Pulling more out of a SLA than it's rated

[Juggernaut]

I was wondering if you could pull 20 ah out of an 7ah SLA 12 volt battery. Is it not possible or would it's run time be like 30 min. what's the most ah "obviously it will not handle it for an hour." what's the most you could pull out of a 7ah SLA battery if you can. Thanks for the help.

 

[Flash_Gordon]

Re: Pulling more out of a SLA then it's rated

Amp hours is a measure of battery capacity. Your 7AH battery should deliver 1 amp for seven hours or 7 amps for one hour.

You do not draw AH but current measured in amps. If you are talking about drawing 20 amps, your 7AH battery would have a very short runtime. It might also tend to be over discharged and possibly overheated.

What are you trying to run? If you are really drawing 20 amps DC, a much larger capacity battery should be used.

Mark

 

[turbodog]

Re: Pulling more out of a SLA then it's rated

I was wondering if you could pull 20 ah out of an 7ah SLA 12 volt battery. Is it not possible or would it's run time be like 30 min. what's the most ah "obviously it will not handle it for an hour." what's the most you could pull out of a 7ah SLA battery if you can. Thanks for the help.

Usually you don't want to discharge a SLA battery at more than 1C rate, where C is the AH capacity. So, that means 7 amps for your battery. If you discharge at a faster rate, you will see voltage droop and lowered capacity. If you discharge at way too high of a rate you will see smoke and flames.


Different batteries have different discharge rates. Nicd and nimh consumer cells usually handle a 2C rate pretty well. This drains them in 1/2 hour roughly.

Pro-grade nicd/nimh cells can handle 20C or more. You would see these cells in r/c racing and usually battle bots. These are top grade cells that don't have much voltage sag. Their internal resistance is low. This helps with the voltage. They also will maintain a high % of their rated capacity even at high discharge rates.

It's not always a well known fact that battery mfgs rate the capacity at certain discharge rates. These rates are usually very, very low so as to make the capacity numbers look as good as possible. Typically as rates go up, usable capacity goes down. Premium cells will hold close to rated capacity as noted above.

You could parallel 2 packs. This would help achieve your needed amperage. You could use a subc nimh pack. These things will deliver 20 amps w/o breaking a sweat.

 

[Lynx_Arc]

Re: Pulling more out of a SLA then it's rated

there is one way you could get 20Ahr out of a 7Ahr 12v battery... using a buck circuit to run something that takes about 2v at 20amps.. you have about 84watt hours in that battery if you could somehow get a 50% effecient buck circuit you could potentially get 20amps at 2v or 40 watt hours out of it. This is all theoretical though and I don't know of anything that takes 2v at that current.

 

[Fallingwater]

Re: Pulling more out of a SLA then it's rated

If the discharge was in short bursts you could probably do it without killing the poor thing too fast. Not at all recommended, though.

 

[Probedude]

The 12V 7AH battery is very commonly used in UPS's.
They pull a lot of amps because of the power conversion.
A 300W UPS needs to pull 300W/12V = 25 amps if the conversion was 100% efficient.

They don't last many cycles at this rate - really don't know what the cycle life would be but I'm sure the data is out there.

Here's a 12V 7AH battery
http://www.csb-battery.com/english/01_product/02_detail.php?fid=4&pid=44

100A/130A discharge for 5 seconds.
The PDF shows the discharge time at various C rates, 3C (21A) is under 10 mins to a terminal voltage of 8V

Here's a panasonic datasheet, has curves for up to 30A discharge rate.
http://www.panasonic.com/industrial/battery/oem/images/pdf/Panasonic_VRLA_LC-R127R2P.pdf

 

[MorePower]

Re: Pulling more out of a SLA then it's rated

there is one way you could get 20Ahr out of a 7Ahr 12v battery... using a buck circuit to run something that takes about 2v at 20amps.. you have about 84watt hours in that battery if you could somehow get a 50% effecient buck circuit you could potentially get 20amps at 2v or 40 watt hours out of it. This is all theoretical though and I don't know of anything that takes 2v at that current.

Your syntax is messed up. Amp-hr refers to capacity, as mentioned previously in this thread. You can't get 20 amp-hours out of a 7 amp-hour battery (that would be like taking a dozen donuts out of a box that only has 4 donuts in it), but you can pull 20 amps from a 7 amp-hour battery.

 

[Probedude]

Re: Pulling more out of a SLA then it's rated

Your syntax is messed up. Amp-hr refers to capacity, as mentioned previously in this thread. You can't get 20 amp-hours out of a 7 amp-hour battery (that would be like taking a dozen donuts out of a box that only has 4 donuts in it), but you can pull 20 amps from a 7 amp-hour battery.

I was getting confused too - and I should know this.
He's correct.

12V 7AH means you can pull 7A in 1 hour, or 1amp for 7 hours. This is at 12V (12V being the key) 7A * 12V * 1 hr = 84Whrs of power. 1A * 12V * 7 hrs = 84 Whrs.

By using a buck switching regulator to downregulate the 12V to 2V and assuming a 100% conversion efficiency you can now do this:

20A at 2V = 40W of power. 40W/12V = 3.33A draw at 12V.
So going into the buck converter is 12V @ 3.33A, the output is 2V at 20A.

So the 20A COULD be pulled out over a period of over 2 hours.

12V 7AH = 84WHrs = 2V 42AH.

7AH is capacity, but in this case of a 12V battery. A 2V 42AH battery has the same capacity. This is why you're charged KWHr's by the electric company.

 

[Lynx_Arc]

Re: Pulling more out of a SLA then it's rated

thanks for answering... I made out an answer post and proceeded to have it ready and hit the reply button and again CPF was.... down. twice this week it died when posting the first time I had triple posts this time..... nothing lol.
most likely though at that current a buck circuit able to do that would cost more than several batteries.