Max battery discharge

[Big Steve]

Anyone tell me what is a reasonable max rate to discharge batteries at in terms of their Ah rating?

 

Putting it another way, if I have a 2kw inverter, how many Ah of battery would I need to run it flat out without knackering the batteries?

[thelonious]

You can discharge lead acid batteries very quickly without harm, this is one of the reasons they are used for starting engines. In this application, you might be able to flatten one completely in only a minute or two.

 

The important thing is not to let them get too flat, and not to leave them flat for long. The voltage should not be allowed to go below 12v and preferably 12.2V, though you can only get an accurate reading after leaving them to stabilise for a while.

 

Your 2kW drain at 12V gives a current of 167 amps. Say you had two fully charged 110Ah batteries that would be 80 amps each which is fine (assuming your wires are HUGE) but you couldn't do it for long. Half the batteries' capacity would be gone in about 45 minutes. If you use more than half, the batteries will start getting very flat and may eventually suffer damage, particularly if it is not quickly recharged.

 

If I've got any of that wrong, I'm sure we'll soon hear!

 

Theo

[Yoda]

About right, lots of variables so most answers will be theoretical.

[Big Steve]

You can discharge lead acid batteries very quickly without harm, this is one of the reasons they are used for starting engines. In this application, you might be able to flatten one completely in only a minute or two.

 

The important thing is not to let them get too flat, and not to leave them flat for long. The voltage should not be allowed to go below 12v and preferably 12.2V, though you can only get an accurate reading after leaving them to stabilise for a while.

 

Your 2kW drain at 12V gives a current of 167 amps. Say you had two fully charged 110Ah batteries that would be 80 amps each which is fine (assuming your wires are HUGE) but you couldn't do it for long. Half the batteries' capacity would be gone in about 45 minutes. If you use more than half, the batteries will start getting very flat and may eventually suffer damage, particularly if it is not quickly recharged.

 

If I've got any of that wrong, I'm sure we'll soon hear!

 

Theo

 

Thanks, that's just what I wanted. Thinking on the lines of 2k inverter to power microwave and hoover, probably never need more than 30 mins day max discharge.

 

Isn't there some rate at which the plates will start to distort?

[chris w]

You can discharge lead acid batteries very quickly without harm, this is one of the reasons they are used for starting engines. In this application, you might be able to flatten one completely in only a minute or two.

 

The important thing is not to let them get too flat, and not to leave them flat for long. The voltage should not be allowed to go below 12v and preferably 12.2V, though you can only get an accurate reading after leaving them to stabilise for a while.

 

Your 2kW drain at 12V gives a current of 167 amps. Say you had two fully charged 110Ah batteries that would be 80 amps each which is fine (assuming your wires are HUGE) but you couldn't do it for long. Half the batteries' capacity would be gone in about 45 minutes. If you use more than half, the batteries will start getting very flat and may eventually suffer damage, particularly if it is not quickly recharged.

 

If I've got any of that wrong, I'm sure we'll soon hear!

 

Theo

 

 

Theo

 

Some errors here I'm afraid. Battery discharge is not a linear process due to a phenomenon called the Peukert Effect.

 

Running the maths, 2 x 110AH batteries drawing a total of 167A would be half discharged in only 17 minutes NOT 45 minutes. ie: about a third of the time that a linear calculation would give.

 

Discharging a battery below 12.2v (50% discharge) will very quickly end its life. leaving it discharged without fairly immediate recharging will end it even quicker.

 

On recharging, because of battery inefficiencies, you need to stuff in about 50% more charge than you took out to get the batteries back to 100% charged. So charging takes a long time.

 

Chris

[Guest]

Theo

 

Some errors here I'm afraid. Battery discharge is not a linear process due to a phenomenon called the Peukert Effect.

 

Running the maths, 2 x 110AH batteries drawing a total of 167A would be half discharged in only 17 minutes NOT 45 minutes. ie: about a third of the time that a linear calculation would give.

 

Discharging a battery below 12.2v (50% discharge) will very quickly end its life. leaving it discharged without fairly immediate recharging will end it even quicker.

 

On recharging, because of battery inefficiencies, you need to stuff in about 50% more charge than you took out to get the batteries back to 100% charged. So charging takes a long time.

 

Chris

That would tally with my findings for a 3kw inverter with a 550AH battery bank. I find that using batteries to run large loads through an inverter is very inefficient and tend to use the genny where possible.

[Big Steve]

Getting the picture now, looks like a genny then.

 

Thanks everyone.

Edited December 8, 2007 by Big Steve

[thelonious]

Theo

 

Some errors here I'm afraid. Battery discharge is not a linear process due to a phenomenon called the Peukert Effect.

 

Running the maths, 2 x 110AH batteries drawing a total of 167A would be half discharged in only 17 minutes NOT 45 minutes. ie: about a third of the time that a linear calculation would give.

 

Discharging a battery below 12.2v (50% discharge) will very quickly end its life. leaving it discharged without fairly immediate recharging will end it even quicker.

 

On recharging, because of battery inefficiencies, you need to stuff in about 50% more charge than you took out to get the batteries back to 100% charged. So charging takes a long time.

 

Chris

 

Interesting, I hadn't heard about that. It's all explained here: Peukert's equation

 

Basically, the capacity of a battery depends on how fast you discharge it.

 

Apparently, batteries are rated at a '20 hour rate', meaning if you discharge them linearly over 20 hours you get what it says on the tin eg. 110 Ah. Faster, and you get less out. At high currents, this loss becomes quite high.

 

Interestingly, if you run them as less than the 20 hour rate (for a 110Ah battery this is 5.5A), you can actually get more than the rated capacity.

