Battery charging- a dumb question

amiers · 10-05-2010, 08:34 PM

why cant you charge a car battery while the leads are connected?

JC · 10-05-2010, 09:38 PM

You can (and I have).

But if the battery's already flat, why not just disconnect the negative lead anyway, and then charge it. There is a chance that a surge could fry your ECU, or melt some wiring and start a fire - it's not a huge chance, but the chance is still there which is why "they" recommend to take the leads/lead off.

ronwest · 10-05-2010, 11:29 PM

Also, a trick that I learned only recently (after discarding many batteries over the years) is that if the battery is dead flat and just won't charge at all, stick it on 6v charge for an hour or two. A lot (all?) modern chargers won't charge if the battery is below a given charge state.
I tried it out on two 12v batteries that had been down the back of my shed for at least a couple of years. Neither would take a 12v charge but both did when primed by a 6v charge.

amiers · 11-05-2010, 06:12 AM

thanks guys. i think i have a small power drain somewhere and the battery is getting a bit weak. too make matters worse i dont drive the old girl much these days. hence i just wanted to give the battery a top up till i get it checked out.

xr6turbo2005 · 11-05-2010, 06:29 AM

if you get a trickle type charger, you can charge the barttery with leads on. just need to connect the negative to an earth on the car and not the battery

mik · 11-05-2010, 11:20 AM

Quote:

Originally Posted by ronwest

Also, a trick that I learned only recently (after discarding many batteries over the years) is that if the battery is dead flat and just won't charge at all, stick it on 6v charge for an hour or two. A lot (all?) modern chargers won't charge if the battery is below a given charge state.
I tried it out on two 12v batteries that had been down the back of my shed for at least a couple of years. Neither would take a 12v charge but both did when primed by a 6v charge.

ooh nice one ronwest , i`ll be giving that a try ;).

Hords · 11-05-2010, 11:42 AM

Quote:

Originally Posted by xr6turbo2005

just need to connect the negative to an earth on the car and not the battery

Why's that?

ronwest · 11-05-2010, 11:46 AM

Quote:

Originally Posted by mik

ooh nice one ronwest , i`ll be giving that a try ;).

yeah, when I think of the batteries I've discarded over the years... :(

Obviously it doesn't "fix" a buggered battery but if you've had a car sitting around for a while and battery won't take a charge this will most likely get it underway.

galaxy xr8 · 11-05-2010, 07:37 PM

Quote:

Originally Posted by xr6turbo2005

if you get a trickle type charger, you can charge the barttery with leads on. just need to connect the negative to an earth on the car and not the battery

Yeah why is that ?.

AMB · 11-05-2010, 10:43 PM

Quote:

Originally Posted by amiers

thanks guys. i think i have a small power drain somewhere and the battery is getting a bit weak. too make matters worse i dont drive the old girl much these days. hence i just wanted to give the battery a top up till i get it checked out.

A battery just slowly loses charge when sitting around doing nothing, even when not connected to anything. Connected to the car, there is a small drain all the time. And if you don't keep a battery fully charged, or close to it, it gets old more quickly. If you're battery is more than a few years old, I wouldn't persist with it, you'll be stuffing around and be lucky to get another year out of it. Get a new battery, buy a good automatic charger (about $80), and keep it connected andfully charged all the time (it uses a tiny amount of electricty for that). the battery will last for many years.

BigCav · 11-05-2010, 10:54 PM

Quote:

Originally Posted by ronwest

Also, a trick that I learned only recently (after discarding many batteries over the years) is that if the battery is dead flat and just won't charge at all, stick it on 6v charge for an hour or two. A lot (all?) modern chargers won't charge if the battery is below a given charge state.
I tried it out on two 12v batteries that had been down the back of my shed for at least a couple of years. Neither would take a 12v charge but both did when primed by a 6v charge.

i work for an auto store and people are constantly bringing flat batteries back to us claiming there dead (like a new battery just dies within a month, people dont get theres probably a reason there old battery died too, but anyway)

and we have the same issue with not getting a trigger signal for want of a better word to start charging, i hook up a second battery in series and put it on 24v charge, works a treat, only need to jump start it like that for a minute or 2 before u can charge it normally

AMB · 11-05-2010, 11:28 PM

Quote:

Originally Posted by BigCav

i work for an auto store and people are constantly bringing flat batteries back to us claiming there dead (like a new battery just dies within a month, people dont get theres probably a reason there old battery died too, but anyway)

I'm guilty of that a few years ago. Old flat battery, got new battery and it also went flat after a few weeks. took it back saying there's something wrong with it. Turned out it was my alternator, it was putting out over 14 volts no problem (so seemed to be alright), but had low current output (not so easy to check).

Amiers, you could check to see how much current is being drawn with a simple multimeter. With everything off, I think about 5-10 mA is normal, someone correct me if I'm wrong. I had a faulty relay once, and it was drawing something like 40-50 mA.

galaxy xr8 · 15-05-2010, 04:44 PM

Quote:

Originally Posted by xr6turbo2005

if you get a trickle type charger, you can charge the barttery with leads on. just need to connect the negative to an earth on the car and not the battery

Hey mate, don't know if you seen but you have been quoted twice as to why you mentioned this , I'm currious to know why you say connect the earth to car not the battery ?.

**GasoLane** · 15-05-2010, 09:59 PM

Quote:

Originally Posted by xr6turbo2005

if you get a trickle type charger, you can charge the barttery with leads on. just need to connect the negative to an earth on the car and not the battery

It doesn't matter if you connect it to an earth on the car or the earth lead on the Battery.

Well, this works in NSW...things might be different Nth of the border

JC · 15-05-2010, 11:33 PM

Quote:

Originally Posted by GasOLane

It doesn't matter if you connect it to an earth on the car or the earth lead on the Battery.

Agreed. When jump starting, definitely use earth to car not battery, but for charging, direct to battery is what you want.

BigCav · 16-05-2010, 01:54 AM

i have never gotten why people say to earth off the body, it makes no sense what so ever

5.0 ED · 16-05-2010, 08:25 AM

In all wiring you have some resistance, some capacitance and some inductance, normally this can act as a small kind of filter to noise etc. Now when you jump start a car and you put the jumper earth onto the car, you are connecting it directly to the same earth as used by the engine management. therefore holding the engine managements voltage more consistantly through the other cars battery.

When you turn the car over the other cars alternator will load up and increase its current output, when the load is remove the voltage can spike, this isn't normally a problem cause the alternator isn't working when you normally would start your car.

Now when cranking you get a voltage drop over the earth cable to the car, you get a voltage drop once again to the body, since the earth is normally joined to the engine and then to the body, its rather a big voltage drop over the cable, when that power is removed you get a reverse spike of power down that cable, due to the inductance of the wire etc. couple this to the spike from the alternator if placed upon a faulty battery around 16volts it'll spike to, reverse voltage on cable around 1-5volts depending on how good the cable is, so you end up with a possible 21volts over the eec, instead of a possible 16volts if it was hooked up to ground of the car.

Now doing this with a battery charger has no real point, its not being cranked at time or anything so it wont hurt it. If your battery is good, and your earth cables are nice and thick it once again wont hurt it.

At the end of it all 99.9999% of the time it wont hurt it. It's just that it could happen, so..................

xr6turbo2005 · 16-05-2010, 10:06 AM

Quote:

Originally Posted by galaxy xr8

Hey mate, don't know if you seen but you have been quoted twice as to why you mentioned this , I'm currious to know why you say connect the earth to car not the battery ?.

well thats what teh trickle charger i have (projecta brand) requires.

actually took one back thinking it wasnt working correctly until i re-read the instructions