Lead Acid Battery Charging Level Indicator.

Hi guys i have a simple 12v lead acid battery charger.it consists of 13.5ac transformer & a bridge rectifier.rectifier output is directly conected to the battery.i'm charging 12v/7A batteries.

without the battery i measured the charger output voltage it shows 12.3vDC.

Question1
when i conect the battery it will immediately rise upto 15v & above.is this normal?

Question2
if i use a voltage regulator calculated 13.6v on the output of the bridge rectifier will the voltage rise to 15v & above or will it maintain 13.6v & never go above 13.6v?

What i want to do is show battery voltage using some led's.i can make a circuit but it will measure the rise up voltage 15v not the actual battery voltage.that's my main problem.

Suraj143 said:

Hi guys i have a simple 12v lead acid battery charger.it consists of 13.5ac transformer & a bridge rectifier.rectifier output is directly conected to the battery.i'm charging 12v/7A batteries.

without the battery i measured the charger output voltage it shows 12.3vDC.

Question1
when i conect the battery it will immediately rise upto 15v & above.is this normal?

Question2
if i use a voltage regulator calculated 13.6v on the output of the bridge rectifier will the voltage rise to 15v & above or will it maintain 13.6v & never go above 13.6v?

What i want to do is show battery voltage using some led's.i can make a circuit but it will measure the rise up voltage 15v not the actual battery voltage.that's my main problem.

Click to expand...

13.6, rectified can be as high as 19 Volts DC.

If you use a 10 Volt Transformer with a full wave bridge, you will get a Better Charge rate for this application.
13.6 Volts Rectified, is TOO HIGH.

The battery is being charged by the peaks of the unfiltered full-wave rectified wave. You need a electronic voltage regulator between the rectifier and the battery, like a PB137, or an adjusted LM317.

The electronic regulator must control the voltage at the battery terminals. For an 12V SLA, the long-term float voltage should be 13.7V +-0.1V.

Ok mike that means if i use a regulator (13.6v) the charging voltage will be fixed 13.6v.

But the hardest thing how to add leds to show battery level?because all the time it will show 13.6v while charging.

Suraj143 said:

because all the time it will show 13.6v while charging.

Click to expand...

You need to disconnect the charger, bleed off the surface charge by taking out 6A for a minute or so, then read the open circuit voltage, then compare it to charts on the Web to convert to %charge. ~12.8 is full charge, ~11.5v might be empty.

Unregulated battery charger is a battery killer.

A regulator, at least regulator I.C. circuitry, needs a smooth D.C. to run from. Some regulator I.C.'s even when supplied their filtered D.C. get freaked out with 120 Hz full wave rectified input on the series pass device. It is a feedback stability issue with the linear regulator and the voltage pulses on the input.

A good lead acid charger is a three stage charger. First is constant current phase, at up to about 20% of rated A-H of battery in current. As battery voltage rises the second stage is bulk phase which is voltage regulated at about 14.3 vdc. Third stage drops back to a float/topping charge of about 13.5 vdc when the stage 2 constant bulk voltage charge current drops to about 10% of its current limited stage 1 current. Some charger also have an equalization mode that raises voltage to 15.5 to 16.5 vdc to peak out the charge state between all the cells. This can be damaging if used too much.

Any LED lights would be used to indicated which of the three stages are in process.

The stage voltages are modified slightly based on temperature.

Here is a charger that I built that charges a 12V 7AH gel-cel battery and turns on a LED when the battery is charged.

Hi thanks for the reply guys.

k7elp60 thanks for the diagram.Your charger circuit I understood.But the LED indicator circuit is much complex.I'll try it to compact.

@ Willbe
If I want to measure the voltage then I have to disconnect the charger & measure the open circuit voltage.Is it ok if I measure the battery voltage without the load after disconnecting the charger?

If I wan to measure the current then I have to do a circuit like k7elp60 mentioned.

But that charging indicator is bit complex.

Hi, Suraj

also see that one :

**broken link removed**

I also remember an Elektor Magazine solution with Leds, resistors ans Zeners ... not very accurate, but simple.

I dig into my archives ...

Also see LM 317 / LM 350 Datasheets : they show SLA chargers apps.

Alain

Hi Suraj143,

you might want to check out the UC3906N (Texas Instruments), an SLA-charger chip. It contains the necessary circuitry to stop charging the battery when it is fully charged, and cuts back on charging current (and voltage).

The chip is reliable only if the battery is being charged under no-load-condition.

Boncuk