Dumb question on battery charging

Submitted: Thursday, Jan 21, 2016 at 14:23

ThreadID: 131411 Views:2350 Replies:4 FollowUps:5

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Battery Power

In this article we explain why you might need an additional power source independent of the starting battery installed into your vehicle and we look in detail at types of batteries,

William 1

I don't want to die wondering so I'll ask a dumb question.

For 45 years whenever I have had to charge a battery I have disconnected the car's battery leads before connecting up the battery charger. Why is it necessary??? Why can't I just connect the battery charger to the car battery c/w all the car leads and thereby not having to reset the clock, Bluetooth and radio security etc.

Paul

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Reply By: Member - Alan H (QLD) - Thursday, Jan 21, 2016 at 14:41

Thursday, Jan 21, 2016 at 14:41

Anytime I have charged a vehicle battery I have never disconnected it. Welding on the vehicle is different. A quality charger will look after the battery and the car.

Alan

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Follow Up By: Member - Alan H (QLD) - Thursday, Jan 21, 2016 at 14:43

Thursday, Jan 21, 2016 at 14:43

There are no dumb questions - it is only dumb if the question is not asked

Alan

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Reply By: Allan B (Member, SunCoast) - Thursday, Jan 21, 2016 at 14:47

Thursday, Jan 21, 2016 at 14:47

.
You can safely leave the battery connected to the car wiring Paul.
Although the voltage of the battery terminals may rise to maybe 15 volts during charge, this is no higher than what the alternator does whilst driving. The car electrics are designed for this voltage, in fact normally rated to 16 volts.

Some pundits will rave on about "voltage surge". Absolute nonsense. The car battery acts as a very effective low impedance suppressor of any brief over voltage.

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Allan

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Follow Up By: pop2jocem - Thursday, Jan 21, 2016 at 15:30

Thursday, Jan 21, 2016 at 15:30

What Allan has said. Your battery is getting charged by the vehicle alternator while you drive around and all the accessory leads are connected.

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Pop

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Reply By: HKB Electronics - Thursday, Jan 21, 2016 at 15:53

Thursday, Jan 21, 2016 at 15:53

You should not have to disconnect the battery with modern day chargers, modern vehicle electrics are very roboust and protected against over voltage. Accessories you add may be another thing but should be able to handle normally charge voltages would could get upto 16V or higher depending on the climate.

Having said that I did recently test a NOCO smart charger and was alarmed at how high the voltage could get in float and pulse charge modes.

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Follow Up By: Member - Peter W (WA) - Saturday, Jan 23, 2016 at 00:29

Saturday, Jan 23, 2016 at 00:29

HKB. Would that still apply to a dual battery system with a Redarc BCDC1225LV isolator and using a Ctek 25amp charger. Vehicle is a 2014 Pajero.

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Follow Up By: HKB Electronics - Tuesday, Jan 26, 2016 at 16:55

Tuesday, Jan 26, 2016 at 16:55

The Ctek output appears to be tightly controlled so I can't see it being a problem.

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Reply By: Member - Alastair D (NSW) - Friday, Jan 22, 2016 at 11:37

Friday, Jan 22, 2016 at 11:37

I think that old wives tail or myth originated in the days when battery chargers were just a transformer and rectifier with no regulation. When the battery was flat the high current caused voltages losses but as the battery got nearly full the voltage could get quite high.

I recently chucked out a museum piece - a carbon pile battery tester with a built in charger that used a selenium rectifier. Open circuit the charger gave out ~25v, with a 100w head light bulb as a load it dropped to about 17v. I would not like to use that on a modern vehicle with all the electronics.

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Follow Up By: Nomadic Navara - Saturday, Jan 23, 2016 at 15:14

Saturday, Jan 23, 2016 at 15:14

It took a while for someone to come up with the reason for the reason for the old "you must disconnect the battery before recharging it." Must be less of us old buggers on the net these days (are many of our generation dying off?)

I disagree with this being a myth. If you still have one of those old chargers around and you wish to use it then disconnect the battery first. Some of those old carbon pile and Tungar tube rectifiers could put out some pretty ropey outputs. A lot of those chargers would put out some furious outputs for fast charging applications (charge your battery in 20 minutes.) It was better to disconnect the battery than.

It's a case of knowing what you are doing. The modern 12 V chargers are well behaved, nol need to disconnect. If you do hapen to have one of the older style chargers then disconnect.

PeterD
Retired radio and electronics technician

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