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Battery charger

Battery charger

(OP)
Hi

At work I am curntly building a battery charger.

I took a 220v to 10v transformer and took out the secondary windings because I wanted to make it a 12v, I rewinded it with 2.5mm flexible cable untill I got my 12v. The transformer works well untill I connect it to the battery, the current rises to anything between 10A and 17A and the voltage rises to 16v, witch according to me is much to high for charging a battery. When I connect a normal load like a motor or light it works 100%. I have even took out some of the windings to drop the voltage to 11v but still gets the same problem.

What may be the cause of the problem?

Any help would be appreciated

RE: Battery charger

The peak voltage is roughly 1.4 times the RMS value. So at 11V you can easily get 16V at the battery. Sounds like a great charger and a little sulfated battery for the voltage to raise that much with 10-17A. Many chargers depend on transformer resistance to work. Just need to pull more wire off.

RE: Battery charger

(OP)
So if I lower the voltage to about 10v it will still be fine to charge a 12v battery? Or should I just put in a voltage regulator to keep the voltage low?

RE: Battery charger

Just in case... there's some rectification/charging electronics between the transformer and the battery right? Because the transformer will produce AC and the battery requires DC. So what is doing the rectification/smoothing for you?

RE: Battery charger

(OP)
Yes life year there is a rectifier on the secondary side and the given voltage is on the DC side, and it is a full wave rectifier with a 100uf cap

RE: Battery charger

Often commercial chargers have a tap on the primary for normal charge and boost. A basic bulk chargeer isn't worth a regulator, assume you are using lead acid batteries. You could put an extra diode or two in series with the output to add a set voltage drop and lower current. Then you could short them out when higher currents are needed.

RE: Battery charger

In that case, I reckon it should work!

The only explanation I can think of is that your 12V battery is suspect (faulty cell?) and is actually sitting at 10V unloaded. When you connect your tranny + rectifier, the 2V differential appears across the battery's internal resistance (0.1 Ohm say) and so current rushes in to it and the filter capacitor. The inrush causes the voltage of the battery to rise steeply. Perhaps you're unlucky and the inductance in the secondary plus the inductance in the cables happens to form a resonant LC circuit with the capacitance of filter cap and the battery and you get an unstable ringing effect.

RE: Battery charger

(OP)
thanks guys!

i solved the problem...

I just took out the secondary turns that was 2.5mm cable and replaced it with a 1.5mm cable. works like a dream!

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