On 6/19/2013 7:07 PM, [email protected] wrote:
It still does not answer or address where the energy comes from that was
lost in raising the cell or cells with the higher resistance over ambient
temperature.
During charging, the charger supplies more energy, since the pack
voltage is higher (due to the resistance).
Perfect 100v 100ah pack (with zero resistance):
- Charger supplies 100v x 10a x 10h = 10kwh
- Batteries get 10a x 10h = 100ah of charge
Same 100v 100ah pack, but with resistance:
- Charger supplies 110v x 10a x 10h = 11kwh
- 1kwh of heat is produced in the resistance
- Batteries get 10a x 10h = 100ah of charge
During discharge, the batteries supply the same amount of energy, but
some of it is burned up as heat (in the cell resistance), rather than
powering the load.
Perfect 100v 100ah pack (with zero resistance):
- Controller loads batteries 100v x 10a x 10h = 10kwh
- Batteries deliver 10a x 10h = 100ah of charge
Same 100v 100ah pack, but with resistance:
- Batteries sag to 90v under 10a load
- Load gets 90v x 10a x 10h = 9kwh
- 1kwh of heat is produced in the resistance
- Batteries still deliver 10a x 10h = 100ah of charge
As an example NiMH cells in the past have a very high self discharge
rate. You can charge up a pack and they lose energy while reaching a full
charge. Cells that have higher resistance or are in a higher state of charge
lose more energy as heat then the others. If you let the pack sit in a fully
charged state they self discharge and the energy loss produces heat. Enough
that it actually helps keep the pack warmer in the winter.
Both lead-acid and nimh have "side" chemical reactions (gassing and
heating) that can occur. For nickel-based cells (NiCad, Nimh, NiFe)
these side reaction starts above 1.45 v/cell, and by 1.54v/cell this
side reaction is consuming essentially 100% of the applied current. For
lead-acid, it starts above 2.27v/cell, and by 2.5v/cell the side
reactions are consuming essentially 100% of the applied current.
The side reactions are strongly related to voltage and temperature. A
small increase in voltage or temperature causes a dramatic increase in
the current bypassed by them.
Lithium cells (mostly) don't have these side reactions, so the
self-discharge rate is much lower. On the other hand, there is nothing
short of destruction of the cell to limit the voltage rise.
--
Don't worry about people stealing your ideas. If your ideas are any
good, you'll have to ram them down people's throats. -- Howard Aiken
--
Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
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