Li-ion cells and cold temperatures

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Please click on the image to enlarge it.

I was curious about the behavior of lithium cells at low temperature. In particular I was expecting capacity loss and voltage sag  compared to the standard test temperatures of 20-25°C.

The tests above were conducted on a single cell, a Samsung ICR18650 26F, discharged at 1C.

The voltage sag and loss in capacity are lower than I expected. I am planning to do some -10°C tests to have even more data to compare.

Mixing Battery Chemistry : Li-ion and Li-po

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I have about fifty brand new li-ion cells (18650 format) in my lab and I am tempted to build another battery pack for my bike .

The cells in question are 25 pieces of LG ICR18650C2 ( 2800mAh) and 25 pieces of Panasonic NCR18650B (3400 mAh) . 50 cells are not enough to make a battery composed of 20 cells in series , because of the low discharge capacity ( C rating). To buy more cells would be the easiest solution , but considering shipping , customs and cost of the cells (at least 2-3 dollars per cell with a minimum order of 100 pieces) is not a cheap solution.

Looking on the net, a well-known seller of RC toys and products ( HK) has very affordable prices for lithium- polymer cells. There is also a warehouse in North America and the shipping is not that expensive compared to the li-ion 18650 cells.

So I decided to mix the two different battery chemistry’s , Li-ion and  Lipo to get a battery with adequate capacity ( about 8- 10Ah is what I aspire to ) and discrete C -rating. On average I use between 4A and 10A on my bike on a level surface , with peaks of 35A (rare) when I am in a hurry.

I could not find any feedback on the feasibility and the behavior of the cells, so I decided to do some testing on my own.

The two types of cells have the same charge voltage ( 4.2v , 4.1V if you want to conserve battery life, prolonging the useful life cycles ), but different levels of low voltage , Lipo does not like to be discharged under 3.4v ( under load) , while with the li-ion can reach 2.5v ( under load) .

I have done several tests and the LVC mixing the two cells  (li-ion  and lipo connected in parallel) seems to be 3.0v . This way, at the end of the discharge LiPo cells are still cold woth no signs of “stress” , while the li- ion batteries are hot as usual , a typical behavior of this chemistry.

How  the tests were performed.

Cells used :

– Ah Turnigy 5.0 20C ( Lipo )
– LG ICR18650C2 ( 2800mAh)
– Panasonic NCR18650B (3400 mAh)

The cells were charged all at the same voltage and then connected in parallel . This way I obtained an 11.2Ah nominal battery . Then the battery was subjected to a discharge test using the CBA4 battery analyzer .
The discharge tests were performed at 5A, 10A and 15A continuous , stopping the discharge at 3.0v .

The tests compare two kind of batteries:

  • “Mixed” Li-ion +  Lipo battery which is 11.2Ah nominal.

And

  • “Pure” Li-ion battery which is 11.4Ah nominal.

 

Here are the results :

Let us look at the behavior of a battery composed of only 18650 cells with similar capacity (11.4 Ah nominal vs 11.2 Ah  nominal), discharged at the same current with the cutoff at 2.5v :

Now, we directly compare the test results of mixed cell with the “pure” li-ion cell of similar capacity.

As can be seen from the graphs the lipo cell contribute for the most part of the graph, trying to limit the voltage sag. When they are at the end of their capacity (3:45-3.60v on the graph), you can notice a drastic drop in voltage because only cells li-ion are contributing to the load.

E-Bike improvements

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It’s finally warm here in Canada and I have started to use my e-bike to go to work. I introduced some improvements to keep things tidy and easy to use.

Starting with my e-bike computer I used an old printer cable (with many cores) to re-wire the connection between the LCD screen and the main board. I also added a toggle switch that powers off the unit while I am at work or when the bike is not in use, avoiding battery draining.

IMAG0189

 

Everything looks so much tidier now!

I also re-wired the main board with 10AWG cables.

IMAG0211

 

The bike now uses the Hybrid battery (see previous post), 54V nominal and about 9.5Ah (with 7.5Ah of usable capacity to preserve cycle life). I was able to shrink wrap the battery in black PVC tubing, making it safer and more professional looking.The battery is secured to the frame with “backpack” straps.

IMAG0212

 

I am still working to improve the bike.

Next things to do:

  • Use the main battery to power up the e-bike computer (now is powered with an external battery)
  • Introduce a plug for recharging outside the wooden battery frame, weather insulated, and protected ( with a fuse).
  • Reduce the amount of wires.

