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Can I leave the battery connected when I charge it?


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While charging a Battery in the 60s you just left the Battery connected to the car.  When they started to use more electronics in cars in the 80s we were told to disconnect the Battery when changing it.  I asked Mr goggle how to do it and all I got was one result of an RAC forum in 2007 where they recon that with modern electrics you can leave the Battery connected! None of the other results were any help at all and didn’t answer my question, just talked about removing the negative terminal first, etc.

Consequently, my question is, can I leave my Battery connected to my 1998 Corolla when I charge it?

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18 minutes ago, olddriver said:

Consequently, my question is, can I leave my battery connected to my 1998 Corolla when I charge it?

The answer to that is generally yes provided that:

1/ You connect the Battery charger to the Battery before applying power. This is because the open circuit voltage at the terminals of the charger could be (relatively) high. Even then the risk is low if you were to dab them across the Battery. Connect it first and the risk is zero.

2/ The charger is of a type that does not take the Battery voltage to high. No modern charger should do that, old ones will. Even then the risk once again low.  

3/ When charging has finished disconnect the mains to the charger before disconnecting the Battery clips.

 

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7 minutes ago, Mooly said:

The answer to that is generally yes provided that:

2/ The charger is of a type that does not take the battery voltage to high. No modern charger should do that, old ones will. Even then the risk once again low.  

 

Thanks very much, yeah, the battery charger could be 30/40 years old, it has a low and high setting but perhaps better to disconnect the Battery.

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Your car is 22 years old so MAYBE an old Battery charge will be ok.

if ever you change to a newer car I would say ditch that old Battery charger and buy a modern computer controlled charger that charges real slow as required. You may consider buying a modern charger even for the current Battery.

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5 hours ago, olddriver said:

Thanks very much, yeah, the battery charger could be 30/40 years old, it has a low and high setting but perhaps better to disconnect the battery.

You could do, or you could just monitor the Battery voltage during charge. Once full charge is approached the voltage will climb to perhaps 16 volts or higher. When that happens the Battery is also at risk of damage if left like that for a long time.

 

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What you need as cat lover has said is a modern Battery charger that will switch to a float charge once the Battery is fully charged it will then maintain a full charge. Although I tend to charge my corolla Battery occasionally to full charge then disconnect as I do not like to leave it connected in the garage unattended.

 

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I'll just add one more thing...

An old Battery can appear fully charged (and indeed it is) but because of its age its capacity is well down. In other words a new 60Ah battery will have its rated capacity, a ten year old one may be down to just 10 to 20Ah.

Think of it like adding pebbles to a fuel tank. One might be half full of pebbles and the other has none and yet both tanks can filled full of fuel and appear 'full'.

If you find you have a real need for charger then it might be better to invest in a new Battery instead. The Battery in an older car can survive on surprisingly few miles over a year without showing any signs of being low on charge.  

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Thanks Mooly, but my Battery is only a few years old and because of the lockdown I have not been using the car very much, so thought that I would  give it a charge.

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Fair enough although it may make less difference than you might think. Our old 2003 Corolla spent a couple of years doing as little as 5 to 600 miles in a year, often stood for 7 to 10 days in winter before doing just a mile or so. The Battery was fine during all that time and never seemed down on charge when starting.

From a practical point an external charge can initially seem to make a big difference. You will think it turns over really quickly, a big big difference... but it doesn't last, the Battery will be back at its previous level which is dictated really by the condition of the Battery within only a day or two.

As soon as you start the car the alternator has way more current delivery than any Battery charger and will quickly bring the Battery to within shouting distance of its potential fully charged state. Hours of driving or an external charge will improve on that but as mentioned, it generally won't last. 

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