| |
VK3XX > TECHNI 06.08.04 10:47l 65 Lines 3390 Bytes #999 (0) @ WW
BID : C71345VK3XX
Read: OE5AKM GUEST
Subj: Re: Charging car batteries.
Path: DB0FHN<DB0MRW<DB0HOT<DB0ERF<DB0FBB<DB0GOS<DB0EEO<DB0RES<DK0WUE<HA3PG<
ZL2TZE<VK5UJ<VK3AVE<VK3FRS
Sent: 040806/0934Z @:VK3FRS.#MEL.VIC.AUS.OC #:32556 [Kilsyth] $:C71345VK3XX
From: VK3XX@VK3FRS.#MEL.VIC.AUS.OC
To : TECHNI@WW
Hello in particular to Dick VK3ABK and others interested in this topic.
I think that this is quite a difficult situation bearing in mind some of
the junk that is sold as battery chargers! If a battery is disconnected
from the car electrical system then it will tolerate these battery
chargers. The only times that I have been involved with flat batteries I
have taken the battery into the shack and put it on my stabilised 13.8 v
20 A PSU. I would say that would be quite safe to do in situ in the car
too.
It is "fast" chargers which can put up to 18/20 volts on to a flat battery
which are really dangerous. You might get away with that on an old car
unless the lights or radio were switched on but I wouldnt dare to do it on
a modern car with even a basic trip computer fitted never mind an engine
management computer. Those wouldnt last 5 milliseconds with one of those
really primitive chargers. I reckon the fuse would last longer than the
computer! However I dont think the use of the stabilised PSU would worry
the computer at all since it has to suffer 14.4 volts(plus a safety
tolerance of say 10%) produced by the car alternator in normal operation.
If the battery is down to around 11 or 12 volts you can expect that the
PSU will contribute a bit of current limiting too on first application. I
find that with my PSU the inrush current would be around 15/20 amps and
the voltage may sag a bit below 13.8v but that doesnt matter as it picks
up after the current has decreased a bit.
The only trouble with using a 13.8V PSU is that the battery never really
becomes fully charged. It is usually sufficient to get a start if the
engine is in good shape. The alternator will then bring it up again after
a few kilometres of driving.
The main reason that I think that manufacturers recommend disconnecting a
battery is that they have to consider the risk that a user might give the
battery a heavy overvoltage charge and then disconnect while charging and
the battery is gassing without first turning off the charger. The spark
created could then cause an explosion.
Having said that I think that suggestions for modern cars such as having a
small battery that can maintain the computer supply while the battery is
disconnected are quite good and would be vital with a common or garden car
battery charger. But on a modern car I would not use one in situ. I would
however be quite happy to use my 13.8v PSU though and at that voltage the
battery would not produce any significant gas. I use a car battery in the
shack with a smaller 6A PSU set accurately at 13.8 v for all my low
voltage radio equipment. The PSU runs at about 100mA trickle charge and
the PSU takes the transmitter current peaks and the steady part of any
load. It also helps the stability of the 13.8 volt system when running 100
w of SSB and CW. I just cant see any movement on my digital meter with my
20 A PSU switched on as well! This assists to minimise distortion in the
final transistor amplifier which isnt all that linear anyway!
So you can add this to the advice on this topic most of which seems to be
pretty valid. It has also amazed me how many people have wondered why a 12
volt car battery hardly gets any charge from a 12 volt supply used as a
charger!
Cheers and
73 - GORDON, VK3XX @ VK3FRS
Message timed: 19:32 AEST on 06 Aug 2004
Read previous mail | Read next mail
| |
