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Charging  Empty Re: Charging

Post by Admin on Tue Apr 16, 2019 11:22 pm

Hi Jalex,
Interesting question.

In order for a cell to achieve it's storage capacity the charging voltage must be equal to the fully-charged voltage of the cell. If this is not achieved then there will be very little energy stored, if any at all.

If you connected a 12v battery to a 6v charger (transformer/rectifier) then there will be no charging current at all.

If you connected a 12v battery to a 6v regulated PSU then the reverse voltage at the controlling device can burn the PSU.

As a cell charges and absorbs energy, the terminal voltage will rise. This is true of all cells, irrespective of technology. In the case of a lead-acid battery, then an empty cell could be as little as 1.95 volts (any less and it could be defective). If the voltage is less then there is no energy in the cell.

When you pass a controlled current through the cell then the current is converted into chemical energy. The terminal will rise to typically 2.3v. If the cell is "float charged" then you need to regulate the charger voltage to 2.3v per cell to prevent overcharging.

If you do not regulate the voltage and continue passing current , then the voltage can rise to 2.45v, but at this stage the cell cannot hold any more energy, so the applied power will be turned into heat and continued charging will destroy the cell.

But if you discharge most cells below their "nominal voltage", such as 1v for our lead-acid cell, then there is no energy stored. In fact, many cells that are so deeply discharged may not recover at all.

So to answer your question, if you use a 6v charger to charge a 12v battery then the battery will not be charged. No energy will be stored and the battery could even have permanently damaged cells if the voltage were so low:

If you had a pair of 2v cells to get 4v, then you discharged the battery to 2v, then it is very unlikely that the two cells will be identical. One of the two cells may still have 2v across it's terminals and the other 0v. A 12v battery is even worse. The weakest cell that becomes empty will be charged backwards by the other cells causing permanent damage.

If you have a 12v battery and only a 6v charger, then look closer at the battery. Do you have access to the bridges between the cells? If so then you can charge three of the cells at 6v. When they are full then you can use the charger to charge the other string of 3 cells. That way you can charge a 12v battery with a 6v charger.

If you have a wet-battery (12v) then you can open one of the acid or vent-plugs and use a stainless-steel spike to make a contact with one of the plates. If you have 1/2 the battery voltage on a plate with respect to any terminal, then you have found the centre-tap of the battery. If not then try the next plate. You will probably get away with this a few times, but if you short two plates then you could get a violent discharge, so be careful (push thin plastic either side of the plate you need to make contact with).

Oops!!! This response is evidence that if you ask an engineer a question, then you had better be prepared for a long answer.

The short answer to your question is "no".

Very best regards from Harry - EA/SM0VPO

They say that money cannot buy you happiness. But if you want to prove this by experiment, then I volunteer to be the test-rabbit :-)

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Charging  Empty Charging

Post by Jalex2 on Tue Apr 16, 2019 7:29 pm

Hi Harry

I have a question maybe you could answer
Lets say you have a 12 volt battery and only a 6 volt charger.
Will the charger charge the battery to 6 volts and can you used that 
charged 12 volt battery to replace a 6 volt battery of the same AH rating.
 This is just something I wondered about and have no intention of doing LOL
  Thanks Jim


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