Battery voltage choice

[cgrenier]

Hi guys, I'm planning my first robot and did a lot of reading so I'm now planning my electric needs. I will use 2 (maybe a third in close future) 6v servos, Raspberry Pi and other similar voltage (5v) boards as far as I can tell.

I was planning to feed the servos off a 6v battery directly and solder a LM2940 on the way to the boards.

I read on this forum that when feeding servos AND controllers one should use 7.2v battery. Why is that ?

Should I use 2 batteries in order to maintain power stability on the Raspberry Pi if/when the servos happen to suck the battery empty ?

How complicated need it be to charge the battery(ies) without removing from the robot by connecting a power supply to a circuitry based on a BQ2000TPN-B5 let's say.

Thanks for your advices

[knossos]

There are several things you have to consider here. 

First of all you need to consider the drop-out voltage of your voltage regulator.  Voltage regulators take an unstable input voltage and output a lower, stable voltage.  The minimum voltage difference between the input and output voltage is called the drop-out voltage.  The drop-out voltage can be found on the datasheet for the voltage regulator (it is also dependent on current and temperature).  For simplicity, lets use a dropout of 1V.  With a 6V battery and a 1V dropout, you would have a 5V output.

So we're good right?  Not quite. This brings us to the next thing we need to consider, battery voltage after discharge.  As a battery discharges, the voltage drops.  If the battery begins at 6V, as it gets depleted the voltage decreases which means your voltage regulator would no longer be able to output 5V. 

In addition, if you have a spike in the current draw, the voltage can dip temporarily, once again interrupting your 5V output.

If your 5V output to your microcontroller (or in your case the Raspberry Pi) drops to low it can cause it to reset.

For all these reasons, and more, it is better to go with a higher battery voltage (although too high of a voltage has other problems associated).  Keep in mind though, that a 6V servo wouldn't be able to run directly from a 7.2V battery.

[cgrenier]

Thanks for this information knossos. I thought the 0.5v dropout lf the lm2940 was saving the day but will all you explain I understand it would sooner or later lead to unstability on the computer side.

Since 7.2v is somewhat high a voltage for the servos , I guess I should add a switched voltage regulator or another component between it and the battery. Would you please suggest a way to achieve that ?

[Gertlex]

It's definitely a tricky thing to work with... 6 V isn't convenient to obtain, IMO.  I often go with setups with two battery packs, too.

For your setup, I'd be tempted to do a 5 AA NiMH battery pack for the servos (nominal voltage 6V), and a 7.4 V LiPo battery for the electronics, with a 5 V regulator (e.g. this switching one).  9V NiMH batteries  (really 8.4 V) are a possibility too, for the electronics, but most of them can't do a very large current draw.

Disclaimer: I don't use hobby servos any more... I use Dynamixel servos, for which an 11.1 V LiPo Battery (12.6 V at full charge) works nicely. With the 7.4 V battery for electronics, I just need one charger... All of my NiMH batteries sit around unused.

[knossos]

One thing to watch though, with switching regulators is the high frequency noise.  One way to combat this is to step down the voltage using a switching regulator and then use a standard linear regulator for a regulated voltage with less noise.  Also, keep leads from the switching regulator as short as possible.

[knossos]

My current setup is using 7.2V NiMH battery packs (like those used for RC cars) and run a 5V regulated power source for the servos and a second 5V regulated source for the electronics.  Yes, you do lose a little torque and speed running them at a lower voltage, but it allows me to use the same 5V regulators to power either electronics or servos.  In the future I will be switching to a 12/24V system (I will be switching to a larger chassis with wheel chair motors).  When I do, I will be adding an adjustable  switching regulator to step down the voltage to feed to the standard regulators.  This will allow me to keep the rest of the power distribution the same with no changes.

[cgrenier]

Ok, so am I getting close if I choose to use a 7.2v NiMH battery pack, linear 5v regulator to the electronics (low current, not too big a voltage drop, therefore reasonible energy loss and having the advantage of not producing noise like switched regulators seems to do) and switched 6v regulator (better efficiency considering the potential higher current demand) for the servos ?

I could afterward add a second, same voltage battery pack (in parallel, or each standing alone to separate electronics from servos), without modifying the circuitry all that much.

Also, how big a mess am I getting into if I plan charging batteries without unplugging them from the robot wiring ?

[knossos]

IMO you don't need a switched regulator for the servos, a standard regulator would be fine.  As for charging batteries, a simple switch would allow you to charge the batteries without unplugging them from the batteries.  My setup currently allows me to run up to 4 packs in parallel.  When running multiple packs though I would recommend adding reverse voltage protection and over current protection.