Hybrid lipo/18650 pack, different capacity ?

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Hillhater

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Random thinking here, so stick with me a little... :eek:
We know 18650's are the go for light weight and compact size.
But, they leave something to be desired in the discharge ability (power) , voltage sag, and cost areas (if you want the powerful ones )
I guess most of us understand the "Hybrid" pack principle of parallel connecting a high discharge pack (lipo or ultra cap), with a similar capacity , but poor discharge ability, pack of lifepo4 or similar.
So a hybrid pack of Lipo and 18650 should be no problem ?
The voltages will obviously need to be equal...but here is the killer question......do they actually need to be the same capacity (Ahr) in each chemistry ?
EG could it work with a 5Ahr Lipo , and 20 Ahr of cheap (low power) 18650's ?...both at say 36 volts.
The theory being, that the 40c Lipo would feed the peak current surges during acceleration etc, and as its voltage falls with capacity drain, the parallel 18650 pack would "recharge " the Lipo by balancing the voltages between the packs ?
My logic is that you can buy capacity (Ahr) in 18650 very cheap, but any reasonable discharge rated 18650 is either lower capacity or expensive, often both !

So, first question is, what problems paralleling 2 packs of same voltage but different capacity and discharge rating ?
 
I can't answer your Q's, due to my limited knowledge. But I can tell you I read a thread about hybrid pack. And it was 18650/lipo.
Like you they wanted to get a higher discharge rate by adding in a few high C rated lipo's. Iirc voltage needed to be similar but not amperage. I do think there was a little back and forth on how well it would work, but I am fairly sure the OP did choose to go through with his plan. Surely he can tell you more, if it is worth considering or not. I don't have that thread in my bookmarks, otherwise I would give you a link. But I am sure you can find it via search.

If you could use 2 dollar cells or less for 18650's and then add some hi rated Lipo cells you might end up getting a pack with hi discharge rate at a reasonable cost.

Here are more:
https://endless-sphere.com/forums/viewtopic.php?f=6&t=70148#p1061393
https://endless-sphere.com/forums/viewtopic.php?f=2&t=71673
 
Thanks for the links,..
It seems Ohbse proved the theory, but still using similar capacity packs , whilst Motobikewheels built a pack exactly as I was thinking .. Low Ahr cap Lipo, paralleled to a high capacity 18650 pack.
But I couldn't find any feedback as to if he had any issues.
However , it would seem that it's viable, and the savings in size, weight and cost, look interesting also.
 
Hillhater said:
Thanks for the links,..
It seems Ohbse proved the theory, but still using similar capacity packs , whilst Roadrash built a pack exactly as I was thinking .. Low Ahr cap Lipo, paralleled to a high capacity 18650 pack.
But I couldn't find any feedback as to if he had any issues.
However , it would seem that it's viable, and the savings in size, weight and cost, look interesting also.
Click to expand...
I see a problem in pairing their SOC and LVC, where many 18650 have considerable capacity at 3.1V (or even under) and can be discharged to 2.5V, Lipos will fall the cliff at 3.6V doing no useful work, raising IR, heating, trying to unbalance and taking damage. 3.6V is nominal voltage of many 18650 cells (it means half the rated capacity mark).
 
Ahh, ! ..Thanks,
Good point, and very relavent if you need to use all the capacity.
However, looking at the capacity mapping thread. https://endless-sphere.com/forums/viewtopic.php?f=14&t=54202&p=1141402#p1141402
...it does appear that many of the 18650's have 80% of their capacity above 3.5v...especially if you are not pulling max discharge from them..
But yes, much care would be needed to select the best cell to match the Lipo working voltages.
 
I really hope you will hammer til hybrid pack into shape and that you can do a write up in this thread as things move along.
I am finding myself with a currency that have plummet the past months and everything just keep getting more and more expensive for me. If there ever was a time to save as much as possible that time is now. If I can copy your project, and save a few bucks in doing so, yet ending up with a high performing pack I would be very happy.


I wish there was great working smaller sized and cheap super caps available. Then we could have any C rate :twisted:
 
As long as the lipo packs are not over discharged, it will work fine. Both lipo and the 18650 (presumed to be lithium manganese of some type) charge to 4.2v.

I'm assuming you mean to stiffen the c rate of the 18650 pack, with a smaller size lipo. Again, it will work fine. In parallel it will behave as if it's one large pack, with the lipo helping to lessen sag under load. I look at it like this. take 20 ah of 2c cells, and 5 ah of 20 c. You don't end up with 25 ah of 20c, but you do get better than 2c out of it. Effectively you get 5c or something better than 2c. If it was half 2c and half 20c, you'd end up more like a 10c pack.

