Do you want the Road Tax system to change? Do you have thoughts on the potential changes to the Road Tax system? | No thanks
The right way to charge? - Dwight Van-Driver
It's that time of the season when I have to assist Mr Lucas and give him a dose of electricity via a charger now and again.

Some books tell me that I should always disconnect the battery from the vehicle before charging and others say not. Even HJ's recommendation, the Airflow Battery Conditioner/Charger, condones charging without removing the battery as there are extension clips to allow a quick clip connection without opening the bonnet.

So I ask, what is correct? Do I do any damage in not disconnecting the battery, and if so, what?

Re: The right way to charge? - Andrew

The short answer is that it shouldn't make any difference if you charge with the battery on or off, however, I would always remove the battery.

Long answer :-)

A charger commonly kicks out about 14-15V on a half wave rectified DC signal, that is the DC voltage rises and falls from 0-15V at 100Hz. If you have a *good* charger you may well get a constant DC of 15V.

By keeping the battery connected you are potentially subjecting all the electronics to this rather nasty 0-15V waveform. The electronics should handle it but essentially what you are doing is powering up/down each electronic module 100 times a second.

If you remove the battery, you will obviously loose your radio code, but also the engine management may loose some (non-vital!) information. If you have a newish vehicle it probably has some form of adaptive control and 'remembers' how you drive and adjusts engine/gearbox control accordingly. This information is often backed up by the battery and is lost when battery is disconnected. Therefore the the car may seem a little 'different' for the first 1/2 hour or so.

On a safety note, if you are charging the battery it will 'gas'. I feel a LOT safer when the battery is out an in a well ventilated area rather than in the engine bay.

Hope this helps

Re: The right way to charge? - Dave N
Be careful disconnecting the battery, as a lot of electronics rely on it, and some don't like the power surge when you reconnect.

As for the gassing as it charges, doesn't that happen anyway when the car is driven?
Re: The right way to charge? - Tomo
Following up on Andrew, I use a variable voltage, regulated and smoothed power supply which I have for other purposes, set to 15V to charge. Since the input from the vehicle alternator/rectifier must be "dirtier" than this, I see no reason to disconnect, and if you have a superior sort of charger with similar characteristics that applies. It should not "boil" the battery either.

With the ordinary unsmoothed half wave, with the top of the waveform "chopped" at the battery voltage and all sorts of odd harmonics, I do agree with Andrew that solid state electronics may suffer and the battery would be better disconnected.

("did not used to have this sort of trouble with contact breakers and valves and things old timer mumble-mumble......"!)
Re: The right way to charge? - Honest John
Use an Airflow and you don't need to disconnect the battery. That's the whole point. But don't expect to be able to re-charge a duff battery with one. It's for maintaining a charge, not restoring a charge from nothing:

Re: The right way to charge? - John S

Following your recommendation, and having just got a new battery for the Minor, I bought an Airflow unit. I just leave it connected permanently and it seems fine, and holds the volts at 13.8 so it starts like a dream.

Apparently max. charge rate is only about 1.5A so, as you say, it would take while to recharge a flat battery. However, I believe that a long slow charge is the best hope of geting some life from a neglected battery, so perhaps worth a try if you have the time.

No real concerns about damaging electronics on the Minor.


Re: The right way to charge? - Marman
I have an Airflow, it's excellent, but a fortnight after buying it last year, I saw a similar thing in Halfords that did the same sort of job at half the price, whether it is as good I cannot say. But I can really recommend the Airflow
Re: The right way to charge? - THe Growler
On this point, what of using booster cables to start engines with dead batteries?
?impact on delicate electronics?
Re: The right way to charge? - Dwight Van-Driver
Thanks all.

Still flummoxed as to what is exactly right but doesn't seem to be that important otherwise it would be explicitly stated.

Handbook says disconnect; AACar Care Book - doesn't matter; Airflow (purchased some months ago following HJ's recommendation) - leave coupled and for Xmas Claus bought me a Haynes for the Disco which says disconnect.????

Re: The right way to charge? - Dave N
You have to be careful if you're using jump leads due to power surge. It's recommended that the negative lead goes on something like the engine hook, rather than the battery, but many manufacturers don't recommend their use. Land Rover certainly don't, as all sorts of things can blow up. They only recommend charging the battery with a low output charger.
Re: The right way to charge? - Simon Saxton

I followed HJs advice & have utilised the Airflow charger successfully for several years with success.The company produces quality goods.


Ask Honest John Right column

Value my car