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I am building a charger for NiCD batteries and wondered if it was possible to accept the battery in either orientation and rather then protecting the battery detect the battery's polarity and switch the charging current polarity so it can be charged either way around? Thanks in advance!

Thanks for your responses! The idea is to do it for a single NiCD AA battery. It seems that most charging chips do check the polarity as a safety measure. Is there any reason they don't use the result to change the charging polarity rather than cutting the charge altogether. I guess its not really any more difficult?

Thanks again guys. I have been told by the person doing the electronics that it's going to need 2 power relays and a fair bit of additional feedback circuitry which makes it work. this pretty much rules it out. Does anybody have any smart solutions?

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  • \$\begingroup\$ You can first detect the battery polarity and then switch on the charger. This will probably require a microcontroller and mosfets. \$\endgroup\$ – Indraneel Jan 21 at 23:06
  • \$\begingroup\$ Can be done. Value of doing so is questionable unless there are major constraints on reinserting. Adding a "battery inseretedincorectly / stupidlyt / by a moron" LED may be enough. Detect polarity - if correct, proceed. If reversed, light LED. (A reversed indicator MAY be able to be a diode, resistor and LED. Maybe a transistor and an extra r or few. \$\endgroup\$ – Russell McMahon Jan 22 at 10:43
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    \$\begingroup\$ Just to be clear, you don't want to hear about any solutions that need a "fair bit" of circuitry...is that right? \$\endgroup\$ – Elliot Alderson Jan 30 at 13:27
  • \$\begingroup\$ Yes, I'd just assumed that there would be some simple way of reversing the positive and negative terminals depending on the polarity detected. This way it could charge directly for the chip in any orientation. I cant really work out how a single DPDT Switch or Relay but I'm pretty new to all this. Thanks for your help! \$\endgroup\$ – tommarshallandrews Jan 30 at 17:13
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To answer your question, yes it is possible. The choice of exactly how to do this would depend on the specific battery involved and what other design constraints you face.

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An H-Bridge is often used to control a motor, but it should also be able to do what you want, so you'll need to sense the polarity, and then use a microcontroller to turn on the appropriate switches. That should be less expensive and fewer parts than two power relays.

The Diodes, Inc. DMHC3025LSD-13, is a Logic-Level switchable set of complementary N-MOSFET and P-MOSFET, and it looks like something that might do it. Here's a screen shot of the datasheet for it:

Screen shot of specifications part of datasheet of Diodes, Inc., datasheet for part # DMHC3025LSD

You would connect the battery between the "P1D/N1D" and "P2D/N2D" terminals.

Please note that I have focused mainly on learning power electronics, and have not actually done this, so you'll need feedback from electrical engineers who know the complications, rules-of-thumb, and gotchas! Perhaps ask another question, "what should I watch out for?" Best of luck, and please come back and let us know how it worked out.

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Yes it's possible. the simplest implementation would probably be to build a negative resistance. (this is an op-amp circuit)

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