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I wanted to make a docking station for my robot. My idea was simple - robot drives in, batteries are disconnected, pins that I attach to my Ni-MH charger's electrodes are inserted into battery pack, and start charging. Like on this picture:

Diagram

But I've read on the internet, that chargers look at batteries temperature, to know when to stop. Is it going to work? Or are there any alternatives to dock my 4 Ni-MH battery pack?

These are 4 batteries, 2400 mAh, connected in series, 1.2V to generate 4.8V.

EDIT: I have also read about super fast charging, something like this: http://www.batteryspace.com/ch-v2880superfastnimhbatterychargerhomecarwithlcdmonitor.aspx. Will this work?

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  • \$\begingroup\$ "Hydride" takes a capital "H". \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 19 '14 at 12:23
  • \$\begingroup\$ @IgnacioVazquez-Abrams Corrected :) And any comment on my design? Won't the batteries explode? \$\endgroup\$ – Michał Aug 19 '14 at 12:33
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    \$\begingroup\$ Find a charger that does delta-V sensing instead of temperature. \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 19 '14 at 12:35
  • \$\begingroup\$ @IgnacioVazquez-Abrams Ok, thank you for suggestions :) \$\endgroup\$ – Michał Aug 19 '14 at 12:38
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    \$\begingroup\$ Put the charger circuit in the robot. The external contacts should just be a source of low-voltage AC or DC power. \$\endgroup\$ – Dave Tweed Aug 19 '14 at 12:46
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You don't have to design a battery charger from scratch, there are special purpose ICs which handle the charging algorithm for you, like delta-V, timeout, etc as are mentioned in other answers. Here are some places to start. The links point at NiMH since you mention that, but these vendors also make ICs for lithium-ion batteries: Linear Technology, Maxim, Texas Instruments.

If you want to buy a working charger circuit board, you might want to switch from NiMH to lithium-ion or lithium-polymer for more choices. Such as available from Adafruit.

You'd want to put the battery and charger board together in the robot so that the dock power pins would be a simple DC voltage input (unmodified output from wall adapter) rather than a battery charger.

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  1. Add connector for thermoresistor into charger dock.

  2. Make thermoresistor go into contact with batteries when gobot goes to dock.

  3. Install charger circuit into robot, not dock (by Dave Tweed).

  4. Use delta-V charger (by Ignacio Vazquez-Abrams).

  5. Use timed charger (require fully discharged battery).

  6. Use slow (0.1C 10 hour) charger.

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From what I've read you measure the voltage from the battery to gauge charge, not temp. Temp is a backup to avoid overheating and rupturing faulty cells.

In order to measure the voltage while charging you have to build a pulse charging circuit. You feed the battery short, fairly high current pulses of DC, then measure the battery voltage between pulses. When the battery reaches the target voltage, you stop.

Getting the algorithm right is tricky. I believe that smart chargers taper off the charge rate as the battery gets close to full. You should read up on smart battery chargers.

Also note that in order to do this right you should really measure and charge each cell separately. That way if there are differences in capacity or charge level between cells then you don't undercharge some cells (bad) and overcharge others (disastrous, can ruin cells.)

Why not buy an inexpensive smart charger, dissect it, and move it's electronics inside your robot?

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