[chbot] Big batteries

[Robin Gilks]

>
> Depending on your application, telecom backup batteries may not be
> what you want... you may already be aware of this, but there are quite
> different flavours of lead-acid batteries. They are all based on the
> same chemistry, but there are huge differences between the plate
> designs.
>
> There are specific batteries for cars (lighting / starting / ignition
> = LSI), which are designed for high currents but don't like being
> cycled. Then there are traction / deep-cycle batteries that are
> designed for being cycled over their full capacity, but they don't
> tolerate quite as high currents as the LSI batteries (I don't know
> whether the ones used for storage in solar-electric applications are
> simply from this category, too, or whether they are again specific to
> their intended use). And then there are backup batteries like the ones
> used in telecom applications, and these are optimised for being
> floated fully charged for long periods of time, without being cycled
> much at all. This is just a very superficial overview, but the point
> is that if you are using a lead-acid battery for an application
> contrary to its design, it will not perform well, and it may shorten
> the battery's life significantly.
>
> Kind regards,
>
> Helmut.

Don't get me started on battery types and uses - I'll ramble on for hours
- its amazing what I've learnt from both books and experience in the four
years I've been playing with wind turbines!!

One thing is for certain - as Hadley rightly says - don't expect to use
more than 20% of the capacity if you want more than a door stop in a few
months time, as I found to my cost with a couple of 150 amp-hr truck
batteries.

Its not just that 20% or so that is the battery killer, but also charge
and discharge rate - most deep cycle batteries will discharge happily at
5C (where 'C' is the amp-hr capacity at the 20 hour rate) but can only be
charged at 1C. Hence my looking for something about double my existing 450
amp-hrs as my maximum charge current often goes well over 50amps (2C).

All lead-acid based chemistry batteries have the same problem - they can
convert lead oxide and (metallic) lead to lead sulphate quicker than the
reverse, it mostly depends on the electrolyte medium. Absorbed glass mat
(AGM) such as the telecom style standby batteries are not as forgiving
during charging as flooded types used in cars and golf carts but are more
efficient, have lower self discharge, generally of better build quality
and no outgassing during operation (so safer).

The higher capacity also results in a lower internal impedance so that
helps regulate the output voltage of the turbine generator.

Enough ramblings for now....

Cheers all

-- 
Robin Gilks