Best of both world mixing li-ion and lipo

I have a really nice high c rating 6s lipo battery with only 3300mah. As you might know, that doesn’t offer a lot of range. At the same time, I have a bunch of free 18650s(salvage from laptop) that I have left from building my ebike.
These 18650 are rated for 2c (they are usually 1-2ah) and there is no way I am putting 6s25p to give me 100amp draw.

So, what if I I just say make a 6s4p lion and parallel them with my lipo pack so I have range AND torque(high amp) when I need to.

Like @lowGuido would say: “Do it for science” :wink:

I have a lion blow up next to me before and now I am scared. But I might, I just wonder have anyone in here done that b4.

I dont think so.

Normally everyone would advertise you to not mix different C rating because the higher rated cells could potentially overdischarge.

You should not use different Ah rating --> same effect.

But i can imagine that the higher c and ah will equal the liion batterys but i dont know.

You can use diefferent C rating batterys in a pack as long as they have same capacity and min/max voltage.
But nothing best from both worlds, cause you’ll be limited to the C rating of the weakest battery in your pack.

You can do everything you want :wink: I just said that nobody would advertise you to mix batteries![quote=“Nordle, post:5, topic:5705”]
But nothing best from both worlds, cause you’ll be limited to the C rating of the weakest battery in your pack.
[/quote]

Could you explain that in a bit more detail?

@edwardlui531, I think it should work.
Higher C rating akkus have lower internal resistance, so if the need for lots of amps arises the amps will come from the Lipos. If the lipos voltage sags they will be supported by the 18650. Similar functionality like a capacitor.
Don’t think its dangerous to parallel these akkus as long as they have the same voltage when they are connected.

The worst thing that could happen is you destroy or weaken your 18650 because they are also providing amps.

And of course, if you mix different C rating akkus in series, only draw as much current as the weakest can handle and monitor the individual cell / pack voltage.

Do it and tell us how it went. :yum:

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When i said ‘‘you can’’, i meant ‘‘it would work totally fine’’…
…better?

More detail here:
2 batterys, both 3V-4.2V, both 1000mAh, one is 5C (x1Ah= 5A) the other is 10C (x1Ah=10A)
now we wire both batterys in parallel to get 2Ah, but the C rating stays 5C, cause the weakest cell in our parallel group has 5C. 2Ah x 5C = 10A. 10A is what you could draw from this example battery.
Draw more, for example 15A, would be 7,5A per cell, but as you can see above our weakest cell only can handle 5A. Puff magic smoke inc.

ok, let’s all agree that @edwardlui531 should do this … :grin:

@Nordle, just for the sake of it, I have a correction.

Current will not be symmetrical because batteries with different C ratings also have a different internal resistance.

So in the example of the OP, his 6s4p pack can maybe supply 16 Amps, the lipo maybe 40 Amps (no C rating mentioned). In theory the total system can supply 56 Amps, however when we pull those Amps we don’t know which pack will supply how much. But we know it will not be symmetrical. In order to calculate it we have to measure the internal resistance.
It could also be that even if the motor only draws 40 Amps (below the limit of the lipos) too much current comes from the 18650.
As mentioned, the biggest risk is that the cheap 18650 are aging more quickly than normal.

Oh, and btw, some charger can measure the internal resistance of an akku pack. If the resistance of the lipo is significantly lower than the resitance of the 18650 pack and total current does not go over the limit of the lipo then everything is fine.

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However, i still think as long as you don’t exceed the lower C rating you’re fine. I read a long thread on ES about this just can’t find it atm…

My opinion, it’s a bad idea to mix different types of batteries together in series or parallel.
You could be playing with fire.

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I completely agree with @Namasaki. Don’t do this. You spare yourself from having to buy a second $30 LIPO, but that’s the only benefit.

If you really want to use your 18650s, you can buy a few more salvaged laptop batteries on Ebay, fully test them, and build a pack with the ones you already have.

I think proper respect is due here; when you use LIPOs it’s better to play it safe.

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To be very honest, I don’t see any problem combining lipo and liion as long as they are in parallel. The higher c rating will provide the burst when needed. Battery connected in parallel have a tendency to equalize on discharge, so they will naturally balance out each other. Same to capacitor, they also actually a battery, only short burst connected in parallel. Hence I don’t see the problem doing so, correct me if I am wrong.

When I built my ebike battery, I already knew what I got myself into. Thanks for looking out for me tho, I really don’t want to burn my house down.

Again, according to my calculation, I will need 6s25p to get me 100amp draw.

Yes that is exactly what I am thinking. Now, upon connecting them in parallel on the big terminal, should I also connect them via balance lead?

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I’m curious, why would you need 100a capability.
I’ve done some real life testing with 6s and dual motors going uphill and pulling less than 40a from my batteries.

Report when you have some more info on this.

I also played with the idea of connecting lipos and liion together… Although, what seemed safer for me, was to make a ‘‘battery switch’’ to be able to switch between lipo and liion.

Although, Im also scared of the low discharge / c rate of the liions… Im also planning on using laptop batteries with 2c rate, that would give around 4A for 1 cell (2250mah original capacity)… which is not very bright, 16A total (for 6S 4P connection)

So, if you do find a way and it works, that would be awesome! Please give all info you find on this topic, if you find that someone has done similar!

screenshot of my single motor bench amp draw test, you can see it peaks at 56 but I put a lot of presure. I plan on doing dual motor, so I say 80-100amp is probably a guide line. Also I live in san francisco which have some steep hills.

that means you will also need to switch in the bldc bat min(regen) setting which I don’t think you can change vesc on the fly.

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Running dual motors doesn’t double the load, it devides the load between the motors.
So you have 23a peak per motor

You should never mix batteries of different chemistry