Good thing I don’t use a BMS then 
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I’d really like to revisit this topic before I do something stupid (again.)
It took me a year, and >1k mi to realize that the pack I built for my mountainboard, out of Model 3 “Energy” cells, couldn’t come close to properly feeding the 6374 motors I used. The reason I’ve never seen more than a 60A draw from my pack, which once suggested to me that your pack doesn’t need to be able to feed max motor amps x2, is because that’s all the cells are capable of delivering!
Since I built it using Batteryblocs, I’m free to reconfigure at will.
Here’s the burning question: If I replace one or two 10A cells in each 6P group (It’s a 10S pack now, will be 12S after the rebuild) with a P42A (45A max draw) to increase ampacity from 60A to (in theory) 50A + 45A=95A or 40A+90A=130A, is this going to help keep temps of the Energy cells down, or am I just shooting myself in the foot?
Per the above, I’m not worried about what happens under constant load, but rather what happens when I let off after skating hard for a while, or if I maintain the heavy load long enough to deplete the one power cell. If the current draw from each cell is proportional to the internal resistance or ampacity, I’d expect the power cells to be drawn down more easily, then for them to draw current from the energy cells when I’m coasting, similar to the way my range extenders charge the internal pack when I’m coasting. What I’m worried about is the voltage differential growing the longer I’m on the throttle, to the point where when I finally let-off, the energy cells will dump all 40-50A into the power cells to equalize them, vastly exceeding the 4.2A recommended max charging rate per cell.
Aside from the simulations above, has anybody (here) actually tried mixing power + energy cells within the same pack, have something to add?
Since the experiment is so easy, I may try it anyway, just like to hear some BTDT first.
I think you are over rating the p42a.
In a 6p configuration if you replace 1 cell, you’ll need to add the amps of all 6 cells together then divide by 6. By adding 1 cell to a 6p you do not magically get an extra 45a from that p group,
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I’ve done some actual testing but you’ll have to run your own experiment to know for sure. Trying to guess what your results might be from tests I’ve run with different cells and conditions can’t be done.
Paralleling cells like you outlined can work. But as you mentioned it depends on how hard you use them and the duty cycle of your high current draw.
If you rate the Model 3 cells at 10A then rate the P42A’s at 30A to provide the same level of stress to each. Replacing one Model 3 cell with a P42A won’t increase your current handling by that theoretical 20A IMO as the current sharing will affect things. Too much to think through, brain hurts. 
Run the experiment with two cells and measure the current from each cell during low and high discharge current levels. It’s the only way to know for sure.
Soflo, I think we’re both mostly wrong… If I followed your math, I’d only be able to draw 15.83A out of the whole pack. ((5x10A + 45A)/6 = 15.8333A) I realize I won’t be drawing 10A from each Mod3, nor 45A from the lone P42A, but I expect to draw “somewhat” under those 2 figures, and must be able to draw more than 60A from the whole group of 6 cells. Sounds like the 30A approximation below is useful. Bottom line is that cell mixing is an advanced form self-flagellation. I’ve decided to practice battery eugenics, stick with power cells for my main packs, play games with disposable range-extenders only.
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Mooch, would similar lessons from this experiment apply if paralleling two packs of the same cells, but different size, to extend range? i.e. a 12s2p in parallel with a 12s3p (of all brand new P42A if we want to get real specific)
I’m out of my element here, but I’m thinking this is somewhat analogous to a large capacity cell in parallel with a medium capacity cell, of similar discharge curves.
[I ask totally selfishly bc I’m planning on doing this. I was convinced it was safe and effective at one pt. But I’m having second thoughts]
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The different sizes wouldn’t affect anything. The condition of the cells and interconnects would though.
Directly paralleling different p-groups just makes a bigger p-group. How the cells interact, the levels of inter-cell current flow as they are discharged and charged, depends on how close all the cells in that group are matched. If they are wildly different then you will have a lot of current flow within the p-group as they all constantly try to balance each other out. This creates more heat and ages the cells faster.
This is how any p-group works though, whether all in one pack or two groups of cells paralleled together.
I think the safety wouldn’t be directly affected any more than any other setup since a bad cell, one that was about to fail, would be an issue no matter how it was used in a pack.
Yes, but subsequently acting as one large cell. The same as any p-group.
Safe? Absolutely not. There is no such thing as “safe” use of any li-ion cell. There is always some risk. 
However, we can considerably reduce that risk by using good pack building practices and having a good BMS in place.
