Battery charging countdown timer required.

[Donwill]

Recently we have bought a few portable gardening battery operated machines which require the battery to be charged for a set time otherwise the battery overheats and can be suffer damage, one is 8 hrs the other 5hrs. I assume they have constant current chargers, the charging current not being dependant on the back voltage of the battery.

I require a timer which would plug into a 13A socket which has a simple 10 or 5 position rotary switch on the front graduated in say 1 to 10 hrs with a big button on the top which will switch it on, it will then count down and switch off.

At the moment SWMBO is leaving bits of paper with time-off reminders all over the house and I would like a more elegant solution. There are plenty of cheap standard on/off timers about but has anyone come across a countdown timer as described and where can I get one.

Don

[Bill]

In message , Donwill writes

If you feel adventurous and competent then.

When I needed something similar years ago I modified a 24 hour mechanical time switch, the sort with lots of little push pegs to set

1/4 hour slots. This was modified to run through itself so that once started it would run until it was time for it to turn off and then it would disconnect the load and itself from the mains. It maybe possible to do it with the newer digital readout timers but I found the mechanical ones quite adequate

[Bob Eager]

I use one of the cheap rotary mechanical on-off timers for this purpose. NOt quite as elegant as what you describe, but it does the job.

This is the kind where you press in segements for the time you want. I have a block of 4 hours pressed in. It's permanently plugged in, but the socket is usually 'off'. When I want to charge, I turn it on, rotate the dial until it goes click, and I get four hours. Come back next day and turn it off. Now, if you don't come back, in another 20 hours it will go on, but I can live with that.

I'd probably just use 2 of those in adjacent sockets.

[Dave Plowman (News)]

This might do:-

[Andrew Gabriel]

If these are standard sealed lead acid batteries, then I suggest you get a proper microcontroller-based charger for them, and ignore the crap charger they came with (it's only the really crap chargers which have to be disconnected after a certain time). A collegue bought one of those jumpstart-with-inverter units with a 17Ahr SLA battery in it. A few weeks later, he accidently charged it for several days, and that was the end of it. I also have one (but with a compressor). I charge it with a proper SLA charger, and not the crap wallwart it came with which has no regulation or timing circuitry in it at all.

Aldi had a very nice charger for SLA batteries a few months back. It was £12 or £14. Sadly, they don't have it anymore, and it's more like £50 anywhere else.

[PCPaul]

The thing to look for is a charger that switches to 'trickle charge' or 'maintenance mode' once the main charge is done.

To answer your original question, you should be looking for 'one shot' timers, such as the DMB51CM24, £14.00 from , datasheet at .

[Peter Parry]

not quite what you wanted but once you set a countdown time it remembers it and pressing a button activates it.

or DIY :-)

[dennis]

The old Smiths timers had pegs you inserted.. you just inserted an off peg when you wanted it off and forgot about it. It would never switch on again unless you put an on peg in.

[Harry Bloomfield]

on 13/07/2008, Andrew Gabriel supposed :

Mine has a rather good relay operated charge circuit. Each time it is powered up, it brings the battery voltage up to a set point then the relay drops out with the battery fully charged.

[Harry Bloomfield]

Donwill formulated on Sunday :

For my chargers which include a proper means of control, I have in my garage a time clock which powers a 4 outlet strip. That is set to come on for an hour or so each day.

I recently put my compressor on a one shot circuit. Reset it runs for 5 minutes then shuts off - that way my compressor is never left on and forgotten. The one shot is just an electronic relay with a dial on the front to set how long it runs for. You can get these in ranges able to be set for seconds, minutes, or hours from places like Farnell, RS and CPC.

[Dave Plowman (News)]

My awfully cheap Lidl one switches off when charged.

[Pete C]

:)

Buy her a 'parking timer' keyring?

cheers, Pete.

[Andy Dingley]

They _don't_ require a set time, that's just the easiest way to warn against over-charging. An intelligent charger, usually sensing the battery EMF, sometimes temperature as well, is always a better idea. For one thing, this "ideal" time varies widely depending on conditions such as state of charge, battery age and temperature.

They're not expensive these days. I've just bought a couple this weekend: lead-acid gel for a tenner, NiCd / NiMH for 15. Doing on a simplistic timer is obsolete.

[Ian Stirling]

These are integrated chargers - you plug the device into the charger. And yes, they do, as a simple resistor is cheaper than the pound or so of stuff that'd be needed to do a proper charger.

[Ian Stirling]

I note that teh surprisingly good 1.99 12V battery charger of a couple of years back was 12.99 this year.

[Dave Plowman (News)]

Do you mean the 5 amp SMPS car one? Which goes to a maintenance charge after the fast one? Never saw that at 1.99 - it's excellent value at 13 quid. Other places charge 3 times that for the same thing.

[Ian Stirling]

Yes. I swore loudly next year, as I was intending on getting 5. I hadn't imagined that it could be that good at 2 quid, so I diddn't examine it closely at the time.

[Dave Plowman (News)]

I keep a close eye on Lidl stuff via their website - and never saw it advertised at that low price. As it was I bought several as Xmas presents.