Uninterrupted Li-Ion Battery Power for Mobile Robot

[k1200]

Hi,

I'm in the process of building a mobile robot to demonstrate an agricultural spraying use case. The robot has two motors that drive the skid steer (6v @ 2A each) and a water pump (12v @ 4A) taken from a car windshield washer pump. I have an Arduino equipped with Wifi, GPS and a few other sensors (IMU, LEDs) which is providing PWM signals to a cheap-o 4-axis DC motor driver.

The whole setup uses a smartphone power bank (1 x 5v, 5000mah) for the Arduino and its electronics and a pack of 4 x AA NiMh batteries for the 4-axis motor driver.

Its a pain dealing with many batteries, have no ability to detect charge levels, and in general very messy with lots of cables. I've also noticed that the powerbank for the Arduino is unable to supply 1.5A consistently with WiFi running and GPS running. Not a long term solution...

I was thinking it would be fantastic if a single battery pack could manage all the power reliably for me.

I recently found and broke open a 10,000mah Li-ion power bank and found five 2Ah cells connected in parallel. I'm now thinking of rebuilding a single rechargeable battery pack from these batteries!

Is there anyway I can achieve what's in the diagram using off-the-shelf electronics from Digi-Key or other supplier?

What I want as features are:
- Uninterrupted power for motors and Arduino from battery and recharge at the same time from wall supply (like a smartphone)
- Same battery should provide stable 2A @ 5V to the Arduino and electronics without drops while the motors are running
- Same battery should power the three motors irrespective if one, two or three are running at anytime.
- Battery should provide feedback on its charge level

Is this even possible? I'm no expert in battery electronics and afraid that I might create a fire if I'm not careful . Given that the battery cells will drop their voltages very low due to the high loads, I'm even wondering if the Li Ion batteries will survive or be able to recharge at all.

Many commercial robots like Roomba and Drones from DJI and 3DRobotics have internal rechargeable batteries but I'm not sure how they manage power the motors and electronics at the same time. Any help is much appreciated!

 

[wiredsim]

That is fairly easily accomplished, but it appears you may want to bump your understanding of how lithium batteries work. Check out battery university if it's still around.

The key point I think you need to know is that lithium-ion battery cells are typically 3.7 volts nominal which is the average charge over a complete discharge cycle. Fully charged they are 4.2 volts and typical cutoff voltage is 3v. This means that the USB battery pack you are using has circuitry that takes the voltage from the batteries and converts it to a stable 5V, no matter the voltage from the battery.

This is relevant because you need to manage the batteries voltage levels at charge and discharge. Since you are talking about a 12 volt load at the high end for the components you listed that would require 3 cells in series. One option is getting a 3s BMS and then running a 12v-5v and a 12v-6v converter off from that as well as hooking directly to the 12v load. The only issue is obtaining a 3s BMS with a high enough current capacity for all of the load of the motors.

The other option is you keep the batteries in parallel and hang DC boosters off from that output. But no matter what you have to protect the cells from running too low or over charging.

 

[k1200]

Wiresim,

My know-how on Li-Ion battery engineering is rudimentary to say the least. Definitely Battery University This part appears straightforward:

3S in series
Splits in-> 12v to 5v
-> 12v to 6v
-> 12v to 12v

You say its hard to find a suitable BMS because of the 8A peak load. I came across this interesting article from Linear on battery chargers with power path management:

http://cds.linear.com/docs/en/product-info/Battery_Management_Soulutions_PowerPath.pdf

Their line of "Switch Mode PowerPath Power Managers " are stated to work for >1.5Ah >2W "large" batteries. Would one of them work?

I'm considering something like the SSOP-16/LTC4006 for my setup, i.e. with a 3 cell series battery pack.

The only info I can find on the internet is if Li-Ion batteries can even recharge while supplying heavy loads like motors. E.g. at full load - 8A, wouldnt the battery voltage would be pulled too low to even start charging?

Maybe I'm over-engineering this problem!

 

[amberwolf]

If the battery voltage goes low enough that you can't (safely) recharge it, then you aren't using the right battery.

You either need enough parallel cells of the less-capable type to be able to sustain the C-rate (current) you are pulling from them, with the Ah (capacity) to supply that current for the entire time you need them to, or you need to use larger cells (higher Ah) with a higher C-rate.


As for the BMS, there are probably hundreds of thousands of existing BMS boards, some of which would do what you're after, so that you don't have to try to design and program one from chip-level on up. But you need to first figure out what it's going to have to handle (voltage, current, etc) and what capabilities it must have (balancing? separate or integrated charge / discharge ports, etc.), before you can go looking for one.



Most likely you would want to design the system so that the highest current load will operate directly from the battery voltage (via the BMS), and then use DC-DC converters to create the other voltages from the main battery voltage.

 

[k1200]

Hi,

After a bit of research inspired by your suggestions, I've come up with a basic schematic of my "planned" battery system.

I realized that to support a few additional DC motors, I must use a battery with a high C rating. Atleast 20A continuous.

My load rating updated is now: 5A min and 20A max (continuous)

I bought a Turnigy LiPo 10C battery - http://bit.ly/2qepUGg
and a BMS with balancer - http://ebay.eu/2qYKW8Q

and will hook it up like so:


Note : The power supply will not be used concurrently with the load for the time being until I can find a "smarter" BMS with power path management.

The big problem with these high C rating batteries from HobbyKing is the lack of documentation. Neither does Turnigy provide any on their website, and there was no attachment in the box when the batteries landed yesterday.

The main concern I have is the BMS is designed for an 18650 cell - but I have NO CLUE what cells are used inside the Turnigy battery. Am I making a good move. I have a feeling I'm going to be advised to keep a fire extinguisher handy! :lol: :wink: