When is battery Full

[Clearsky]

Is there any way to tell using a multimeter if a battery is full?.<br /><br />I have a standard 12V car battery we use as aux power (lights etc) and rather than dig it out each time and put on the charger can I simply check it with a multimeter, and if so what readings would indicate a full charge?<br /><br />Thanks

 

[Triton II]

Re: When is battery Full

Hi clearsky,<br /><br />My Yam digital instruments have a battery voltage indicator which reads between 12.8 and 13 volts when the battery is fully charged (the battery itself has an "eye" which shows a green iris when it's fully charged.) These figures are ignition on, engine stopped.

 

[Clearsky]

Re: When is battery Full

Thanks Triton II,<br /><br />Does your "eye" go off as soon as the charge drops below 12.8-13 volts?. The reason I ask is, I was wondering if the output is fairly constant until it's dead, eg will it pump out 12-13 volts constantly and then suddenly die when flat, or does it slowly go down from 13-12-11-10 etc until it's flat.

 

[ricksrster]

Re: When is battery Full

It goes down gradually, 13-12-11-10-9. Under 9 and things quit working. you can add a selector switch plus an isolater to charge both batteries while you drive your boat. it takes longer that way and you should still recharge after you get home.

 

[Paul Moir]

Re: When is battery Full

10.5v at room temperature and a battery is effectively flat. The eye is a hydrometer which is a much better way to determine charge. I have a bookmark here somewhere... here it is:<br /> http://www.windsun.com/Batteries/Battery_FAQ.htm <br />Have a look at the colourful table about 3/4 of the way down. Naturally you have to let the battery sit a long time after a charge before you can take a reading since charger's "float" will make the battery temporarily read high. So generally the design limit for low voltage on 12v devices is 10v, but of course some work much lower than that.

 

[Texasmark]

Re: When is battery Full

Specific gravity is the actual measurement. We measure volts because it's easy, but the specific gravity determines the acid/water content of the solution which is determined by the charge.<br /><br />Around here I use 12.75 volts as full under standard temp (75 degrees) and pressure (14.7 psig...roughly sea level).<br /><br />Before you make the measurement, you need to have let the battery sit for 24 hrs to stabilize either from charging, or adding water.<br /><br />Other way I do sometimes is watch the charger. When the amperage gets to around 2 amps I consider it full.<br /><br />Mark

 

[Clearsky]

Re: When is battery Full

Thanks everyone, If it drops below the 12.75 volts I'll put it on the charger just to make sure.<br /><br />Grant