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, 2017 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\$ Aug 15, 2017 at 10:42

2 Answers 2


This is my first post here and I just stumble in your post, as I like to search about car battery chargers. Answering two questions and to suggest a way to improve your charger with minimal changes (at the end).

Q1: Can selenium rectifiers be replace by silicon rectifiers?

A1: Definitely YES, in fact, most repairs like that should replace. It will be an upgrade, making any charger work better. BUT care should be taken to size properly the rectifier selected. I believe the chosen KBPC2510 bridge is robust enough for your charger, as it is rated for 25A and the front plate of it says 10-12Amp.

Q2: Why the voltage seems to be higher than expected, as you measured and said: “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.”

A2: Your older charger seems to be made essentially with a transformer and rectifier. It doesn’t have any filtering capacitor or regulation components. This case might be confirmed as you might have measured the “open circuit” voltage using a multimeter in DC voltage, when the average DC voltage is measured, which can be somewhat different than the real RMS value. A rectified sinusoidal waveform with 11.9V of average value, has a PEAK value that is Sqrt(2) times this: V_peak = 1.414 x V_avg = 1.414 x 11.9V = 16.8V But a bridge rectifier has a voltage drop in two diodes (2 x 0.7V), so the maximum expected charging voltage is:

V_max12 = V_peak - 1.4V = 15.4V

If you repeat this for the 6V settings, you will find:

V_max6 = (6.9 x 1.414) - 1.4 = 8.3V

These small differences between what was calculated above and your measured values could be caused be [Average x RMS] and small variations in diode performance. So the battery behaves as large capacitor, and battery voltage is close to peak calculated voltages.

POTENTIAL PROBLEMS with this kind of charger - and how to alleviate them:

  1. Does not have short circuit protection. This might become worse if older transformer is oversized and could deliver short-circuit currents bigger than 25-30A. This would result in overcurrent for modern bridge rectifiers, less forgiving to abuse (and spanking) than original selenium ones.
  2. Does not have reverse polarity protection. This could also create severe overcurrent, damaging Si bridge rectifier.
  3. Does not limit the current for deeply discharged batteries.
  4. The final charging voltage might be too high, prone to battery gassing and shortening its life.

Possible contingency solutions, while preserving its original and antique simplicity & robustness:

  1. Install 12V headlamps in series - Installing 2 or 3 highbeam lamps in parallel among them (each about 50W) while in series with one of charger’s cabling, could limit the maximum charging current to less than 8A or 12A. This protects against short circuiting the terminals= lamps light up in 90% full brightness. This also protects if terminals are connected with reverse polarity in the battery, the lamps would turn on even if charger is unplugged (turned-off) or lamps would light up “overly bright” if charger is already turned-on. This also limits final charging current and allow more time to turn-off charger when desired battery voltage is reached.
  2. Install additional rectifier diodes in series - Measured 15.4V to 15.8V is somewhat too high to charge most 12V batteries. Final voltage should be about 14.4v for bulk charging. One way to achieve this is to use the same 0.7V to 1.4V of voltage drop by the use of another bridge rectifier, selecting
    • from negative to positive (2 diodes in series, working in parallel) to get 1.4V drop, or
    • from shorted AC terminals to positive terminal (2 diodes in parallel) for 0.7V.

Note for Overheating management:

Both rectifier bridges should be properly heatsinked, maybe even with forced convection (fan cooled.) Heat generated by headlamps would benefit from forced convection too.

I hope these comments help you and other fellows.

  • \$\begingroup\$ Thank you fore the detailed explanation. So, what I ended up doing was little different. So, as per the bridge rectifier's data sheet it should be 1.1V drop. But I don't want add too many rectifiers so I found a DC-DC step down converter with 10A capacity. I have tuned the output to be around 13.86VDC (I sometimes increase it to 14.4V if the battery voltage is too low) and set the maximum current pot to 3Amps. It can be ramped up to 8 Amps but at that point I need a cooling fan to dissipate the heat. So, for not totally dead batteries I use this setup now...cntd in next comment \$\endgroup\$ Apr 20, 2020 at 16:18
  • \$\begingroup\$ ...contd BTW on the transformer I found on the label it said 14/9VAC it can output. Since I repair, rebuilt and test starter motors for bikes I have a bench supply made with a 50Amp rectifier and UPS transformer that outputs 13.25VDC and can easily handle 35Amps. So, I use that for a completely dead battery and then once its up I use the other charger to continue. \$\endgroup\$ Apr 20, 2020 at 16:22
  • \$\begingroup\$ Regarding your 1st reply: Typical Silicon Rectifier has 0.7V per diode, near its specified / rated current. This would be 1.4V for a diode bridge. That 1.1V is somewhat lower than I would expect, but could happen in much lower currents. \$\endgroup\$
    – EJE
    Apr 24, 2020 at 15:40
  • \$\begingroup\$ Another point: using a stepdown switch power supply module can indeed reduce the voltage, but it would expect to receive an smooth voltage from rectifier, I mean, filtered by electrolytic capacitor(s). Otherwise, it will turn on&off 120 times per second, and its small input is not enough to compensate that. My guesstimate would be to use a 4700 uFd x 25V (or better: 40V) at the output of rectifier. It would be good up to those 3A your module is managing now. \$\endgroup\$
    – EJE
    Apr 24, 2020 at 15:47
  • \$\begingroup\$ About your 2nd reply: If I understood correctly, as you repair repair starter of motorcycles, sometimes you have some very weak or “dead” battery, right? AFAIK, these batteries are typically iwith capacity in the 5Ah to 20Ah range. I’ve studied and tested some charging methods. Fast charges are an extreme resource, which could be done at maximum 1C (5A for the smaller 5Ah to 20A for 20Ah one) - and for a short time (15-30minutes max), monitoring its temperature and voltage, and terminating this high current if it starts “fissing”/“gassing”/“boiling” (you got the idea). Continue... \$\endgroup\$
    – EJE
    Apr 24, 2020 at 15:57

Selenium rectifiers have a much softer I/U characteristic than silicon 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 silicon 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\$ Aug 15, 2017 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, 2017 at 12:22

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