# When to use multiple batteries vs a UBEC

When should you use multiple separate batteries vs a single battery with multiple UBECs?

I'm trying to design the power system for a small 2-wheeled robot. Aside from the 2 main drive motors, it also has to power an Arduino, a Raspberry Pi and a couple small servos to actuate sensors.

• the motors are each rated for 6V with a peak stall current of 2.2A
• the Arduino uses about 5V@100mA
• the Raspberry Pi uses about 5V@700mA
• the servos each use 6V and have a peak stall current of 1.2A.

So the theoretical max current draw would be 2.2*2+.1+.7+1.2*2 = 7.6A.

Originally I was planning to use three separate Lipo batteries:

• one 12V using a step-down converter to power the main drive motors for [email protected] peak
• two 3.7V lipos each with step-up converter (rated for 5v@3A) to handle the servos and logic separately

Then I discovered UBECs, which sound too good to be true, and they seem to be both cheap (<\$10) and efficient (>90%) and able to handle my exact volt/current requirements.

Should I instead use a single high-current 12V lipo with three UBECs to independently power my drive motors, sensor motors and logic? Or will this still suffer from brown-out and power irregularities if a motor draws too much current?

What am I missing?

Any quality power device should list its load regulation capability, which is typically in a datasheet as a voltage range coupled with a maximum output current. You can expect normal voltage fluctuations to occur within this band.

However, when you include a motor in the scenario, you need to be aware that voltage can spike up/down if you suddenly remove/apply current to the motor because the motor windings act like (are) an inductor. Typically a flyback diode is used to prevent/limit the spikes caused by motor operation; see this thread for a schematic of an H-bridge driving a DC motor with flyback diodes.

Another thing I would like to point out is that you are adding currents, which could be a troublesome thing to do - while 5V and 6V seem very close, 6V is actually 20 percent more than 5V. I would suggest that you convert your "current"@"voltage" to the power required by each circuit, then use that power rating to find a suitable supply.

You said:

the motors are each rated for 6V with a peak stall current of 2.2A

the Raspberry Pi uses about 5V@700mA

the servos each use 6V and have a peak stall current of 1.2A.

But I would suggest:

Assuming 2 motors and 2 (a couple) servos, your total power requirements are:

$$\begin{array}{ccc} P_{\mbox{req}} & = & (2*13.2) + 0.5 + 3.5 + (2*7.2) \\ P_{\mbox{req}} & = & 44.8W\\ \end{array}$$

Now you can take your power requirement and use that for any power converter/battery eliminator. Divide your power requirement by the nominal pack voltage to get your current requirement for a specific battery pack. Divide the current by the battery capacity (in Ah, or remember to convert current requirement to mA) to get the estimated run time on a particular battery pack.

All of this is guidance to help you select a proper BEC, because yes, as long as your battery provides enough current and you can tolerate the voltage swing, it should work fine.

There are a few things you should consider when designing your system.

• Motors are usually rated for the stall current, but sometimes generate out-of-spec currents. Higher current draw may happen when the motor turns backwards and back-currents occur when switching the motor off.

• When using LiPos with high discharge rate, those currents are usually absorbed by the battery with low voltage fluctuations, due to its low output impedance.

• BECs usually but not always have a low-pass filter. Sometimes those filters are enough to handle spikes, sometimes not. Sometimes those filters also help filtering spikes backward.

With those in mind, it is usually enough to have separate BECs for logic/application processors and motors. Use a high-current rated battery (usually most LiPos are at least 15C, so a standard 2200mAh is usually enough). Do not chain them and try to avoid connections and wiring before the BECs.