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Temperature Affects Battery's power and max discharge rate!
#1
This weekend I was testing new batteries, and I found something really fascinating unexpectedly: 
Battery temperature seems to have an effect on battery power, a pretty big one too!

I flew around a course for 8 times, each lap I was doing a 5 seconds full throttle run, and cruised the rest of the lap at 40-50% throttle.
Interestingly, on 6 different packs I was testing, I found something unusual: the middle 3 to 4 laps had the most powerful punch outs! (3rd, 4th, 5th)

You would have thought the 1st and 2nd laps should be the most powerful right? Since it's fully charged! But no, the power output wasn't the strongest in the first lap, it improved in second lap, and the max power finally peaked in 3rd lap...

The difference wasn't obvious when flying slow, but in punch outs, I was clearly able to draw a much higher current like 10-20% more. There seems to be a higher discharge rate when LiPo was warmer.

I think the reason behind this is the temperature of the packs... Before take off, LiPo was sitting outdoor at 10-15 Celcius degree, and the LiPo gradually warmed up from 1st and 2nd laps...


Here are 3 of the packs that I ran... read it from bottom to top, 1st lap is at the bottom of the graph...




So here is the take away, warm up your LiPo batteries before flight might boost the power immediately Smile
  • less voltage sag at punch-outs
  • higher discharge rate and hence more power!
This is especially important for those who race!
Don't be a LOS'er, be an FPV'er :) -- Blog - Facebook - Instagram - Twitter - Google Plus
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#2
I have noticed the the voltage sag is highest when the battery is fully charged or close to empty. It also looks like the internal resistance is at the highest when fully charged
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#3
(11-Apr-2017, 04:00 PM)oyvinla Wrote: I have noticed the the voltage sag is highest when the battery is fully charged or close to empty. It also looks like the internal resistance is at the highest when fully charged

Interesting Smile
I've heard and found the opposite: IR is lowest when fully charged,  guess I need to re-do the test at some point!
Don't be a LOS'er, be an FPV'er :) -- Blog - Facebook - Instagram - Twitter - Google Plus
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#4
Temperature has a large influence on the internal resistance of a LiPo. The colder they are, the worse they perform.

Here's a graph showing discharge curves from an 850mA/h 1S LiPo at various temperatures:

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#5
I'm not sure how I post pictures from my phone, but here is some IR vs voltage data from a charge cycle I did with a gaoneng gnb 120c/240c 4s 1300mah battery. I'll just post the data from one of the cells, but the other cells were almost identical:

3.86v - 3mohm
4.20v - 13mohm
3.56v - 10mohm
3.80v - 3mohm
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#6
The internal resistance of a LiPo is lowest when the cells are fully charged.

The problem is that actually measuring internal resistance with any degree of accuracy with the kind of equipment most of us use is actually quite meaningless. The contact resistance of the various plugs and connectors, let alone the wires themselves is very difficult to measure without four-wire measurement systems.

Take anything that your charger might tell you with a huge pinch of salt!
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#7
(11-Apr-2017, 06:43 PM)unseen Wrote: The internal resistance of a LiPo is lowest when the cells are fully charged.

The problem is that actually measuring internal resistance with any degree of accuracy with the kind of equipment most of us use is actually quite meaningless. The contact resistance of the various plugs and connectors, let alone the wires themselves is very difficult to measure without four-wire measurement systems.

Take anything that your charger might tell you with a huge pinch of salt!

Yes, i don't trust the numbers as my charger shows different ir on each side (skyrc dual charger). But i use it mostly to get an indication about how a battery will perform. The it tendency has been the same with multiple batteries for me. I always put my batteries on the heated floor in the bathroom before performing the it reading, otherwise it will be very different from time to time
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#8
(11-Apr-2017, 07:02 PM)oyvinla Wrote: Yes, i don't trust the numbers as my charger shows different ir on each side (skyrc dual charger). But i use it mostly to get an indication about how a battery will perform. The it tendency has been  the same with multiple batteries for me. I always put my batteries on the heated floor in the bathroom before performing the it reading, otherwise it will be very different from time to time

That's a good idea to get the batteries to a predictable and relatively high temperature before measuring.

