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GM7HUD > TECHNI 05.08.04 15:15l 52 Lines 2869 Bytes #999 (0) @ WW
BID : 72804_GB7ESX
Read: GUEST OE7FMI
Subj: re: Charging car batteries.
Path: DB0FHN<DB0THA<DB0ERF<DB0FBB<DB0GOS<DB0EEO<DB0RES<ON0AR<WB0TAX<GB7ESX
Sent: 040805/1222z 72804@GB7ESX.#31.GBR.EU $:72804_GB7ESX [Witham, Esx]NNA V3.1
Dick VK3ABK asks about disconnecting a car battery before charging it...
My current Honda manual says you should disconnect the battery as did many
other owners manuals.
The battery cant tell whether it is being charged by the alternator or by a
charger. The alternator normally produces a higher voltage than the battery
when it charges it. When you stop the engine, the battery voltage will drop
but for some time the battery voltage will be higher than normal. This state
of affairs doesnt damage the alternator so applying a charger to the battery
shouldnt cause any damage as long as the maximum voltage is limited.
And that is where I think the problem lies. My battery charger is a cheap
nasty piece of junk my late father gave me 25 years ago. It uses a metal
rectifier and off load the voltage is 18V :-( There is no overcharge cutout
or timer on it so it is possible to overcharge the battery if you leave it
connected too long. This and the fact the voltage can rise above the normall
14.4V charge level is why most manufacturers suggest to remove the battery
before charging.
However, disconnecting the battery on modern cars often lands you in a world
of pain. Some car engine management systems take many miles to relearn about
the engine they control after they have their power removed. Then there is
the problem of anti-theft coded radios/CD players needing to be reset, RKE
fobs need to resynchronise. All sorts of irritating things to be reset.
(Some Vauxhall car electric window fuzzy logic controllers need reseting
after a battery disconnecton aswell.) It's bad enough having to disconnect
a battery to charge it without having to jumpthrough hoops with all the toys
and gizmos aswell.
I use two solutions to the problem. For charging the battery I have a 10A
regulated 13.8V PSU. This wont fully recharge a car battery but it can be
connected up with having to disconnect the battery and left in circuit
forever. It can charge a battery sufficiently to start a car even on a cold
day with a cold engine (it gets to -12C regularly here in the Winter). To
preserve all the settings etc when having to change a battery I havea lead
consisting of a cigar lighter plug and fuses to connect to a small 7Ahr SLA
battery. Plug this into the coigarlighter socket, connect upthe battery
and then you can disconnect the main battery. The current drain on my car
is about 200mA so you get several hours before you need to reconnect the
main battery.
I've used the small SLA battery trick many times when having to change out
faulty main batteries and the regulated PSU has been used during Winters on
my XYL's car which does lots of short stop/start journeys without
problem.
Make sure you use fused leads as car batteries will produce thousands of Amps
when short circuited leading to elevated temperatures in the leads :-)
73 de Andy GM7HUD
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