1.5V AA rechargeable lithium

[Tim Streater]

I've been buying cheapo AA from Amazon - the type that cost about 20p each when you get 50 or so in a pack. I'm using them for the sensors on the garage doors that they have to prevent them closing if there's an obstacle. Trouble is they only last a couple of weeks, and, having two doors, I seem to spend a lot of time changing them.

So, I wondered about rechargeable like these:

Anyone any experience with such? Charging is simplified as any USB socket, more or less, would do, and the constant 1.5V would be better than the 1.2V you get from NiMH, as I think the door unit jobbies are a bit voltage sensitive.

[The Natural Philosopher]

I've looked into these - they are 3.7v cells and a lot of electronics to make them look like 1.5V. They are not cheap. They are only about three times the capacity of a stock alkaline cell so you would still have to change them quite frequently.

Those are the facts I have to offer.

[Spike]

It might be worth your trying out a set of these primary cells first, to see what sort of life you get from them:

I use them in security and wildlife cameras, and they give good results in terms of life.

I've never yet found that cheap batteries are worth the aggravation.

[Martin Brown]

You might be better off replacing those sensors with something less grotesquely power hungry.

The reviews are less than stellar. It is a standard Lion battery and some switched mode PSU electronics to make 1.5v. Big question is how well does it manage the discharge. How quickly does it kill the cells?

Grumbles on the reviews include does not hold charge. If they were any good and available in AAA I'd be interested in a pair for my desk phone which requires a strict 2.5+v to operate the LCD caller display.

As it is I burn a couple of the cheapest and nastiest ZnCl cells every month or two to keep it fed. Fully charged NiMH last a couple of days before their terminal voltage is insufficient to drive the LCD.

[Roger Hayter]

I had some 3.7v cells which were down-converted to 3.3v. They are available for voltage sensitive equipment designed for primary lithium cells like CR123 cells. Unfortunately, I found that when first connected to a load they briefly (I couldn't be bothered to check on a 'scope) supplied 3.7v before the converter started working. This meant that the torch I wanted them for switched itself off before they did so. I don't know if other voltage converted cells do the same, but it could be catastrophic to some electronic devices to supply 2 1/2 times the rated voltage.

[Tim Streater]

Such as? And how would I do that?

[The Natural Philosopher]

how about light weights hanging off micro switches?. as te door clises, the weights contact stuff and the switches stop the door. No batteries needed.

That is why you are in a DIY group.

My BIL has a small garage and quite a medium sized car. he hung corks on strings that contact the windscreen when its time to stop...

[Tim Streater]

I'm coming to that conclusion. I'll see how I get on with better batteries. I did wonder about getting a lithium 3.xV rechargeable and wiring it in, but I'd also then need a charger

[Tim Streater]

Something mechanical is going to need to whack the bottom if the door as it unrolls downwards, and in any case doesn't solve the central issue, which is that the unit using the AA is sending a continuous radio signal to the centrally mounted control box which decides whether to run the door motor or not. And the central unit says "No" if if doesn't detect the signal.

So to replace my "power hungry" sensors, I'd have to research or reverse-engineer whatever protocol or signals this system uses, and then breadboard my own rig to do the same. And hope that at the end of the day it used less power. And then times 2 as there are two doors.

Seems to me I'd need a lot of test kit for such a project.

[T i m]

<snip>

By 'whack' that could mean 'touch lightly'?

If you had a light bar (fibreglass tube?) suspended across the width of the door(s) and hanging down sufficiently to allow the door to stop once indicated, then (as has been mentioned), add a microswitch and line to a central position, that could be used to tell the door to stop?

So you have power in the garage to run the door motor but no power to run the sensors? Or are the sensors on the door itself (so it's the wiring that would then be an issue, not the powering as such)?

Oh no, like one is supposed to do when say learning all about politics? ;-)

Or, if the sensors are on the (moving) door not frame, power them with a rechargeable and rig a pair of sliding contacts that connects them to a mains powered charger when the door is down / shut?

Hey, what else do you have to do (now most people here are running spellcheckers)? ;-)

Cheers, T i m

[Theo]

Unless it's dire I would expect the cell lifetime to be OK - if it's any good it should have a low voltage cutoff and temperature control.

There's a DC-DC converter in there to turn the 3.6v lithium into 1.5v. That takes power - maybe that's OK since lithium has a higher power density. But the problem is deciding when to run it, and in particular the quiescent current draw. I wouldn't be surprised if there's a constant battery drain just to run the DC-DC even if there's no load (how would the battery know there's a load unless it presents a voltage at the output?).

So while it might be OK for a battery you take off charge and use on a high current load until it's flat (in a motorised toy, for example), I can't see it working in anything with 'standby' where you expect it to run for days/weeks/months/year at low load.

If it's a desk phone I'd be looking at an external PSU, or maybe a single protected lithium iron phosphate cell (if a voltage range of 3.5-2.5v isn't going to kill it) or lithium iron (if 4.2v is OK - these are more common).

Theo

[charles]

I did that in our garage so that SWMBO doesn't take up my 'walk space'

[The Natural Philosopher]

I have been gifted half a dozen nicd/nimh fast chargers surplus from te noughties aeromodelling days. They take 12v and fast charge (20 mins)

4-7 series nickel cells

If anyone wants one..

Personally since weight is not an issue I would be putting in NiMh rechargeables ...the end point of those - 1v - is about the same as an alkaline cell

[The Natural Philosopher]

So dismantle the control unit and take wires to the door - a microswitch can be activated bu the weight of a cork... on a suitably long lever..

I spect you have it already

[Chris Green]

So have I but my brain is worn out now and I've become rather dim.

[Tim Streater]

This is a constant drain situation with probably a lowish power, but enough to drain a crap AA in two weeks. It was in my mind to look at a separate 3V or so battery I could mount next to the unit, possibly such as:

and they do a charger too, although not cheap.

[Theo]

Have you got space to mount a solar panel? If they don't have mains, it might be worth thinking about using that to top up a battery (eg 3x NiMH) or a beefier battery that's regulated down. That means you don't need to worry so much about conversion losses, and you aren't removing the battery regularly to charge it.

Theo

[T i m]

<snip>

Given this is only an issue when the doors are closing and not once closed or opening ... and assuming the safety interlock only works during close and not also the opening cycle, how about an external switch in series with the battery(ies) cutting the power when the doors are fully shut?

Cheers, T i m

[Tim Streater]

Yes, the garage doors are mains operated. It's the damn safety sensor that wants 2 x AA.

[Joey]

Looks like a very quick and dirty design. Why cant you mains power the safety sensor with a plug pack and some sort of flexible wiring between the plug pack and the sensor ? Say a big loop and a counter weight that keeps the wire down as the door raises ?