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Topic: Buck Converters and Wall Warts (Read 183 times) previous topic - next topic

mdr78

Hi folks,

I was having some trouble with some Lead Acid Battery Packs - they kind you bring camping or use during power outages. The batteries in them where dead, so I replaced the internal cells but couldn't get them to charge afterwards.

The input voltage on the side of Battery Pack read "12v 0.5a", which made sense because the internal cells, where 12v cells.

So I set my Buck Converter (more of a cheap Bench PS really) to "12v 0.5a" and tried to charge them. But no matter how long I charged the cell, the Pack's charge indicator light would never go green.

Later I found the original Wall Wart for the Battery Packs. It read "12v 0.5a" output - so I set the Battery Packets charging with it, and things were good, the charge indicator light went green!

So I  initially decided that my Buck Converter was junk, but then investigated a little further with my multi-meter. I found that the Wall Wart was actually outputting 16-18v 0.5a. So I set my Buck Converter to a the same voltage and voilĂ  the Buck Convertor worked perfectly.

So here is the question ... why on earth would the Wall Wart transformer describe itself as "12v 0.5a", when in reality it is transformer with much higher output voltage?

What am I missing here?

Ray K

Grumpy_Mike

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So here is the question ... why on earth would the Wall Wart transformer describe itself as "12v 0.5a", when in reality it is transformer with much higher output voltage?
Because the 12V is produced when 0.5A is being drawn. These are known as unregulated supplies and used to be quite common but are now becoming a rarity.

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So I set my Buck Converter (more of a cheap Bench PS really) to "12v 0.5a" and tried to charge them. But no matter how long I charged the cell, the Pack's charge indicator light would never go green
That is because it was a good regulator and 12V is not sufficient to charge a Lead / Acid battery.

Koepel

It is dangerous to overcharge lead-acid batteries.
The batteryuniversity has a number of pages about charging lead-acid batteries. This page is about using a power supply: http://batteryuniversity.com/learn/article/charging_with_a_power_supply

mdr78

Because the 12V is produced when 0.5A is being drawn. These are known as unregulated supplies and used to be quite common but are now becoming a rarity.
That is because it was a good regulator and 12V is not sufficient to charge a Lead / Acid battery.
So in this case < than 0.5A is likely to be being drawn at a higher voltage ?

I have never come across this before.
I was always told - never use a higher voltage than indicated.

Ray K

Grumpy_Mike

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So in this case < than 0.5A is likely to be being drawn at a higher voltage ?
Yes that is the case.

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I have never come across this before.
You probably have not been about too long either, it was very common 30 years ago. These things are normally heavy due to the transformer.

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I was always told - never use a higher voltage than indicated.
Good advice in the right place, but battery charging is different. If you think you can charge a 12V battery with a 12 supply then you are wrong.

mdr78

Yes that is the case.
You probably have not been about too long either, it was very common 30 years ago. These things are normally heavy due to the transformer.
Good advice in the right place, but battery charging is different. If you think you can charge a 12V battery with a 12 supply then you are wrong.

well I was just following the instructions on the back of the pack.
But it makes perfect sense - now thanks.

Electronics appears to be full of non-absolutes :-)

Grumpy_Mike

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Electronics appears to be full of non-absolutes :-)
The thing is that their are so many possible variations hard and fast rules are few and far between. However there are some, like get the polarity correct. Do not over stress a component. Don't wire anything up with the power connected. If a project sounds easy you might just be able to do it, if it sounds a bit tricky forget it.

But there are many rules with qualifications like:-
Never connect a voltage higher than supply voltage to a chip unless the data sheet says you can.

However as you get to understand the topic rules become not so important.

slipstick

OTOH there's also a rule that says that no battery has ever been charged by applying to it a voltage lower than its existing voltage. 12V nominal LA batteries often get to over 13.5V when fully charged. And to get them charged the charger has to be able to supply at least that.

Also the confusion over the charger marking is actually very simple. Chargers are usually marked with the markings of the battery they're intended for. So a 12V charger is intended for a 12V nominal battery. That's always been one of the differences between wall-wart chargers and wall-wart power supplies.

Steve

Wawa

#8
Feb 13, 2018, 11:56 pm Last Edit: Feb 13, 2018, 11:58 pm by Wawa
~12volt is the "battery flat" voltage, and ~12.8volt is the "battery full" voltage.
So you can't charge a 12volt SLA battery with a regulated 12volt supply.
More than 12.8volt is needed.

SLA  batteries can be (should be) charged with a constant voltage.
The charger should be set to <=13.6volt (13.8volt for flooded cells).
But 13.4volt or 13.5volt is safer if you leave the charger connected for some time.
Some current limiting could be needed.
Leo..

TomGeorge

Hi,
You may be better off getting an SLA charger, it will do all the necessary charge processes needed to get your batteries charged and maintained.

https://www.jaycar.com.au/6-12v-fully-automatic-lead-acid-battery-charger-800ma/p/MB3519

Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

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