# 500F supercap charging causes circuit undervoltage (pre-charge)?

I have a 24V Generator starter system. Which controls all the engine parameters and includes a starter and it's supercap of 500F.

I'm supplying it with two redundant 24V 1kW sources. First DC/DC converter is directly connected to the management system (about 1A). Second DC/DC converter is connected to the supercap.

The management system starts and after a selftest and about 30s toggle the supercap contactor for loading.

Both converters should be in redundant mode, if one fails, the second should be able to compensate the loss and reduce the supercap inrush current impact.

However, when supecap is completely empty and starts to charge, the voltage is going under 16V (24V - 33%) and unfortunately the management is restarting causing troubles.

Do you have any ideas how to smooth the voltage at least till 18V? For the motors there are some soft starters. For the pre-charge there some Resistor+Contactor based circuits, but need a voltage monitoring. Unfortunately I have no Digital Inputs available.

Is there any on shield solutions?

simulate this circuit – Schematic created using CircuitLab

• Firstly HOLY capacitor Batman!! Thats huge. Secondly sounds like the convertors need a soft start as the empty cap current is looking like a short circuit to them. A dirty quick fix would be to put more resistance in your wire connections to the Cap. Maybe try making the wires twice as long. A more efficient solution would require more dollars. – crowie Sep 19 '16 at 10:14
• Charge the supercap via a resistor. 1 ohm would limit the charging current to 24A but the (short term) power rating would have to be 24*24*1 W. Maybe 10 50W(continuous) 10 ohm resistors in parallel on a heatsink would be close enough. Once the capacitor voltage is above 20V, switch a contactor across the resistor to save power. – Brian Drummond Sep 19 '16 at 11:01
• @crowie, right, 500F are necessary to start a Perkins 1104d-44tg2 diesel motor. 50-60kW, 4.4L. Genrally a 1-2.2 Ohm Resistor could make a deal. – Eldar Musin Sep 20 '16 at 11:43
• @BrianDrummond, R1 is already a 2.2Ohm 300W. Sounds like I could just connect both converters in parallel and to cable the resistor in series with the Supercap. Sure for the first charging seconds, the poor resistor have to work, but as the voltage raises and the current falls, the dissiped energy will be quite low. Thanks for the idea. Have to check the MTBF of the resistor – Eldar Musin Sep 20 '16 at 11:47
• Right, at the moment R1 only isolates one DC/DC converter. If they don't like being paralleled, giving each its own resistor is another option. – Brian Drummond Sep 20 '16 at 12:55

Batteries often have capacities of >> 10k Farads and on cars will demand full current which can burn out Alternator diode bridges if undercharged completely and results in currents > 100A for brief periods in cars.

The smart solution is to fix the 1kW power supply to react properly until the output voltage is in normal range.

A band-aid solution is to raise the impedance to Cap with a lossy InRush Current Limiter (ICL) that operates at >150'C with a starting resistance in the range of 2 to 4 Ohms that has a negative temperature coefficient as it rises 150'C.(NTC) This can be Metal Oxide radial disc.

Shop for

• Product Index > Circuit Protection > Inrush Current Limiters (ICL) below 25A at major component distributors

Remember this is a bandaid solution not a permanent one.

Add protection to disable the Solenoid (or equiv) switch to load if Cap Voltage is too low.

• Thanks for the ideas. I'm using a DC/DC DDC8630 from Powernet, and will give them a feedback cencerning the derating and a modification idea. And why I can't use an NTC as a long term solution? – Eldar Musin Sep 20 '16 at 11:55

Use a wide input range automotive style (robust) DC/DC converter to boost your 12V-24V rail to 28V and then have a further linear or switching regulator to give you clean 24V for the control at all times.

Alternately I recommend that you have a separate supply for the control if it is always going to be powered from mains. There is no point having the control at the mercy of the charging PSUs. You can have a redundant pair if you want for that.

• I'm using a 1kW 110Vdc to 24Vdc (nominal 26Vdc) power converter DDC8630 from Powernet. Unfrotunately over 41Amps it derates with no limit instead of giving a power limitation. I will think about a separate power supply, cause it's really 1A@24V. The idea to separate the power supplies hurts me because of the place, but at the same time it's only 1A@24Vdc. I'm getting back with a solution – Eldar Musin Sep 20 '16 at 11:50