What happens when I serialize NCR18650B to Li-co?

[parajared]

I am considering serializing a 10.2ah 3.7 volt NCR18650B (li-ion) 6s (22 volts) to a 10ah 3.7 volt Hobbyking lipo (li-co) pack 6s (22 volts) to make myself a 44 volt pack with the intent to have higher discharge ratings than just one single chemistry.

99% of the time I would be trucking along at 9-10 amps which would be less than 1c discharge for my 10ah pack, however I also enjoy discharging 30-40 amps on occasion for that really steep hill or occasional joyride and I speculate that 10ah NCR18650B would not handle that high of discharge.

For the last five years I have been building 6s 10ah lipo bricks because my rc charger only goes up to 6s, so these wouldn't be hardwired together, the serialization would happen through an external serial wire I built.

What would happen if I serialize NCR18650B to Turnigy lipo?

 

[teslanv]

The 18650 cells will be the current bottleneck of the system. If you have a method to limit current for the whole system it should be fine if you set it to the discharge rating of the 18650 cell limit.
The two halves will probably not stay balanced cell wise, so that would be a PITA to manage.

 

[parajared]

Wow,
Thanks for the fast reply.
So basically the li-co pack would go flat and I would still have energy left on the li-ion pack, correct?

 

[teslanv]

You don't want either pack to go flat. But the smaller amp-hours half would always be slightly lower voltage per cell than the larger half.

 

[amberwolf]

I am considering serializing a 10.2ah 3.7 volt NCR18650B (li-ion) 6s (22 volts) to a 10ah 3.7 volt Hobbyking lipo (li-co) pack 6s (22 volts) to make myself a 44 volt pack with the intent to have higher discharge ratings than just one single chemistry.

To get higher discharge ratings, you would need to *parallel* two packs of the same voltage. If they are both nominally 3.7v/cell, then they will need to have the same number of series cells.

The one with the lower internal resistance (higher c-rate) will then handle the majority of the high current draws, but the other pack will provide some additional current, increasing the total c-rate by whatever it's c-rate is.

Because the packs are directly paralleled, the higher Ri one (lower c-rate) will then "recharge" the other to keep them both at the same voltage (really it just discharges slower; they are both discharging at the same time).

Somewhere around here, there are threads on paralleling different chemistries, with more specific descriptions of what exactly happens in there.


If you put a lower-c-rate pack in series with a higher-c-rate pack, you get a lower-c-rate pack in total. Same thing with capacity--the lowest Ah pack determines the total Ah of the entire series packs. You basically "waste" the "extra" c-rate and ah of the better pack, as it's unusable with the lower-c-rate / capacity pack.

 

[999zip999]

There's no hidden magic. Buy the best cells run them at half or less rated. And the money spend on quality cells should last a long time. Many great cell choices at great prices and prices will go down in the future. The time of blue label no name cells are loosening ground. Hobby King should be stepping up quality control too.
A good cell matched in battery that started out balanced and matched. Not mismatched, some of these and some of those. Who wants a hobby that's all headache. Did I mention the higher risk of fire. Or go ride your bike.
Don't go looking in the junk draw for a few old D batteries for a flashlight.

 

[teslanv]

Dogman and Amberwolf make great points. Namely if you have a bunch of 18650 cells and want to pair them to some LiPo, the best way do do this is to make groups of cells where you have a combination of 18650 cells AND LiPo in Parallel, so that the low resistance of the LiPo supports the high resistance of the 18650 cells under high current loads.
So for example, if you had 2 bricks of 6S-5Ah LiPo and 36 individual cells of 18650's from which to build a 12S total battery, then for each 6S group, you would parallel together a 6S LiPo pack, and a block of 6S3P 18650 cells. And then serially connect that group with the second group of 6S of identical configuration.

 

[parajared]

Yeah, I use the 18650 packs for FPV airplanes and the li-co packs for e-biking and my li-co is starting to wear out (about 450 cycles and 4 years of use). I haven't used my battery all winter and the weather is starting to get nice here in Az. On my ride last weekend my hobbyking lipo would only charge to 4.15 volts per cell and my 20ah pack only gave me 7.8 ah before hitting what I consider to be low voltage cut-off time (3.70 volts per cell). I have pitched my 20ah pack, but I have two 6s 10ah hobby lipo packs that are still in good condition.
Since I already have everything hard soldered into 10ah packs I will do what Amberwolf was saying and parallel first. This would make a 20ah lico/li-ion easy enough. Parallel my 6s 10ah li-ion to 10ah 6s lipo to make 20ah then serialize the whole shebang to 12s.

Thanks for getting me pointed in the right direction fellas.

 

[parajared]

NCA ended up not working that great paralleled to li-co. I did 10ah 12s NCA paralleled to 10ah 12s li-co. What would happen is whenever I would hit the throttle the li-co would take most of the load and the NCA would sag; the problem being is that it would spring back a little out of balance every time. Multiply this by every time I apply and let off the throttle and halfway through the ride NCA would be sitting at some pretty wacky voltages, the worst being 3.36 volts on one cell and 4.01 volts on another after about a 2 hour ride.