Adding extender battery in parallel.

Just running this through my mind...I have a Sur-ron electric bike coming and I was going to build another 60 volt pack with maybe 7 cells in parallel (16S 7P). The thing is the 2 cells I can get may not work...One is the BAK cells which are 3000 mah but only 2C (3C burst) and the other is EVE at 2600 but 3C continuous. The cells in the bike are Panasonic PF cells at 3000 Mah and 10 amp max discharge. If I add the extender pack in parallel with the EVE cells, then I dont think it will work, because the capacity of the EVE and the PF cells are different. I think as the pack gets drawn down, the EVE cells will begin discharging faster and the Panasonic cells will then begin charging them to keep the same voltage putting strain on the PF cells. Maybe the water filling different size tanks analogy is the one to use in which case it should be fine...any comments would be appreciated...

Search around here for paralleling packs. Different capacity packs in parallel can work fine so long as they are the same nominal voltage and are at an identical voltage when connecting them in parallel. I ran parallel packs for a couple of years. Charging can be tricky depending on each packs BMS. I just charged each pack separately.

Someone please correct me if I'm wrong but it will work best if both packs aren't drastically different in terms of internal resistance. The load will be shared proportionally. The packs with lower resistance will carry more of the load. You'll also want to make sure the BMS current limits are the same. For instance: I had one BMS cut power while discharging. The other pack ended up carrying the full load for a brief period until the other BMS turned on. At that point; there will definitely be a rush of current from one pack the the other to balance the voltage between the two (not ideal).

I'm running a parallel pack on my Sur-ron made from Panasonic CGR18650DA cells, which are similar to the PF cells. My pack is 16s12p and about 30Ahr, so nearly equal to the stock pack.

You can see some details here: https://endless-sphere.com/forums/viewtopic.php?f=10&t=101441

I wouldn't worry too much about current sharing. The capacity of your extender pack is not as important as the internal resistance. Ideally, you want to have the internal resistances of the cells close to minimize charge shuttling between packs, but I don't think it will make a huge difference if they are off a bit.

My setup has worked perfectly and I'm very happy with it.

Ok thanks for the input...Ill find what the R is of the EVE cells. I already know what the PF cells are, as I made many packs using these. So if I combine 18 in parallel, there would not only be more range but each cell will see less load, and hence less battery sag at higher amps.I have a BAC 4000 coming and so at about 140 amp draw, each cell would only be seeing about 7-8 amps at full power for a couple secs and then at a constant 50-60 amps, its only 3-4 amps each which is nothing. I looked up these cells panasonic cgr18650da cells and they are only 2450 mah...thats a bit of a difference from the PF cells which are 2900. Well, if you have had no issues then mine should be ok at 2600 mah, as theyre closer in capacity to the PF cells.

By the way I checked out your battery extender build...nice job...I am going to build something integral with the back seat area, that will look super cool,but not alot of area to work with, hence the 7P deal...but once I get it here then I can start designing something...

Under the seat will be a good place. Well protected and the seat is too low to start with if you have long legs.

The capacity of the cells isn’t so important. You want the voltage sag under load to be close. Even if they aren’t I don’t think it’s going to matter that much.

Yea, untill I get the bike, I wont know but looks like I can design something around it. I def have short legs so thats not a problem, lol. As for voltage sag, generally speaking the higher the C rate the lower the sag, as my vendor says the EVE cells are 5C (2600 mah) so actually more discharge capacity than the Panasonic PF cells.

Just properly parallel at the cell string level to mitigate these concerns of mismatch and charge shuttling.

Well..lol...the cells I am using have a very similar resistance and are close in capacity, so should be good. The cells I want to use which I am getting at a good price are EVE 2600 5C. There is another cell also an EVE 3000mah 3C continuous. These are guaranteed grade A but 60 cents more per cell. I will test out the first cell, document and report here. Im excited to get the bike and design a cool looking extender pack. I will also be using a 150amp BMS with this pack. I will probably also replace the BMS in the Sur-ron original pack with a better one (if I can get it to fit) with a 150 amp BMS so that you dont have to bypass the load side of the BMS which in my opinion is dangerous.