Rechargeable batteries worthwhile for smoke alarms?

Hexane

We have five wireless smoke alarms which are battery-powered and each take a 9V battery (the rectangular type). At present buying cheap supermarket non-branded alkaline non-rechargeable replacement batteries for these works out at about £6 a year on replacements because the batteries last about two years. So of course these devices have a very low power draw. I don't think we have any other commonly used devices that use 9V batteries of that physical size; we do have various devices that take AA and AAA batteries and we already have some rechargeable batteries and two chargers for those.

I've seen mention that rechargeable 9V batteries vary in their construction, with the result that it's recommended to get the charger and the battery from the same manufacturer.

Argos will sell me a battery charger that claims to recharge 9V batteries (as well as other types) for £25  ( https://www.argos.co.uk/product/9800830?clickPR=plp:5:5 )

and will also sell me rechargeable 9V batteries, from the same manufacturer, for £8 each ( https://www.argos.co.uk/product/9804070?clickPR=plp:4:5 )

So buying the batteries for all five smoke alarms, and the charger to charge them with, would cost a total of £65.

So that would mean a payback time of more than ten years, for which the charger and the batteries would need to last that long and the batteries would need to cope with being recharged probably at least six times each.  And I would need to charge the batteries "for free" from my surplus solar generation (which is easy to do.)

Recharging the batteries every two years (or even half that) isn't significantly more of a chore than buying new non-rechargeable batteries every two years.

The batteries and the charger both only have a one year guarantee.

If this payback time was for a solar panel installation, it wouldn't be seen as particularly bad. And a solar panel installation can't easily be taken away when one leaves the house, whereas these items can. (Assuming they have a use at the new house.)

So, is it worth it?

And also, are there reasons that rechargeable batteries are particularly unsuited to very low power draw applications such as (wireless) smoke alarms?

Hexane

Slightly messing up the payback calculation here, is Amazon offering "CITYORK 9V NiMH/NiCD Li-ion Battery Charger with 4 Pack 300mAh 9V Ni-MH Rechargeable Batteries" for £16.99 (around a quarter of what I would pay for the named brand offering from Argos) and offering 9V non-rechargeable batteries for less than a pound each when bought in larger packs. That changes the payback time to roughly seven years...if the products can be relied upon.

I will mention that one of my old AA/AAA battery chargers still works despite being twenty years old, although it's not been in regular use that whole time, and certainly none of the batteries that came with it have survived that much regular use.

QrizB

Most (not necessarily all) household rechargeable batteries have a high enough self-discharge rate that they'll need charging every 2-3 months. The exception would be something like an Eneloop or a lithium-ion battery (neither of which I've seen in 9v sizes).

Also be aware that few 9v rechargeable batteries actually give 9 volts. Some are 7.2v and some are 8.4v. The 8.4v ones usually work where a 9v one would but the 7.2v ones can be more problematic.

matelodave

A 9v alkaline battery has a capacity of  around 1700mAh whereas the the batteries you are suggesting only have a capacity of 175mAh which is ten time less - you could end up having to recharge them every three months or less.

I use these in my smoke alarms https://docs.rs-online.com/fabf/0900766b81549eff.pdf and they last around 10-11 years (our preset one is eleven years old and the smoke alarm is still function (it detected the burnt toast yesterday)

Other makes are avaialble, Varta, GP, Energiser - https://www.batterystation.co.uk/battery-types/lithium-batteries/9v-lithium-batteries.html

Note they are non rechargeable lithium, not rechargeable lithium-ion

Martyn1981

I've got three smoke alarms with rechargeable 9V equivalents, but they only last about 9-12 months, and of course always go off in the night (when it's colder) and you stumble around trying to work out which one is beeping.
Not sure I'd go down the rechargeable route if I hadn't already, might be better to have a stronger and longer lasting 'proper' battery in them.
Sorry to be so 50:50 about it.

Coastalwatch

We've only one smoke alarm and just put one of the longer life Puracell batteries in the hope it does actually last longer. We've been here five years now and fitted that new one last year.

I guess only time will tell.

No doubt we should probably have one in each room but, as neither of us smoke and we've no gas appliances then reckon the risk is somewhat less than it might be otherwise.

Rodders53

Smoke alarms have a finite life before replacement is needed...  See the expiry date on the sticker.  Typically around 10 years or thereabouts.
Ionisation ones (no longer sold/recommended) due to depletion of the radioactive material. Others simply down to sensor aging and altered efficacy / response times to smoke (fire).  

Even mains operated interconnected alarms, where the battery is just a backup for power cuts.

Smoke alarms are available with 10 year life lithium cells included. In both standalone and mains versions.  Consider buying them when you next need a new smoke alarm.  The lifetime cost is probably lower, and certainly more convenient.

QrizB

Rodders53 said:

Ionisation ones (no longer sold/recommended) due to depletion of the radioactive material.

Considering that the half life of americium-241 (the most common isotope in smoke alarms) is 432 years, depletion is unlikely within the lifetime of anyone reading this sentence.

I test my alarms occasionally with smoke, rather than just the self-test button. I tested the one in the kitchen this morning (by accident) using the toaster!