I've just had a nearly heated discussion with my son in law (SIL)
about the power from rechargeable batteries and we both agreed in the
end that we really weren't that sure of our positions !

Regardless of the battery type, if you have a say a 1.4AH and a 2.5AH
battery driving the same device, I'm of the opinion that the actual
power output of the motor is going to be same during the 'flat' part
of the battery capacity period and that only as the battery begins to
reach the end of its charge capacity will the power start to drop off
- true or false ? The voltage will be the same but is the internal
resistance and therefore the maximum current output going to be the
same at full charge regardless of the capacity ?

Interestingly I discovered, before reading the instructions (!), that
my new Ryobi drill Li-ion batteries will switch themselves off before
they reach the end of capacity and that I don't have to make a
considered judgement on the critical point like my old Bosch with Ni
Cd batteries.

Rob

On Fri, 19 Jun 2009 13:05:29 -0700 (PDT), robgraham
wrote:

I've just had a nearly heated discussion with my son in law (SIL)
about the power from rechargeable batteries and we both agreed in the
end that we really weren't that sure of our positions !

Regardless of the battery type, if you have a say a 1.4AH and a 2.5AH
battery driving the same device, I'm of the opinion that the actual
power output of the motor is going to be same during the 'flat' part
of the battery capacity period and that only as the battery begins to
reach the end of its charge capacity will the power start to drop off
- true or false ?

Substantially correct, assuming a 1 amp draw the 1.4AH will last 1.4
hours, the 2.5 will last 2 and a half hours. During these respective
time both will deliver an equal current

The voltage will be the same but is the internal
resistance and therefore the maximum current output going to be the
same at full charge regardless of the capacity ?

That depends upon the battery and manufacturer, it isn't related to
capacity. It may well be that a larger capacity battery will have
better performance (lower internal resistance) but this would be
related to improvements in manufacturing process between one and the
other rather than intrinsic qualities.

Interestingly I discovered, before reading the instructions (!), that
my new Ryobi drill Li-ion batteries will switch themselves off before
they reach the end of capacity and that I don't have to make a
considered judgement on the critical point like my old Bosch with Ni
Cd batteries.

All Lithium batteries do this, as well as limiting charging. If they
don't they have some rather interesting characteristics which involve
explosions.

"robgraham" wrote in message
...
I've just had a nearly heated discussion with my son in law (SIL)
about the power from rechargeable batteries and we both agreed in the
end that we really weren't that sure of our positions !

Regardless of the battery type, if you have a say a 1.4AH and a 2.5AH
battery driving the same device, I'm of the opinion that the actual
power output of the motor is going to be same during the 'flat' part
of the battery capacity period and that only as the battery begins to
reach the end of its charge capacity will the power start to drop off
- true or false ? The voltage will be the same but is the internal
resistance and therefore the maximum current output going to be the
same at full charge regardless of the capacity ?

Interestingly I discovered, before reading the instructions (!), that
my new Ryobi drill Li-ion batteries will switch themselves off before
they reach the end of capacity and that I don't have to make a
considered judgement on the critical point like my old Bosch with Ni
Cd batteries.

Rob

I don't know the answer to the underlying question you are asking which I
think is "is the internal resistance of a battery related to its capacity?"
however in practical usage the power is determined not by the battery
internal resistance but rather the load so you are correct the power output
of the motor will be the same during the flat part of the discharge curve.

On Fri, 19 Jun 2009 22:29:57 +0100, Calvin Sambrook wrote:

I don't know the answer to the underlying question you are asking which
I think is "is the internal resistance of a battery related to its
capacity?" however in practical usage the power is determined not by the
battery internal resistance but rather the load so you are correct the
power output of the motor will be the same during the flat part of the
discharge curve.

For a given load I would expect a larger battery to have lower internal
resistance and therefore to be able to deliver higher current and
therefore power. I have absolutely no idea whether the difference is
likely to be noticeable or not.

--
John Stumbles

Who's *really* behind all these conspiracy theories?

In article
,
robgraham wrote:
Regardless of the battery type, if you have a say a 1.4AH and a 2.5AH
battery driving the same device, I'm of the opinion that the actual
power output of the motor is going to be same during the 'flat' part
of the battery capacity period and that only as the battery begins to
reach the end of its charge capacity will the power start to drop off
- true or false ?

The peak current output of a battery is nothing to do with its capacity.
More to do with the quality of the cells. Cheap cordless drills can often
have their performance dramatically improved by changing the battery cells
to a better make.

--
*When a clock is hungry it goes back four seconds*

Dave Plowman London SW
To e-mail, change noise into sound.

robgraham wrote:

I've just had a nearly heated discussion with my son in law (SIL)
about the power from rechargeable batteries and we both agreed in the
end that we really weren't that sure of our positions !

Regardless of the battery type, if you have a say a 1.4AH and a 2.5AH
battery driving the same device, I'm of the opinion that the actual
power output of the motor is going to be same during the 'flat' part
of the battery capacity period and that only as the battery begins to
reach the end of its charge capacity will the power start to drop off
- true or false ? The voltage will be the same but is the internal

Basically true all other things being equal.

