A battery draw question

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Model T Ford Forum: Forum 2012: A battery draw question
Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Moorehead on Thursday, September 13, 2012 - 05:16 pm:

If a battery is rated a 600 amps, how long does it take to run this battery down? For instance, if a light bulb is connected to the fully charged 600 amp battery, and the bulb draws 6 amps, will the bulb burn for 100 hours before going completely out? (6 X 100 = 600) I have always been curious about cranking amps and amp loads and wondered just how that amperage draw can be determined into actual time.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Royce in Dallas TX on Thursday, September 13, 2012 - 05:37 pm:

I think you are talking about the Cold Cranking Amps rating of a battery. If the battery has a CCA rating of 600 amps that means it will (when new) deliver 600 amps maximum for at least 30 seconds when the outside temperature is 0 degrees Farenheit.

There is another standard called the battery Reserve Capacity. It is a number of minutes the battery can deliver 25 amps and still maintain voltage above 10.5 Volts.

I don't believe the battery manufacturers post a spec that answers your question directly.

I have left the headlights on once for 18 hours. The two 6 volt bulbs in series were just as bright after 18 hours as they had been the day before. My battery in that car is a 12 volt group 24 with 800 CCA rating. I bet it would run the headlights for a couple days.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Cameron Whitaker on Thursday, September 13, 2012 - 05:43 pm:

600 amps is usually the cranking amperage, that is, it is roughly the maximum amount of current the battery can put out for a short length of time without melting something inside it. If you tried to draw 600 amps from it continuously, it wouldn't last long.

However, the battery's actual capacity is substantially different, and often, it's hard to find out on car batteries. My guess would be somewhere between 50 to 80 amp-hours. So if we assume 60 amp hours, your light bulb would go for about 10 hours before running the battery down.

Now if you REALLY want to be technical, car batteries won't last very long if you keep running them down and charging them again. The lead plates they use are porous to maximize surface area. This gives the battery a lot of short-term current capacity (which is good for starting), but not so much for long-term draw.

That's where a Deep Cycle battery comes into play. The lead plates in them are solid, and because of that, they can be discharged fully (which means to about 8 volts for a 12 volt battery) and charged again, over and over. Some car batteries, such as Odyssey batteries, are deep cycle batteries, and can still put out a lot of current for starting.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Moorehead on Thursday, September 13, 2012 - 05:44 pm:

Does it may any difference as to the voltage of the battery, 6 or 12? All things being equal, would one last longer than the other? I know recently my GPS ran the battery flat when left on during the recent Ohio Jamboree. That was a 4 day period.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Cameron Whitaker on Thursday, September 13, 2012 - 06:26 pm:

The 6 volt will theoretically have twice the current capacity, at the cost of half of the voltage. So a typical 6 volt may be around 100-160 amp hours.


Top of pagePrevious messageNext messageBottom of page Link to this message  By JohnH on Thursday, September 13, 2012 - 07:39 pm:

My T battery (a deep cycle Century 12A 6 volt forklift type) is rated at 660A CCA, and 106Ah capacity.
The amp hour capacity of lead acid batteries is usually quoted at the 20 hour rate. For this battery, it means you could draw 5A for about 20 hours. Drawing higher currents will effectively reduce amp hour capacity. Drawing 160Ah will flatten the battery in considerably less than one hour.
As Cameron says, a 6 volt battery of any given physical size has about twice the Ah rating of the same size 12V type. However, it is important to remember that the actual power available is still the same. One could argue that one advantage of 6 volt batteries is the thicker plates.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Moorehead on Thursday, September 13, 2012 - 08:41 pm:

Thanks for the explanation. I knew someone would explain in real terms.


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