How do I tell if my battery is charged?

[KrillBee]

I have an old Sears battery charger. It charges at 6 amps and 12 volts. It has a meter that reads 0-8 DC amps. I plugged in my car battery due to some car startup behavior that lead me to believe it was low on juice (I don't drive much). Initially it was at about 2 amps. From what I understand, the meter is supposed to get closer to 0 the more it gets charged. It got down to about 1.8 amps after 2-3 hours of charging and I shut it off because I found my voltmeter and wanted to see what my battery tested as on that. My battery showed about 13 volts.

Which meter do I trust, the amp meter on the charger or my handy volt meter?

 

[thescreensavers]

How old is the battery?

 

[KrillBee]

How old is the battery?

2.5 years

 

[Eli]

A fully charged lead acid battery will read ~13.2V. This should settle down to ~12.7V after the surface charge is shed.

Those old types of chargers aren't intelligent, and indeed rely on the batteries tendancy to draw less current as it becomes charged.

Throw on your headlights, and measure the voltage with them on. That will tell you quickly the condition of your battery.

Anything less than 12.2V with your lights on, and your battery either needs to be charged, or isn't holding it's full charge.

 

[LTC8K6]

Yes, put a load on it and measure the voltage.

Or drive over to an auto zone or advance auto and they'll check it for free for you.

 

[cardiac]

This is one of the tools I seem to use the most. Everyone always wants to borrow it. For $20 it does come in handy:

http://www.harborfreight.com/automo...6-volt-12-volt-battery-load-tester-90636.html

Load tests the battery and measures alternator output.

Bob

 

[exdeath]

The ammeter on the charger will show 0 (or very close to it) when the battery is fully charged, as that means there is no long current flowing into the battery.

 

[exdeath]

This is one of the tools I seem to use the most. Everyone always wants to borrow it. For $20 it does come in handy:

http://www.harborfreight.com/automo...6-volt-12-volt-battery-load-tester-90636.html

Load tests the battery and measures alternator output.

Bob

Yup, you know it's the right tool for seriously testing a charging system when 75% of it's surface is covered in vent holes and heat sinks