Help me with this automatic battery charger.

[maxeemo]

Hi guys. I'm going to make an automatic battery charger for my 6V 4.5Ah SLA battery. This is the diagram that I've found;

**broken link removed**

I built this on breadboard, and it seems to work well, but...

As it mentioned in the document, charger starts charging the battery with charging current of about 466 mA, and current decreases as voltage of battery increases, and becomes approximately equal to zero when the battery is fully charged.

My problem is that the charging current starts from about 250 mA, and now that the voltage of battery has reached to 6.9V, it is about 70 mA. And after 48 hours, my battery isn't fully charged! Is this normal? If not, what is the problem? Is this a good circuit to use? Are all those capacitors necessary? Is there any better and faster circuit on the web? This one is the result of my search, I didn't find anything simpler and more useful.

Sorry for bad English.

and thank you beforehand.

 

[ronv]

It sounds like you may not have set the voltage in the correct place. What did you set the voltage to, and was it before or after the diode.

 

[tosborn]

You said, "after 48 hours, my battery isn't fully charged." You might want to describe how you determined this. IIRC a fully-charged 6v lead-acid battery at rest will have a voltage of around 6.3v. Are you expecting the fully charged voltage to be greater than this?

 

[Boncuk]

A lead-acid-battery always draws current - charged or uncharged when connected to a battery charger.

The only point to determine end of charge is the battery voltage. (Use a cell voltage of max 2.2V per cell for a full charge).

End of charge at a battery voltage of a 6V-battery is well within safe limits (if charged to 6.9V).

If charging continues the battery will still increase voltage up to 7.2V (2.4V per cell) when it starts gasing!

An SLA-battery has no pressure relief valve and gas pressure will eventually destroy the battery (like a small bomb!)

Even if the overpressure safety valve pops open the battery will be dead after that since it will lose battery water (sulforic acid gel) and spill the acid over the place.

Car batteries (wet) have refill caps to add distilled water if the battery loses liquid. They contain pressure relief valves for each cell. Batteries lose acid (water) due to overcharging or in hot environments like deserts or tropical areas with high ambient air temperature.

The circuit looks OK to me. Just make sure charging terminates at maximum allowable full charge battery voltage.

Boncuk

 

[maxeemo]

It sounds like you may not have set the voltage in the correct place. What did you set the voltage to, and was it before or after the diode.

I've set it to 7.2V. It was after diode, right on charging terminals.

You said, "after 48 hours, my battery isn't fully charged." You might want to describe how you determined this. IIRC a fully-charged 6v lead-acid battery at rest will have a voltage of around 6.3v. Are you expecting the fully charged voltage to be greater than this?

I was waiting for voltage of battery to reach the set voltage (7.2V). But the voltage of battery is stuck on 6.9V and doesn't increase anymore. As what you said, yes, it is fully charged. But I was expecting the circuit to do its job and continue the charging to set voltage.

A lead-acid-battery always draws current - charged or uncharged when connected to a battery charger.

...

The circuit looks OK to me. Just make sure charging terminates at maximum allowable full charge battery voltage.

Thank you Boncuk. Your post was really useful to me. Yes, voltage on charging terminals is set correctly.

-

After all, although the circuit doesn't do its job (as mentioned in document), it seems that it charges the battery in a safe way. The only problem is charging process takes too long.

 

[ronv]

I would "nudge" the voltage up to 7.2 volts after the battery is fully charged. It sounds like the voltage across the diode is very small when you set it with no load and is a few thenths higher with just a slight load. Even this small change will make a big difference in how fast it will charge.