Portable Li-Ion Battery Pack for Mobile Robot Project from PowerBank Batteries

[k1200]

Hi,

I'm building a mobile robot to demonstrate an agricultural spraying robot. 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.

I'm using a smartphone power bank (1 x 5v, 5000mah) for the Arduino and its electronics and 4 x AA NiMh batteries in a pack for the motor drivers

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.

I recently found and broke open a 10,000mah Li-ion power bank and found five 2Ah batteries 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 scared 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.

I know a lot of mobile robots like Roomba, Double and others have internal rechargeable batteries but I'm not sure how they manage the same issues and power the motors at the same time, particularly with Li-Ion batteries.

Any help is much appreciated!

 

[itsmoked]

I'd estimate your sketched setup would only get your robot about an hour running time. Is that adequate?

When you reach for Li-ion batteries you're exiting the "run the batteries till they're dead" realm and you're going to have to manage them or pay the consequences. Those consequences being rapidly damaged batteries and possibly a spontaneous fire when you're not looking.

With Li-ion batteries you need to:
NEVER overheat them
NEVER over discharge them
NEVER over charge them.

Think Goldie Locks "Just right".

You can no longer have 'just batteries' you will need a "battery system" which can be complicated like each of your other subsystems.

Normally to reduce risk Li-Ion packs are assembled with a battery management system that does what's called "fuel gauging". It will include a coulomb counting system that keeps track of how much charge has been put into or pulled out of the pack. It also has to mange each single battery also. Normally as a pack of series batteries are charged ones having less capacity reach 'full charge' before others. Those that reach full charge are by-passed from further charging. This goes on until the full pack of 'individual batteries' is fully charged. In the process the pack's amp hour capacity is noted and limited to the lowest capacity battery so as discharging use is underway the pack is limited to its weakest member, so that weakest member never gets over-discharged in operation. This weakest member's capacity needs to be continuously updated and remembered cycle by cycle.

If you crack open a laptop battery you find a circuit board that runs all this fuel-gauging, bypassing, temperature monitoring, and communications out to the laptop so the user can see how much usage time remains and the pack remains protected.

You cannot buy 'the system' from Digikey but they do sell the components that are used make battery management systems.

At the very least you should not use a "broken open pack" as likely those batteries are unprotected and can easily be under and over charged. Most single Li-Ion batteries (18650) now come with a tiny circuit board in them that disconnects them on over-charge. If you make a bunch of these up into a single parallel pack they can disconnect themselves as charging is completed.

Normally when building a ground supported robot that will work in a place like an ag field, one would probably select lead acid batteries. They allow you to avoid a "battery management system" entirely. Are more robust, don't catch fire, cost less, are trivial to find good chargers for, and can easily have their voltage monitored for State Of Charge (SOC) functions. Of course they weigh a lot more than Li-Ion but they're well understood by farm workers and since the robot is not handheld or airborne the 3 to 10x cost and complexity inherited by Li-Ion use is easily traded off.





Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

5V*2A + 2*(12V*2A) + 12V*4A = 82W, which is more than 20A at full power.

It's likely that your battery bank isn't even capable of supplying that much current. While it might, and that can be easily checked, such a high draw typically degrades the ampacity of the battery. RC racing batteries, however, are intended for, and fabricated for, handling high current applications. For example:

http://www.ebay.com/itm/Gens-ace-70...037391?hash=item41c3fd634f:g:x-cAAOSwnHZYZEty

is supposedly rated at 35A maximum draw against its 7Ah ampacity.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

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[k1200]

itssmoked, IRstuff,

Thanks a ton for the advise! Some clarifications are due:

The motor values are the peak stall currents and I'll be running them most likely at 50% current.
The motors wont be running all at the same time under nominal conditions but I wanted to set the upper threshold for the battery. In this sense I could go from 1 hr to 4 hours of duty time.
My robot is currently a desktop demonstrator (I usually work on desktop prototypes and then scale up if validated!) and I want to use the same system for other prototypes.

If I went for a battery pack with 30% more capacity than the required load, would I then have a reasonal baseline?

The question remains is building the circuitry where I have no experience. I looked up drone batteries and AFAIK, the batteries seem like they have chargers on a "tender" and then hooked to the drone platform. I'm want to have a rechargeable battery with power path management so I dont have my controllers and electronics shut down when the battery goes too low. At the same time, the battery needs to provide stable voltage to all the loads.

Maybe a big ask? Are there any open source, documented designs of similar chargers used in laptops or other places. I wouldnt mind outsourcing the design but need to find a supplier who could do this for me and trying to estimate the cost/complexity.

 

[itsmoked]

Since you're using just a single Li-ion cell I'd just use a bench-top smart charger capable of charging the pack. Look around, there are many.

And then I'd monitor the battery voltage and when it gets to within, say, 10% of cut-off voltage shut down your machine. If you actually need to retrieve the machine before it has to shut down then you will need a fuel gauging system that will allow you to more closely estimate the SOC.

However, SOC is hamstrung if it isn't part of the charging system since it won't have charging info unless it was in the charging circuit.

Here's an example:
[URL unfurl="true"]http://www.linear.com/docs/44018[/url]

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

Seems to me there are plenty of Li Ion and LiPO chargers on eBay that any number of them might do the job, like:

http://www.ebay.com/itm/Venom-Pro-D...949097?hash=item2ca96b6669:g:VDIAAOSwyjBW3H8w

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[k1200]

Itsmoked, IRStuff,

The option of a LTC2943 for battery monitoring + an off the shelf smart charger seems like a good option.

I've decided to go for something like this:

10Ah 12 to 14V battery
DC to Dc conversion and regulation to 12, 6 and 5v

I'll see if I can find a charger as suggested by IRStuff. For now I'm going to avoid the power path management since demostrating my robot is a higher priority.

Thanks a ton for the pointers - I feel like I know a lot more about Li-ion now than I ever did! batteryuniversity.com is a great location for material.

 

[itsmoked]

Carry on, to success!

Keith Cress
kcress -

http://www.flaminsystems.com