But heres the catch 22. Lipo and the 18650 have very different voltages when 95% discharged. You can't take your lipo down to as low voltage as the 18650's without harming them. So you'd have to run your hybrid pack no deeper than the lipo can stand, generally about 3.5v resting a good place to stop.

Then, you could unplug the lipo, and continue on with lower c rate to finish the discharge on the 18650's.

So, it will work, but be a bit of a pain in the ass. You either have to do the disconnect, or lose a lot of the capacity of the 18650's.
 
So a better way might to to make a hybrid pack out of 2 different 18650 batteries? If you have some hi C rated cells like LG HE 2 at 20A and half the pack of cheaper cells you would still get the benefits of those hi C rated cells, without ruining your financial status. Both brands of 18650 batteries will have the same low volt cut off so voltage should not be a problem if hybrid pack are made up from two different 18650 cells?

Gone looking for suitable match of 18650's :wink:
 
All good points,..
..but as I said earlier, with many of the 18650's, you are not missing much capacity if you stop at 3.5 volts, especially since it won't be heavily discharging.
From my experience with 18650's, once you get down to that voltage level, the performance/speed has dropped so much it's game over anyway !
 
I built my 18650 out of Samsung 29E in 4P but I didn't realize how bad the voltage sag would be as I had become accustom to the robust stable voltages you get from lipo.
I also understood why more folks like 25R as its more resistant to voltage sag under load.
My solution was to buy even more 29E's and build a 7P pack, I also went from 11S to 12S while I was at it.

You can see here below with 7Amp draw the 29E falls into that more icky 3.7volts area while the 25R is hanging more around 3.8.
1H0fvI


After almost doubling the about of cells in parallel (4p to 7p) the amount of amp draw per cell is closer to around 3amps for my riding and you can see the voltage sag has significantly reduced for the 29E. I wrote this last line in red as sometimes its easy to skip it when its surrounded in images and its an important note :wink:

Mj9Ho6


The performance is a lot better, before under 4P when I rode at max power draw mode (PAS level 5) the ebike controller would cut out but now it holds its ground with no issues. I haven't actually done any serious testing with a volt/amp meter attached to see if it dips but the battery status always shows full bars even when I am drawing 1200watts on the LCD display.. The LCD display I use does show watts, actually I am pretty sure it shows volts as well if I remember how to flick it on, I only been using my new pack for the last couple of days really so I can only say so much about it.

Depends on your usage needs but what I done might work for you, some people need even more power so it goes past their heads while others need less..
 
macribs said:
So a better way might to to make a hybrid pack out of 2 different 18650 batteries? If you have some hi C rated cells like LG HE 2 at 20A and half the pack of cheaper cells you would still get the benefits of those hi C rated cells, without ruining your financial status. Both brands of 18650 batteries will have the same low volt cut off so voltage should not be a problem if hybrid pack are made up from two different 18650 cells?

Gone looking for suitable match of 18650's :wink:
Click to expand...
That would work also, but you won't be getting either the cost advantage of cheap Lipo, or the much higher C rate of it.
(20-40 C Lipo is cheap compared to the best 18650's which are pushed at even 10c , and still sag noticeably )

Thebeastie.... What you did is effectively add a lower capacity, parallel pack , but of the same cells.
But, indirectly, you have confirmed how the current is split between "packs" (4cells & 3 cells). of different capacity.
Effectively it splits in proportion to the IR, as Ohbse saw last year.
So, a low IR Lipo ( Nanotech would be nice) booster pack should shield the 18650's from high current peaks.
 
[strike]Hmm then I guess the next thing to look at would be supercaps?
How many supercaps would be needed to have a significant impact on C rate? What would it cost and how much extra weight and bulk with those supercaps add?[/strike]


Just saw prices on them supercaps. Probably better to do a second battery for that price. Them caps are heavy too. Guess we will have to wait a few years before prices and sizes drops to where it is doable for e-bikes. For now a second battery in a back pack or if you got large enough space dump it all in the frame and go nuts.
 
It's a good idea and one that I'm planning on putting to the test in full scale on my next project.

Plan at this stage is for 28s 12p of Samsung 30q in parallel with 28s of 4.5ah HK Nanotech A-spec G2 which have a ludicrously low IR.

The 18650 pack will end up weighing about 16.3kg and a pack IR of ~70mOhm, the nanotech about 5.2kg with IR of 36mOhm. Combined 24mOhm, <22kg and capable of 20kw continuous, peaks of 35kw.