 

I am also working on a “cell balancer”, I have done a few tests and I should be able to have a pcb ready for more testing next month..I will keep you posted!

 

 

Cell-log LVC “BMS” project.

I decided to make a Battery Managemet System using the cell-log 8s/m.

Those little cell/battery voltmeters have an alarm port that can be triggered at a selected voltage (battery pack voltage or cell voltage). I made a simple circuit exploiting this feature.

In the video you can see my prototype circuit to cut the load using the cell-log alarm port. When the threshold voltage is reached the alarm port closes the circuit and the mosfet opens the circuit.

I used a 12v 50W light bulb as load.

The battery is a 5s4p 18.5v 6Ah li-mn.

The alarm is set to go off if any cell goes under 3.79v. You can see the light bulb flashing as the alarm start going off. This is due to the fact that the voltage of cells goes up once the load is cut.
The alarm(s) can be set to go off to any value within the cell-log specs.

The next stage of the project is to cut the load permanently (no more flashing) as the fist alarm goes off. Then I will add the possibility to connect more cell-logs to manage a bigger battery.

The beauty of this system compared to a standard ebike bms is that  there can be no current limit (depending on how/where the load is “cut”) so it’s suitable for high power setups.

A big “Thank you!” to my friend Adriano for helping me out with the circuit.

Arduino E-bike computer is alive!

In the last few days I made some improvements and changes to my arduino e-bike computer.

It displays various informations given from the sensors:
+Battery voltage
+Battery temperature
+Amps
+Watts
+Ah used (stored in eeprom)
+Speed in Kph
+Distance in Km (stored in eeprom)
+Km left with the current battery.

Pushing a button I can see the maximum values of the data.

Pushing another button the data stored in the eeprom can be reset to zero.

I want to thank my friend “Ccriss” for helping me (a lot) with the code!

Soon there will be other upgrades!

E-bike goes 45 Kph!!

I recently purchased, at a great price, some “defective” battery packs used in power tools. The cells used for those batteries are Sony SE US18650VT, and they support a 10C (!) maximum discharge rate.

So I recovered some of those batteries and I made a new battery pack.

I then decided to make a 15s4p battery pack (55.5v 6Ah nominal) and test the performance on my e-bike.

The cells should be charged at 4.2 Volts, so the pack would read 63.0 Volts when fully charged. My controller has 63 Volts capacitors so to be safe I charged the battery to 61.5Volts.

It is highly recommended that you always check & double check the specifications and the components used in your controller to avoid any damage to components, bike and most importantly people (including you!).

So, after checking that everything was safe and that the batteries that I recovered were truly recovered I put the pack on the bike and went out for a ride.

I was truly amazed! The bike has a much quicker acceleration and reaches 42-45 kph (depending on incline and wind) very soon, I wish I could measure this acceleration in seconds to let you understand, maybe I will make a video.

Here some other data of the trip:

Average speed:

 

Distance:

 

Energy used during trip:

 

Power peak during trip:

 

Ah used during trip:

*The maximum speed on the first photo (49kph) was reached downhill, top speed on the flat without pedalling is 45kph.

Anti-spark system / Inrush current limiter

Today I decided to make an Anti-spark system for my bicycle.  The problem was that every time I connected the battery to the controller there is a big spark on the connector. This happens because the electrolytic capacitors in the  controller want to drain as much energy as they can as quick as possible to charge themselves, so there is a big “rush” of current flowing on the wires hence the spark.

One may say: “can’t you just leave the battery connected adding just two more wires for the charger?”. Well, I can do that, but my bike is a work-in-progress and I need to remove the battery often.

Here below there are a couple of videos (sorry the quality is low) showing the difference between with and without the Antispark system in place.

Antispark OFF:

 

Antispark ON:

 

The  Anti-spark is simple to make.  In my case (check what you need it for and don’t just copy me, it can be dangerous!) I used a 380 Ohm 3.75W resistor that was sitting around in my little lab. The resistor is soldered to the green wire you see in the video. Instead of closing the circuit connecting the two red wires I close the circuit with the green wires. This allows the current to flow slowly through the resistor from the battery to the capacitors. Then I can connect the red wires safely and without any spark. Note that you will need a “power” resistor because the high current flowing trough it will need to dissipate power in the form of heat. The more the power (in Watts) a resistor is rated, the more heat it can tolerate/dissipate. In other words a common 1/4W resistor could burn, don’t use it!

Bike data: The battery is 36v nominal (42v fully charged) 24Ah Li-ion. The controller is 36v 30A.

P.S. Remember to vote!! Here