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I have a 24 V lithium-ion battery which will pump as Iᵐᵃˣ 40 A and I need to connect a capacitor directly to the 24 V battery which will be 10000 μF(10 mF).

The usage of it is to keep the system stable if it pulled the entire 40 A.

Is it safe or does it need to be charged in a specific way?

Note: There would be BMS attached to this circuit so I am afraid that the BMS would consider it as short circuit.

Davide Andrea
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Hazardous Voltage
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2 Answers2

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No, it's not OK.

This is a common problem in traction batteries for vehicles. It is solved through precharge.

In a vehicle, the precharge circuit consists at the minimum of:

  • A precharge resistor, to limit the inrush current (R1)
  • A contactor (high power relay) across the precharge resistor (K2) to bypass the resistor during normal operation

Additionally, the precharge circuit may have:

  • A precharge relay (K1), to keep the load from being powered through the precharge resistor when the system is off
  • A contactor in line with the other end of the battery (K3) to isolate the load when the system is off

Precharge circuit

In the typical precharge circuit, the precharge resistor is on the positive terminal of the battery, though it could just as easily be on the negative terminal. While you are free to use any designators you wish, the ones in this schematic (R1, K2, K2 and K3) appear to be industry standard, so you are encouraged to use them as well.

Since your battery is low voltage (24 V), you may not require galvanic isolation. In that case, you can skip the precharge relay (K1) and the negative contactor (K3).

Consider connecting the capacitor directly and permanently to the string of cells (before the BMS). In that case, use a precharge resistor to charge the capacitor, then connect the capacitor permanently to the string of cells.

Davide Andrea
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One low tech solution putting most responsibility with the user is to use two connectors, one equipped with about 0.6 Ω series resistance, to be plugged in first, maybe smaller than the main power connector, contact material optimised for high wear resistance rather than low contact resistance.
I didn't have much luck finding connectors with a low number of contacts including more than one 'leading' contact, and most promise mere milliseconds between contact of leading to regular contacts.

greybeard
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  • I thought of relying on battery wires or something too but would NTC solve it, in the link you had put it stated this solution but would it be effective for such thing would it work because we are speaking of milliseconds here if not micro – Hazardous Voltage Jul 11 '23 at 10:18
  • To work well, the load would need to be almost constant, and the NTC to get HOT. – greybeard Jul 11 '23 at 10:30
  • thought of that too, so can I use a pre charge circuit based on a transistor that charges the capacitor slowly? would it be power efficient in normal cases? – Hazardous Voltage Jul 11 '23 at 10:42
  • `slowly` takes me back to something to add to the question: How does the load get switched / what voltage profile/timing is acceptable? Time constant of 10 mF\*0.6 Ω is a mere 6 ms. – greybeard Jul 11 '23 at 11:09
  • it would be 24 VDC as stated above the load would be an inverter(it has a soft starting feature) it will be switched via high current MOSFET, no problem for the timing – Hazardous Voltage Jul 12 '23 at 04:08