Electronic – Supercapacitor in parallel with Lithium 3.6V Primary Cells

supercapacitor

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I have a circuit I would like to implement but need some advice first. It seems basic enough, but not having worked with either Lithium-Thionyl batteries or supercapacitors before I want to make sure.

  1. There are 2x 3.6V/17Ah primary, non-rechargeable batteries in series to get 7.2V. The peak current of the SAFT 33600 is 400mA for 100ms pulses, continuous is 250mA.
  2. The batteries feed a DC to DC converter to get 12V
  3. There a peak currents needed for short burst, thus the addition of supercap(s): there are 2x 5F/5V supercaps in series (160 mOhm Ri)
  4. A schottky diode will be added from the batteries to protect them.

From this, the supercaps are paralleled between the schottky diode cathode and the DC-DC converter. My concern is this scenario, with the initial charge at zero, before the batteries are inserted, the first time "charge".

I think that once the caps are charged, they reach equilibrium in voltage, and from that point will supply the current "peaks" (about 300ms of 2A) while being used, and getting a trickle charge when not. Note that the load is shown as a simple resistor, but it is a peak-cycle circuit that requires burst peaks as stated.

The question then is, does the current flowing from the battery into the caps need to be controlled when the caps are not initially charged? If so, adding a limiting resistor is not a first choice, I don't want to lose any energy unnecessarily during regular use.

Best Answer

There is a major misconception in this statement & question

The question then is, does the current flowing from the battery into the caps need to be controlled when the caps are not initially charged? If so, adding a limiting resistor is not a first choice, I don't want to lose any energy unnecessarily during regular use.

A capacitor charged via a non reactive path from a fixed voltage will always dissipate energy in the resistance. This is true whether you charge very slowly via a large resistance (which may include the internal resistance of a low maximum current capability battery), or if you charge with no "apparent" resistance and vast currents.

As a consequence of the above the answer is that not only must the current be controlled but if you wish to minimise energy taken to charge the capacitor you must use an active converter of some sort - in this case actually or effectively a buck converter. Whether the energy gain is worth the effort is up to you.

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