battery questions
Thread Starter
Drifting
Joined: Apr 2002
Posts: 1,720
Likes: 19
From: Manhattan Beach Ca
battery questions
Car had to be jumped so I measured voltage at 14 something when running. When engine shut off it read 11 volts and wouldnt start again. Double checked alt. by pulling off neg while running and it kept running so knew most likely battery. Went and got a new one.
Now, laying both side by side I tested voltage and both new and old read the same, around 13 volts. Stuck new one in and fired right up.
My question is why do the amps break down if voltage remains the same? I'm obviously not an EE. THX
Now, laying both side by side I tested voltage and both new and old read the same, around 13 volts. Stuck new one in and fired right up.
My question is why do the amps break down if voltage remains the same? I'm obviously not an EE. THX
Burning Brakes
Joined: Jul 2001
Posts: 854
Likes: 0
From: Foxfield, Co
The only true test of a battery is to put a load on it. I bet if you measured the voltage of the batteries during starter cranking the bad battery voltage would drop way down, while a good battery wouldnt drop down much at all. You can see this by turning on your headlights. They will stay on during cranking with a good battery and go dim or out during cranking with a bad battery.
Race Director
Joined: Nov 1999
Posts: 10,643
Likes: 8
From: Foxfield CO 1970 Convertible
Think of this - surface area is related to the amperage, and the number of plates (regardless of size) is related to the voltage. Even a tiny battery can put out 9 volts if it has the correct number of plates (surfaces). The big plates in a car battery are what provide the surface area for more amps. As the battery gets old, the plates get either eaten away or clogged and covered with deposits which leaves a smaller surface area exposed and therefore less amperage.
So a clogged battery with gobs of deposits will still put out 12 volts or so, but since the surface area is smaller, the amps are reduced. The reason your voltage was less with the old battery was probably related to bad connections, corroded cable, or bad ground. You probably got good connections when you installed the new battery.
So a clogged battery with gobs of deposits will still put out 12 volts or so, but since the surface area is smaller, the amps are reduced. The reason your voltage was less with the old battery was probably related to bad connections, corroded cable, or bad ground. You probably got good connections when you installed the new battery.
Le Mans Master
Joined: Sep 2000
Posts: 5,635
Likes: 4
From: Orlando Floriduh
Lead acid batteries produce 2.2 volts per cells so the 12 Volt battery commonly used in cars actually has 13.2 volts. Now the bigger the battery the more AMPS it produces. When it runs down there is voltage but no Amperes so it may light a flashlight bulb just fine but not start a car.
Instructor
Joined: May 2004
Posts: 199
Likes: 0
From: Santa Monica CA
Ohm's Law, to answer your question. V=IR
Voltage, V, is a potential. It represents the difference in charge between the positive and negative terminals. Molecularly, this is an imbalance of electrons. So electrons want to flow from electron rich (negative) to electron poor (positive) places, which we describe as current, I. How much resistance they encounter along the way is R.
Voltage is fixed on a battery, and the more resistance you have in the circuit, the less current will flow.
Dave
Voltage, V, is a potential. It represents the difference in charge between the positive and negative terminals. Molecularly, this is an imbalance of electrons. So electrons want to flow from electron rich (negative) to electron poor (positive) places, which we describe as current, I. How much resistance they encounter along the way is R.
Voltage is fixed on a battery, and the more resistance you have in the circuit, the less current will flow.
Dave
Instructor
Joined: Sep 2004
Posts: 168
Likes: 0
From: Houston Texas
I've found that one of the best ways to check a battery is to charge it then use a specific gravity meter. A bad battery will show a bad cell. The weak cell will show a lower charge level than the others. I weak cell means repalce the battery. It will never be useful. Be careful though. This doesn't work well if you just added distilled water to the battery because it hasn't had a chance to mix with the existing fluid and the specific gravity reading (or charge level) will be wrong.
