Home-made battery charger

[Warren]

Can an old AC adapter be used to charge a bike's battery assuming that it is 12 volt and under the maximum charging rate for the battery? I have a few old AC adapters around and cant be bothered spending $15 for a proper charger at bunnings. I am assuming that it is connected positive to positive and negitive to negitive.

[Ixion]

You mean DC adaptor. Battery charger must be DC.

Any 12V DC source will charge the battery to at least some extent (the extent may be not worth bothering with, though). But a discharged battery is a pretty massive load, so I suspect most of the little plug in adaptors would fry themselves.

[MattRSK]

Yeah I dont think that you should have a problem. time to charge would depend on what sort of current the charger can put out. Also would have to be a DC charger.

[FlyingDutchMan]

You'll need a current limiter of some kind - otherwise the fuse inside the AC pack will blow instantly. You'll need something like 16v plug pack, with a 1A current limiter then connected to your battery. And yes +ve to +ve, -ve to -ve. But for $15 I'd go get a charger.

BTW. a 12V plug pack, doest provide a nice solid 12v - the voltage averages 12v. If you put a large load on it, expect the voltage to drop below 10v. And it can peak up around 14v under no-load. Cheap and nasty those things are IMO.

[paturoa]

yes,

Check the open circuit voltage from the adapter - will need to be at least 14v for a 12v battery

Then check the max current rating for the adapter.

Then divide 12 by that number, eg if you have a 250ma adapter then 12/0.25 = 48

Now get onya bike down to an electronics store and buy at least a 1watt resistor of at least 48 ohms.

Connect the resistor in series and you'll be away. You will likely have to leave overnight as will be charging at less than 250ma. For example the above would charge at about 200ma (to start with). So a 20amp hour battery, from dead flat would take 100 hours to charge.

Yes positive to positive, just like jumper leads.

[skelstar]

What Ixion said. A very flat battery would make a very flat DC adaptor (ie melted).

[bungbung]

...Now get onya bike down to an electronics store and buy at least a 1watt resistor of at least 48 ohms...

That's not going to work, your figures are for a 12V drop across the resistor, forgetting about the battery to be charged.

To charge a battery, you need to apply more than 12V, for a lead acid cell (2V) you need between 2.3 and 2.4 V to charge, so for a 12V battery you'll need 13.8 - 14.4V ideally.

Your current limiting resistor will drop some of your usable charging voltage as well. The value of the resistor will be dependant on the internal resistance of the battery. (between 10 to 30 milliohm for a wet battery) You'd be better off with a regulated supply, but a 16 - 20VDC @ 300ma or more would be a good start.

For a 18V @ 300mA supply, you may want to limit to 250mA, dropping 3.6V (to give a 14.4V charing voltage) V/I=R, the resistor needs to be 14.4Ohms 1W or more.

[Paul in NZ]

Jeeze, just buy a battery charger - you usually only ever need one per life time. It's an investment plus it shaves 3 sec off your lap times. (a mig welder or a 3 phase compressor shaves 4 seconds each and adds 1 cm to your willie.

A good battery charger needs to limit current and voltage to optimise the charge. Even if you had a who bank of those plug into the wall jobbies, chances are they are configured for a NiCad or Lithium battery which requires a completely different charge cycle than a good old lead acid.

Put it this way, whats a battery cost these days? You REALLY want to blow one up?

Paul N

[slopster]

I once charged a car battery with 2 cell phone charges wired in series but it took 4 days to charge

[White trash]

It's an investment plus it shaves 3 sec off your lap times. (a mig welder or a 3 phase compressor shaves 4 seconds each and adds 1 cm to your willie.

Mr Me's got all three of those and his wife tells me he's got the smallest willie around. So who's lying here Paul?

[bungbung]

As a Guzzi owner, presumably Paul is happy with the 'dimensions of his unit'

[Paul in NZ]

As a Guzzi owner, presumably Paul is happy with the 'dimensions of his unit'

Always been comfy with me unit - it's other people that complain...

[Paul in NZ]

Mr Me's got all three of those and his wife tells me he's got the smallest willie around. So who's lying here Paul?

And you never wondered why he has all that stuff now? Christ, you should of heard her go on when all he owned was a rusty bastard file and a broken hacksaw blade...

I mean do the math, a one cm improvement on a one cm willie is still pretty small despite being 50% bigger. For me its merely the difference between knowing that the water in the bottom of the urinal is cold to knowing it is cold and deep.

Paul N

[Falconer]

I bought a arlic charger a few years back for $18. Charged at 1/4 of an amp. the sign on the charger said 4 amp! There are good cheapies but some are bad. Most chargers I have come across are about 14 volt and as the batteries charge the voltage increases and when the battery is charged they drop back to a trickle due to the battery V = to charger V. the system is reliable and works well. Not much need for electronic stuff in a simple charger.

[Bonez]

I bought a arlic charger a few years back for $18. Charged at 1/4 of an amp. the sign on the charger said 4 amp! There are good cheapies but some are bad. Most chargers I have come across are about 14 volt and as the batteries charge the voltage increases and when the battery is charged they drop back to a trickle due to the battery V = to charger V. the system is reliable and works well. Not much need for electronic stuff in a simple charger.

I always thought chargers charge at what the battery requires-ie basicly if it was F$%% it would attempt to charge at 4 amps. Could be wrong..............