🚀 go-pugleaf

Bhoot Nath wrote:
> 
> Three of the six smoke alarms in began chirping a few days back. I
> presume that this means that the 9 volts batteries must be replaced.
> 
> So, we thought, rather than buying alkaline batteries, why not buy
> rechargeable ones. We already have a recharger that works with
> AA/AAA/9V etc. After a few searches at yahoo.com, excite.com &
> mysimon.com we found out that cheapest mail order is about $7, about
> the same as the nearest Walmart.
> 
> 1. Should we get Nickel Cadmium (NiCad) or Nickel Metal Hydride (NiMH)
> batteries for a smoke alarm? Is there a reason to prefer one vs other?

Neither.

> 2. We looked at an EVEREADY pack and it was marked "9 volts (Use 7.2
> volts)" ... Does this mean it would deliver only 7.2 volts? If so, is
> it safe to use it for a smoke alarm? If not, are there any rechargeable
> 9 volt batteries that deliver 9 volts?

A fully charged, fresh 6-cell Nicad will deliver about 6*1.4V=8.4V.
After the initial charge wears off (a week or two) it'll be down to
6*1.2V=7.2V.

Ni-Cad batteries will typically have a few months of shelf-life
between charges.  This means that you'll have to charge the batteries
every few months even if there were no drain.  Typical "9V"
Nicads have a capacity of 100 mAH or so, compared to several
hundred mAH for a alkaline 9V, so you might not even go a few
months between recharges.

Typical *overall* life of the Ni-Cads in this sort of use will be
a few years.  Knowing that an alkaline 9-volt will last 6 months
or a year, are you really saving any money by going to Ni-Cads?

Ni-Cads are a real winner in high-drain frequent cycle applications,
like portable audio equipment, flashlights, cellphones, kids toys, etc.
Here their short (a few weeks to a few months) charge retention and high
peak current capacity make them suitable.  For a low drain application
like a smoke alarm they're poor choices.

> 3. On a web site, I also saw a reference to "Super long lasting,
> industrial strength 9 volt alkaline batter"  for $20, about 10 times
> what a regular 9 volt battery costs. Any one has any experience with
> this kind of battery? WOuld it last 10 years in a smoke alarm?

There are lithium batteries in a 9-volt package that offer excellent
shelf life and more capacity than an alkaline.  These are sold under
Kodak and a few other brand names, and indeed they cost a lot more
than alkalines.

I would stick to the manufacturer's recommended alkaline batteries
if I were you... and don't forget to regularly test the alarms!

Tim.