Good afternoon,
I’m in the process of choosing the right battery for our Magni robot. Since we are considering using lithium batteries, I would like to know the maximum amount of power the robot can draw from the batteries.
Thanks a lot!
Jan
Here is some related information very important to your thoughts about lithium battery usage. Our MCB board cannot be connected with any battery system that delivers any more than 30 volts. Sorry about that as I know it makes the nice lighter weight lithium choices limited and we are looking at that for future feature. Today 30V is the absolute high end battery voltage limit or the board will be damaged.
That being said, I did a few measurements today to answer this a little better. and have edited this reply with todays numbers.
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Sitting stationary with motors powered up I am seeing a typical 0.5 amp usage which is most likely the Raspberry Pi. If I disable motor power it remains about the same, thus I think it’s the Pi.
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If I manually apply very significant torque to the wheels when they are trying to hold a zero position I see 2 to 3 amps. I push wheels to the floor for traction and also back at the same time so it keeps traction and am applying a great deal of torque to both wheels.
For flat floor or mild ramp usage and steady state driving it will then average under an amp or so. For cases where the robot is struggling or in acceleration with high payload is where more current will be used of course.
Surges is the area I don’t have a good characterization on yet. I do have a bench ‘torture test’ that is used to apply extremely harsh transient conditions for motor reliability testing.
So perhaps tomorrow or very soon I can run this test where I have each wheel attached to a 10 lb weight. Then I run some special code that goes forward a high speed then reverses at high speed for as long as I want (one form of the torture test). I will apply my current meter on this setup and feed it into the scope and add that as a fairly close to worse case transient case to round out these numbers for you. There will be much larger spikes in current is my guess and that may help you to some extent.
Today I did the experiment I described in prior post and include a picture here showing current remains under 5 amps for this case. This case is each motor has a 10 lb weight firmly attached to it for inertia then we run both wheels in reverse at what would be 1 meter per second. Then we switch to 1 meter per sec forward on both wheels. There were other cases but this is the worse I saw. The current that is being measured is with a probe that goes to an oscilloscope
Now on to the topic of lithium technology batteries which is a very good question and thanks for bringing it up. This comes up from time to time and I feel we need to look more into this sort of solution to help users who want to use those batteries. Today we have no ‘official answer’.
If in any way you can just stick to lead acid that is what we can say we are qualified to deal with.
On our documentation pages we discuss battery choices way up top of our top page and it points to here: https://learn.ubiquityrobotics.com/need_to_know
Please consider this below now my best attempt to assist in finding a solution. No matter what solution is used as said earlier the batteries must always be less than 30V absolute max or not EVEN when on a charging system.
Next our charger is a Lead Acid technology charger and I am sure is not safe or perhaps even appropriate for any Lithium cells so you would have to look into that side of the issue.
I have not dug deep into this but for a different hobby of mine I use a LiFePO4 type of chemistry where the batteries come out to be much closer to lead acid types of voltage ranges. These come in 12.8 or 25.6V cells and in a quick web search I see several vendors. One that looks to have a good selection is the PSL series from Power-Sonic seen here https://www.power-sonic.com/batteries/psl-series/
So again, We have not qualified these and if in any situation they could get over 30.0 volts it would destroy our board. So this is a starting point to investigate.
I’ll have to look more myself.
Thank you for your very good question.
Mark
Hello Mark,
Thank you for your detailed and very good answer and for the measurements you made! That helps me a lot.
In fact, I am thinking about whether the LiFePO4 batteries would fit. As an example I have chosen this battery:
They have a final charge voltage of 28.8V, i.e. less than 30V.The maximum discharge current is 20A (continuous) and 40A (peak). This discharge current is significantly lower than the discharge current of a lead acid battery. This is the reason why I ask for the maximum current the robot draws. After your measurements I am optimistic that this battery can be used in the robot.
These batteries contain a battery management system and can be charged with a simple charger for lead batteries. Therefore I think that they can be easily installed in the robot.
Thank you very much for the good answer!
Jan
In the link above I mentioned battery choices. We have not done well in describing the size of the battery compartment in detail so I had put more detailed information on this page due to your query and also it was something on my ever growing ‘to do list’.
If it was not there when you looked before, yu can see battery size info here in middle of this page: https://learn.ubiquityrobotics.com/need_to_know
I’ll be saving your pdf and looking at this with interest. We have had other customers wanting to use the lighter lithium tech batteries and as I mentioned I have been using the LiFePO4 7 to 10 amp hour cells for powering of astronomy gear so was curious to locate candidates for Magni.
Stay Healthy,
Mark