Serially connecting 2 x Li-Ion & LiPoly Batteries & Motor Sh

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kigster
 
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Serially connecting 2 x Li-Ion & LiPoly Batteries & Motor Sh

Post by kigster »

Hello!

I recently purchased two Li-Ion batteries (2500mAh 3.7V each) and the USB charger https://www.adafruit.com/products/259. I would like to use them to power an autonomous vehicle with 4 DC motors, running off Adafruit Motor Shield V2 (pictured below).

Right now I am using 9 x 1.5V rechargeable batteries (so 12V total) to power the 4 DC motors connected to the shield, and 1 x 9V battery to power the board.

I have several possibly trivial questions regarding the "power" setup, and I apologize if they are super noobish. I am new to this whole hardware thing, although I've been doing software for decades.
  • 1. The DC motors I have need 6-7V, so I was hoping to put the two batteries in a series to provide this voltage. But on your Li-Ion guide it says it's not "recommended", but then the explanation has some reference to imbalanced charging. I am not charging them, I am using them :) Both batteries are now at 4.1V exactly. And I plan to charge them one at a time using the supplied USB charger. Why can't I double the voltage by connecting the two batteries together and using them as one?

    2. If I am powering Motor Shield with a separate 9V battery, do I need to worry about connecting the grounds of the two separate power sources, or does the shield takes care of that for me?

    3. I would like the batteries to charge faster. I've been trying to understand the instructions and figure out exactly which resistor I need to solder over to get 1000mA charge (so 2X faster). In some places it says R4, and on the board diagram (when opened in Eagle) R4 seems to be the one marked "THERM", while some other instructions say the resistor to solder over is the "PROG" one. On the back of the board it says that charge rate is 1000/PROG so I decided that it's PROG, and then soldered a 2K Ohm resistor as seen on the picture. Did I do it correctly? How long should I expect it will take to charge batteries to full?
Here is the bumper car :)
motor-shield.jpg
motor-shield.jpg (188.12 KiB) Viewed 437 times
And here is the USB charger with soldered resistor:
charger-1000mA.jpg
charger-1000mA.jpg (218.18 KiB) Viewed 437 times
Many thanks in advance for any feedback and help! I love your site :)

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adafruit_support_bill
 
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Re: Serially connecting 2 x Li-Ion & LiPoly Batteries & Moto

Post by adafruit_support_bill »

the explanation has some reference to imbalanced charging. I am not charging them, I am using them
You should not use them in series unless the charges are balanced. See this guide for details: https://learn.adafruit.com/multi-cell-lipo-charging
do I need to worry about connecting the grounds of the two separate power sources, or does the shield takes care of that for me?
The shield takes care of that.
I decided that it's PROG, and then soldered a 2K Ohm resistor as seen on the picture. Did I do it correctly? How long should I expect it will take to charge batteries to full?
That is correct. You should be able to charge them in about 3.5 hours.

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Franklin97355
 
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Re: Serially connecting 2 x Li-Ion & LiPoly Batteries & Moto

Post by Franklin97355 »

Did you remove the 1K resistor first or just solder over it?

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kigster
 
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Re: Serially connecting 2 x Li-Ion & LiPoly Batteries & Moto

Post by kigster »

The two batteries were charged separately, but each was 4.1v almost exactly - within 0.01v of each other.

Does that work?

I already attached them and they seem to work great.

Second: I soldered a 2k over existing one to bring total parallel resistance to 1k.

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Re: Serially connecting 2 x Li-Ion & LiPoly Batteries & Moto

Post by adafruit_support_bill »

If you allowed the charge cycle to complete for both cells with the same charger, they should be 'balanced'.

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kigster
 
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Re: Serially connecting 2 x Li-Ion & LiPoly Batteries & Moto

Post by kigster »

Hi, so I did not charge them in parallel, I charged them one at a time, but they were very close.

After that, after just a bit of use serially, one of the batteries shut off and showed 0V. Eventually it came back to life with 4.06V, while the other one had 4.11V. So I am guessing they were not balanced.

I configured the parallel circuit using a breadboard as shown on the photo. Could you please confirm that this is sufficient, and that this will produce balanced charge, and that I should not expect another shutoff.

Thanks!
charging_balanced.jpg
charging_balanced.jpg (235.57 KiB) Viewed 384 times

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adafruit_support_bill
 
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Re: Serially connecting 2 x Li-Ion & LiPoly Batteries & Moto

Post by adafruit_support_bill »

Be sure to allow the charge cycle to run to completion. As the cell approaches full charge voltage, the charger switches to a trickle-charge for the final phase. You need to let it complete the full charge cycle.

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kigster
 
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Re: Serially connecting 2 x Li-Ion & LiPoly Batteries & Moto

Post by kigster »

Got it, thank you very much for your help!

They are now exactly the same voltage, so I am going to try again :)

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kigster
 
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Re: Serially connecting 2 x Li-Ion & LiPoly Batteries & Moto

Post by kigster »

Unfortunately I continue having problems with these Li-Ion batteries.

Let me try explain what I tried and what's happening.

