluid in the battery and nothing else.

The densitiy is dependent on the ratios of chemicals in the fluid, these rations depend on the percentag of charge of a NORMAL battery.

Now if the battery is defective due to age or sulfation, then the chemicals in the battery are not correct, if too much water ewas added or if water is needed the readings could be incorrect.

The only way to certify the battery is good is to insure it is fully charged, then place a known load on the battery and measure the time it takes to get to a discharged condition, this will not cause harm to the batery if it is in servicable condition.

One also could estimate conditions via measurements, a battery that has low terminal voltage and will charge at a very low rate (use 30 amp charger and only draws 5 or so) indicates the resistance of the battery is high, the plates may be sulfated and the current may go up if the voltage is kept on the battery.

A fully charged lead acid battery will measure 2.25 volts across each cell while floating, if one pulls the caps you can insert a meter probe into the acid and measure the voltage between cells, all cells should be real close, if one is higher or lower than the rest then the battery should be replaced.

Your voltage is too low, was this measured while charging or after charger was disconnected?

If it was while charging then your output from your charger is too low, if current limited then the battery may be slightly shorted.

You need to confirm the charging current from the charger with an ammeter, it should not exceed 1% of the battery capacity for float service, so about 1 to 2 amps when fully charged.

Next, disconnect the batteries and measure the foltage from the charger, it should be greater than 14 vdc unloaded, this voltage is dependent on the type of charger, it should not be below 13.5 as this is the optimum float voltage for a 12 volt lead acid battery.
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Old 12-26-2010, 11:03 PM   #10
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A Hydrometer is going to tell the densitiy of thefluid in the battery and nothing else.
I personally, have yet to see a battery displaying good hydrometer readings that was in poor condition or under-charged. OR a properly charged one that displayed low hydrometer readings.

Now, maybe I live a sheltered life out of the mainstream, but I still feel that use of a good hydrometer, along with a good multimeter, are 2 basic parts of a pretty dern good battery maintenance team - and far better than just one of them alone...
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Old 12-27-2010, 09:29 AM   #11
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I think an argument can be made that a battery with good hydrometer readings can still have trouble holding a load hence the load test however bad hydrometer readings is a very good indicator of a bad battery.
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Old 12-27-2010, 12:56 PM   #12
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I think an argument can be made that a battery with good hydrometer readings can still have trouble holding a load.
I suppose the argument COULD be made - but in my 74 years, *I'VE* never seen it happen...

USUALLY, a battery with internal problems will ALSO show low hydrometer readings - rare exceptions? Possibly - but nothing I have seen as common or typical!

Still, the multimeter is my first testing tool - probably because I use it so commonly in my other electronics interests...
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Old 12-27-2010, 02:09 PM   #13
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Specific gravity is the ratio of sulfuric acid to water, where water has a specific gravity of 1.0. It's an indication of how much sulfuric acid is in solution. The lower the specific gravity of the electrolyte the less sulfuric acid is in solution with the water meaning sulfation has occurred. That's why the recommended time to test the battery's specific gravity is after charging.


Sulfation is the normal accumulation of a soft pasty substance called lead sulfate accumulating on the plates as the battery is discharged. The lead sulfate is still soft immediately after discharge and can revert back to lead and sulfuric acid if the battery is immediately recharged. If the battery is not immediately recharge the lead sulfate will crystallize and the sulfation is permanent.


Permanent sulfation prevents a charge from reaching the plates reducing the charge on the plates where it accumulates. Consequently after charging, the battery voltage will indicate its fully charged but will perform at a reduced capacity, making the battery act as a much smaller battery than its rating.
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Old 12-27-2010, 04:36 PM   #14
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Before you buy new batteries be sure your charger is bulk charging to 14.8 volts (for lead acid battery) before going into absorbtion mode. If not then you are not fully charging the batteries and the new ones will not get fully charged either. Poor charging kills batteries faster than neglect. Automobiles have always charged to 14.8V . Get a volt meter and check it out. I do this as Trojan reccommends to.
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