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4s8P x4 packs for ebike pack?
#1
Do people add car fuse to the battery pack as a fail safe in case of short or overpowering the pack during discharge?
Since I am using used cells i want a extra safety feature to keep all 4 batteries safe incase 1 battery is having an issue and draws more amps from the other batteries. I plan on using glass fuse of 2A on each cell to buss bar, but want to add a car fuse of maybe 25A for each 4S8P battery.
4s8P x4

motor will likely be
48V1500WR
Rear
1500W / 48V
34 mph
48 N.M

Thanks,
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#2
(06-28-2019, 07:46 PM)JBonesaw Wrote: Do people add car fuse to the battery pack as a fail safe in case of short or overpowering the pack during discharge?

The fuses are for safety, therefore are for shorts. But a heavily overworked motor could blow the fuse as well. The fuse needs to be rated high enough that the motor can operate within its amp range without false popping.

(06-28-2019, 07:46 PM)JBonesaw Wrote: Since I am using used cells i want a extra safety feature to keep all 4 batteries safe incase 1 battery is having an issue and draws more amps from the other batteries. I plan on using glass fuse of 2A on each cell to buss bar, but want to add a car fuse of maybe 25A for each 4S8P battery.

Not sure what you mean here. Do you plan on using cell level fuses, or fuses between packs, or both types?

(06-28-2019, 07:46 PM)JBonesaw Wrote: motor will likely be
48V1500WR
Rear
1500W / 48V
34 mph
48 N.M

Not with 4s you won't be. You need at least 14s to run 48VDC.

Or are you planning on having 4 packs of 4s8p, and then serial the 4 packs to make 16s8p??
If this is the case, no point in adding a fuse between each pack. Amps are based on the parallel, not the series. So if a cell decided to dump and blow a fuse, then the rest of the fuses will follow suit as you go from, for example, 25A on 8p (3.125A / cell), then 25A on 7p (3.571A / cell), then 25A on 6p (4.166A per cell), etc etc. You'd have a cascade effect. This will happen if a fuse just breaks because of jostling around or bad solder connection

Also, 8p?? Are you sure? For 25A?? On used cells? I hope these are powertool cells and not non-powertool cells. Otherwise you're asking for trouble
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#3
[Not sure what you mean here. Do you plan on using cell level fuses, or fuses between packs, or both types?]
Yes to both cell level fuse of 2 amp glass fuses but also want a car fuse for each of the 4 batteries 4x 4s8p connected in series.

[Not with 4s you won't be. You need at least 14s to run 48VDC.

Or are you planning on having 4 packs of 4s8p, and then serial the 4 packs to make 16s8p??
If this is the case, no point in adding a fuse between each pack. Amps are based on the parallel, not the series. So if a cell decided to dump and blow a fuse, then the rest of the fuses will follow suit as you go from, for example, 25A on 8p (3.125A / cell), then 25A on 7p (3.571A / cell), then 25A on 6p (4.166A per cell), etc etc. You'd have a cascade effect. This will happen if a fuse just breaks because of jostling around or bad solder connection

Also, 8p?? Are you sure? For 25A?? On used cells? I hope these are powertool cells and not non-powertool cells. Otherwise you're asking for trouble]

yes to 4 packs @ 4s8p. was thinking of going 10p but i think the rear wheel cargo bag will not be big enough to go that long. plus it is a lot of extra weight on the back tire.

I don't know the exact math to figure out the peak output for the motor. (1500w/48v) = 31.25A and divide that by 16 = 1.95 or round up to 2amp per cell.
I think my 2 amp glass fuses will be probably too much if it is dispersed over 8 cells in parallel.
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#4
Take a motors rating and triple it's wattage for surge at start up. So the above 1500W could do close to 4000W on initial start. That'll drop quickly as the torque lowers as the vehicle is moving. So you will need to calculate at a higher amp draw than 2A/cell with your current setup. Take that 25A/pack and size it as you'd you pull 75A/pack. So if you still want to pull 2A/cell (which isn't recommended with used cell), then you'd need close to 40p to handle the amp load. Might even go to 60p just to make sure the energy requirements are met.

