My battery just died yesterday (actually left the lights on). Took it out for charging today but my new automatic charger for detect such low voltage to begin the charging process. So, I had to hook it up to my great old Lucas charger (attached pic below) but only to notice there was no defection in the ammeter. I checked with my multi-meter and found the output was like just 0.24V, opened the case and checked the transformer an it was fine. Looks like a blown rectifier-selenium rectifier I guess (pic below). After a futile search found these rectifiers are no longer available, so finally got a KBPC2510 metal bridge rectifier. I've installed it on the battery charger and measured the out put at 6V and 12V setting gave me 6.9V/11.9V open circuited and when connected to the battery the outputs rose from 6.8- 8V for 6V battery and 14-15.8V for the car battery as it got charged .

Is this because of the metal bridge rectifier? I haven't had the chance to measure the volts with the old rectifier before. Are these voltages bad for these battery? battery charger[![selenium rectfier 1[![se rect[![metal bridge rectifier]2]3]4

  • \$\begingroup\$ 15.8 V seems a bit much for a 12 V lead acid battery (14.4 V should be the limit, if I'm not mistaken). Selenium rectifiers have a higher voltage drop than modern ones, so maybe the circuit was dependent on that to get the right voltages. Or the voltages where so high to equalize the cells and was designed for batteries where you'd fill water in them, I think those had higher charge voltages. \$\endgroup\$ – Arsenal Aug 15 '17 at 9:29
  • \$\begingroup\$ @Arsenal I think you are right. Most of the batteries in those days are those flooded cells. So, slight water loss was tolerable. But nowadays with those maintenance free it might not be a good idea. \$\endgroup\$ – The_Vintage_Collector Aug 15 '17 at 10:42

Selenium rectifiers have a much softer I/U characteristic than silicium ones.

Selenium Diode I/U characteristic

You can simulate the soft slope behaviour on the right by putting a few ohms resistor in series with each silicium diode. For your application a single resistor at the rectifier output may suffice. Mind the correct power rating and cooling for that resistor!

  • \$\begingroup\$ mate unfortunately I don't have a spare power resistor with me. I do have a 12V/15CP signal flasher bulb with me. Will that be sufficient? Its resistance is around 1.7 ohm when I measured plus the voltage seems to be slowly rising only when I checked just now. \$\endgroup\$ – The_Vintage_Collector Aug 15 '17 at 10:45
  • \$\begingroup\$ Yes, 0.5Ω to 2Ω is the correct range. Check the current and voltage while charging the battery. \$\endgroup\$ – Janka Aug 15 '17 at 12:22

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