Author Topic: Limit inital current of devices  (Read 1661 times)

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Offline Spike101

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Limit inital current of devices
« on: January 08, 2014, 10:46:04 am »
Hello,

I am working on a project where we use 4 PC-Engines ALIX boards (x86 boards), which are all powered by an 12V-18Ah battery. Each board draws about 300mA current. When everything is running, the system draws about 1.2A. Until now, I was using a Lead-Acid battery, however I switched to a LiFePo4 battery. With the Lead-Acid battery there was no problem. But with the new LiFePo4-Battery the BMS of the battery does not allow turning on all 4 boards at once. It seems that when switching it on, the boards draw more current, and the BMS of the battery disconnects the battery via a P-Channel FET in order to protect it. According to the manufacturer of the battery, the current draw is limited to 8A, before it switches off.

When i connect only 2 boards and a relay-coil to the battery, and the other 2 boards via the contacts of the relay-switch, it works. So it seems, that this small delay introduced by the relay is enough to make it work. But there has to be some better solution than that. I also tried using a series resistor. That does work, but it dissipates too much power, so unfortunately, that's not an option. But sice it works, it seems, that at least the boards don't relly need the high current in order to start, they just take what's available...

Is there some better way to limit the maximum current that the system can draw from the battery?

thanks in adavance,
Sebastian
 

Offline alxnik

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Offline SeanB

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Re: Limit inital current of devices
« Reply #2 on: January 08, 2014, 01:31:12 pm »
Current sequencer, basically a small board with 4 mosfets on it to switch each board on at 0.1 second intervals. What is happening is that the internal capacitors on the boards are all being charged up at the same time, and this current is too much for the BMS to handle. Otherwise a low Dc resistance inductor in the power line to each board will provide current limiting for the initial pulse. Try 10mH as a starting value, the inductor can be the common one used in car audio as a power supply choke.
 

Offline NANDBlog

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Re: Limit inital current of devices
« Reply #3 on: January 08, 2014, 02:48:46 pm »
Most hot swap controllers have the required functionality to complete this task. Head to TI/AD/Linear/ONsemi for one.
 

Offline Spike101

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Re: Limit inital current of devices
« Reply #4 on: January 08, 2014, 05:44:36 pm »
Thanks for all your great suggestions and information.
I ordered a few components today, and will try it tomorrow. I'll let you know if it did work.
 

Offline peter.mitchell

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Re: Limit inital current of devices
« Reply #5 on: January 09, 2014, 01:44:09 pm »
you could also slow down the current sense loop just a little with a cap so that small spikes don't put it over.
 


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