You didn't mention what "type" of cells you are using. Are they powertool cells or other types? Are they LiFePO4's or other lithium based cells?

Fusing between packs is not needed and doesn't really serve much of a purpose. You only need a fuse (except for cell level) at the ends of the string. So if you plan on pulling a continuous 25A, with a possible 75A surge, then 100A fuse would probably cover it. If your motor will only surge up to 40A, then a 50A fuse would be fine as well.
Remember, a fuse is only needed for dead short protection. A 20p pack can do at least 40A dead short, if not closer 120A or even more, depending on the cell types.
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#5
(07-01-2019, 06:55 PM)Korishan Wrote: Take a motors rating and triple it's wattage for surge at start up. So the above 1500W could do close to 4000W on initial start. That'll drop quickly as the torque lowers as the vehicle is moving. So you will need to calculate at a higher amp draw than 2A/cell with your current setup. Take that 25A/pack and size it as you'd you pull 75A/pack. So if you still want to pull 2A/cell (which isn't recommended with used cell), then you'd need close to 40p to handle the amp load. Might even go to 60p just to make sure the energy requirements are met.

so based on your 4000w max load i would take 4000/48v=83.33amp.  then divide by 16 to get 5.2A on a string level. would it also then by divided by 8 to get .65A per each 18650 battery or does it not work like that? I see several pre buit packs that are usually 14s4p. I figured 8P would reduce strain on the individual 18650s.

You didn't mention what "type" of cells you are using. Are they powertool cells or other types? Are they LiFePO4's or other lithium based cells?

Cheap laptop packs. This is more of a experiment on the cheap and if it works even for short distances then I will look into higher amp and higher storage.
Currently they are an average of 2.35 ah with a total or 19180mah per string of 8. I am uncertain of C rating on these cells but I would not feel safe pulling more than 2c for only short bursts of time.


Fusing between packs is not needed and doesn't really serve much of a purpose. You only need a fuse (except for cell level) at the ends of the string. So if you plan on pulling a continuous 25A, with a possible 75A surge, then 100A fuse would probably cover it. If your motor will only surge up to 40A, then a 50A fuse would be fine as well.
Remember, a fuse is only needed for dead short protection. A 20p pack can do at least 40A dead short, if not closer 120A or even more, depending on the cell types.

When you say end of string do you mean right between the electric controller and the XT90 battery junction or between each 4s8p battery? 100Amp fuse would trip if i lost connection to one of the 4s8p batteries because it would essentially be 14.8V less or 12v if it was 1/4 of 48v. 4000w/36v =111Amp. I think I would even want something closer to 80Amp maxi fuse since I am overvolting the motor. or would it be 20amp fuses on each 4s8p battery? Sorry I am sure there are some videos I can watch on this but most people are just build 15s packs with BMS and iI want something that will just fit evenly over the back wheel saddle bag and I could remove the saddle bag when I park the bike somewhere in the public to charge or just for protection sake of theft. 
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#6
4000 / 48 = 83.3A / 8p = 10.4125A per cell

So you plan on 4x 4s8p connected in series to give a total of 16s8p. Even if you went 10p, it wouldn't be enough for the cells to be in the safe zone.
Under normal load, lets figure even at 2000W / 48V = 41.6A / 10p = 4.16A per cell.

A string is a pack that is at a specific voltage by having series connected cells within, or multiple packs that are connected into series to make the specific voltage. In your application, you have 2 types of strings. You have a pack string, 4s8p, and then you have full string (or operating string) of 16s8p.
So when I refer to not needing them between the packs, I am meaning the pack strings. Each one should be connected directly to the next pack in the string. No need for a fuse there. The large fuse should be in line with the full, operational voltage, string. Preferably on both the Pos and Neg legs.

If you lost connection to one of the 4s8p packs, it won't trip any other fuses. They are in series, not parallel.

Please read the FAQ (located on the main page) to get a better understanding of how series, parallel, pack and strings are referred to.
Proceed with caution. Knowledge is Power! Literally! Cool 
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