Always Disconnect Battery When Using Charger?

[SubLGT]

In my Subaru 2005 factory service manual there is a warning to always disconnect the negative cable from the battery when using an external charger, to prevent damage to the alternator diodes and other electrical modules. I always follow that advice.

I am curious if anyone here has damaged their vehicle electrical system by leaving the battery connected during charging.

 

[davglt]

as long as the polarity is correct I have never had any issues, I never disconnect the battery on any of my vehicles

 

[gizzsdad]

I have never disconnected the battery, and never damaged anything. I should add that I use a BatteryMinder, which gives me a choice of 2, 4, and 8 amp. I rarely use more than 4 amps.

 

[NewC6]

Always best to follow manuf. recommendations. On one of my cars, it says to NOT hook directly to the neg terminal. Reason being it has an AGM battery from the factory and there is a device on the neg. terminal that monitors the battery. If you hook directly to the neg. terminal, you bypass the device and it could damage the device also.

 

[The_Jeff]

If your battery is totally completely shot it won't absorb the poorly rectified output of the charger and it could plausibly bust some other stuff.

Seems like legalese they put in there so if you blow up something expensive under warranty they could say "toldya so".

 

[meep]

stuff happens.

boost mode can apply waaay too much voltage IF the battery is weak and can't absorb it. dirty chargers can have voltage spikes that are unkind. REAL battery desulfators can mess with the ECU.

Desulfator story. I own a couple of homebrew desulfators. They are more powerful than anything storebought, and will peak at 60V (for a short µsec...) if the battery is not accepting a charge (with a half-decent batt the peaks are lower and too quick to see with slower scopes anyway). That said, running it long term, along with a trickle charger, killed the charger after a couple of years.

I decided one weekend to run it on my late model jeep with an mid-life battery. wouldn't expect any nasty peaks since the batt was still decent. probably 36 hours. First time I ran it afterwards, the very moment I pressed a steering-wheel control for the EVIC, the whole vehicle shut down hard. starter would not respond to the ign switch until I pulled power and let it sit.

lucky it didn't brick the ecu/bcm or whatever got messed up.

 

[HerrStig]

Originally Posted By: meep
stuff happens.

boost mode can apply waaay too much voltage IF the battery is weak and can't absorb it. dirty chargers can have voltage spikes that are unkind. REAL battery desulfators can mess with the ECU.

Desulfator story. I own a couple of homebrew desulfators. They are more powerful than anything storebought, and will peak at 60V (for a short µsec...) if the battery is not accepting a charge (with a half-decent batt the peaks are lower and too quick to see with slower scopes anyway). That said, running it long term, along with a trickle charger, killed the charger after a couple of years. 60 volts at what frequency?

I decided one weekend to run it on my late model jeep with an mid-life battery. wouldn't expect any nasty peaks since the batt was still decent. probably 36 hours. First time I ran it afterwards, the very moment I pressed a steering-wheel control for the EVIC, the whole vehicle shut down hard. starter would not respond to the ign switch until I pulled power and let it sit.

lucky it didn't brick the ecu/bcm or whatever got messed up.

 

[OneEyeJack]

Disconnecting the battery is not difficult. On my wife's Volvo you'd have to re-enter the radio code once it's hooked up, again. Other cars might have similar requirements and you can buy a small device that you plug into the cigarette lighter to avoid loosing your cars memory.

 

[dwcopple]

Originally Posted By: gizzsdad
I have never disconnected the battery, and never damaged anything.
+1, 2 amps for me

 

[Colt45ws]

Ive charged mine up to 12 amps with battery connected. Ive also run the stereo with another charger connected at 30 amps. I have 13k uF capacitance on the amplifier feed, which should help with unclean power.

 

[Zero]

Originally Posted By: davglt
as long as the polarity is correct I have never had any issues, I never disconnect the battery on any of my vehicles


This.

Another reason to not connecting to the negative terminal is the mere chance that when you make your connection, it will create a spark and IF enough gas is present, it will ignite.

More legal advice than anything, IMHO.

 

[Whimsey]

I don't disconnect the negative terminal but I put the negative battery tender connection on the engine or body. The car sits for 4 to 6 months during the winter and I've never had a problem.

Whimsey

 

[SubLGT]

The two risks I see with not disconnecting the negative cable is that if the charger has an internal component failure resulting in a high output voltage, then not only the battery will be damaged, but the ECU and BCU will also be damaged, and that will be an expensive repair. There is also the risk that high input voltage surges to the charger (such as from a lighting strike on power lines, or utility crews working on power lines) will pass through the charger and fry the ECU and BCU.

 

[diver1972]

Originally Posted By: SubLGT
I am curious if anyone here has damaged their vehicle electrical system by leaving the battery connected during charging.
I easily overlooked that part of the directions and did that a couple times on the same vehicle with no ill effects noticed.

 

[zzyzzx]

Personally I don't do not disconnect the negative battery terminal, and have never had an issue with this.

 

[andyd]

Every so often a door is left ajar and the battery needs a charge. Marina's car I have a charger and a jump pack.

If I jump start Marina's car (of course) It will randomly default the display to PPPPPP. There's a simple procedure to re-set so no big deal. I f I charge the battery at 2 amps over night it doesn't happen.

 

[gaijinnv]

In our fire house, all the emergency vehicles are on "Battery Minder" type trickle chargers 24/7.

All the battery cables remain connected.

Never had a problem.

 

[aualtima3.5]

Originally Posted By: NewC6
Always best to follow manuf. recommendations. On one of my cars, it says to NOT hook directly to the neg terminal. Reason being it has an AGM battery from the factory and there is a device on the neg. terminal that monitors the battery. If you hook directly to the neg. terminal, you bypass the device and it could damage the device also.


Good advice. Fluids may be able to be fudged with a replacement later, but electrical... don't jack with it unless you know for a fact it will work.

This post is a good example of why you should follow mfr recommendations.