The complication comes from the fact that many better quality packs are
also higher capacity. So you may also see an improvement in performance
with the higher capacity pack. However that is not *because* its higher
capacity, only its a better quality[1].

(I have a some DeWalt 14.4V packs - one is 1.3Ah and the others are
2.0Ah - its notable that performance is better from the higher capacity
pack in two ways; peak current delivery is better (more torque), and
matching is better - so you get a wider "flat" part of the discharge
curve with a pronounced "cliff effect" at the end on the bigger packs,
than with the smaller one)

resistance and therefore the maximum current output going to be the
same at full charge regardless of the capacity ?

Interestingly I discovered, before reading the instructions (!), that
my new Ryobi drill Li-ion batteries will switch themselves off before
they reach the end of capacity and that I don't have to make a
considered judgement on the critical point like my old Bosch with Ni
Cd batteries.

Yup, LiIon cells need much more "management" when charging / discharging
and have to be treated differently to other rechargeable technologies.
Hence why the tools / chargers etc are made with different contacts and
fittings from the older technology versions.

[1] Quality being a nebulous term, but implying a number of attributes
including lower cell internal resistance, better cell discharge curve
matching etc. Note also that different battery chemistries will have
differing minimum internal resistances and hence a knock on effect on
peak current delivery.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Jun 19, 9:05*pm, robgraham wrote:
I've just had a nearly heated discussion with my son in law (SIL)
about the power from rechargeable batteries and we both agreed in the
end that we really weren't that sure of our positions !

Regardless of the battery type, if you have a say a 1.4AH and a 2.5AH
battery driving the same device, I'm of the opinion that the actual
power output of the motor is going to be same during the 'flat' part
of the battery capacity period and that only as the battery begins to
reach the end of its charge capacity will the power start to drop off
- true or false ? *The voltage will be the same but is the internal
resistance and therefore the maximum current output going to be the
same at full charge regardless of the capacity ?

Interestingly I discovered, before reading the instructions (!), that
my new Ryobi drill Li-ion batteries will switch themselves off before
they reach the end of capacity and that I don't have to make a
considered judgement on the critical point like my old Bosch with Ni
Cd batteries.

Rob

It depends on the ratio of battery internal resistance to load
resistance. If the load R is much higher than batt R then the same
terminal voltage will be present on both batteries under load. This is
usually how things work.

But if the battery R becomes a significant percentage of the total
Rbattery + Rload, then the 2 batteries woudl give different terminal
voltages under load. This occurs when the load current is high, making
battery internal resistance significant.

For a given cell design, a larger capacity cell has a proportionately
larger conductance, and R = 1/conductance. However IRL 2 different
batteries can have differing internal details, and their internal Rs
may have other than that relationship.


NT

Calvin Sambrook wrote:


I don't know the answer to the underlying question you are asking which
I think is "is the internal resistance of a battery related to its
capacity?"

I agree, this seems to be the underlying question.

however in practical usage the power is determined not by the
battery internal resistance but rather the load so you are correct the
power output of the motor will be the same during the flat part of the
discharge curve.


Actually, by virtue of the maximum power transfer theorem, maximum power
is transferred when the load impedance equals the source impedance.

Assuming a pure resistive load (and an ideal battery) then max power is
delivered when the load resistance is equal to the battery's internal
resistance.

AIUI, during the "flat part of the discharge curve", the internal
resistance of the battery stays about the same. At the end of the "flat
part of the curve" the internal resistance rises as the battery is
depleted leading to a reduction in available power.

To address the underlying question, the volume of a notional water tank
is akin to the Ah capacity of the battery; the height of the water
column is akin to the voltage of the battery and the diameter of the
outlet tube is akin to the internal resistance of the battery.

As you can imagine, increasing the volume of the water tank does not
*necessarily* change either the voltage or the diameter of the outlet
tube - it just (to horribly mix a metaphor) makes the "flat part of the
curve" last longer for a given load.

On Sat, 20 Jun 2009 19:27:40 +0100, Dave Osborne wrote:

Actually, by virtue of the maximum power transfer theorem, maximum power
is transferred when the load impedance equals the source impedance.

Assuming a pure resistive load (and an ideal battery) then max power is
delivered when the load resistance is equal to the battery's internal
resistance.

Indeed, and if the system were set up for maximum power transfer then
you would only need a much smaller battery, with much higher internal
resistance to match that of the load. However with as much power being
dissipated in the internal resistance of the battery as is used in the
load it would be a toss-up whether the battery would explode due to
overheating before it ran flat due to its lower capacity.

In the case of battery-operated equipment you want *sufficient* power
delivered to the load in such a way that *most* of the available *energy*
in the battery ends up in the load with as little as possible dissipated
in the battery itself.

--
John Stumbles

The astronomer married a star

In article ,
Dave Osborne wrote:
Actually, by virtue of the maximum power transfer theorem, maximum power
is transferred when the load impedance equals the source impedance.

That's fine in theory - but pretty well everything works best when the
source impedance is as low as possible. We're not really interested in
efficiency in this case.

--
*Someday, we'll look back on this, laugh nervously and change the subject

Dave Plowman London SW
To e-mail, change noise into sound.