>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.
>
>
>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.
>

> 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
>
>
>

> 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