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

 

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