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Battery charger voltage

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New Dexter
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Joined: Fri Feb 12, 2016 9:41 am

Battery charger voltage

Post by New Dexter » Thu Nov 29, 2018 6:36 pm

I have an electrical item that has a 6V battery fitted. Apparently lead-acid but it's tucked inside. I might have mislaid the charger but have found one with a plug that will fit. However, it is 7.2V. Now I know that 12V batteries, when charged fully, show a voltage of a little over 13.2 V but it is correct to use a 7.2V charger on a 6V battery?

Sydsmith
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Location: Aberystwyth Wales

Re: Battery charger voltage

Post by Sydsmith » Thu Nov 29, 2018 7:33 pm

ND that's a difficult question to answer without more details.

In the ordinary sense it would be fine to connect a 7.2 volt charger directly across a 6v lead acid battery, as you rightly point out there needs to be a slight over voltage because, as the battery reaches full charge and the voltage in the battery equals that of the charger, charging stops and if the charger has the same output voltage as the battery it will stop charging before the battery reaches full charge.

However a lot of items which use rechargeable batteries have some sort of control system to avoid over charging, and there are two ways you could damage the equipment by using the wrong charger. The main one if the polarity of the output from the charger in respect to the charger socket, get it wrong and it could cook the input circuit of the equipment, the second is voltage of course but in this case it is unlikely to be damaged by 7.2 volts if the polarity is correct.

The third consideration is current output v charge required. If the equipment uses heavy current the original charger will be up to the job, your replacement may not be, it will work given the proviso above but it will take considerably longer than the proper charger to reach full charge. In a perfect world and in round figures a 10 amp hour battery charged at 1 amp will take about 10 hours to charge, but if the charger is only capable of say a quarter of an amp (250 ma) then it will take 40 hours to charge.

Hope that helps. Syd

New Dexter
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Re: Battery charger voltage

Post by New Dexter » Fri Nov 30, 2018 6:55 pm

Thank you Sad, you have confirmed some of my thoughts but I decided to check to see if this forum's gurus had better information

New Dexter
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Re: Battery charger voltage

Post by New Dexter » Fri Nov 30, 2018 6:56 pm

Predictive text. Bother I typed Syd and it gave me sad.

[Edit: For future information, you can edit your posts. Just use the pencil icon at the top right of your post (it only shows for your own posts.) ]

Sydsmith
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Re: Battery charger voltage

Post by Sydsmith » Sat Dec 01, 2018 9:11 am

No problem ND, having been in retail for almost 50 years before I retired I have been called much worse, glad to have been of help.

I am intrigued as to what it is you are charging that is fitted with a lead acid battery. Unusual in this day and age.

New Dexter
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Re: Battery charger voltage

Post by New Dexter » Sat Dec 01, 2018 11:16 am

It's a crane scale and the battery is stated to be lead-acid. I have had the scale for over a year and while it isn't used regularly it's a handy piece of kit. However, I haven't charged it once yet.

Sydsmith
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Joined: Sun Feb 14, 2016 11:15 pm
Location: Aberystwyth Wales

Re: Battery charger voltage

Post by Sydsmith » Sat Dec 01, 2018 1:47 pm

That's interesting and as I said unusual in this day and age, most are as you will know are lithium or something similar these days.

One thing I should have mentioned is that most rechargeable equipment is fitted with a diode within the unit to prevent any chance the charge polarity is incorrect. Syd

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theoldman
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Location: Bacton on Sea, Norfolk UK

Re: Battery charger voltage

Post by theoldman » Sun Dec 02, 2018 6:00 pm

To charge a 6v battery, you must have a voltage of at least 7.....................................
Normal for Norfolk

New Dexter
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Re: Battery charger voltage

Post by New Dexter » Sun Dec 02, 2018 7:26 pm

Thank you, that confirmed my suspicions that i had found the correct charger.
While we are on the subject of charging batteries. I have a 096 battery that I have had in one vehicle for some time. The car hasn't run for at least two years because of a fuelling fault and thus the battery has been sitting under the bonnet and as the result of my neglect it hasn't been charged on a regular basis.
I decided to remove it from the car at the weekend and have placed it on charge. The charger is a Kenlowe with the facility to check the voltage of a battery before and after charge. It started off charging at 1A and gradually it rose to 5A but even though I left it on charge for the afternoon there wasn't much alteration in the charge required. Strangely the voltage check revealed 12V, which was an improvement on the start voltage.
Today I repeated the exercise and managed to persuade one, perhaps two of the cells to show signs of life. However, the voltage check wasn't a good result.
Clearly failing to pay attention to a battery is not a good idea for future use. Sadly this battery, a Yuasa, was new in 2012 and my neglect might not have been good for its future
I have seen it stated that it's not a good idea to charge a battery sitting on a concrete floor so I have made sure that the recent charging activity has resulted on the batteries being charged on wood to insulate them

Sydsmith
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Re: Battery charger voltage

Post by Sydsmith » Mon Dec 03, 2018 11:04 am

Lead acid batteries suffer from positive terminal/grid corrosion, scales/particles of lead migrate from the positive plates and will, because they are conductive, eventually form a conductive sludge in the bottom of the cell and short out and kill a cell or cells in a battery. The migration and subsequent damage is made worse when the battery is left for long periods not fully charged, so the moral of the story is keep you battery topped up.

Well worth investing in a smart charger they are available for less than £30 and will keep a battery at optimum voltage and preserve the battery.

In simple terms, when first connected to a charger the battery in a static state will take time to absorb the charge, and without complicating the issue causes the charge current to rise slowly after first connecting reaching optimum level after a few minutes.

Not sure about the concrete/wood argument, can't think of any good reason why there should be an advantage, on the contrary, when charging a battery will generate heat that is damaging to a battery, positive plate migration increases with heat, on face value then it should be an advantage to charge on concrete to dissipate the heat. Car batteries in hot climates rarely last more than a few years. But I stand to be corrected. Syd

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