Advantages of having batteries in parallel

[PTrider]

Is it true that you can get more miles/km out of a 264wh battery then you can out of 2x 132wh batteries? (assuming you could swap them)

[dmethvin]

Remember that these are all battery packs built out of 18650 cells. The cells are just put together in series to get the right voltage, which is 16 in series (4.2 volts at full charge, total of 67.2 volts) for every EUC I know except the Ninebot. Then you start putting cells in parallel to get a higher capacity and current rating. Since the pack usually has a BMS to manage the charging and prevent over-discharge, it's usually less expensive to have a single pack that uses one BMS. If you have two packs with two BMS then there are some complications since each pack will want to equalize with the others through the discharge lines, circumventing the BMS to some extent.

[PTrider]

22 minutes ago, dmethvin said:

Remember that these are all battery packs built out of 18650 cells. The cells are just put together in series to get the right voltage, which is 16 in series (4.2 volts at full charge, total of 67.2 volts) for every EUC I know except the Ninebot. Then you start putting cells in parallel to get a higher capacity and current rating. Since the pack usually has a BMS to manage the charging and prevent over-discharge, it's usually less expensive to have a single pack that uses one BMS. If you have two packs with two BMS then there are some complications since each pack will want to equalize with the others through the discharge lines, circumventing the BMS to some extent.

So a 528wh unicycle uses 16s4p instead of 4x 16s1p? Is that what you are saying?

Isnt there a way to force the batteries current flow to be one way only making it impossible for the batteries in parallel to charge through the output wires?

[esaj]

32 minutes ago, PTrider said:

So a 528wh unicycle uses 16s4p instead of 4x 16s1p? Is that what you are saying?

Well, to my knowledge, yes and no...   It depends what you mean by 16S1P and 16S4P. For example, I have four separate 16S1P-packs, each with their own BMS. If I put them in parallel, is it 16S4P or 4 * 16S1P? I don't know if there's a convention that says xSyP means that there are y cells in parallel behind a single BMS or if you could call 4 separate 16S1Ps a 16S4P? You probably wouldn't get more mileage out of a four separate 16S1P vs. 16S4P (assuming 16S4P means the cells are in parallel behind a single BMS), as the capacity is still the same, the extra BMSs just add some redundancy (a failure of a single BMS won't cut the power entirely) and is easier to wire, as you don't need to bring the balancing cables from a single BMS to all the packs (like the Firewheel-packs do, even for the 528Wh and the 696Wh (F779), they're all wired behind a single BMS with LOTS of wires going back and forth between the packs).

Quote

Isnt there a way to force the batteries current flow to be one way only making it impossible for the batteries in parallel to charge through the output wires?

Probably could be done with power-diodes (they'd have to be able to withstand very high currents when discharging), but it's not a good idea, you couldn't brake with the wheel anymore, as it would have to dissipate the braking energy somehow else if it cannot charge the packs... trying to dissipate that much power in the motor or the half-bridges would probably burn the motor or mosfets... most likely it would start coasting when you try to brake (it would open the high sides to let the power dissipate into the packs, but the diodes would stop the current from flowing), and the coils/mosfets would start to heat up, as the energy isn't going anywhere but is being dissipated only when the brake-sequence closes the low-sides to "short" the motor to build up the magnetic field in the coils). At least that's what I've understood of regenerative braking, mostly based on this: http://electronics.stackexchange.com/a/56187  It speaks of a single phase, but to my knowledge, similar technique can be used over multiple phases.

[PTrider]

You have 4 16sp1 packs and you use one at a time or you have them wired together?

Im strugling to understant whats the best way(and safe) to increase my milage:

a) use several packs but connect them one at a time 

use several packs connected in parallel

c) use several packs and connect the output of one to the input of another

???

[esaj]

5 minutes ago, PTrider said:

You have 4 16sp1 packs and you use one at a time or you have them wired together?

Im strugling to understant whats the best way(and safe) to increase my milage:

a) use several packs but connect them one at a time 

use several packs connected in parallel

c) use several packs and connect the output of one to the input of another

???

They're all wired together in parallel. The best way (IMHO) is to put all the packs inside the wheel in parallel, that way you have the highest peak output if (and when) you need it. It could also give longer mileage than swapping the batteries on the go, as the stress per pack will be lower (with 4 packs in parallel, 1 pack needs to give out only 1/4th of the total power, vs. single pack which needs to give it all). Now I've got to go, dinner & Force Awakens ->

[PTrider]

Thanks 4 the advise

 

[Keith]

5 hours ago, PTrider said:

Is it true that you can get more miles/km out of a 264wh battery then you can out of 2x 132wh batteries? (assuming you could swap them)

This seems to have gone off track somewhat (except for esaj's reply which is correct) so to bring it back and explain.....

In practice, probably no you wouldn't go further because you'll have extra power available and, therefore, will be bound to use it :-)

In theory, if you accelerate at the same rate and cruise at the same speed you will get further out of a 264Wh pack, than two 132Wh packs used one at a time made from the same cells.

the reason is that the internal resistance of the battery pack is halved if they are in parallel, this means that for a given current draw the voltage at the wheel will be slightly higher. Since Power = Volts times Amps, for a given power, if voltage is higher current required is lower (further increasing the voltage at the wheel). Whether you have two sequential packs or two parallel, total capacity in mA hours is the same. So if you draw less current with parallel packs you will operate for longer. 

I'm assuming that either way you are carrying the total weight of all packs, but even if you picked up the second 132Wh pack half way along I doubt the small weight saving would have much impact.

BMS equalise on a per cell basis and are directly connected only to the cells in the pack they manage. Two,or more, packs with their own BMS are not, in anyway, going to "fight" each other, or react to each other's discharge rates. Providing all packs in parallel are first connected together at the same state of charge this will not change. Should one BMS over current shutdown be lower than the others then you could get a cascade failure (one shuts down, the remaining ones suddenly pull too much current and they shut down). The sooner under-voltage and over-current management is ripped out of BMS and moved to the control board to handle in a controlled way on all self balancing devices the better, protecting the battery at the cost of the user is crap.

it goes without saying, I hope, that batteries must never be connected together in parallel unless they are at damn close to the same charge level (I.e. Same voltage) if a significant difference in voltage exists between packs, very high currents, capable of burning wiring, screwing up BMS,etc, will flow as the packs try to equalise. Once connected there is no reason that the packs should not be charged in parallel too through their charging connections.

There is absolutely no reason that current would flow from one battery to another through the discharge wires, providing they are only first connected together at the same state of charge (voltage). Even if one pack is of a lower capacity than the other (say a 132Wh pack in parallel with a 264Wh pack) then the load will pull 2/3rds of the total current from the 264Wh pack and 1/3rd from the other. Voltage of both packs will (and must) remain the same.

[WakefulTraveller]

I'm a bit confused by all of the terminology used here. One of the battery packs in my MCM4 bit the dust (a damaged cell), but I had another battery pack ready to replace it. This battery pack however does not have a balancing cable to connect the packs together from one BMS to another.

Are there any significant disadvantages to not having this cable when the two packs are connected in parallel?

So far I haven't experienced any real problems.