Would a 7.2V 2000mAh Ni-Cd Battery Pack be an ok power supply to used with this setup? im not sure if that is too much ampage or not.
Botboard with Bs2 connected to a ssc-32 controlling 12 servos
7.2V is usually a 6-cell pack and when fully charged will be closer to 8V than 7V.
Some servos tolerate this better than others.
So a question a to help determine your answer is what servos are you using?
im using a combination of S3003 standard futuba servos and T-Pro MG995 Metal Gear High Speed & Torque Digital Servos. they both say 4.8v- 6.0v but i was hoping they could handle a little extra voltage
i also will be using a PIR motion detection sensor
dunno about your particular servos I guess. very generally speaking, and I say this as a very general comment, analog servos handle higher voltage better then digital ones. your mileage will vary.
ok. well ill try it out. will the PIR sensor be ok?
thanx
It’s best to use the proper votage. I dammaged a servo using 9v @ only 300ma It did not dammage it right away, but repeated abuse eventually killed the servo. I would get a DVM and drop the voltage by using a few diodes in series to be safe or get one of these:
I can only comment on the experience I have had.
I have used HS-422, 475, 645 at 7.2 (8) vdc. The 422’s held out the best. The 475 and 645 became pretty shaky. No matter how they perform though, you must understand the life expectancy will be decreased…
I would expect the 3003 would do ok, but the 995’s would probably not fair as well.
thanks for the advice guys. i cant really afford to buy new electronics right now, but i do have a B50k potentiometer. Could i put that in series with my battery to lower the voltage?
Maybe tap the battery? A string of diodes would work. The pot can’t carry the current, and is much too high a value. You could use ohm’s law to calculate a dropping resistor.
Alan KM6VV
Hi,
The cheapest way would be removing one cell from your battery.
…Or using an UBEC…
This will not work. While you can use resistance to lower voltage 50k is WAY to much. Find, study, ohms law.
Servos like 6vdc. Your battery will supply 8vdc. Your servos may require 4 amps. So your resistor will need to drop 2vdc across it, and do so at 4 amps…
R (resistance) = E (volts) / I (current)
So the resistance required is .5 ohms, not 50k. 
However there is another reason this won’t work. Going back to ohm’s law…
W (watts) = E (volts) x I (current)
So the power required for the resistor is 8 watts! Most pots are less than 5 watts.
This is real heat dissipation.
And there is one last reason this won’t work. As the current changes the voltage drop changes as well. This is a really bad method for lowering the voltage.
Hi there,
There’s a good Wikipedia article about Ohm’s law. I also started working on a calculator based on Ohm’s law a while ago. haven’t worked on it in a while, but if anyone is interested I could finish it.
Sorry for going off-topic BTW.
i know this is a sort of cover up solution, but could i maybe run down the battery to 6v everytime i use it by having it run something like an electric motor or lightbulb for a little bit ?
im aware that this is a Nickel-Cadmium battery and therefore is very vulnerable to battery memory. but as long as i always let the battey discharge all the way after the 6volt drop the battery should be able to last for at least 10 months right ? (and of course i wouldnt over/under charge the battery)
im entered in several highschool animatronics competition that arent until march and then june. i just want the battery to last until then.
thank you
No, you don’t want to “run down the battery a little bit”. For a cheep way, use some 1N2004 or similar diodes in series with the motor. Each diode will drop about .7V.
Alan KM6VV
how come in the botboard users manual they show a diagram of a hexapod with a 7.2vdc powert supply?
Quote from EddieB:
"7.2V is usually a 6-cell pack and when fully charged will be closer to 8V than 7V.
Some servos tolerate this better than others. "
I don’t know too much about batteries, but the batteries typically used with lynxmotion electronics are the Ni-MH batteries. In this case, you are using Ni-Cd batteries, which for some reason, will not work good for powering servos.
pretty sure all the servos used in those examples are HS-422 or HS1422CR analog which are more forgiving (albiet with lower torque ability) than digital servos.
Yes that’s it exactly. The HS422 servos worked pretty well on the old EH2 walkers using 7.2vdc batteries.
Wowy, Except for the obvious improvements with NiMH over NiCad they have very similar operating properties as far as voltage goes.
ohhh ok. that makes sense.
Well… i double checked those digi servos and it turns out that there 3.5-8.4 volts. so according to you guys, the analog servos should be more forgiving. so i think im good until march.
Ps: how does ampage effect servo speed. if i use 300mA instead of 1400mA will they move slower?