I am trying to power a bulb filament that has a rating of 3.0 V and 2.5 A.

First I made sure the filament worked by powering it with a variable power supply and setting the voltage at 2.5 V:

enter image description here

This was successful in heating the filament and it was drawing a maximum of 2 A.

When trying to do the same with a battery power supply, the filament wouldn't turn on.

The battery pack consists of two sets of NiCd batteries rated at 1.2 V and 1000 mAh connected in series. Those two sets are then connected in parallel.

enter image description here

My thinking is that it should supply the same amount of power because the series circuit will sum the voltage to 2.4 V. Furthermore the parallel circuit will sum the current to 2000 mAh.

I am unsure where I went wrong and would appreciate more insight on how to properly power the bulb.

  • \$\begingroup\$ That spring contact to the battery will have some serious voltage drop at 3.5 A. Wires look thin too. \$\endgroup\$
    – winny
    Mar 15, 2023 at 8:59

1 Answer 1


Furthermore the parallel circuit will sum the current to 2000 mAh.

2000 mAh is the battery capacity, not the output current capability of the battery. A parallel connection does indeed sum the capacities. It will also sum the output currents. Unfortunately you've not told us what the maximum output current is for those cells.

Usually, AA NICds ought to be able to output an amp or more.

Check the contacts in the battery holders for high contact resistance. Are the cells fully charged, and not at the end of their useful life? Check each cell individually with a DMM.

  • \$\begingroup\$ Hello, thanks for the reply. I did check each battery in advance, and I measured the final output voltage of the battery pack to be the expected amount. As for the high contact resistance, I soldered some thicker wire, and it still did not work. Unfortunately, I was not provided with the maximum output, so I can't assess that. What would you recommend I try next? \$\endgroup\$ Mar 15, 2023 at 9:07
  • \$\begingroup\$ @import_hill Check what happens to the voltage when you connect the bulb. With healthy NiCD cells, you should still get something close to 2.4V. \$\endgroup\$
    – Simon B
    Mar 15, 2023 at 10:57
  • \$\begingroup\$ @SimonB I connected a voltmeter to the battery and found that the voltage drops to the mV range when connected to the load. I believe that this is the issue, although I'm not sure how to fix it. For more context, these batteries are brand new so I don't think it is the condition of the batteries. \$\endgroup\$ Mar 15, 2023 at 13:34
  • \$\begingroup\$ If the batteries a brand new from a reputable source, return them for a refund. If they are brand new from a flaky internet marketplace (you know who they are) then put it down to a lesson learned, and buy reputable next time. \$\endgroup\$
    – Neil_UK
    Mar 15, 2023 at 13:55
  • \$\begingroup\$ how heavy are they, NiCds tend to feel quite dense? If it's a cheap knock-off, they may well be lighter than expected. \$\endgroup\$
    – Neil_UK
    Mar 15, 2023 at 14:29

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