8 Servos with 6.5v AA Battery pack

Hi guys, I am making a project which requires 8 servos and 2 brushless motors. I am having a 11.1v LiPo for the brushless motors, but I am confused which battery to choose for the 8 servos. Please recommend me a suitable battery pack. Thanks, here's the data sheet for the servos.

Datasheet: https://robu.in/wp-content/uploads/2015/10/sg90_datasheet.pdf

8 servos will use quite some current. No data for current in the link You provided.
Voltage is 4.8 - 6.0 volt. An old fashioned 6 volt lead battery?

You can add a step-down (buck) regulator powered by the 11.1 volt lipo.

5 volts will work very well. Get a buck regulator that can handle 150 % of your max current requirement.

To keep things conservative, to keep things cool and to meet what are sometimes optimistic specs a bit more than halfway…

Start here. Cheaper ones can be found; you might get what you pay for, 'nuff said:

You could use more than one, of course.

a7

SG90 stall current is around 600mA. So 8 of them may need up to 4.8A total. You will probably get away with a 5V 5A DC-DC buck converter connected to the 3S Lipo. If you really need maximum speed and power from the servos then maybe 6V 5A but in my experience SG90s work well and last better at 5V.

Alternatively if you really want to use a separate battery for the servos then 4 x AA NiMH rechargeables e.g. Eneloops, should work. Note: rechargeable AAs can handle more current than the common alkaline AAs.

Steve

The servos will work well with 5 volts as noted. It is essential that the power supply has enough current capacity so the 5 volts is, well, 5 volts.

When using a buck regulator, especially if you cheap out, testing under load is strongly recommended.

I would not proceed on a “getting away with” basis - plan for generous excess current capacity, and still you want to check and make sure you’ve been generous enough.

Buck regulator specifications are more like aspirational, boast regulator specifications even more suspect.

There is no substitute for a direct careful test.

a7

But won't it consume the power of the battery? and I am already using 2 ESCs. I am making a model airplane, 2 servos for the ailerons, 1 for the rudder, 2 for the elevators/stabilizers and 3 for the landing gears

Here's the datasheet also:

Everything in the air will need power.

A flight battery has to be selected on the basis of weight, voltage, maximum current and desired flight time. All those factors trade off against each other.

So whether you use one lipo and buck regulators or fly an additional separate battery system for the servos, you still have the same constraints. You still have to fly all the stored power your electronics will use.

Buck regulators can be quite efficient. IMO using one battery will simplify things.

a7

Sure thanks but can i use PCA9685 - 16 Channel 12-Bit PWM Servo Motor Driver I2C Module? Actually I am connecting this to raspberry pi 4 not arduino due. Sorry for asking this on the wrong forum, but before i was trying it with arduino due but I think its harder than raspberry pi

I don't see why not, and I don't see what that has to do with where the power for the servos comes from.

a7

I've built dozens of model aircraft using a single Lipo. And you don't need lots of current for the servos because 3 of them will hardly be used (landing gear servos!).

The disadvantage of using separate batteries is a) additional weight and b) you need to remember to keep BOTH batteries charged.

BTW I wouldn't use NiCd batteries. Old technology now replaced by NiMH for almost all purposes.

Steve

As quoted in #11, not a large demand in model aircraft and your 6V 700mah nicad pack should do fine. It might be old tech but the tech has worked well for the last 50 years or more without too much drama. We would run as many as 8 servos at a time on some aircraft in those days no problem.
Things like cell memory and leakdown over time were things to be aware of but if managed didn't create any worries.
Just be aware that some of the older Futaba receivers back in the '90s didn't like 6v as it made them too sensitive. These days with 2.4g no worries though.
Rule of thumb with model aircraft........any more than the bare essentials between the receiver and the servos is a breaking point. Many would even remove the switch and have the battery plug directly into the receiver through an access hatch.

By definition. :rofl:

If you are using an esc it will usually have a substantial bec as part of the module, perhaps around 5A to drive servos/ receiver.
There is usually not much demand for servo power in an aircraft as the control surfaces only move small amounts. Only larger draw will be when using retracts.

Maybe a BEC, maybe substantial. I have seen and heed general advice not to use ESC BECs.

If you do use an ESC BEC, just make sure it isn't a linear regulator on the ESC (most are), as that would mean substantial losses to heat. At least.

a7

please recommend me a battery that I should buy