[oclug] Batteries

[Greg Franks]

Gorm Jensen wrote:
    >> In the not-too-distant future, my 3 UPS units will need new
    >> batteries.  Is there a technical reason not to buy a car
    >> battery, which has 10 times the capacity for half the price of
    >> an exact replacement?

>>>>> "Raj" == Raj  <thukral at pobox.com> writes:

    Raj> three reasons I can think of right away: 1. Car batteries
    Raj> give off fumes and need to be run in a ventilated area - the
    Raj> fumes are corrosive, explosive, poisonous!  I may be wrong on
    Raj> this and the newer car batteries may be sealed but I doubt
    Raj> it.  

They give off hydrogen gas (sometimes) which is why you want to have
them in a well ventilated area (ka-boom!).  The electrolyte is
corrosive (sulfuric acid), but if you're not going to tip them over,
you don't have a big problem.  Lead-acid batteries are used in diesel
submaries.  If they outgassed poisonous/corrosive fumes, one would
certainly have a problem.  The UPS probably has a lead-acid "gel-cel"
which gets rid of the liquid problem.  (The UPS's I've seen have
them).

    Raj> 2. There will be a problem with the charge rate and
    Raj> characteristics - since the battery capacity is higher, it
    Raj> should theoretically need a higher charging current and it
    Raj> may also have different charging characteristics from the
    Raj> original.  Which means the lower capacity charger built into
    Raj> the UPS may not charge the batteries to their full capacity
    Raj> or take too long to charge them (best case) or even
    Raj> overcharge the battery which would lead to reduced life and
    Raj> capacity and may also damage the battery (worst case).

Lead-acid batteries self regulate in that when they charge up, their
back-emf goes up, so current goes down (that's why when your car's
battery is fully charged, the voltmeter shows 14V).  They survive in
an automobile environment (and other equally horrible conditions
(i.e. boats :-)) quite well.  Old cars did not have fancy voltage
regulators or charging circuits.  Lead-acid batteries can also be
trickle charged, so they will eventually reach full capacity.  Don't
deep-discharge them though -- they don't like that at all!   

Nicads don't have this property, so they need fancier chargers.
Lithium Ion batteries are even more finicky.

    Raj> 3. There may be no warranty on the battery if you use it in
    Raj> an application it was not designed for.

    Raj> cheers!  --Raj.

-- 
   __@               Greg Franks              <|       _~@ __O 
 _`\<,_         Ottawa, Ontario, Canada        |O\   -^\<;^\<, 
(*)/ (*)                                       (*)--(*)%---/(*)
          "Where do you want to go today?"   Outside.