Using A Battery Charger

[Nemo2000]

In the Owner's Manual for my RAV (Mk2) it states that the Battery should be disconnected before using a Battery charger "in order to avoid damage to the electrical circuits". Why should this be the case, given that surely the alternator is charging the Battery anyway whenever the engine is running?

Incidentally, I used a battery charger on a number of occasions before discovering that instruction !! so I wonder what harm I might have done?

[jedi134]

I leave the Battery connected all the time, never done any harm.

[IanXT4]

I have always charged my car batteries while still connected and have never had a problem.

My guess is that car manufacturers have no way of knowing what you are going to connect and therefore cover themselves with a "disconnect clause".

Modern car electrics are incredibly robust, a normal Battery charger should not cause any problems.

Any advice given is meant to be helpful and in good spirit. No liability is accepted for loss , damage or injury. WORK SAFELY

[jedi134]

There was a post on here where a guy with a 4.1 rav4 connected his car Battery the wrong way round, and all that happened is his fuses blew

I would have thought toast but no, as far as I know all it ok.

[Jamesbelfast]

Modern electronics don't take kindly to volatge surges so my recommendation is to disconnect the Battery. Whilst the risk may be small it hardly worth the risk of damaging an expensive component. The other advantage is that when disconnected there is absoluteloy no drain on the Battery so the charger will have the best opportunity of recharging. It's similar to trying to fill a water tank that has a small leak - if there is no leak then it will fill faster.

Some manufacturers recommend switching on the sidelights when disconnecting or disconnecting a power source as it helps reduce the risk of a surge.

[IanXT4]

Modern car electronic systems are built to incredible standards that would put some older military systems to shame.

Here is an example of one of the many protection circuits that are built in.

http://www.vishay.com/docs/88490/tvs.pdf

Looking at this spec sheet they seem to indicate that the industry specs design for a voltage spike of 65 - 87V caused by a load dump. Based on this I suspect that your average home car charger is pretty safe.

FROM THE SPEC SHEET

"What is Load Dump?

The worst instances of surge voltage are generated when the Battery is disconnected when the engine is in operation,

and the alternator is supplying current to the power line of the vehicle. This condition is known as “load dump”, and

most vehicle manufacturers and industry associations specify a maximum voltage, line impedance, and time

duration for this load dump status, as shown in Figure 5."

I would be more worried about damage to your home electronics via mains surges than problems with in circuit car charging :D

[jedi134]

Yep your car Battery will be acting as one huge capasitor to absorb any spikes from the charger I would think.

Plus chargers these day have more electronics in them than was used to sent men to the moon.

[anchorman]

I studied auto electrics some years ago at college. We were always taught the old fashioned way to disconnect the earth lead before connecting an external charger. The reason was to prevent an electrical spike in voltage as the charger was connected in the on position as this could cause a spark. The spark induced a higher current which was enough to damage semi-conductors. These modern smart chargers seem to do away with the risk but frankly I cannot get out of the habit. The only consideration is for those with a tracking device as it will result in you getting a phone call if the Battery is disconnected without warning them first.

[firemac]

Modern car electronic systems are built to incredible standards that would put some older military systems to shame.

Here is an example of one of the many protection circuits that are built in.

http://www.vishay.co...s/88490/tvs.pdf

Looking at this spec sheet they seem to indicate that the industry specs design for a voltage spike of 65 - 87V caused by a load dump. Based on this I suspect that your average home car charger is pretty safe.

FROM THE SPEC SHEET

"What is Load Dump?

The worst instances of surge voltage are generated when the battery is disconnected when the engine is in operation,

and the alternator is supplying current to the power line of the vehicle. This condition is known as "load dump", and

most vehicle manufacturers and industry associations specify a maximum voltage, line impedance, and time

duration for this load dump status, as shown in Figure 5."

I would be more worried about damage to your home electronics via mains surges than problems with in circuit car charging :D

I have to ask: in what circumstances could a Battery be disconnected with the engine running & the alternator charging - apart that is from separation of one of its leads??

[Mistermena]

I have always charged my car batteries while still connected and have never had a problem.

My guess is that car manufacturers have no way of knowing what you are going to connect and therefore cover themselves with a "disconnect clause".

Modern car electrics are incredibly robust, a normal battery charger should not cause any problems.

Any advice given is meant to be helpful and in good spirit. No liability is accepted for loss , damage or injury. WORK SAFELY

You've nicked Anchorman's WORK SAFELY bit! I hope you've paid him his royalties?

[IanXT4]

" I have to ask: in what circumstances could a Battery be disconnected with the engine running & the alternator charging - apart that is from separation of one of its leads?? "

Well there are two scenarios I can think of:

1) Someone changes the Battery and forgets to properly tighten the terminals (probably a relatively common event statistically)

2) Simple user error/stupidity, I have seen some amazing situations occurring when people have a dead Battery a little knowledge and a set of jump leads !!

For car manufactures its all about managing risk, if they can design the electronics (cost effectively) to survive the worst case they reduce their warranty claims and increase profits.

[firemac]

" I have to ask: in what circumstances could a battery be disconnected with the engine running & the alternator charging - apart that is from separation of one of its leads?? "

Well there are two scenarios I can think of:

1) Someone changes the battery and forgets to properly tighten the terminals (probably a relatively common event statistically)

2) Simple user error/stupidity, I have seen some amazing situations occurring when people have a dead battery a little knowledge and a set of jump leads !!

For car manufactures its all about managing risk, if they can design the electronics (cost effectively) to survive the worst case they reduce their warranty claims and increase profits.

Ah, yes. Logically, Captain, you are correct.