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'[EE] calculating mAh for a battery'
2011\02\08@135049 by

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 <micro_eng2yahoo.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

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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.

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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.
Em 9/2/2011 22:40, Brian Gregory escreveu:
> The correct unit is mAH, the capital H stands for Hour.

Isn't "H" for Henries?

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Henry :-) (but seriously).

> Isn't "H" for Henries
>> 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
>
> "How many volt ?" just doesn't sound right
>
Good point but "12 volt battery" does
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
> I guess that's why they decided that units should be treated like any
> other noun -- it just makes sense.
When "volt" is a noun.
"How many volts ?" - noun

"12-volt battery" - adjective. 'battery' is the noun

"How many 12V batteries ?"

Committees eh
>> 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 ?