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12/6/2019 7:27:02 PM
Posted: 2/27/2007 9:16:06 AM EST
specifically, is a 12V, 7.2Ah rechargable lead acid battery a bad choice for an application that will draw 8.3A DC?

and, when the battery says " 12V 7.2Ah 20HR " what does the 20HR mean?
Link Posted: 2/27/2007 9:24:38 AM EST

Originally Posted By gringopistolero:
specifically, is a 12V, 7.2Ah rechargable lead acid battery a bad choice for an application that will draw 8.3A DC?

and, when the battery says " 12V 7.2Ah 20HR " what does the 20HR mean?


How long is the draw at 8.3 A? A few seconds is ok, a minute might not. The problem is battery heating (yes, draw heats the battery as does charging).

the 20 HR is the rating for constant draw at the advertised capacity. For that battery, that is a 0.36 A draw for 20 hours. If you have a 3.6 A load, it will deliver significantly less than 2 hours of duration due to heating and other losses.

Link Posted: 2/27/2007 9:28:40 AM EST

Originally Posted By Keith_J:

Originally Posted By gringopistolero:
specifically, is a 12V, 7.2Ah rechargable lead acid battery a bad choice for an application that will draw 8.3A DC?

and, when the battery says " 12V 7.2Ah 20HR " what does the 20HR mean?


How long is the draw at 8.3 A? A few seconds is ok, a minute might not. The problem is battery heating (yes, draw heats the battery as does charging).

the 20 HR is the rating for constant draw at the advertised capacity. For that battery, that is a 0.36 A draw for 20 hours. If you have a 3.6 A load, it will deliver significantly less than 2 hours of duration due to heating and other losses.



I rank Keith_J right up there alongside MacGuyver for all things technical! There appears to be no limit to his intellect!
Link Posted: 2/27/2007 9:47:46 AM EST

Originally Posted By Keith_J:

Originally Posted By gringopistolero:
specifically, is a 12V, 7.2Ah rechargable lead acid battery a bad choice for an application that will draw 8.3A DC?

and, when the battery says " 12V 7.2Ah 20HR " what does the 20HR mean?


How long is the draw at 8.3 A? A few seconds is ok, a minute might not. The problem is battery heating (yes, draw heats the battery as does charging).

the 20 HR is the rating for constant draw at the advertised capacity. For that battery, that is a 0.36 A draw for 20 hours. If you have a 3.6 A load, it will deliver significantly less than 2 hours of duration due to heating and other losses.



the draw at 8.3 amps might be a few seconds, it might be a minute or two.

how did you come up with the 0.36 amp number?

it should run for about 45minutes at 8.3 amps right?

Link Posted: 2/27/2007 9:53:26 AM EST
It's the deep discharge that's a problem for lead acid batteries. If you overspec them greatly so you only discharge them 20% of the way, they'll typically last for years. The datasheet for the particular battery may contain more information about specific usage versus expected life. I know that with a set of SLA batteries that I use at work that if I discharged them 80% of the way each day, they only last 45 days. After doubling the number of batteries, they now last more than nine months between replacements. So by doubling my upfront cost I more than tripled the battery life. I saved money in the long run. Don't think that just because something is called deep discharge that it doesn't hurt the battery.

For the rating of 7.2Ah 20HR, the Ah stands for Ampere-hour, and the 20HR means it is the 20 hour Ah rating. The 7.2Ah means approximately 7.2A for 1 hour or 1A for 7.2 hours. The capacity of a battery depends on the discharge rate(power output). The formula is t = C/I where t is the discharge time in hours, C is the current capacity rating, and I is the current draw. Because you're pulling a relatively high current compared to the battery capacity, it would actually be less because if a battery is discharged at a relatively high rate, the available capacity is lower than you would expected.z
Link Posted: 2/27/2007 9:56:48 AM EST

the draw at 8.3 amps might be a few seconds, it might be a minute or two.

Then it will be fine for years assuming it's charged between discharges. Is that the case?

As to my life comment, I'm talking about quality Panasonic or equivalent Japanese batteries. If you buy cheap Chinese ones they will probably fail in less than a year. I buy a lot of them for UPS's, and buying quality batteries is worth every penny extra.z
Link Posted: 2/27/2007 10:03:56 AM EST
The amp-hour capacity labeled is only for the 20 hour rate. For a 7.0 A*H battery, that means it will deliver 7/20 amps for 20 hours. Drawing 8.3 amps will probably only deliver 15 minutes, about 2 A*H of capacity.

Deep discharging will reduce the life to about 100 cycles where light discharge (say running that 8.3 Amp load for only 5 minutes, then fully recharging) will deliver over 500 cycles.

Link Posted: 2/27/2007 2:16:40 PM EST

Originally Posted By Keith_J:
The amp-hour capacity labeled is only for the 20 hour rate. For a 7.0 A*H battery, that means it will deliver 7/20 amps for 20 hours. Drawing 8.3 amps will probably only deliver 15 minutes, about 2 A*H of capacity.

Deep discharging will reduce the life to about 100 cycles where light discharge (say running that 8.3 Amp load for only 5 minutes, then fully recharging) will deliver over 500 cycles.



THANKS FOR THE HELP. I like to make stuff myself, and I bought a "car" spotlight that runs on the cig outlet. I snipped the wires, and souldered them to the leads on a deer feeder battery. I wont use it a lot at one time. the max I would probably use it is 5-10 minutes. MAX.

this isnt meant to be a every day thing. just sort of when I am looking at the deer and bigfoots.

keith J and Zoom, thanks for the help.
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