Im having some confusion with the calculation of lipo batteries and Im fairly new to them. I am looking for reliability and accuracy over speed but all three would be good. I need a battery between 12 and 24 volts (closer to 24) so to my understanding that is 6 cell which is close enough. Then I need to calculate amps. Each motor need 5 +- .2 amps which means between 4.8 and 5.2 amps. If I did the math right I need 40 amps with all 8 servos. I also know you find amps of lipo batteries by taking the mAh and dividing it by 100 then multiplying that by the C rating. I think that volts is what burns out a servo so I need the volts to stay near 24 but can the amps be anything over 40? Can I have any amount of amps as long as its past 40 for the servos to function or will that also burn the servo out? Also another problem I have is that almost all 6 cell batteries have a lot of amps. For example on amazon one 6 cell battery is 4000 mAh with a C rating of 60 which is 240 amps. Will that burn out all my servos or am I doing the math wrong with misconceptions?
Am I doing something wrong or do I need to keep looking to try to find a battery with 6 cells but a lower amps.
I really need help if someone can guide me on finding the right lipo battery.
The formula is (mAh/1000) * C rating. The rest of your math is correct.
But your thinking is backwards. You start with what is the maximum load, then you can calculate what battery you need. (Looking at the specs in the link you provided, that's a lot torque from a servo).
Higher voltage is what determines the torque available, so to get 380kg, you need to use the highest voltage the servo is spec'd for. That would be 24V.
The specs say the max current for the servo is 5-Amps. So if all eight servos are moving under load at the same time, you need 40-Amps from the battery. If your project would allow it, using individual batteries for the servos might be more efficient. At least you won't have to use #10 wire.
So, you need 24V at a minimum of 40-Amps. Look at the Turnigy 7S, 5000mAh 60C battery. this battery can provide up to 300-Amps continuous, but you only need 40 so it will last longer.
You can never have too many amps available in your battery. The load will just consume what it needs.
But 40-Amps from one battery would mean really short endurance, which is why I suggested that each servo have its own battery. Using the same Turnigy battery for each servo just increases your endurance by 8X.
Yes, a 7S battery produces 25.9V. Your servo spec says it can handle up to 30V (but it doesn't say for how long). But at 5-Amps the battery will discharge to below 24V pretty quickly.
Something else to consider is how much torque do you really need? That spec'd 380kg will only be available while the battery is almost fully charged. If you only need, for example 190kg, then you will get that as long as the battery is > 12V.
Thanks This helps a lot. But what exactly do you mean by endurance what is that measuring and what exactly will go down? Also I would prefer to use just one battery but it depends on what you mean by endurance. Also the more torque the better that why Im using 24 volts but ill try a 6 cell just to be safe.
Thanks Again.
hert:
Thanks This helps a lot. But what exactly do you mean by endurance what is that measuring and what exactly will go down? Also I would prefer to use just one battery but it depends on what you mean by endurance. Also the more torque the better that why Im using 24 volts but ill try a 6 cell just to be safe.
Thanks Again.
Endurance is, in brief, "how long does this thing work?"
40 Amps out of one battery will discharge it pretty quickly. 5 Amps out of eight batteries will last 8X longer.
In reality, what voltage is the minimum acceptable? From that you can calculate how long the battery will provide at or above that voltage.
Using the battery I referenced above, you can expect it to provide 40 Amps for about 7.5 minutes.
Secondly if they do draw max current continuously they will overheat.
I suspect this unit will simply shutdown on over-temperature, cheaper units just cook themselves.
To get a feel for endurance you will need to measure the actual average current consumption in realistic conditions.
The datasheet for that servo suggests the current is limited to 5A, but its not absolutely clear. If so this means
the max torque is independent of the supply voltage. The speed of response will depend directly on voltage.
And finally make sure your LiPo cell has protection circuit, or at least a fuse.
SteveMann:
Yes, a 7S battery produces 25.9V. Your servo spec says it can handle up to 30V (but it doesn't say for how long). But at 5-Amps the battery will discharge to below 24V pretty quickly.
Don't forget that 25.9V is only the nominal voltage. Fully charged it will be 29.4V which is perhaps pushing your luck. By the time it reaches 24V (only 3.4V/cell) it will already have used around 90% of capacity and be heading into the danger zone.
which I think is a safe option. How do I calculate how long the battery will last? Someone also said that after 90% capacity it will enter the danger zone. Im fairly new to lipos but is this dangerous? What does this mean?
Your battery choice looks good (I'd have preferred a better know brand but they can be a lot more expensive).
You can only calculate how long a battery charge will last when you have a reasonable idea of the AVERAGE current that the motors take when the device is working. E.g. if the servos only work for 1 minute in every hour it will last a long time, if they're working every minute then a lot less time.
"Danger zone" is just a dramatic way of drawing your attention to the idea that Lipo batteries should not be run down too low before recharging or they sustain damage. Exactly how low is a bit debatable, somewhere between 3V and 3.4V per cell but the point is that YOU are responsible for checking it. BTW the danger is not anything like fire or explosion, it's just that your battery loses the ability to be recharged.
Thanks, One more quick question does this mean I can only run my lipo to about 18 volts since its a 6 cell before having to recharge it. The servo motors can go all the way down to 12 volts is there anyway I can get it down to there.
If you run your lipo down to 12V, that's only 2V/cell, it will probably never take a full charge again. That would make it a very expensive single-use battery.
Anyway there is almost no charge left at 18V so after reaching that point it would drop very rapidly to 12V so you would get very little extra runtime, just kill your battery.
Why don't you breadboard your project using a bench-top power supply? This way you can test every other component of your project and measure the current requirements of the servos as well as their average duty cycle. As I said before, you need to know the load requirements before you even think about selecting a battery.
Of course, a 24 Volt, 40 Amp power supply is going to be real expensive. But your 40 amp requirement is assuming that all servos are operating all at the same time, all the time.
A pair of sealed lead-acid lawn tractor batteries would be a lot cheaper for testing. That would reliably provide 24 Volts at 40 Amps for a long time.