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I have a battery application. With an 11.1 V battery input, I need to maintain a 12 V, 2 A output as the battery discharges.

I bought a 1500 mAh, 11.1 V LiPo battery pack with a 35 C rating and I fully charged and balanced it.

I also got an LM3481 SEPIC mode dev board.

The test setup

Battery feeds the input of the boost converter, the output of the boost converter goes to an electronic load.

Load is a 1.2 A constant current load.

The problem

The test starts off fine, but quickly becomes out of control. The battery input current keep going up and up, within 7 minutes and battery current reaches almost 4 A and rising. The output stays at 12 V at 1 A.

What I think is the issue

I think this is due to the internal resistance of the battery. Once the current starts to flow, the voltage drop across the internal resistance will cause the input voltage to drop, therefore the boost convert will have to draw more current to keep the output current going, then the resistance would drop even more voltage, causing more current to be drawn. This just keeps happening in a "runaway" unstable system.

Possible Solution

Regulate the input current. Put a max input current based on the current load. If the input current gets too high, control the boost converter via a DAC to lower the output voltage, and keep the input current at that max value.

But that max input current draw will depend on what the load is. If the load requires an input current of 500 mA, I would want the input current to limit to be around 500 mA, but if the load changes, so does that set point.

Questions

1) Is this a common problem using boost converters with batteries?

2) Did I just buy a bad battery?

3) What would work for this application?

4) This battery that I bought seems like it has really high internal resistance. Like 1.65 Ω (for 3 cells in series). I put on a 3 A load and the battery voltage drops very low!

Sorry I don't have exact numbers on that, I didn't write it down. What is a normal internal resistance for a LiPo battery? For a 35 C rated battery, there is no way it would be able to provide 35 times its capacity and have any voltage left over for the load. It is a 1500 mAh battery.

Extra Tests

I put it on my charger to get some metrics that it gives me

When I put it in balancing mode. I get this reading for each cell

Cell 1 = 4.06

Cell 2 = 4.06

Cell 3 = 4.06

Then I put it in discharge mode.

Open (no load) voltage 12.06V

Then it goes to 500mA load The voltage immediately goes down to 11.18V

While discharging at 500mA it also gives each cell voltage

Cell 1 = 3.48

Cell 2 = 4.03

Cell 3 = 4.03

Cell 1 is not balanced at all, even though the balancing mode reads that it is.

Should LiPo battery drop that much voltage at 500mA load?

12.06-11.18 = 880mV

880mV/500mA = 1.76 Ohms of internal resistance.

I tested both batteries. They came in a two pack. Could both be bad? Is Floureon a bad brand?

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Some thoughts:

What if you tried cooling the battery pack with a fan (and avoid cooling the evaluation board and/or load)? If after the "7 minutes" the input current is lower than "almost 4 A" it is almost certain due to the temp of the battery.

If the internal resitance is indeed 1.65Ohm, note this is also very close to the UVLO of 4.25V (11-4*1.65 = 4.4V)

I read something in http://www.ti.com/lit/ug/snva461a/snva461a.pdf at section 4.5. Maybe that is applicable to this situation?

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  • \$\begingroup\$ During the test I felt no warmth coming from the battery at all. Not sure if that means anything. You would think a battery rated at 35C would be able to handle a few amps. I read 4.5 as well. This happens after the output is steady and then I turn the load on. That was for a start up state. And yes, eventually the boost stopped working all together because the input was too low, but it should not have gotten there in the first place. \$\endgroup\$ – user2676271 Oct 14 '18 at 18:06
  • \$\begingroup\$ If the voltage drop would be 6.6V at 4A, the battery would dissipate 26.4 Watts and you should notice the battery warming up. I rather think the battery is bad and the voltage vollapses as soon as some current is drawn. You might try using non-rechargable batteries in series to get about 11V and check what happens then. \$\endgroup\$ – user201205 Oct 15 '18 at 18:09
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Sorry I never posted the answer, I thought I did. Anyway, it turns out it was the cables I was using from the battery to the electronic load.

There was a very bad connection on the plugs and was causing all of the problems. Threw those cables away and put new ones in there and everything is right in the world.

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