On Fri, 06 Jan 2006 19:02:40 +0000, Steve Firth wrote:

The Natural Philosopher wrote:
On Wed, 04 Jan 2006 15:35:15 +0000, Steve Firth wrote:

Dave Plowman (News) wrote:
In article ,
Steve Firth wrote:
All of Drivel's theories are blown away by the poor thermal
characteristics of current battery technology. The high charge density
lightweight batteries have much lower thermal efficiencies than
lead-acid batteries.
I find it interesting that many car makers are going to expensive lengths
to save weight - extensive use of aluminium etc. But still fit lead acid
batteries - where cost really isn't a consideration.

It's because lead acid batteries don't need complex charge controllers,
work across a wide range of environmental temperatures and provide huge
cranking currents time after time. They are big, rough and tough and
also extremely efficient compared to any of the alternatives around.
Where they fail is on charge per kg which is where Lithium based
batterys win. Where Lithium batteries fail is on charge/discharge
cycles, efficiency and the need for complex charge cycles and careful
management of discharge.

Actually, most of that is utter tosh.

Sadly, not so.

Lithium bateries have almost excatly te same charge characteists as Lead
Acid - constant current with a contsant voltage limit.

200-1000 cycles or about 3 years, whichvere is first, is teh cycle time.

A 70Ah LIPO battery will, on current technolgy, provide about 2KA without
exploding for maybe 5-10 seconds. It will certainly provide 1KA
continuously.

And can be charged at up to 70A..more than most alternators can deliver.

This is not theoretical knowledge: This is how we fly these *******s.

Sure and by doing so you get a relatively poor round trip efficiency
from the battery.

We get about 90% like this. I guess whether that poor, is down to your
definition.

At more modest rates, say 1 hour discharge - its beyond my capacity to
measure the difference between whats gone in and what comes out.


If you want to get the best out of Lithium batteries,
i.e. to get a round trip efficiency of 90% then both charge and
discharge need to be controlled as described, otherwise more of the
energy is going into waste heat. It's possible to get Lithium batteries
down to 50% efficiency if you trea them like lead acid batteries.


Its not. They will destroy themslves first :-)

It is almost IMPOSSIBLE to fly LIPOS at less than 90% efficiency round
trip.

The internal resistance is way lower thanlead acid, and very comparable
with nickel cadmium thses days.

To run them at 50% efficiency implies currents high enough to melt the
electrodes and to instantly explode the battery.


You can treat a battery rough if you're not bothered about efficiency.

The downside is the cost - $1000 currently, though in volume they should
be no more expensive than lead acid ultimately - and safety. The high
discharge rates are achieved by using a fairly aggressive and inflammable
organic polymer, and it can and does, ocasionally go pop under adverse
contitins (over temeperatire, shorting, over charging) and cause a seriosu
fire hazard.

So can a lead acid if course.

Ther are other, heavier and lower charge/discharge rate batteries coming
along that are less flammable. These should be suitable for car usage.

"coming along", "should be".

Yes inded. Look at te new range of batteries descroibed in te URL someone
posted as another thread. OK its pure advertsing, BUT these cells are going
into poro pwqower tools this year. We can see how good they are.