Same specs from JUST 30q cells ends up being 40kg and 840 cells with way more capacity than I require.
 
Ohbse said:
It's a good idea and one that I'm planning on putting to the test in full scale on my next project.

Plan at this stage is for 28s 12p of Samsung 30q in parallel with 28s of 4.5ah HK Nanotech A-spec G2 which have a ludicrously low IR.

The 18650 pack will end up weighing about 16.3kg and a pack IR of ~70mOhm, the nanotech about 5.2kg with IR of 36mOhm. Combined 24mOhm, <22kg and capable of 20kw continuous, peaks of 35kw.

Same specs from JUST 30q cells ends up being 40kg and 840 cells with way more capacity than I require.
Click to expand...

considering doing the same. looking forward for your results!
 
An instructive discussion, thanks guys.still unclear about some things that seem undiscussed- like equalising voltage of both packs after charging separately, but any hoo, mixing types seems hard.

off question (as is a new build suggestion) but on topic, WE presumed, ebikers, dont need 30c, but 1c and falling sucks, 2c would be civilised and 3c worth having - for faster recharge too.

all that can be had from simple lifepo4 pouch packs. 3c on paper, 2c in packs & steady volts and output for a true rated output and capacity. 18650 limn ratings are BS - u cant use the bottom 20% nominally stored for starters.

my theory is the mainstream pouches can do 3c, but conservative assemblers limit them to 2c.

Ping batteries boast better than most c rates for lifepo4 & their optional chargers - like 3c discharge & 4c charge? - & their trick seems to be using 5 amp pouches & some paralleling - eg. a 10ah pak is 2x 5ah modules in P.

I am not keen. intuitively, i prefer as few and as big pouches as possible, & settle for 2c if i must, but am not convinced 3c is impossible using mainstream pouches - even if fewer than 2000 recharges result.

anecdotally, i have a reputable factory ebike, 350w motor & controller, kosher 12ah panasonic li-mn cell 12ah bottle battery - i can draw full power after a full charge for a mere few minutes. after that i am down a bar when apply full throttle. yet it is rated as being able to produce 430 watts (36vx12a)?
 
dogman dan said:
As long as the lipo packs are not over discharged, it will work fine. Both lipo and the 18650 (presumed to be lithium manganese of some type) charge to 4.2v.

I'm assuming you mean to stiffen the c rate of the 18650 pack, with a smaller size lipo. Again, it will work fine. In parallel it will behave as if it's one large pack, with the lipo helping to lessen sag under load. I look at it like this. take 20 ah of 2c cells, and 5 ah of 20 c. You don't end up with 25 ah of 20c, but you do get better than 2c out of it. Effectively you get 5c or something better than 2c. If it was half 2c and half 20c, you'd end up more like a 10c pack.

But heres the catch 22. Lipo and the 18650 have very different voltages when 95% discharged. You can't take your lipo down to as low voltage as the 18650's without harming them. So you'd have to run your hybrid pack no deeper than the lipo can stand, generally about 3.5v resting a good place to stop.

Then, you could unplug the lipo, and continue on with lower c rate to finish the discharge on the 18650's.

So, it will work, but be a bit of a pain in the ass. You either have to do the disconnect, or lose a lot of the capacity of the 18650's.
Click to expand...

stiffen the c rate (of 18650s) is well put.

may i ask how the problems you cite with lipo, would be affected were the boost pack, say, a 5ah 2 or 3c lifepo4 pack, where afaik, its a ~constant 3.3v per cell til ~dead empty?

also, to be clear, i presume a given is having several meters onboard to monitor / decide when stuff should get switched in or out?
 
cycleops612 said:
all that can be had from simple lifepo4 pouch packs. 3c on paper, 2c in packs & steady volts and output for a true rated output and capacity. 18650 limn ratings are BS - u cant use the bottom 20% nominally stored for starters.

my theory is the mainstream pouches can do 3c, but conservative assemblers limit them to 2c.

Ping batteries boast better than most c rates for lifepo4 & their optional chargers - like 3c discharge & 4c charge? - & their trick seems to be using 5 amp pouches & some paralleling - eg. a 10ah pak is 2x 5ah modules in P.
Click to expand...


What about the 20C Li-NMC (Nickel Manganese Cobalt) pouches from szwestart? I remember one ES member praising them in another thread. The praise goes on for about one more page in that thread.