I am now completely comfortable charging both batteries in a balanced way. My setup seems to work, and it brings both batteries up to 4.17V. When I connect batteries in series, I get, as expected, approximately 8.35V.

I am trying to use them to power my car (this one: http://www.amazon.com/gp/product/B00GSIRHEU/) , which has 4 DC motors connected to Adafruit Motor Shield V2. I also have Uno connected to the shield, powered by an additional 9V battery. The VIN jumper is not present on the shield, so the two power sources are independent of each other.

My test sketch runs all 4 motors at 50% speed, and then at 100% speed. These motors run really fast.

I tried the following:
  • 5 x 1.5V batteries: this setup works. I see about 6V from the pack, and during motor rotation I see current go from 0A, to about 500mA during constant rotation, and spiking to about 900mA during acceleration.
  • 9 x 1.5V batteries: this works great, and motors run a bit faster with a lot of power. The current seems to fluctuate very similarly to the above, while the total voltage is much higher: about 12V.
  • 2 x Li Ion batteries, balanced, with total voltage of about 8.35V. I have connected red from battery #1 to black of battery #2, and then the remaining red and black are connected to the car's power input (I am not using the USB charging circuit for this, just using two batteries directly). I see current draw up to 400mA, during 50% rotation, and then as soon as the acceleration begins, one of the batteries shuts down, and the power is completely cut off. As soon as I plug the battery into the charging circuit, it switches on, and shows 4.2V charge.
So I am unable to use these batteries to power my car, even though if I understand correctly, they are supposed to be able to support current up to 2.5A.

Also please note that it appears that it's always the same battery that shuts down. Which makes me think it might be an issue with that particular battery.

Just for the record, I am using Triplett 1101-B multimeter, switched to measure 10A current, and when I see a reading of, say, 0.95 I am interpreting it as 950mA.

Please let me know if you have any suggestions on what to look at next.

Konstantin

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Franklin97355
 
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Re: Serially connecting 2 x Li-Ion & LiPoly Batteries & Moto

Post by Franklin97355 »

My recommendation is to get a pack designed to be used as you are using yours. Something like this http://www.nitrohouse.com/Common-Sense- ... 13872.html
and a charger like this http://www.nitrohouse.com/Hitec-X1-MF-8 ... 16753.html

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kigster
 
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Re: Serially connecting 2 x Li-Ion & LiPoly Batteries & Moto

Post by kigster »

Thank you for the recommendation... This pack would provide up to 5000mAh, which is very high current. It would be the right solution if the problem is insufficient current draw. But I am not convinced that it's the problem.

I am now able to run all 4 motors at all speeds on only 4 x 1.5V rechargeable batteries. It runs like a charm. You would think that 2 x 2500mAh/3.7V Li Ion batteries would provide much higher current than that.

So I am back to thinking it's a faulty battery, or a faulty protection circuit on the battery, that over-estimates the current being drawn.

By the way – I am writing a library for driving autonomous vehicles using either Servo motors or DC motors, and it includes Adafruit Motor Shield adapter. You can see the source here: https://github.com/kigster/BackSeatDriver

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Franklin97355
 
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Re: Serially connecting 2 x Li-Ion & LiPoly Batteries & Moto

Post by Franklin97355 »

So I am back to thinking it's a faulty battery, or a faulty protection circuit on the battery, that over-estimates the current being drawn.
Possibly, but you are using the Adafruit batteries in a way they were not designed to be used so you may have to accept the fact that it just won't work.

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kigster
 
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Re: Serially connecting 2 x Li-Ion & LiPoly Batteries & Moto

Post by kigster »

How exactly am I using them in a way that they are not designed?

I followed the instructions on how to charge them while keeping them balanced, your tutorial specifically mentions that this is required for applications that require more than 3.7V. I read the tutorial before I bought them, and it gave me confidence that these batteries would provide me with sufficient power in a small form factor. Now that I am unable to use them for my project I am really confused and frankly frustrated that you would say something like this.

I have a lot of faith in Adafruit, and I am happy to work through DIY solutions, but please don't tell me I am using the battery in a way it was not designed without backing this up with explanation. That's just really inappropriate.

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adafruit_support_bill
 
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Re: Serially connecting 2 x Li-Ion & LiPoly Batteries & Moto

Post by adafruit_support_bill »

What franklin is trying to say is that the cells are not guaranteed to work in that way. These cells are sold individually, not as matched pairs. Due to normal sample-to-sample variability, we can't guarantee that any two arbitrary cells will have parameters matched closely enough to function in a balanced pack.

Factory-built balanced packs use cells that have been carefully matched for both charge capacity and internal resistance characteristics. If there is a significant mismatch between discharge characteristics of the cells, one can end up doing the bulk of the work.

The fact that one cell is shutting down earlier than the other does not mean that cell is defective, or even less potent than the other cell. It just means that it is the one that ended up doing all the work.

If both cells are performing normally as individual cells, I'd say what you have is just an incompatible pair. If one is failing as an individual cell, we can replace it. Otherwise, I can offer you a discount on another cell of the same type. If you contact [email protected] with a link to this thread we can arrange that.

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