Lead acid's limited charging current Vs. transformer's current rating

[Willen]

I know that generally we need 10% current if battery's capacity (Ah) is 100. I need to limite current almost 1A limited current for 10Ah battery. But if I have a transformer (with exactly suitable charging voltage) already rated 1A output, then also I need to use current limiter?

First I thought I don't need limiter then. Later I thought battery takes all available current, so battery might cause like short circuit to secondary coil of the transformer due to over current drain. Being confused!

 

[NorthGuy]

Transformer is AC, but you need DC for the battery.

If you do have a rectifier attached, battery will take nearly all current that it can and in doing so, it'll drag the voltage down, which will naturally limit the current.

It is possible that the transformer, rectifier, or battery might get damaged in the process.

 

[Mosaic]

Full wave rectify with a 3A rated bridge or diodes, add a bulk electrolytic of > 1000uF and measure the unloaded voltage output. Hopefully you have around 17 to 20VDC unloaded.

You seem to want a 'taper' charging system based on the transformer balance of current and voltage. This depends on the VA rating of the transformer.

For an output of about 14V @ 1A, that's 14VA. You will have rectification losses along the way so a 17V x 1A = 17VA would be better.
The prob with a taper charger is that as the current drops the battery voltage rises and can cause the battery to go into its gassing phase which happens easily in the hot months as temperature can affect battery chemistry a lot.
Too much gassing corrodes the +ve plates and dries out & concentrates the electrolyte leading to early battery demise.

Therefore an unregulated 'dumb' charger must not be 'forgotten' when charging.

There are many 'smarter' solutions online if you can do a bit of electronics.

 

[audioguru]

I know that generally we need 10% current if battery's capacity (Ah) is 100.

I think you are talking about an old fashioned lead-acid or Ni-Cad battery.
The modern Lithium-Polymer (Li-Po) batteries that power my electric radio-controlled model airplanes produce an output current of up to 70 times their capacity rating and can be charged at 12 times their capacity rating.

 

[Mosaic]

The title of the OP post did specify 'lead acid'

 

[audioguru]

I forgot to read the title of this thread.
I have NEVER used an old-fashioned lead-acid battery except to start my car engine.

 

[KeepItSimpleStupid]

1 Amp is the rating of the windings. You can loosely consider the transformer the fuse if not protected in some way. There are "energy limiting" transformers which will withstand a continuous short.

The "short circuit current" is dependent on losses in the wires, windings and Rds (on) of the diode. The diode forward voltage also has to be subtracted.

Furthermore, rectification raises the output V. i,e, There are equations used to predict the DC output when using full or half-wave rectification.

Spend some time at www.batteryuniversity.com.

 

[dr pepper]

1/10 C as you mentioned charging current is a guideline for maximum battery life, you can charge a battery faster (depending on type as to how much) but the life is reduced, power tools usually charge at a high rate, and car batteries can be charged at around 100a from a medium sized alty even though the battery may only be 80aH.

 

[Willen]

Full wave rectify with a 3A rated bridge or diodes, add a bulk electrolytic of > 1000uF and measure the unloaded voltage output. Hopefully you have around 17 to 20VDC unloaded.

You seem to want a 'taper' charging system based on the transformer balance of current and voltage. This depends on the VA rating of the transformer.

For an output of about 14V @ 1A, that's 14VA. You will have rectification losses along the way so a 17V x 1A = 17VA would be better.

Um..... You guys making me more confusing. I am basic guy in English and electronics too. OK lets say I have 8VA very general linear transformer and if I added a rectifier + high value capacitor to charge 6Ah 6V lead acid (don't say no capacitor needed to charge a battery, and don't say you need exactly 7.999V not 8V hehe). I just need basic and general concept! So now If I connected the output from the bridge which is normally 8V (don't sai it is not 8V, but it is 8.001V after rectifier ) to the 6Ah 6V battery, then is it generally valid? Or just within few minute I will get the secondary coil burnt?

 

[NorthGuy]

Transformer can only do so much. Then the core saturates. When you load it beyond that point, its voltage will go down. Meanwhile battery voltage will go up because you're charging it. So they will become equal (minus two dide drops, which is far greater than 0.001V). If the transformer wire is not thick enough, it'll burn.

As the battery gets close to the full charge, the current will go down and the voltage will go up. For the 8V rated transformer, it might be as high as 10V (11V is the peak voltage for 8V RMS, minus diodes). If you apply such voltage to the 6V battery for a prolonged period of time, it'll boil out.