Current Limiting Technique

[stube40]

I am setting up a brief experiment that involves a switch an array on twelve 12V lead-acid batteries into a test inductor load at 150V with a limited 40A current.

This test will only be conducted for about 10 seconds and I dont want to spend too much time/effort worrying about current limiting methods. Hence, I was going to spec a high-wattage resistor around 4Ohm, connect it in series with the batteries and throw it in a bath of liquid nitrogen to keep it cool. Since the test will run for only 10 seconds, I thought I would get away with this.

Anyone disagree or have any better suggestions?

[Alex Tsekenis]

You should spend time on this since you are dealing with 12 lead-acid batteries in series.

The issue I see is that your cables and coil that have some resistance will heat up too thus changing your current limit.

Use liquid nitrogen for your resistor will affect its resistivity so you will have to use a higher resistance value to compensate.

Does it have to be KEPT at 40A maximum or you want any value up to 40A? In the first case you should use a current source, in the second a fuse.

Whare do you get your liquid nitrogen from?

[stube40]

You should spend time on this since you are dealing with 12 lead-acid batteries in series.

The issue I see is that your cables and coil that have some resistance will heat up too thus changing your current limit.

Use liquid nitrogen for your resistor will affect its resistivity so you will have to use a higher resistance value to compensate.

Does it have to be KEPT at 40A maximum or you want any value up to 40A? In the first case you should use a current source, in the second a fuse.

Whare do you get your liquid nitrogen from?

I'm expecting to connect the battery array to a simple circuit with a 45mH inductance and current limiting resistor. The connection will be made with a relay and then I'll expect the current to ramp linearly up to 40A in about 13ms, after which I want the resistor to stop the current rising further.

The liquid nitrogen is just from the local cryogenics supplier - it's fairly simply to get hold of over here in Oz.

[Hero999]

What's the capacity and internal resistance of the batteries?

You should take it into account because the voltage will probably drop below 12V per battery when the 40A load is applied.

I hope you connect a transient suppressor to the coil, other wise an arc will form across a mechanical switch or a semiconductor switch will be destroyed.

[indulis]

Don't forget the inductance of the wiring.

[stube40]

What's the capacity and internal resistance of the batteries?

You should take it into account because the voltage will probably drop below 12V per battery when the 40A load is applied.

I hope you connect a transient suppressor to the coil, other wise an arc will form across a mechanical switch or a semiconductor switch will be destroyed.

I've trawled the data sheet but there's no mention of the internal resistance.

[Hero999]

There will be an internal resistance, even if it is not mentioned on the datasheet.

The easier way to work out the internal resistance is to measure the unloded voltage, apply a load and measure the voltage again. The internal resistance is equal to: (Vunloaded-Vloaded)/Current.

[Alex Tsekenis]

Also remember to record the state of the charge level of the battery as internal resistance will increase as the battery is drained.