A Battery System for Poppy

english
electronics
Tags: #<Tag:0x00007fae8a521d00> #<Tag:0x00007fae8a521b98>

#1

Hello everybody,

I hope you are doing well. I’m new in the forum and I’m looking forward to learn many things about Poppy. Our team is working with the robot Poppy in a project to add a battery system to it and make the manipulation from the patient an easier task. We have made some measurements of the DC consumption of the robot and we found the following data.

Using the two energy entries, the DC current will reach a peak of something close to 5 Amps at 12 V. Even if this current peak won’t last long, the batteries must have the right characteristics to handle it. Also, LEDs will be added to the robot as part of the project (most likely LEDs strips like this one: https://www.adafruit.com/product/2329), which means that the peak might be higher than 5 Amps. We have not done tests with LEDs yet.

The decision our team is facing right now, is to choose the batteries for the robot. As we don’t have much experience with batteries, it would be great to hear from someone else with more experience, mainly on what criteria we need to consider in order to choose the batteries. And who knows, maybe from someone who has worked in a similar project.

Any advice or recommendation is welcomed.

Thank you in advance, we appreciate your help very much :slight_smile:


#2

In principle you have to go for LiPo batteries. For the AX/MX motors you need the ones that are called “3S” meaning they have 3 cells in series. With the nominal voltage of a LiPo cell of 3.7V the 3S batteries will provide 11.1V. In reality the voltage starts at about 4.2V/cell when fully loaded and is not recommended to let it go bellow 3.3 - 3.4 V. You should be fine with them.

One of the important features of the LiPo batteries is that they can provide very high discharge currents. That is indicated in the “C” characteristic of the battery. That shows how much multiply of the battery capacity (in mA) can deliver. For instance a be terry that is 2200mAh and has a 25C rating (pretty standard) will be able to give you sustained current of…(be prepared) 55A, albeit only for a few minutes (2.4 to be more exact = 60/25). Of course if you use the battery at 5A you will get 10 times more time.

The only thing left is to choose the shape, dimension and the capacity based on the limitations you have for the robot (where you want to place them etc.). Keep in mind that they add weight to the robot (not that little) and the higher you place them the more you will stress the lower part of the body. Have look on Hobbyking website to have an idea what you have on offer - you will be surprisingly overwhelmed.

The design that I have for my robot uses 2 batteries (I use 2S batteries as the robot uses XL-320 servos) right down in the soles:

And here you can see how the battery fits in.

It simply slides from the back and it fits snugly between several walls.
This way the impact on servos is minimal (only when you need to lift the foot up there is some impact) and as an added bonus the heavier soles increase significantly the stability of the robot.

To inject the power I use some small boards that are on the inside of the shins. They act as unidirectional current gates (not allowing current to flow into the battery when is discharged for instance and another battery is inserted in the other leg):

The two pins that are on the back are where the battery connector is going in.

And this is a picture of an earlier version (smaller batteries and feet without the added DOF for ankles) where you can see how all things come together:

A very important benefit of this is that you can swap the battery one by one when they are low, without the need to shutdown and restart the computer on the robot (which is a big hassle). The hot swap models above ensure that once a new battery is inserted the current doesn’t “rush” in the remaining battery.


#3

Thank you very much for a very complete answer sonel. It is really interesting how you specifically locate the batteries in order to give more stability to the robot, also the boards you use as unidirectional current gates is something we were not considering.

You mentioned at the beginning that we basically have to go for LiPo batteries. I read that there is a risk of incineration of this type of batteries if they are not charged correctly. Is this something to be worry about? also, what is the disadvantage of using other types of batteries? The high discharge rate “C” is the main factor to choose a LiPo battery?


#4

Sorry for the late answer. I completely forgot about this discussion and neglected to reply.

To answer your congestion, the LiPo batteries are nowadays ubiquitous: all devices that need relatively high power use LiPo batteries. The reasons for this are:

  • They are lighter than other batteries.
  • They have one of the best energy density (meaning the amount of energy per unit of mass for the battery).
  • They don’t have memory effect and are relatively easy to maintain / charge / discharge. A battery that is charged but unused can keep its charge for many months.
  • They can provide massive currents - I’ve mentioned earlier that “C” rating used in the LiPo batteries. There are many batteries that can deliver 20-50A currents, something that some model boats, aircrafts or trucks are taking advantage fully.
  • They come in so many shapes and forms that is very convenient to find something that will be suitable for your particular need.
  • Because the hobby market is so big their prices have come down continuously and it is very easy to find good reliable batteries at very competitive prices.

#5

Hello Sonel, don’t worry about the delay :). At the end we decided to choose LiPo batteries. We were planning to use 2 batteries of 2200 mAh each, but we decided we are going to try using 4. All these 4 batteries will be integrated in Poppy’s legs, two per leg. I will update the post when we integrate them to let the forum know how everything goes with that.

Also, we tried to find what the small boards you mentioned (“To inject the power I use some small boards that are on the inside of the shins”), but we could not find them. We are going to use some diodes that were recommended to us for someone that has been working on integrating batteries on Poppy.


#6

The boards was made by me - and I haven’t yet made them for sale. Here is the schematic of the latest version that includes also a LED that indicates if that particular branch provides power or not.

SPR1704.pdf (17.9 KB)

With simple diodes you need first to make sure they are suitable for the current you’re aiming (look for something at least twice the planned current - ex. 10A). And the problem with them is that they tend to have very high forward voltage - up to 1V. This has two drawbacks: you loose that voltage for robot (ex. if the batteries are 14V you will only get 13V - which becomes a problem towards the low end of the battery voltage) and secondly the 1V x 5A = 5W of power that will be dissipated (and wasted) by the diode, hence they might run very hot.

An alternative is to use Schottky diodes that have lower forward voltage - in the range 0.3-0.6 V. There are some models with lower forward voltage (towards 0.2V) but they tend to be expensive. There are also some “smart diode” devices like this that behave like a Schottky diode but in reality are a complex device. I’ve used them before (the pictures for the Poppy Seed that I posted some time ago are with this device) - but they are even more expensive and quite heavy.

The design above is very simple and cheap and should work fine up to 7A per channel.