All this assumes that the packs are paralleled at the p-group level and not just at each pack’s main pos/neg leads. Joining the main leads together parallels the packs but only as independent packs. Any differences in the packs will result in current flow between the packs as they try to balance each other out. The p-groups from each pack will not be paralleled and you will need two BMS’.
Each pack’s p-group needs to be connected directly to the other pack’s matching voltage p-group to actually parallel the packs completely and to be able to use a single BMS.
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Sorry, I should have been more clear - I’m specifically talking about paralleling two independent packs by joining the main leads together. This electrical connection would only happen when both packs are 100% charged. No BMS on the discharge. Packs are individually charged with their own BMS.
Mechanically / functionally, this is much easier to achieve (less wiring) than actually connecting each p-group’s pos & neg to the second pack to form one single cohesive battery.
Similar to paralleling slightly different capacity 12s lipo pouches.
Ahhh….okay.
Hugely easier. 
But it results in two packs that will “go their own way” and each will need to be monitored and separately balanced as necessary.
The huge mismatch in pack capacities means that there will be a lot more current flow from one pack than the other as the discharge proceeds and the higher voltage pack tries to keep the lower voltage pack charged to the same level (same voltage).
They will indeed act the same way, conceptually, as the individual cells I used in my simulations. As long as neither pack ends up trying to supply too much current, the cells are in decent condition, and they are kept balanced when charged, it seems you should be okay paralleling them like that.
But I cannot guarantee anything as I have no idea of the actual condition of your cells and packs. You’ll have to carefully test the setup at a low current level first, checking voltages/temperatures/current levels, to make sure the current being supplied by each pack is within expected levels.
Naturally, I am going to recommend only paralleling same-capacity packs that have cells of the same age and to use a BMS for each pack. Good luck though!
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Hmm, I was planning on doing something similar with one of my builds. Would you recommend first just draining one pack, then switching to the other pack mid-ride?
My initial logic was that in paralell each cell would see less load, so I’d get a bit more capacity…
Of course no guarantees 
But just for the details - all brand new P42A cells bought at the same time from an Authorized Molicel/NPE US distributor.
Will do! I’ll report the results
Your input is always greatly appreciated
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If I may, I think the key for trying a multiple parallel pack setup is to always ensure both packs are at the same voltage when electrically connected. The only time you can do that is at 100%, or don’t ever connect them (swap one for one).
And the more different the packs are from each other - capacity, internal resistance, chemistry, manufacturing, age, cycles, etc etc - the higher the risk
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I know. Just wondering if it’s better that each cell sees, say, a 20A burst load for 5 seconds every 10 seconds, or a 10A burst load every 10 seconds, but in the downtime small current leaking from one pack to the other to balance it. Surely it can’t be that bad?
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Agreed. Paralleling the packs reduces the per-cell current, increases efficiency, and would result in a bit more riding time.
How much pack-to-pack current flow there was would be determined by the condition of each pack. With identical packs though, of about the same age and number of cycles, I think that current flow wouldn’t be of any concern.
I recommend monitoring it though as we have no guarantees.
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i’ve had a 12s3p where each p was 4.2Ah,
smacced a 12S6Ah pack on top, was ok
(though these were lipos so all i really had to do was add a P group in the wiring harness, no so much like putting a 12s2 p in parallel wirh a seperate complete 12s3p, though that’s sorta similar?
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Well…umm…huh?
ya i’m hust as confused 
in other words, i has 8 6s lipos, 6 of which were 4.2Ah and 2 that were 6Ah
had em in series pairs so it makes 12S
so it’d be
4.2-4.2 ]
4.2-4.2 ] all in parallel in a box
4.2-4.2 ]
and then as a range extender, put in parallel but between the box and the esc instead of being directly in the parallel chain? with the others
6.0-6.0
idk parallel is weird explaining when not dealing with whole brick/complete bog boy pack layouts 
anyways
i had no problems with it, worked just fine
at one point i was tenpted to run lipo with liion but that would likely be a very very big nono cause voltsge curves
this winter break tho, gonna try having an extended range pack in my backpack and see if running a cable down to the box caused any issues (assuming i don’t eat it and catch fire)
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I’m doing it, two hoverboard packs parallel with my main pack, will get back with how it works as soon as I can finish and test it
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What are the configurations of each battery in your setup?
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10S5P of VTC6 on the main pack and each hoverboard pack is 10S2P of some 2200 mAh cells
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