The biggest problem though is the balance connectors. As the connectors age and the contact surfaces oxidise, the resistance of the connectors will also change. With modern batteries having IR values in the single milliohms, trying to measure that through a connector that might have more resistance than the cell itself is tricky to say the least.

This also makes it hard to trust the readings even as an indicator rather than an absolute value as the IR reading can rise due to factors that have nothing to do with the battery cells themselves.

We're nerds though, and nerds love numbers. Smile

Maybe "IR" should actually mean Irrelevant Reading instead of Internal Resistance, just like MIPS means Meaningless Indicator of Performance instead of Millions of Instructions Per Second.

Big Grin
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#9
(12-Apr-2017, 08:43 AM)unseen Wrote: ...The biggest problem though is the balance connectors. As the connectors age and the contact surfaces oxidise, the resistance of the connectors will also change. With modern batteries having IR values in the single milliohms, trying to measure that through a connector that might have more resistance than the cell itself is tricky to say the least...

While I agree a number of things affect IR measurements, resistance of the balance connector contacts and wiring shouldn't be one of them.  Battery internal resistance isn't a resistance that can be directly measured. It's more of a calculated estimate.  The typical approach is to measure cell voltages at say no load, measure the voltages again at a fixed load current or some load with measured current, and then use Ohms law to calculate the resistance based on how much the voltage output changes for the change in load current.  Assuming the load current is through the normal battery output leads and the voltages are measured on the balance connector, resistance in the balance connection won't have a significant effect. EDIT: Nor will resistance in the main battery leads, connector, or wiring to the charger or whatever is applying the load since they aren't included in the voltage drop measurement.
Kevin B.
Quads:
Custom 110mm FPV, NanoQX w/DX6i
Other: 3D printing (printer buildThingiverse), electronics, AVR microcontrollers
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#10
(12-Apr-2017, 06:06 PM)Helibus Wrote: While I agree a number of things affect IR measurements, resistance of the balance connector contacts and wiring shouldn't be one of them.  Battery internal resistance isn't a resistance that can be directly measured. It's more of a calculated estimate.  The typical approach is to measure cell voltages at say no load, measure the voltages again at a fixed load current or some load with measured current, and then use Ohms law to calculate the resistance based on how much the voltage output changes for the change in load current.  Assuming the load current is through the normal battery output leads and the voltages are measured on the balance connector, resistance in the balance connection won't have a significant effect. EDIT: Nor will resistance in the main battery leads, connector, or wiring to the charger or whatever is applying the load since they aren't included in the voltage drop measurement.

What you say is true. As long as the resistance of the test equipment does not vary between taking the initial 'no-load' voltage measurement and the 'on-load' voltage measurement, the resistance of the entire test setup is constant and eliminated by being the same at both measurement moments.

I still have my doubts about the ability of the A-D converters and supporting circuitry on any hobby grade charger to measure IR accurately though. No hobby grade charger has temperature compensation or even 0.01% components in the measurement circuits to ensure any kind of accuracy or repeatability.
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#11
I learned this from racing ground RCs. Same as the tests show. We always do a couple warm up laps before the race starts. I do the same when getting ready for high speed passes too Thumbs Up Ive never seen it graphed out so clearly though. Nice job yet again Oscar! Wink Thumbs Up
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#12
The hardcore heli pilots pre-heat their batteries to about 45-50°C, even in the summer... Big Grin

PS: In my personal experience it gets especially bad below 10°C... funny though that some of my batteries with lower C-rating suffer less in cold temperature. The effect can probably vary a lot from brand to brand...
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#13
i've seen the performance drop below 10 deg celsius... and a huge drop when there was -25 Smile

different batteries different effect nimh died almost instantly on -25... lipo lived a bit longer...

so preheating lipos is a thing...
<trollmode>ok so today ill create a campfire and will throw batteries in for 5 minutes to preheat them, Konrad we will preheat your batteries first... and lets breath in the fumes to get even faster...</trollmode>

i'm aware of the lower end limiting the performance, has anyone validated other side of the equasion ? when batteries are too hot and they loose parameters?
All the best
Grzesiek (Grisha/ Greg)

Curently flyable: Nox 5, Nox 3, Minimalist 112, Scrap at the moment... nothing is flyable
Bench / in progres: fixing Nox 5, Nox 3, Minimalist 112, Scrap
thinking about building: 450

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