Post by thread:[EE] calculating mAh for a battery

So in general, given a NiHM cell, the mAh capacity is the amount of current it can deliver over a given time into a set load, correct? I have some unmarked AAA cells, so I charged them with my commercial charger and set up a fixed load (power resistor 75ohm 5W) and then proceeded to log the current it was delivering over time. The curve was not really what I expected, it showed a slow discharge for about an hour and then a very sharp drop to where it then flattened out. Am I measuring this incorrectly to find the mAh capacity of the cell? From what I can see, it tells me that I have a 275mAh to 310mAh cell (those are the starting points to where it dropped off sharply)

On 9 February 2011 07:50, alan smith <spam_OUTmicro_eng2TakeThisOuTyahoo.com> wrote: > So in general, given a NiHM cell, the mAh capacity is the amount of current it can deliver over a given time into a set load, correct? > > I have some unmarked AAA cells, so I charged them with my commercial charger and set up a fixed load (power resistor 75ohm 5W) and then proceeded to log the current it was delivering over time. The curve was not really what I expected, it showed a slow discharge for about an hour and then a very sharp drop to where it then flattened out. > > Am I measuring this incorrectly to find the mAh capacity of the cell? >From what I can see, it tells me that I have a 275mAh to 310mAh cell (those are the starting points to where it dropped off sharply) > > > Alan, were you discharging them as a single cell or with the 2 cells connected in series? If the latter, then the first drop-off point would represent the end of one cells capacity, with the other providing the second flat area. (At lower current as you have a resistive load). This would also indicate that the lower capacity battery is being reverse polarised - probably not good for it. OTOH, if this represents a single cell discharge and is the same for both cells, then it's a bit more interesting - how about some figures. Initial voltage, time to first drop-off (1hr ?) , second plateau voltage etc. Could your measurement setup be indicating a voltage when open circuit or connected to a high impedance? The figures you give don't quite line up. With a 75ohm load on a 1.5V cell, the current is 20mA. if the drop-off is at one hour, then the capacity ois 20mAhr. Even if series connected to 3V, this only gives you 40mAhr. RP

Em 8/2/2011 16:50, alan smith escreveu: > So in general, given a NiHM cell, the mAh capacity is the amount of current it can deliver over a given time into a set load, correct? Correct. > I have some unmarked AAA cells, so I charged them with my commercial charger and set up a fixed load (power resistor 75ohm 5W) It would be better to use a constant current sink to do such measurement. Cell efficiency varies with the discharge rate. In your case you had a high discharge rate at first that gradually diminished with time. The cells deliver more total charge when discharged more slowly. You could measure the total charge at several discharge currents for comparison purposes. With the constant current sink you just need to measure the time until the cell voltage drops to a certain set-point (usually 1V/cell or 0.9V/cell). The total charge is the product of the time by the constant current. > and then proceeded to log the current it was delivering over time. The curve was not really what I expected, Did you integrated the curve to obtain the total charge? > it showed a slow discharge for about an hour and then a very sharp drop to where it then flattened out. That's exactly the behavior of most batteries. > Am I measuring this incorrectly to find the mAh capacity of the cell? From what I can see, it tells me that I have a 275mAh to 310mAh cell (those are the starting points to where it dropped off sharply) You must measure the charge until the voltage drops below the set-point. Isaac __________________________________________________ Fale com seus amigos de graça com o novo Yahoo! Messenger http://br.messenger.yahoo.com

alan smith wrote: > So in general, given a NiHM cell, the mAh capacity is the amount of > current it can deliver over a given time into a set load, correct? Battery capacity in this context is the definite integral of the current over time from fully charged to discharged. For mAh the current is in units of milliamps and the time in units of hours. In other words, pretty much exactly what "mAh" says. > I have some unmarked AAA cells, so I charged them with my commercial > charger and set up a fixed load (power resistor 75ohm 5W) That would discharge it at about 16mA initially, going down proportionately as the battery voltage goes down. It would be about 12mA when the battery is close enough to discharged to end the test. > and then > proceeded to log the current it was delivering over time. The curve > was not really what I expected, it showed a slow di > > Am I measuring this incorrectly to find the mAh capacity of the cell? I can't say since you didn't say exactly how you were computing mAh. Do the integral and see what you get. > From what I can see, it tells me that I have a 275mAh to 310mAh cell Then it should have taken almost a day. Did it? Why not just show the graph. > (those are the starting points to where it dropped off sharply) That makes no sense. The mAh can only be measured over a whole discharge cycle. Capacity doesn't "start" somewhere and then drop over a test, and you can't tell what the capacity is until the test is complete. The starting point has very little to do with capacity. Surely there are references out there that explain this rather basic concept. ******************************************************************** Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products (978) 742-9014. Gold level PIC consultants since 2000

In-Reply-To: <.....491782.57229.qmKILLspam@spam@web39307.mail.mud.yahoo.com> Alan, > So in general, given a NiHM cell, the mAh capacity is the amount of current it can deliver over a given time into a set load, correct? > > I have some unmarked AAA cells, so I charged them with my commercial charger and set up a fixed load (power resistor 75ohm 5W) and then proceeded to log the current it was delivering over time. The curve was not really what I expected, it showed a slo- w discharge for about an hour and then a very sharp drop to where it then flattened out. > > Am I measuring this incorrectly to find the mAh capacity of the cell? From what I can see, it tells me that I have a 275mAh to 310mAh cell (those are the starting points to where it dropped off sharply) > The correct unit is mAH, the capital H stands for Hour. e.g. a fully charged 1000mAH cell should be able (approximately anyway) to supply 1000mA for an hour. More generally an x mAH cell is able to supply a current i mA for x/i hours.. (That's x divided by i) For a NiMH cells the end should be taken as when the voltage drops to somewhere in the region of 1V which usually means almost all the energy has been used up and the voltage will plummet towards zero pretty quickly after that. Brian Gregory. briangKILLspamcix.compulink.co.uk

>> Isn't the plural Henries? Like Volt -> Volts? > > If Volts and Amps and Coulombs and Farads and Newtons and ... > were correct, then Henries might be too, although arguably it would > be Henrys Lux ? Lumen ? You'd have to say that the unavoidable common useage would be to pluralise with an 's' something that is a quantity with identifiable units "How many volt ?" just doesn't sound right Rules like this are arbitrary anyway. People make them, not the electron

Justin Richards wrote: >> "How many volt ?" just doesn't sound right >> > Good point but "12 volt battery" does. I guess that's why they decided that units should be treated like any other noun -- it just makes sense. I'm not a native English speaker, but "how many men are in your crew?" sounds as right as "i have a five-man crew". This corresponds to "how many volts?" and "a twelve-volt battery". Gerhar

>> If Volts and Amps and Coulombs and Farads and Newtons and ... >> were correct, then Henries might be too, although arguably it would >> be Henrys > Lux ? > > Lumen ? I don't understand your point. The units lux and lumen are not proper names so are assigned lower case for symbol or name. What do you suggest as the plural for lux? If you suggest luxes you'd be liable to be out of luck :-) (or lux or luxes)