I've read good things about the chemistry but I never heard of this szwestart company. They are also jumping on the Graphene train we saw around christmas from Hobbyking. And that kind of make em not so trustworthy afaic. :)

Think those puches I linked to will do a fair job? anyone tested those? Or know of other trustworthy supplier?
 
craigsj said:
So this is my first post and I'm resurrecting a thread so apologies for that. I've been thinking about this and it seems there's a consideration that I have not seen anyone comment on yet.

Let's say you have the example given here of two packs with a 10:1 ratio of capacity and a 2:1 ratio of ESR. Let's also start with the packs perfectly balanced, then place a load on the parallel combination. We'll call pack A the low ESR LiPo and pack B the bulk pack with higher ESR.

Initially we will have 2A of draw from pack A for every 1A from pack B due to their ESR ratio. That's fairly straightforward but it seems that's as far as anyone takes it.

Over time, pack A will see its capacity consumed at 20x the rate of pack B. That means its voltage will drop more quickly AND its ESR will rise. Both these things will shift the load toward pack B and result in more sag at the load, just as you'd expect and you'd want. Problem is that it may not happen fast enough.

Now let's remove the load. Due to a voltage imbalance that now exists between the packs, an equalizing current flows from pack B to pack A and it's value will be (Vb - Va) / (Rb + Ra). As long the delta V is low enough this is a good thing and just what you'd want.

The problem is that a little delta V can produce a lot of equalizing current. In the example in this thread, the 30Q's in pack B would be capable of huge sustained currents vastly in excess of the charge currents of pack A (and that's typically what you'd have). There's perhaps a 10x or greater difference between what pack B can deliver and what pack A can accept. What this means is that, barring any additional elements in the design, you are limited ultimately by how great a difference in pack imbalance you can tolerate due to the maximum charge current your LiPo can accept. Fortunately, part loads alleviate some of this problem but regen makes it worse!

I've been looking for information on how great the charge current of LiPo can be as a function of voltage but no luck so far. I'm afraid it doesn't improve as the cell empties.

Like the design here, I had planned a 10:1 capacity ratio but with an even greater ESR ratio of 4:1. I believe it will work fine but am concerned with sustained heavy loads (which is what the hybrid design intends to enable). I will be monitoring cell voltages of the two packs to see how big the delta gets. In my case I can tolerate only 0.08V per cell safely.

This approach is safer with lower capacity ratios and lower ESR ratios, naturally. I'd like lower than 10x capacity but it doesn't work for me.
Click to expand...

Dunno, but it sounds good, inquiring thinking on a fascinating, important topic.

I hope this isnt dumb, but u seem to assume long sustained big draws on the hybrid pak, whereas methinks much (non freeway/mountainous) urban use is bursty, in which case, imbalances are quickly rebalanced by modest amp draws by the newly lo volt pak.

so r u saying e.g. 5 amps of 6C lipo and 50 amps of 3C lifepo4?

It would clearer if u worked thru the numbers for such an example of what u seek.

it does puzzle me. a cascading hybrid array of storage of differing properties, seems so simple and attractive, yet so elusive.

u would think there is a role for smart bmsS & controllers, to govern power draw in a way that limits the voltage imbalance that can develop. U get what u want mostly, but are governed at times.
 
Could you not charge the 2 chemistry separately have them both feed the bike and isolate the packs from each other with diode. Have LVC cut out contactor on the lipo to keep it safe.
 
I remember somebody doing something like this a long time ago and it seemed to work. I don't see any issues with paralleling the two different pack types as long as the voltages are matched before connecting. It should be great for short bursts. Doing the math should be possible to tell what is the maximum time of high rate discharge you can do and maximum duty cycle. As the packs get more discharged, I would think the maximum burst time will drop. As long as the average C rate (averaged over the entire discharge time) for the combined packs is less than what the Li-ion part of the pack is rated for, heating should be not be a problem.
 
I did not noticed any problems with 1C charge rate LiPo's taking short down hill 3C regens repeatedly, right after 10C up hill.

From other view point, in this case, it adds to the packs equalization current. Big hill, is worst case scenario, because it usually goes up before going down. :?
 
Regen raises overall voltage in the circuit and now your LiPo pack is much smaller. Your bulk pack V raises to its resting V, which is much higher in compassion to loaded while equilibrating (considering high IR) and only then takes some charge eventually, in lower proportion (IR).
Problem comes, when you sacrifice some capacity of power pack to fit bulk pack.

I only have LiPo's now, but any partial replacement to low IR bulk pack will put more load on remaining part.
Does it look better now? :)
 
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