powering servos with li-ion battery - Page 1
Forum section: electronics / beginners

author: Chal7401
author_rank: Newbie
timestamp: April 28, 2017, 11:42:22 pm
content: hi everyone, I'm new so I don't know if I'm posting this in the correct section, please let me know, anyway, I'm trying to power a pair of servos with a li-ion battery, is an ncr18650b and the servos are towerpros mg995, I'm stepping up the voltaje to 5v with a MT3608 wich is also powering an arduino and a hc-05, the arduino powers 2 7segment displays controlled in matrix and a gy-521(MPU6050), the battery it self also powers a SIM800L, the servos only get power when I need to move them so I'm using a logic mosfet(060N03L) to control the connection and disconnection, the problem is that everything shutsdown when I connect the servos, it is not a short but I think that the protection of the battery is responsible of this, maybe inrush current, both the mosfet and servos work just fine, my question is, how can I prevent this from happening without removing the battery protection(in case that is the problem)?, thanks for reading.

author: james_s
author_rank: Super Contributor
timestamp: April 29, 2017, 12:00:49 am
content: You will need to determine what is shutting down, and how much current everything is drawn. Also are these digital or analog servos? One of my hobbies is flying RC aircraft and I've found that digital servos tend to require much more beefy power sources, they draw current as a series of short duration spikes and at least the inexpensive ones have horrible decoupling. I used them on one of my planes and I had to add some decoupling caps in the servos and use a beefy switchmode BEC regulator otherwise the receiver would go nuts whenever a servo moved.

author: Chal7401
author_rank: Newbie
timestamp: April 29, 2017, 12:22:11 am
content: thanks for the reply, I havent measured the current drawn from all the components but I measured the current draw from both servos under load (1.03 Amps) and the SIM800L(20 mAmps) but I've read that it can have peaks of up to 2 Amps when connecting although those don't seem to shut everything down, and they are digital servos, when i connect them everything shutsdown and the battery gets like locked and I have to remove it, wait a cupple of seconds and put it back inside to get it working again.

author: james_s
author_rank: Super Contributor
timestamp: April 29, 2017, 12:26:59 am
content: It sounds like you might need a bigger battery, or smaller servos. Analog servos are definitely going to be less of a challenge from a power standpoint. A lot of people get hung up on analog vs digital but neither one is necessarily better. Digital servos tend to have somewhat better low speed torque but there are good and bad examples of both.

author: SeanB
author_rank: Super Contributor
timestamp: April 29, 2017, 01:48:35 pm
content: You might want to try a separate boost converter for the servos, so the switching loads do not affect the micro power supply. Add a lot more input capacitance to the converter as well, so it has a lower impedance supply might also help, but a second supply ( preferably one with a shutdown that disconnects the load) will definitely help.

author: james_s
author_rank: Super Contributor
timestamp: April 29, 2017, 08:53:01 pm
content: You probably don't even need the run the servos off the boost convertor, most servos were designed to run off a 4.8V NiCD pack, and they will work fine from around 4V-6V. Higher voltage results in more torque and faster speed but that may not be important.

author: Audioguru
author_rank: Super Contributor
timestamp: April 30, 2017, 12:20:35 am
content: A Lithium battery cell is 3.2V when dead and is 4.2V when fully charged. Are your servos designed to be powered by one Lithium cell or an old 4.8V Ni-Cad battery?Is the battery cell fairly new and fully charged?Do you have a pcb designed for the current or a Mickey Mouse solderless breadboard?

author: james_s
author_rank: Super Contributor
timestamp: April 30, 2017, 06:30:28 am
content: Most of the time these days the servos are powered by a 5V regulator but most of them will work fine off a single cell lithium ion or a 2 cell LiFePO4. It's not that critical in most cases.