As there are no other requirements (cost?  :-) ) then I would go for
individual SLA cells.  Four will give you 8V.  Pay careful attention
to charge and discharge and you'll likely have a very reliable, long
lasting, and inexpensive solution.

It may be worth looking at Li-Ion batteries and their variants due to
their low discharge.  They aren't that much more dangerous than SLA -
it's just that when SLAs leak or explode they usually aren't leaking
or exploding in someone's pocket.  The problem is the cost, both for
the batteries, and the charge control circuitry.

Lastly, consider a generator/motor with a windup spring to store the
1AH you need, coupled with a lithium coin cell or two for memory.  Use
a catch with a low current solenoid to prevent the spring from
unwinding when you don't need the large current.  It would only be
cost effective if you were planning on making thousands, and it would
require a good mechanical engineer, but it solves a whole bunch of
problems you don't have, and creates a bunch of others you don't want.
 Wouldn't that be fun?

-Adam

On 12/15/05, Russell McMahon <apptech@paradise.net.nz> wrote:
> I have a volume application that requires a battery.
> Best choice seems to be "sealed lead acid" (SLA) for reasons listed
> below. Suggested alternatives welcome.
>
> NB: Solutions MUST NOT redefine the problem, suggest alternatives
> which do not meet all criteria etc :-). Often enough when I ask
> similar questions many answers address requirements which I do not
> have. There are reasons for most of the requirements . I will
> partially explain some of them if it seems useful.
>
> Application:
>
> Exercise equipment operated by 'user'. When operating, power is
> provided by user actions via an 'alternator'. When at rest or when
> user speed drops too low (startup, programming, pausing etc) power is
> supplied by a battery. Battery is recharged by user action when
> possible. Current drain is up to 250 mA. Supply internal rail is
> typically 5V but may use the dread LM7805 type regulators so input
> needs to be say 8v min (dropout, headroom, wiring loss, connector
> loss, ...). Higher OK but too much higher may stress 7805 thermally as
> manufacturer probably anticipated a 6 cell battery pack (nominal 9v).
> Equipment is in two parts. Part A draws up to 250 mA as above and its
> design cannot be altered. Part B draws under 20 mA, design is flexible
> and includes power conversion and control functions.
>
> Requirement:
>
> Rechargeable battery.
> Good shelf life under self discharge.
> Lowest practical cost.
> OK recharge rate (faster the better).
> Reasonable capacity - say 1 to 2 AH OK.
> Battery life needs not be optimum but should be reasonable. ie some
> violation of best practice float etc voltages and charge currents is
> OK as long as battery life of say 3+ years is achievable.  Much of
> this equipment gets very little use but some is used extensively. A
> mains supply is a possible option in some cases but cannot be relied
> on. Mass and volume energy densities are not important.
>
> Alternator output is rectified to a smoothed DC rail and may reach 120
> VDC plus in some cases. For reasons not covered here this is to be
> converted to battery charging and equipment supply voltage by a linear
> regulator so dissipation is up to about 30 Watt worst case for short
> periods. This can be handled OK.
>
> ____________
>
> Solution so far:
>
> 12v SLA. Typically 1.2 or 3 AH.
>
> 6v SLA with a converter to lift output to about 8v for "part A" supply
> is a possibility but cost an issue.
>
> 12V SLA requires a voltage rail from alternator of 15v+ to get enough
> headroom to charge SLA.
> Dissipation in equipment part A at 250 mA is therefore about
> (15-5)*0.25 =
> 2.5 watt. A series resistor can be added to drop some of this.
> SLA charging circuit is crude but acceptable (and cheap).
> Self discharge of 10% month - may be less. Varies rather with
> construction and chemistry.
>
> NiCd, NimH have excessive self discharge rates (typically 20% & 30%
> month)
>
> Lithium Ion. Cost?
> Capacity degradation with time regardless of usage.
> Goodish self discharge.
>
> Lithium Ion Polymer - better self discharge than Nickel chemistries.
> Wide voltage swing charged to discharged.
> Price ?
>
> Other?
>
>
>
>         Russell McMahon
>
>
>
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist