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Author Topic: Volt-amp-hour battery description  (Read 10313 times)
Andygodber
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« on: June 02, 2012, 09:31:50 PM »

When a battery's capacity is described in volt-amp-hours, how is it different fom normal Ah?
For example, the UPS (yes, UPS) batteries by APC are often described this way.
The link here shows a 48V battery with 1728 volt amp hours.
http://www.apc.com/resource/include/techspec_index.cfm?base_sku=SUA48RMXLBP3U
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Countrypaul
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« Reply #1 on: June 02, 2012, 11:27:27 PM »

Sounds typical American approach to get bigger numbers. Rather than just say 36Ah, quote the Ah x volts, why not just quote 1728 Watts for an hour?

Paul
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johnrae
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« Reply #2 on: June 03, 2012, 08:44:55 AM »

Apart from producing bigger numbers one possible purpose is to create a theoretical watt-hours concept.  The subject system therefore has a kW-Hr rating of 1.728kWH.  I state theoretical since you will not get 100% out of the battery (without causing severe loss of life/capacity)
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nowty
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« Reply #3 on: June 03, 2012, 02:12:24 PM »

It is an attempt at giving the potential total power output of a battery.

For example my electric bike lithium ion battery has a capacity of 14Ah, however unless you knew that the battery voltage was 37v 14Ah means nothing. It has also been quoted as 518 watt hours which is simply the voltage x current.
Of course what none of these figures tells you though, is at what discharge rate these figures are correct for.

For example my big mother fork lift truck batteries are rated at 1000Ah at a 20hr discharge rate. When I set my system up my off grid system, my Sunny Island required the Ah capacity based upon a 10hr discharge rate so I had to re-calculate this which worked out to be 917Ah.

Going back to my electric bike it has a continuous power output of circa 200w and if used at full power runs out of steam after about an hour, not 2.5 hours as is suggested by the 518 watt hour capacity quote. At the 1 hour discharge rate, the battery is giving little more than 200 watt hours, so less than half of the quoted spec.
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