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[EVDL] A123 M1 Capacity question

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E
G'Day all! I've been lurking for a while, and just had a question
about the a123 M1 cells found in the DeWalt 36V battery packs. This
has almost certainly been covered before, but I can't remember the answer. :)

The a123 website says the M1 cell (found in DeWalt packs) is 3.3V and
2.3AH, yet I've read on web forums and the like that the DeWalt pack
is also 2.3Ah. Surely both can't be correct? I was of the
understanding that, wether connected in Series or Parallel, capacity
was the combined total, ie, 2.3*10=23Ah, for the DeWalt packs, just
like the Voltage is 3.3*10=33V (36V). I understand that Voltage is
different depending on configuration (33/36V for Serial, 3.3V for Parallel).

Cheers.

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Joined
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72,624 Posts
Discussion Starter · #2 ·
Connecting batteries in series increases the voltage. Connecting the batte=
ries in parallel increases the AH.
Beano -- 1981 Ford Escort EV =

EValbum 1010Ted Sanders

> Date: Mon, 24 Sep 2007 01:12:26 +1000> To: [email protected]> From: defau=
[email protected]> Subject: [EVDL] A123 M1 Capacity question> > G'Day all! I'=
ve been lurking for a while, and just had a question > about the a123 M1 ce=
lls found in the DeWalt 36V battery packs. This > has almost certainly been=
covered before, but I can't remember the answer. :)> > The a123 website sa=
ys the M1 cell (found in DeWalt packs) is 3.3V and > 2.3AH, yet I've read o=
n web forums and the like that the DeWalt pack > is also 2.3Ah. Surely both=
can't be correct? I was of the > understanding that, wether connected in S=
eries or Parallel, capacity > was the combined total, ie, 2.3*10=3D23Ah, fo=
r the DeWalt packs, just > like the Voltage is 3.3*10=3D33V (36V). I unders=
tand that Voltage is > different depending on configuration (33/36V for Ser=
ial, 3.3V for Parallel).> > Cheers.> > ____________________________________=
___________> For subscription options, see> http://lists.sjsu.edu/mailman/l=
istinfo/ev
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Discussion Starter · #3 ·
both are 'correct'
capacity is not measured in Ah. it's Wh (watthours or powertime)
so the capacity is the voltage times the amphours (Ah).amp hours means
how many amps you can pull in an our. but the power is watt

the cells in the dewalt packs are in series in which the voltage is
added. when they are in parallel the amp power is added. also the Ah

so when you want to know the capacity of a battery setup you multiply
the voltage and the Ah number.

Dan

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Discussion Starter · #5 ·
Don't be confusing him now. Usually, capacity = Ah, energy=Wh


----- Original Message -----
From: "Dan Frederiksen" <[email protected]>
To: "Electric Vehicle Discussion List" <[email protected]>
Sent: Sunday, September 23, 2007 11:46 AM
Subject: Re: [EVDL] A123 M1 Capacity question


> both are 'correct'
> capacity is not measured in Ah. it's Wh (watthours or powertime)
> so the capacity is the voltage times the amphours (Ah).amp hours means
> how many amps you can pull in an our. but the power is watt
>
> the cells in the dewalt packs are in series in which the voltage is
> added. when they are in parallel the amp power is added. also the Ah
>
> so when you want to know the capacity of a battery setup you multiply
> the voltage and the Ah number.
>
> Dan
>
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev

_______________________________________________
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72,624 Posts
Discussion Starter · #6 ·
that would be like stating the capacity of a cylindrical tank as the
cross section rather than the volume.
as for confusion, that was the source of his original confusion

energy is what it stores. capacity is how much it can store. in joule or
equivalent units (Watthours, kilowattseconds etc)

Dan

Dmitri wrote:

>Don't be confusing him now. Usually, capacity = Ah, energy=Wh
>
>
>----- Original Message -----
>From: "Dan Frederiksen" <[email protected]>
>To: "Electric Vehicle Discussion List" <[email protected]>
>Sent: Sunday, September 23, 2007 11:46 AM
>Subject: Re: [EVDL] A123 M1 Capacity question
>
>
>
>
>>both are 'correct'
>>capacity is not measured in Ah. it's Wh (watthours or powertime)
>>so the capacity is the voltage times the amphours (Ah).amp hours means
>>how many amps you can pull in an our. but the power is watt
>>
>>the cells in the dewalt packs are in series in which the voltage is
>>added. when they are in parallel the amp power is added. also the Ah
>>
>>so when you want to know the capacity of a battery setup you multiply
>>the voltage and the Ah number.
>>
>>Dan
>>
>>_______________________________________________
>>For subscription options, see
>>http://lists.sjsu.edu/mailman/listinfo/ev
>>
>>
>
>_______________________________________________
>For subscription options, see
>http://lists.sjsu.edu/mailman/listinfo/ev
>
>
>

_______________________________________________
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Discussion Starter · #7 ·
Oh great....now you want to redefine energy and capacity...where do I get
off this train?

