What's wrong with Ah's and what the hell is Watt per cell? - Page 1
Forum section: electronics / projects

author: compet17
author_rank: Regular Contributor
timestamp: October 05, 2015, 12:59:05 pm
content: Until a few years ago the capacity of rechargable batteries has been measured in Ampere-Hours only. But things have changed and I find more and more of those "Watt per cell" values.What does that mean "Watt per cell"(WPC)? I mean Watt-hours would make sense...And how can you calculate the Ah's from WPC?A 12V battery has 6 cells. So a 15WPC battery has 90W... what? how long? I'm confused...Does anyone know how this is meant? Is this a concept like "Ohm per quare" which Dave explained in on of his videos?

author: Balaur
author_rank: Unknown
timestamp: October 05, 2015, 02:12:02 pm
content: It's a concept used (mainly?) by the UPScrooksdesigners.The Watt/Cell means the maximum power that the cell can provide during a specified time interval, until it gets down to a specific voltage (per cell - v/c)Let me provide an example, as it's more intuitive:Let's assume a 100 Ah cell with a nominal voltage of 2V.- If you measure the capacity with a C/5 (i.e 20 A) constant load, you may get something close to the nominal capacity (i.e. 100Ah) during a 5-hour test. Let's assume that you have been able to integrate the voltage over the testing duration and compute the power (intuitively 2V * 20 A * 5h = 200Wh). The W/C rating in this case is 40 W/C (i.e. 2V * 20A) for a 5 hour test.- Let's imagine that that cell is used (as the only battery) in an UPS where the ON (main power down) time is not hours, but mainly minutes, as the UPS needs to power a rack of servers with a 1 KW power requirements. The power is only required for a few minutes, until the mains are getting back on or until the servers are powering down. In this scenario, the UPS needs to provide 1KW during a short interval of time.Assuming that the power drawn by the equipment is constant and the voltage conversion is ideal (no losses), then the 1KW means a current approximately 500A on the poor battery cell. The battery will most likely get depleted in 1/5 of an hour. Ideally, that means that the power rating of the cell is 1000 W/C during 12 minutes.Realistically, the internal resistor battery will have a strong impact on the cell output power.The same cell can support:- 40 W/C during 300 minutes (200 Wh)- 600 W/C during 15 minutes (150 Wh)- 1000 W/C during 10 minutes (133.3 Wh)- 2000 W/C during 3 minutes (100 Wh)and so on.An UPS manufacturer will select the battery cells with the W/C rating that will make sure that the overall UPS device is able to provide the x KW during the y minutes required.

author: JacobPilsen
author_rank: Regular Contributor
timestamp: October 05, 2015, 04:01:29 pm
content: Should It be watt-hour (symbolized Wh) per cell.

author: compet17
author_rank: Regular Contributor
timestamp: October 05, 2015, 05:46:48 pm
content: Watt-hours would be logical for the capacity. But the confusing fact is that W(Watt)/Cell is used....The UPS explanation sounds good. I found these WPC values only on UPS batteries. Thanks for that clear answer!

author: SteveLy
author_rank: Regular Contributor
timestamp: October 06, 2015, 08:36:56 am
content: As Balaur mentioned W/cell will be the max power you can safely get from a cell. It's not a capacity rating but replaces what traditionally would have been the max current or amp rating.I think one reason for some manufacturers opting to use watts/cell instead of max amps is because people often get confused about how to convert amp rating per cell into amp rating for multiple cells in various series and parallel combinations. The watts per cell rating makes it easy: no matter how you arrange multiple cells, the power rating of the resulting battery pack will be number of cells times the watt rating of each cell (assuming good matched cells and good connections between them).Also, for many applications the power consumption of the device being powered is easier to estimate / work with than the amp draw from the battery (whose output voltage has quite some range). A typical mistake by "young players" (to borrow Dave's expression; but many are not so young) is to use the nominal or even the maximum voltage rating of the battery to calculate the input amps required from the battery. This can result in the battery being over stressed at the lower end of its output voltage range. A W/cell rating gets around this problem and may be safer for some hobbyist applications.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 06, 2015, 04:18:00 pm
content: Ah is "charge capacity" - another way of counting the number of coulombs the battery stores. Without voltage, it doesn't tell anything about energy storage. If you push 5 amperes through a battery for 30 minutes, you have injected 2.5 Ah to the battery.Wh is "energy capacity", which shows exactly how much energy the battery stores. It is Ah times average voltage. This is the exact measurement how much electricity the battery stores.A is the current capability, or maximum rating how much current the cells can push out without overheating.W is power capability, which takes voltage into account.W/cell is the maximum rating how much power you can safely take out of the cell without damaging it. It is determined by electrode design and thermal design. The battery pack total power capability is "W/cell" times number-of-cells (regardless of the arrangement, parallel or series).These are the only meanings. If someone uses these terms for anything else (such as trying to indicate energy storage capability with "W/cell"), anything they say must be disregarded. In a product, anything "per cell" is strange, because the user is only interested in the complete system, and may not even know the number of cells.

author: Balaur
author_rank: Unknown
timestamp: October 06, 2015, 05:14:33 pm
content: Quote from: Siwastaja on October 06, 2015, 04:18:00 pmNot quite. The W/cell indicates the maximal power that the cell can provideduring a specified time interval.There is not a single W/cell value. The manufacturer provides a table with a W/cell for multiple time interval.Please have look at datasheets of UPS-specific batteries. ExamplehereQuote from: Siwastaja on October 06, 2015, 04:18:00 pmAgain, this W/cell data makes sense for a specific class of applications and I guess that the people in this field are familiar with and know how to make use of this information.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 06, 2015, 05:42:52 pm
content: Yes -- maximum powerin any systemalways varies with the pulse length. That is a detail I left out to simplify the post, as there were problems with very basic units of power, energy and charge. But there are details on everything, and none of the basic quantities listed on my post are "simple" -- they all have "terms and conditions" and varying mileage.But often times, only a single number for maximum power is given, which, without any other specification, would be power that can be used "forever". Often, this number can be exceeded for some time if the user knows what they are doing.With batteries, "pulses" exceeding maximum continuous rating (i.e., rating that can be sustained "forever") can be relatively long, lasting for tens of seconds or even minutes. Oftentimes, the semiconductors in the device limit the real maximum power, as their pulse ratings are rated in micro to milliseconds. For example, a li-ion energy cell could be specified to provide 1C continuous power (i.e., 10 Watt power for a 10 Watt-hour cell), with peak capability of 3C (30 Watts in said case), peak lasting for maximum of 10 seconds.Battery technology often suffers from poor documentation starting from the manufacturer. W/cell indeed makes sense in some contexts, and the general drift towards power and energy units (from A, Ah to W, Wh) makes a lot of sense.