 

The calculator provided on the website above indicates that a 220Ah battery bank discharged at 160A will last about 0.6 hours (36 mins). If you are only taking half, then you end up with just 18 minutes, as you correctly pointed out. Effectively you only have 95Ah capacity at this current.

 

Increasing the battery bank to 440Ah gives you about 240Ah to play with. Not as bad, but still a big loss. Steve would need a lot of batteries to get 2kW with any kind of efficiency.

 

Theo

[bottle]

Ah!!!!! Gibbo's site.

[chris w]

Interesting, I hadn't heard about that. It's all explained here: Peukert's equation

 

Basically, the capacity of a battery depends on how fast you discharge it.

 

Apparently, batteries are rated at a '20 hour rate', meaning if you discharge them linearly over 20 hours you get what it says on the tin eg. 110 Ah. Faster, and you get less out. At high currents, this loss becomes quite high.

 

Interestingly, if you run them as less than the 20 hour rate (for a 110Ah battery this is 5.5A), you can actually get more than the rated capacity.

 

The calculator provided on the website above indicates that a 220Ah battery bank discharged at 160A will last about 0.6 hours (36 mins). If you are only taking half, then you end up with just 18 minutes, as you correctly pointed out. Effectively you only have 95Ah capacity at this current.

 

Increasing the battery bank to 440Ah gives you about 240Ah to play with. Not as bad, but still a big loss. Steve would need a lot of batteries to get 2kW with any kind of efficiency.

 

Theo

 

Basically, the discharge shape of a battery is a curve. Since it's a curve it is difficult to quote a "discharge figure" as it will only be accurate at one point. However, this is what is done and generally it is quoted at the 20hr rate by convention but not by any law or rule etc.

 

If you discharge at the 20hour rate (ie: at about 5A for a 110AH battery) it will indeed discharge by 100% in 20 hours. But there is nothing to stop my selling you a 110AH battery and calling it a 130AH battery but at the 40 hour rate.

 

Most people wouldn't check whether the hour rate quoted really was 20 hours or not (or would not understand the difference between a 20 hour rate and a 40 hour rate). But you see a "130AH" battery for the same price as a 110AH battery and most will choose the 130AH battery whereas in reality they are the same thing.

 

So double check the battery hour rating as it is important.

 

Chris

[sueanddaren]

Basically, the discharge shape of a battery is a curve. Since it's a curve it is difficult to quote a "discharge figure" as it will only be accurate at one point. However, this is what is done and generally it is quoted at the 20hr rate by convention but not by any law or rule etc.

 

If you discharge at the 20hour rate (ie: at about 5A for a 110AH battery) it will indeed discharge by 100% in 20 hours. But there is nothing to stop my selling you a 110AH battery and calling it a 130AH battery but at the 40 hour rate.

 

Most people wouldn't check whether the hour rate quoted really was 20 hours or not (or would not understand the difference between a 20 hour rate and a 40 hour rate). But you see a "130AH" battery for the same price as a 110AH battery and most will choose the 130AH battery whereas in reality they are the same thing.

 

So double check the battery hour rating as it is important.

 

Chris

[sueanddaren]

Don't forget batteries are chemical beasts so you also have to consider the temperature aprox 3% increase for every 5degC and end voltage, the lower the voltage the more you will pull out.

The 20 hour rating comes from the old automotive specs (car batteries) Industrialy reserve power uses 10 hour rates to 1.8Vpc @20 deg C and motive power uses 5 hour rates @30 degC but I can't remember the end voltage.

 

Peukert numbers are OK but don't expect them to be too accurate at ether end (very high rates and very long rates) other considerations such top Lead resistance and paste utilisations come into play.

[Yoda]

Don't forget batteries are chemical beasts so you also have to consider the temperature aprox 3% increase for every 5degC and end voltage, the lower the voltage the more you will pull out.

The 20 hour rating comes from the old automotive specs (car batteries) Industrialy reserve power uses 10 hour rates to 1.8Vpc @20 deg C and motive power uses 5 hour rates @30 degC but I can't remember the end voltage.

 

Peukert numbers are OK but don't expect them to be too accurate at ether end (very high rates and very long rates) other considerations such top Lead resistance and paste utilisations come into play.

 

Steve and Gibbo will love this

[chris w]

Don't forget batteries are chemical beasts so you also have to consider the temperature aprox 3% increase for every 5degC and end voltage, the lower the voltage the more you will pull out.

The 20 hour rating comes from the old automotive specs (car batteries) Industrialy reserve power uses 10 hour rates to 1.8Vpc @20 deg C and motive power uses 5 hour rates @30 degC but I can't remember the end voltage.

 

Peukert numbers are OK but don't expect them to be too accurate at ether end (very high rates and very long rates) other considerations such top Lead resistance and paste utilisations come into play.

 

OK own up! Is that Sue or Darren?

[Big Steve]

Me minds starting to boggle

[Guest]

Don't forget batteries are chemical beasts so you also have to consider the temperature aprox 3% increase for every 5degC and end voltage, the lower the voltage the more you will pull out.

The 20 hour rating comes from the old automotive specs (car batteries) Industrialy reserve power uses 10 hour rates to 1.8Vpc @20 deg C and motive power uses 5 hour rates @30 degC but I can't remember the end voltage.

 

Peukert numbers are OK but don't expect them to be too accurate at ether end (very high rates and very long rates) other considerations such top Lead resistance and paste utilisations come into play.

I've said it all along.

[sueanddaren]

OK own up! Is that Sue or Darren?

 

 

Daren!