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf
Of Dan Frederiksen
Sent: Sunday, September 23, 2007 12:07 PM
To: Electric Vehicle Discussion List
Subject: Re: [EVDL] A123 M1 Capacity question

that would be like stating the capacity of a cylindrical tank as the
cross section rather than the volume.
as for confusion, that was the source of his original confusion

energy is what it stores. capacity is how much it can store. in joule or
equivalent units (Watthours, kilowattseconds etc)

Dan

Dmitri wrote:

>Don't be confusing him now. Usually, capacity = Ah, energy=Wh
>
>
>----- Original Message -----
>From: "Dan Frederiksen" <[email protected]>
>To: "Electric Vehicle Discussion List" <[email protected]>
>Sent: Sunday, September 23, 2007 11:46 AM
>Subject: Re: [EVDL] A123 M1 Capacity question
>
>
>
>
>>both are 'correct'
>>capacity is not measured in Ah. it's Wh (watthours or powertime)
>>so the capacity is the voltage times the amphours (Ah).amp hours means
>>how many amps you can pull in an our. but the power is watt
>>
>>the cells in the dewalt packs are in series in which the voltage is
>>added. when they are in parallel the amp power is added. also the Ah
>>
>>so when you want to know the capacity of a battery setup you multiply
>>the voltage and the Ah number.
>>
>>Dan
>>
>>_______________________________________________
>>For subscription options, see
>>http://lists.sjsu.edu/mailman/listinfo/ev
>>
>>
>
>_______________________________________________
>For subscription options, see
>http://lists.sjsu.edu/mailman/listinfo/ev
>
>
>

_______________________________________________
For subscription options, see
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_______________________________________________
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Discussion Starter · #8 ·
Hi Dominant,

Let me try to explain the issue you are dealing with:
Each cell of a DeWalt pack is 3.3V and 2.3Ah as you quoted.
In a pack, the 10 cells are all connected in series, which
has the effect that the same current goes through each cell.
So, if one cell can deliver 2.3 Amp for 1 hour (2.3 Ah) then
each of the 10 cells can deliver that 2.3 Amp for 1 hour
before they are empty. Since the same current goes through
all of them, they all reach 'empty' at about the same time
after 1 hour of 2.3 Amp, so it is correct that the pack
has the same Ah rating as the single cell.
Only if a pack would have cells in parallel would the Ah
rating increase. For example if it had 2 strings in parallel
then each string could deliver half the current, so it would
last twice as long; or you could draw double the current for
the same time, in both cases the pack would have 4.6 Ah
because two cells parallel have their 2.3 Ah added up.
But then the pack would need 20 cells (2 strings of 10) to
get this double capacity - this is the same thing as having
two complete packs connected in parallel and drawing half
the current from each pack.

Now, what is the advantage if the Ah capacity does not increase
to connect so many cells in series?
The reason is that the power of the pack is defined by the
current multiplied by its voltage: P = I x U
In terms of the DeWalt pack, say that you are want to see
how much power you get when draining a cell and the pack
in 1 hour:
1 cell: Power = 2.3A x 3.3V = 7.59 Watt.
So, after 1 hour the cell has delivered an amount of energy
of 7.59 Wh (Watthour)
The pack: Power = 2.3A x 33V = 75.9W
After 1 hour the pack delivered 75.9 Wh.

So, you see that the power available to drive tools or to
propel a vehicle goes up with either current or with the
voltage. In the DeWalt packs the current capability was
apparently sufficient (the current will be much higher than
in my example, but the Ah can stay the same if the power of
the two stays equal: 23 Amp for 6 minutes is still 2.3 Ah)

If you have 10 cells you can connect them it at least 4
different ways, but you can calculate that each way delivers
the same power and energy:
1. One string of 10 cells, like in DeWalt pack.
The current is the same as for a cell, voltage is 10x as large
2. Two strings of 5 cells (current doubles, voltage is 5x)
3. Five strings of two cells (current 5x, voltage 2x)
4. All ten in parallel (current 10x, voltage same as cell)

The way a pack is connected is a design decision, dependent
on what the motors need that they want to feed with this pack.
In addition, the higher the voltage, to lower the current so
the less stress on wires and contacts, also the losses in
motor controllers tend to be lower at 33V than at 3V while the
controllers are cheaper.

Hope this clarifies,

Cor van de Water
Systems Architect
Proxim Wireless Corporation http://www.proxim.com
Email: [email protected] Private: http://www.cvandewater.com
Skype: cor_van_de_water IM: [email protected]
Tel: +1 408 542 5225 VoIP: +31 20 3987567 FWD# 25925
Fax: +1 408 731 3675 eFAX: +31-87-784-1130
Second Life: www.secondlife.com/?u=3b42cb3f4ae249319edb487991c30acb

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Dominant
Sent: Sunday, September 23, 2007 8:12 AM
To: Electric Vehicle Discussion List
Subject: [EVDL] A123 M1 Capacity question

G'Day all! I've been lurking for a while, and just had a question about the a123 M1 cells found in the DeWalt 36V battery packs. This has almost certainly been covered before, but I can't remember the answer. :)

The a123 website says the M1 cell (found in DeWalt packs) is 3.3V and 2.3AH, yet I've read on web forums and the like that the DeWalt pack is also 2.3Ah. Surely both can't be correct? I was of the understanding that, wether connected in Series or Parallel, capacity was the combined total, ie, 2.3*10=23Ah, for the DeWalt packs, just like the Voltage is 3.3*10=33V (36V). I understand that Voltage is different depending on configuration (33/36V for Serial, 3.3V for Parallel).

Cheers.

_______________________________________________
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