On Fri, 22 Mar 2013 16:57:55 -0000, "Keith W"
wrote:

GunnerAsch wrote:
On Fri, 22 Mar 2013 12:40:50 -0000, "Keith W"
wrote:

GunnerAsch wrote:
On Fri, 22 Mar 2013 09:23:17 -0000, "Keith W"
wrote:


The conventional Batteries are sometimes called Sealed Lead Acid
Batteries but they are actually AGM batteries.

Modern passenger aircraft normally use Nickel Cadmium batteries

Why havent they converted over to NmH?

They have a relatively high self discharge rate and can lose
up to 20% of the energy stored in the first 24 hours. This is
acceptable for hybrid vehicles where the battery is primarily a
temporary buffer to capture the energy from regenerative
braking but not good for a system intended to initiate an
aircraft startup sequence after a week in the hangar.

Keith

Thats not true anymore. Since Sanyo developed the Enerloop NiMH
battery..they are being shipped charged from the factories. All the
makers are using the new tech and have been doing so for at least 3
yrs. Ive got NiMH batteries that I only need to put on the charger
ever 6 months, just to top them off.

Gunner

Trouble is they are not available in the size or capacity to be used
in an electric vehicle, last time I checked the largest was a D size.

Keith

Oh, they are available a lot bigger than that. Just not readily
available to the public.

From the mpoweruk site:

Unlike the consumer applications where NiMH has been almost completely
replaced by Lithium ion, NiMH chemistry is still finding use in
automotive applications where it is the technology of choice for
powering HEVs and where it has accumulated over 10 years of trouble
free service and can thus last for the lifetime of the car. The
operating temperature range for NiMH cells has been extended to over
100 °C (-30 °C to + 75 °C) which far exceeds the temperature range
currently achievable by Lithium cells making NiMH technology ideal for
automotive use. NiMH can handle the high power levels typical in EV
applications, the active chemicals are inherently safer than Lithium
based cells and NiMH batteries don't need the complex battery
management systems (BMS) essential with Lithium batteries.

Early cells were susceptible to memory effect and also suffered from
high self discharge, nearly ten times worse than Lead acid or Lithium
batteries, but both of these weaknesses have been overcome and it is
claimed that the most recent cells can hold their charge for a year.


Advantages
High energy density (W/kg), about 50% better than Nicads, but only
about 60% of Lithium ion.

Low internal impedance though not as low as NiCads

Typical cycle life is 3000 cycles.

Can be deep cycled. (80% to 100% DOD)

Using NiMH batteries, more than 3000 cycles at 100 % Depth of
Discharge (DOD) have been demonstrated. At lower depths of discharge,
for example at 4 % DOD, more than 350.000 cycles can be expected.

Robust - NiMH batteries also tolerate over charge and over discharge
conditions and this simplifies the battery management requirements.

Flat discharge characteristic (but falls off rapidly at the end of the
cycle)
Wide operating temperature range

Rapid charge possible in 1 hour

Trickle charging can not normally be used with NiMH batteries since
overcharging can cause deterioration of the battery. Chargers should
therefore incorporate a timer to prevent overcharging.

Because of potential pressure build up due to gassing they usually
incorporate a re-sealable vent valve

Reconditioning is possible.

Environmentally friendly (No Cadmium, Mercury or Lead)

Much safer than Lithium based cells in case of an accident or abuse
due to the use of more benign active chemicals, a particularly
important property in high power and automotive applications.



Shortcomings
High self discharge rate.

Can be stored indefinitely either fully charged or fully discharged.

Suffers from memory effect though not as pronounced as with NiCad
batteries

Battery deteriorates during long time storage. This problem can be
solved by charging and discharging the battery several times before
reuse. This reconditioning also serves to overcome the problems of the
"memory" effect.

High rate discharge not as good as NiCads

Less tolerant of overcharging than NiCads

As with NiCads the cells must incorporate safety vents to protect the
cell in case of gas generation.

The coulombic efficiency of nickel metal hydride batteries could be up
to 85% but is typically only around 65% and diminishes the faster the
charge although this is projected to improve.

While the battery may have a high capacity it is not necessarily all
available since it may only deliver full power down to 50% DOD
depending on the application.

Cell voltage is only 1.2 Volts which means that many cells are
required to make up high voltage batteries. The competing Lithium
cells typically have 3 times the cell voltage (3.2 Volts to 3.7 Volts)
and a much higher energy density.

Lower capacity and cell voltage than alkaline primary cells.

Limited supplies of rare earth element Lanthanum. Mostly in China.