Hello, battery weight and its output is always an issue in making a robot.
I am building a robot, it uses 12 MG996R servos. I am wondering if a battery like this one:
https://www.ebay.com/itm/Gens-ace-4000mAh-2S-7-4V-TX-Lipo-Battery-Pack-For-Spektrum-DX9-DX8-Transmitter/282835992657?epid=1077050423&hash=item41da56c051. Right now I am using an external PSU, the peak stall current is 3.5A, this battery has a rating of 4000mAh, the battery is only 345g according to the seller, the form factor is just right for my robot and the price is within my budget. Thanks.
The question is not how much power the battery has but how much it can provide.
Will the battery allow you to draw your stall current from it or will it fail under the load?
When looking at batteries try for the manufacturers specs to see how much it can supply (Peak Amps) and for how long (Amp Hours)
Daz
The MG996R has a stall current of 2.5A@6volt, so 12 servos...
The battery has a Ah rating, but not a max current rating (two different things),
so it seems to be designed for relatively low current draw devices (like that remote control).
Get a battery that can handle the stall current of all the servos.
Leo..
Note that a fully charged 2S battery will exceed the voltage rating (7.2 V) of the MG966R.
The stall current of the MG966R is 2.5A, so the battery, and whatever buck converter you use must be able to handle 2.5A x (number of servos that will be moving at once).
af1812:
Hello, battery weight and its output is always an issue in making a robot.
I am building a robot, it uses 12 MG996R servos. I am wondering if a battery like this one:
https://www.ebay.com/itm/Gens-ace-4000mAh-2S-7-4V-TX-Lipo-Battery-Pack-For-Spektrum-DX9-DX8-Transmitter/282835992657?epid=1077050423&hash=item41da56c051. Right now I am using an external PSU, the peak stall current is 3.5A, this battery has a rating of 4000mAh, the battery is only 345g according to the seller, the form factor is just right for my robot and the price is within my budget. Thanks.
Bad idea. Transmitter batteries are always low output, which is why they can pack a lot of capacity (mAh) into a relatively small package. That one is specified with maximum short term output of 2C so no more than 8A, even if the spec is accurate which most are not.
Steve
Thank you for the replies.
Then what with be a good chose? I need a battery that can provide enough current for 12 servos but yet light enough.
Look for a battery that can handle the current, and from the several you find, choose the lightest one.
jremington:
Look for a battery that can handle the current, and from the several you find, choose the lightest one.
If each MG996R consumes 2.5A on stall, my robot needs to have 8 servos in stall to support it self, that makes 20A, how and where can I find a battery with such a large capacity but yet light weight and not cost a fortune?
I failed to see the collation between a battery labelled 5Ah and the actual usage.
Let's say I have a battery rated 2Ah, that is the total capacity of the battery, is this correct?
Assuming the above is correct, now I have a MG996R draws max 2.5A, so does it mean in less then 1 hour, this battery is completely discharged?
For maximum allowed battery discharge rates, look at the "C" rating.
20C means that a battery is rated to be discharged at 20C Amperes, where C is the battery capacity in Ampere hours.
So, a 2Ah, 10C battery can be discharged at up to 20 Amperes. In the very best case, the charge will be exhausted in 2Ah/20A hours, or about 6 minutes. Probably more like 2-3 minutes.
To take into account battery aging and manufacturer exaggerations, most people downgrade the stated battery capacity by a factor of 2 or more.
So putting aside all the other red flags here is a few more.
Specs from the manufactures site (I hope).
Gens ace 4000mAh 7.4V RX 2S1P
Lipo Battery Specs:
Capacity: 4000mAh
Voltage: 7.4V (2S)
Discharge Rate: 1C note the 1C. Meaning no more than 4 amps (1 x Capacity of battery)**
Weight: 151.1 g
Size: 695119 mm
Charge Rate: 1-3C Recommended, 5C Max
Also note the packaging of the battery from the website.
Here.
Is not the same, so it may not be a genuine battery.
Case from the website.
As opposed to the ebay one.
Does not bode well for its authenticity.
For a battery that is light, having the ability to supply 20+ amps and for a resonable amount of time.
Cost is going to be high.
And if you have to regulate the voltage then the regulator to handle that current is not going to be cheap.
Look at multiple power sources for individual servos and you might cut the specs required down but then the cost goes up for more batteries.
Now you know why there are not a lot of battery operated robots walking across the grass.
Daz
Now you know why there are not a lot of battery operated robots walking across the grass.
And so many disillusioned robot builders!