Battery life

[thekman]

I need to run a 1/2HP 24VDC motor...FLA=20...for about 12 seconds with around a 50% duty cycle. I have it powered from a rectified 500VA transformer, but it needs to work when the power goes out. Unfortunately, there is no place for a couple of car batteries. I've not been able to get any data from this thing while it's running, so I don't know exactly what the running current is...it's driving a transmission, so I doubt seriously if it's anywhere near the FLA....
I'm unable to find the formula telling me the curve down from amp-hours to what current is available for a few seconds....I need to know how many 12 second cycles I can run on lower capacity batteries (7-12Ah) The formulae I've seen involve the peukert # and I don't have that, namely because I need to specify a battery based on availability, (my customer needs to be able to get batteries at autozone or walmart)

So is there a fairly generic curve I can use, plotting Ah vs time, for SLA batteries?

 

[mikekilroy]

The formulae I've seen involve the peukert # and I don't have that, .....

So why worry about it? just pick 2x what u want and be done with it?

n is the Peukert number for the battery. That number shows how well the battery holds up under heavy rates of discharge. Most range from 1.1 to 1.4; 1.2 is about average, from:

http://www.csgnetwork.com/batterylifecalc.html

You know that ah rating is valid for way less than rating, so go with that. 1/20th current is typical where a batts ah rating is ver accurate and u dont have to worry about Mr. Peukart...

math: 24v*20a*50%*12sec/3600sec/hr= 0.8ah

1.4x.8= 1.1ah per cycle. how many you want? 10 then 11ah, two cells from walmart.....

that should keep the sump pump going for an hour....

---------------------------------
but maybe not.... googling gets some interesting hits:

got 11 brand "new" 12V 12Ah battery ( usually supplied for electric bike)

I fully charged them and made a desulfatation and equalization charge to 14.5v with all 11 battery connected in parallel with an... "intelligent charger"

They have sit for 12 hour and was somewere to 13.2V and fully charged.

I tested every of them at 0.6C ( 7.5A) for 100W power discharge using my CBA II.

I got around 7-8Ah from each and at that C rate that exactly correspond to 80-90% of capacity. normally if they wold hbe in 100% excellent condition, they would give 8-9Ah

The Peukert curve/effect is incredible on SLA. On A123 lithium cell it is like no effect no the capacity eventhou you discharge at low or hig current...

with SLA it's always a percentage of the nominal C/20 capacity you will get. normally at C/20, you would get 12Ah out of a 12Ah battery.. but as you increase the current, you loose capacity

Ex: at 50A, a 12ah battery would only give like 4Ah.. and at 100A it<s like 3Ah

Doc from:

http://endless-sphere.com/forums/viewtopic.php?f=14&t=16838

--------------

http://batteryuniversity.com/learn/article/calculating_the_battery_runtime

seems to say you will need 5x required when so far over rating....
--------------
Hello Everyone, I should introduce myself. I just bought a Currie Ezip 750 scooter on sale (new) from Walmart and I am throughly enjoying it, and I am hooked on electric scooters. I can see that a brushless motor and Lipo batteries are in my future.

My issue today is that I have the original new 24v 12Ah battery pack that came with the scooter, and I purchased two 12v 12Ah batteries from eBay to make myself a spare (longer range) battery pack. I have gone through @ three full discharge and full recharge cycles to "break-in" my SLA batteries. I am using a Watt Power meter to track Volts, Amps, Wh, Wpeak, Vminimum, and Total Wh & Ah usage.

Now the more that I research scooters I see that the Peukert Effect will come into play here. My 24v 12Ah battery pack will realistically give me 7-8 Ah due to the high discharge rate (scooter draws @ 10-16 amps while cruising) as I am @ 235 lbs. from

http://endless-sphere.com/forums/viewtopic.php?f=14&t=49291

-------------------------------
seems like you really should consider a couple 12v deep discharge batts from walmart instead.

http://www.KilroyWasHere<dot>com