[Part 8] Electrical Design - Power Supply

I think this UPC design has very flexible design in terms of power supply. It basically uses standard AA battery housings meaning if the power is low, I can just swap the rechargeables with regular alkaline batteries on the fly.

One battery pack houses 10 rechargeable NiMH AA batteries each rated at 1.2V 2500mAh. There are four battery packs installed in parallel to give out a total of 12V 10Ah power supply. This is enough to power the device for about two hours.

One important thing that should always be made sure is that the system must be able to handle steady state high current drain situation. The theoretical maximum current output from a OSMC is 160A. Since I’m using two, it is capable of shooting out continuous supply of 320A. Of course this is not physically possible given that the motor is weak and wire gauges are small. But before all this can happen, the standard battery case will melt under this current.

The limiting factor in this case can be anything of the following; battery case, wire gauges, or connections. However, usually it is pretty easy to find out. Basically the maximum current rating is limited by the weakest part of electric circuit. Bearing this in mind, I followed every single point/junctions in my circuit to find out where the smallest diameter of wire runs through and simply measured that diameter. Then there is a tabulated data on the limiting current based on the diameter of wire (copper) and I read the maximum current from there. I got about 36A at the terminal of each battery pack which has the smallest diameter (or cross-sectional area). Since there are four battery packs in parallel my maximum allowable current limit is 144A for the entire system.

Now that this value has been established, I bought a fuse that was rated under this number. I just went for two 40A time-delay fuses for extra safety. The nominal operating current is expected to be around 25A.

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