[ROVERNET - UK] Battery Charging, in or out of the car

Warwick Brooks warwick at regscom.com.au
Thu Aug 16 12:58:20 BST 2007


Charging the battery with a battery charger without disconnecting the
negative terminal (earthed terminal) is okay for a car with a generator but
if you have an alternator it can damage the diodes in the alternator's
rectifier (the thing that converts the AC output to DC).  There shouldn't be
any risk of damage to the radio.

There is no danger, just risk of damage to the diodes from inadvertent
voltage spikes.  On modern cars with computers controlling various functions
the potential damage extends to these too.

You could fit an isolator switch in the earth lead close to the battery and
use that to disconnect it from the rest of the car while charging.

When charging, a lead-acid battery releases hydrogen which is of course
highly explosive.  Hydrogen released from a battery being charged under the
bonnet can freely escape whether the car is being driven or not.  A battery
in the boot however is a different story.  If car is being driven and the
sealed battery box cover is in place then the gas will accumulate inside the
cover until it fills it (from the top down) and is pushed out through the
drain hole in the floor.  There isn't much free space in the battery box -
it's mostly full of battery.

A fully-sealed battery still produces hydrogen but is fitted with vent
valves to prevent pressure build-up.  Modern battery design has reduced
gassing somewhat but t still happens, particularly if you don't have a
regulated battery charger and you over do it a bit.  But having said that, a
good controlled over-charge occasionally is good for the battery's

If the cover is not fitted or is not sealed the gas escapes into the boot
where it will rise extremely rapidly to the highest point and escape through
the nearest gap or porous material.  If you don't have an isolator switch
fitted which gives you access to a battery connection point outside the
battery box you will have the cover off and so the gas escapes into the
boot.  It is extremely unlikely that enough hydrogen could accumulate in the
boot to be of any danger.  Trying to contain hydrogen is like trying to
carry marbles in a string bag.  Nevertheless, I would charge a boot-mounted
battery with the boot lid open.


-----Original Message-----
From: rovernet-bounces at lyris.ccdata.com
[mailto:rovernet-bounces at lyris.ccdata.com] On Behalf Of Brian Humphreys
Sent: Thursday, 16 August 2007 7:11 PM
To: rovernet at lyris.ccdata.com
Subject: [ROVERNET - UK] Battery Charging, in or out of the car


I recently had to purchase a new battery. Not because my old one was worn
out, but because it had been in and out of the car so many times to be
charged (too much sitting in the garage and not enough driving), that the
constant removing of the negative terminal had made the lead post turn into
a conical shape. As a result, tightening the negative terminal meant that
the terminal climbed up the post and off the top!

Anyway, why shouldn't I charge the battery in the car and leave the positive
and negative terminals connected. Is this a potentially dangerous thing to
do, and is there any danger of damaging the modern radio/cd player that I
have fitted.

For info, my new battery is of the fully sealed variety and is mounted in
the boot.

Many thanks,

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