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[Andy A]

The battery has run almost flat. Over the past few weeks of me needing the doors to be open when working on the car.

My question. Is it OK (safe)  to connect a battery charger to the battery when the battery is still connected to the car?

Thanks

[The Red Baron]

i do. dont see a problem. never use boost though to start it. 

[Andy A]

Thanks Red, I'll get it on charge.    if the rain stays off for long enough. 

[flyer 0712]

Yep is not a problem to do that....i have done it many times...leave it for a couple of hours and as stated before do not try to start it while leads are connected if it is an ordinary charger...

[Andy A]

Thanks Flyer.  I'll get it on charge as soon as the rain goes off.

[Bigron]

If you have an automatic charger of the type that Halfords sell, which give full charging current until gas charge state has been reached and then drop back to a maintaining trickle charge, it is best to disconnect the battery as this type of charger has a pulse output which will upset the ECU, maybe permanently.
Any normal, smooth DC charger will be fine left on, with the caveat that if it is a cheapo, basic charger with a 15 volt or higher output, take it off as soon as the gas charge state has been reached as otherwise it will rapidly turn the water content in the electrolyte into its component parts, hydrogen and oxygen. Aside from that not being good for the battery, that combination of gases under the bonnet could lead to an incendiary Omega!

Ron.

[DrAndyB]

My question. Is it OK (safe)  to connect a battery charger to the battery when the battery is still connected to the car?

Connect charger and let it do its stuff and you will be OK.

I put mine on every couple weeks.  I am still on the original VX battery  - had the car from brand new  . 14 years and ticking maybe a record !

I am paranoid now, so I'll pop it on charge too 

[Andy A]

The charger I'm using is this one  http://www.ebay.co.uk/itm/Challenge-Xtreme-Automatic-Car-Battery-Charger-CMT6978-/261200885267. Will this be OK Bigron?

Is anyone else here connecting an automatic charger up without disconnecting the battery without any problems?

Thanks

[DrAndyB]

Looks OK as has automatic fast and slow trickle charge.  Similar in principle to the one I have.

With battery remaining connected it will prevent voltage going too high, and would be within normal parameter of alternator so no/minimal risk of frying anything 

[Andy A]

Cheers Andy, will put it on charge first thing in the morning if it isn't raining. 

[Bigron]

Andy, I am unfamiliar with that particular charger, so cannot comment; Halfords specifically advise against using their charger whilst the battery is connected to the car, for the reason I stated - the heavy pulsing is loads worse than the gentle ripple from an alternator and is very unfriendly to temperamental microprocessors.
The pulsing on and off is for the charger to measure the state of the battery and determine when to stop bulk charging and revert to trickle or maintaining charge.
I believe that most automatic chargers work that way, so for your own safety and peace of mind, I would just lift the negative terminal off the battery and connect the charger directly to the battery.
I understand that you are reluctant to do this because of the inconvenience of resetting things when you reconnect, but I have a 12 volt power supply that I connect to the car first, before disconnecting the battery, to keep the car electronics alive during charging from the Halfords charger. A fiddle, but it saves time and potential ECU damage (we all know how delicate they are) in the long run!

Ron.

[Andy A]

Ron thank you for your reply.

I didn't want to disconnect the battery, because the voltage drop was sensed by the car alarm and would set the car alarm off every time. Connecting a 12v DC power supply to the terminals first would keep the alarm happy (hopefully) and save all settings at the same time.  Better to be safe than sorry. 

I have a few old power supplies here that I can solder a couple of crock clips to. What are the specs of the PSU that you used?

[Bigron]

Hi Andy.
The power supply I used was a bit overkill, at 13,8 volts (the important bit) and 25 amps!
All you need is a 1-2 amp capability and an off load voltage of no higher than 15 volts, better 13-14 volts, as you will no longer have the limiting effect of the car's battery and therefore don't want it to go too high.

Ron.

[CaptainZok]

Ron thank you for your reply.

I didn't want to disconnect the battery, because the voltage drop was sensed by the car alarm and would set the car alarm off every time. Connecting a 12v DC power supply to the terminals first would keep the alarm happy (hopefully) and save all settings at the same time.  Better to be safe than sorry. 

I have a few old power supplies here that I can solder a couple of crock clips to. What are the specs of the PSU that you used?

Turn ignition on then off and you have something like 30 seconds to disconnect the battery without setting the alarm off.

[Andy A]

Ron, you're not into radio are you?

CaptainZok, Thanks for the info.