Building/Combining Battery Packs

[Brendon1128]

I currently have two battery packs that I'm looking to combine to make one, big, long range pack. The battery I'm currently running for my Smart Pie is a 78-cell 18650 Lithium pack, arranged in 13 cell serial (48v) by 6 cell parallel (15ah).

Last year, I was using a smaller battery pack, a 52-cell 18650 Lithium, arranged in 13 cell serial (48v) by 4 cell parallel (10ah). I upgraded to the 15ah pack for longer range and better power output. The BMS on my new pack can put out 30 amps continuous, with 45 peak. The old pack's BMS was half that.

For the past 8 months or so, my old pack has just been sitting on my shelf. Rather than let it go to waste, I'm looking if it would be possible to add the batteries from my old pack in parallel on the grid with my new pack to make a massive 13 by 10 cell (25ah) pack. Has anyone ever tried this, and what would be involved with regards to soldering the connections? Do you have to do it in a certain pattern? Thanks!

[Bikemad]

The easiest way would be to make up a "Y" lead so you can simply connect the output wires from the two packs together in parallel, but you would need to make sure that both packs were charged to the same level (Ideally there should be less than one volt difference).



This would give you the option of using either or both of the batteries to suit the length of your ride.

Making the two packs into one single pack would involve a lot more work, but it could be done, but all of the cell interconnections would obviously need to be linked together between the two packs in the correct sequence, or you would end up with some very big sparks!

Alan
 

[Brendon1128]

Hmm, while the parallel connections sounds good, Im a little worried about what would happen if I accidentally didnt charge them up to the same level. With that option I assume you need to d/c them together and charge them fully each.

 A lot of times when Im on the go, I don't have enough time to do a full charge and leave my house with a 50-75% charge.

With regards to building one, what sequences would I need to follow to properly join/add the new cells onto my existing pack?

[Bikemad]

If they were not charged to the same level you would have to just connect one pack at a time and use them as individual packs until they were both at the same level again. If each pack was equipped with its own voltmeter you could simply use the highest pack until it drops to the same voltage as the lower pack and then use both.

By using both pack together you will be discharging each pack less than you normally would, so both packs would recharge much quicker using two separate chargers.

Building battery packs must be done correctly for both safety and reliability reasons. The cell layout of combining the two packs into one will vary according to how each pack has been physically constructed and it will also have to comply with size constraints of the finished battery.

If you are not 100% sure of what is required I suggest that you leave them exactly as they are, as two complete packs that can be used either individually or combined will be far more usable than two disassembled packs that cannot be used at all. 

Alan
 

 

[Brendon1128]

Ok, makes sense. Thanks Bikemad!

[AdamB]

Alan is right!

Unless you are want to do some very unusual stuff, it's just best to leave the build side alone & connect in parallel through the wire harness! Plus, you can pick & choose to take both or 1 or the other with you depending on the range needed for that trip! You can also connect on series later on if you get a controller that can handle it (96v) this way...

Really the only advantage is if you want to divide them up into small voltage packs that can be combined as above while also used in lower voltage packs like I want! Otherwise it's not really worth it...

Just be certain to ONLY use in combination if all packs are the same voltage (I prefer within 0.1-0.2v max difference & typically insist on identical voltages)! I have to disagree with allowing up to a full volt difference between packs, especially on life batteries... That's a recipe to destroy the batteries!

It's possible that by the time the higher voltage packs get to their low voltage cutoff, that the lower voltage pack can be discharged to unsafe levels! Or worse, that they are overcharged... In my opinion BMSs & Smart chargers can't be entirely relied on to protect out of balance batteries/cells! I've seen plenty of times that they failed! That's why I prefer to measure things myself & monitor all directly!

Adam