Volts vs. Amps

[randy_252]

Reading several post on batteries, I understand that you need the higher range of volts for starting the motor.

Do amps play a part here? Never really saw it mentioned.

If I have a battery that has 12.6 volt, any idea how many amps I should be reading? Or would this be dependent on the "Cold Cranking Amps" of the battery?

 

[rickdb1boat]

Re: Volts vs. Amps

Amps play the most important role. Depending on what your trying to start will determine the amp draw from the battery. If your starter draws 50 amps, then that most likely is what you will read with an amp meter when cranking. The cold cranking amps is what the battery will provide. To be precise, it's the amount of amps the battery will provide at 0* for 30 seconds, while maintaining at least 7.2 volts. The more CCA's the battery has, the more power it has to turn the starter for a given period. When the battery is just sitting there with no load on it, you will read no amp draw...

 

[Silvertip]

Re: Volts vs. Amps

With no load on the battery, it is delivering zero amps so you can't simply stick a meter on it and measure its capacity. Any 12V battery regardless of its capacity or size is a 12V battery. A starter for example will only draw so much power from the battery when attempting to start the engine. A small battery (say 350 Cold cranking amps or CCA) will crank the engine just fine. So will a 600 CCA or a 900 CCA. The 350 however will crank it for a shorter period of time. The 900 would crank it the longest. Look at CCA as a bucket of electricity. a 350 CCA battery is a small bucket. a 900 is a large bucket.

 

[Texasmark]

Re: Volts vs. Amps

My 90 hp requires 120a to start per the book. Even when cold you are only on it for like a minute max.

What you want to know about your battery is what is the terminal voltage when supplying your starting current. If it remains within a volt or two of the "static" voltage (roughly 12.75v), and the cables and terminals are in good shape, it will spin your engine rapidly and you will (should) get a nice crisp start. When it gets low on charge or sulphated up, the terminal voltage drops under (that) load and the engine spins slowly and starting is more difficult.

A 2 cycle outboard is usually easy on a battery. A cold diesel tractor is another story.

Mark

 

[randy_252]

Re: Volts vs. Amps

OK, Thanks. Just didn't understand where the amps came in. Now alot of the post I read make alot more sence.

 

[mikeblas]

Re: Volts vs. Amps

The problem you ran into, Randy, is that battery specifications aren't as simple as we'd like to think.

When you see a "12-volt, 350 amp" battery, you're reading two different units -- but they're measured in two different ways. The battery should nominally supply 12 volts. It might supply a little more, or might supply a little less. That nominal specification is different than the current specification of 350 amps because the current specification is a peak specification. The battery will supply as much current as is asked of it -- up to 350 amps.

There's no way to ask for more voltage from the battery. You can always ask for more current.

 

[jlinder]

Re: Volts vs. Amps

The simplest way to understand electricity,volts, and amps is to use a water analogy. Think of water in a pipe.

The water pressure are the volts.
The amount of water flowing through the pipe is the amperage.
Work you can do is a combination of the pressure and the amount of water. In electrical terms watts = volts x amps.

Now imagine that you have a small, long garden hose. If you are not drawing much water out the end the pressure at the end of the pipe is the same as the pressure at the start of the hose. If you try to draw a lot of water then you don't have much pressure. The longer the hose the more pressure you lose.

Compare this to a small guage long wire. If you draw very little current the voltage at the end is the same as that at the source(battery). Try drawing a lot of current then the voltage drops, just like the pressure in the hose drops in the paragraph above.

In water, if you want to maintain pressure you get a bigger pipe. in the electrical world if you want to maintain voltage with a heavy current draw you get a bigger wire.

(The amount of voltage you lose in the wire is known as the voltage drop)

Hope this helps.

 

[Texasmark]

Re: Volts vs. Amps

Hey j, you are on it my man. It's all physics, just a different form of matter to be controlied. Water is the easiest way to teach someone basic electrical theory.

Mark

 

[jlinder]

Re: Volts vs. Amps

It's a very old analagy. Add to that batteries are pumps, capcitors are storage tanks, inductors are flywheels, switches are valves, etc.

It makes it a lot easier to understand when you can visualize something physical.