Modular Battery build

shabtronic

1 mW
Joined
Sep 27, 2021
Messages
10
Hi Folks

I'm attempting to make a battery for my e-bike. I have 2 spare 48v (54.4v max) 12.5AH batteries and want to make a 72v or 80v max - haven't decide which yet - controller can handle 90v max.

In total I have 130x18650s from the two 48v packs - don't know what spec they are yet - havn't got that far.

I was thinking about making 4 x 5S 6P batteries, that I could serial together. The reason for doing this is cheaper bms and charge units, simpler to make and much more flexible mounting options on my ebike e.t.c.

Has anybody done this?

What are the pros and cons of doing it this way?

or is this just a really dumb idea?

thanks!
 
I have done this, and do plan to do it again.

There is a repercussion of the extended bussing of the pack structures, and how they work when combined. A good electrical knolege is what you need, basically, all cell interconnections must be strong enought o handle all the intended current without signifigant voltage drop, or the like. In the wires.

For instance: M current bike is 15aH, 20s. 1 pack. 20s, 1p

I plan to go .. with a much more powerful controller in the same bike. This will be either 20s or 24s. 20s, 1p, 25Ah, <or> 24s, 1p, 25Ah...

The space for the battery is available for either configuration with the new battery.

I plan to build an 20s... and run if this is what I like, I will use, but.. I have the space available for the 4s on top of this. This will amake fundamentally a 24s battery. the 20s balance plug will hold 25 spaces.. and the 4s balance lead taps will be available to insert into the 20s balance header. Pre planning allows this. The BMS connections will be easy, pole, terminal connections. The modified bus ( interconnect) will be a thic copper strap to keep losses down and not lose the voltage ( that I do not want ot lose, under load. )

This should provide me with 20s or 24s capability with one 20s pack, and a sub-pack of 4s, with strong interconnects that are available for use .

( battery end taps, battery cell boosters, cell end switched, old names for this system, circa 1900 similar concept in old electrical design implementations)

Requires operator / builder/ configuration plan. This is where you simplify, for safety and reliability, cost.

One sub pack on another is not a big deal. 10 different sets of cells in parallel and series mess, mud, conglomerate, is not a good idea. You do not want, {(3) x 4aH, 2p, 2s packs ( x10)} for a 20s, built with (10) 2s pack... 6p for 12Ah, 20s. Keep parallel groups to a min, max the p-group for that string-#, and do not have 3 packs where you could use 1... parallel... It would be better if the packs were 2s, 6p x (10) for a 20s.

You see the losses when you parallel a bunch of smaller cells with long wires in between.... and then serial-ize those connections ( big mess? ) for more voltage.. that is a waste in current capability of the bussing en a spot for bottle necked current and heat creation(s).
 
Hey thanks for the reply and info!

I'm trying to decode your info here:

One sub pack on another is not a big deal. 10 different sets of cells in parallel and series mess, mud, conglomerate, is not a good idea. You do not want, {(3) x 4aH, 2p, 2s packs ( x10)} for a 20s, built with (10) 2s pack... 6p for 12Ah, 20s. Keep parallel groups to a min, max the p-group for that string-#, and do not have 3 packs where you could use 1... parallel... It would be better if the packs were 2s, 6p x (10) for a 20s.

are you saying 10 packs of 2s 6p is far far better than 3 x 10packs of 2s 2p?

You've got me super confused here (which is not difficult :) )

thanks
 
Put all your paralleling together first

So say 20S is your voltage, 6P groupings is your foundation at the 1S level.

Ideally each of those groups should have a similar State of Health and Ah capacity so you have a chance of keeping them balanced without superheroic efforts.

Getting 4 separate 5S BMS that will work at up to 84V will be "a challenge"

So 20S could be 4x 5S strings, makes it easy to use inexpensive Hobby chargers for balance charging, no need to use a BMS for that, keeps things very simple.

If you want LVC based on per- cell/group 1S voltage, get a single BMS at 20S and wire so it is easy to remove/replace. Either "protective only" no balancing, or in effect that function easily disabled.

Or just LVC at the pack level, say at 68V will be good for longevity, and very likely none of your groups will be too far below the average 3.4Vpc.

 
Thank you for taking the time to reply folks - I understand now (I think :) )

I should build:

30 sets of 1s 6p (because the parallel groups are more efficient due to the short wiring)

then 4 sets of

5s of the 1s 6P groups/strings

Theses are then my 4 battery modules.

Which I then 4s with external cables.

So 20S could be 4x 5S strings, makes it easy to use inexpensive Hobby chargers for balance charging, no need to use a BMS for that, keeps things very simple.

Are you saying I should charge the four [5s 6p] modules individually? (that's what I planned)

It doesn't matter about low voltage cutoff protection for the modules, if everything is initially setup ok - my controllers low voltage cutoff on the pack should suffice.
 
shabtronic said:
Hey thanks for the reply and info!

I'm trying to decode your info here:



are you saying 10 packs of 2s 6p is far far better than 3 x 10packs of 2s 2p?

You've got me super confused here (which is not difficult :) )

thanks

YES. That is waht I am saying ( and john too)...

You are not confused at all, this is correct.
 
DogDipstick said:
shabtronic said:
Hey thanks for the reply and info!

I'm trying to decode your info here:



are you saying 10 packs of 2s 6p is far far better than 3 x 10packs of 2s 2p?

You've got me super confused here (which is not difficult :) )

thanks

YES. That is waht I am saying ( and john too)...

You are not confused at all, this is correct.


Hey thanks for your reply - I'm starting to "get it" now I think!

When I dismantle my two 48v packs - I should mark the batteries in their existing parallel groups - and try to keep them to same groups when I make my 6p strings. I'm thinking that these would have been matched for capacity/characteristics by the folk who made the 48v packs (maybe this is wishful assumptions :) ).
 
shabtronic said:
Are you saying I should charge the four [5s 6p] modules individually? (that's what I planned)
I am not saying you should, you said you wanted to which is why a gave you that layout.

The advantage of charging 5S sub packs separately is the ability to use cheap hobby chargers, when balancing is required, they do a better job than cheap BMSs.

If you want to bulk charge at 83V that's quicker and easier than four separate charge sessions or breaking the pack down

but then you might need to buy separate active balancers when balancing is required, adds to costs at 20S but may be the best way.

If you trust your charger(s), and can get and keep the cell/groups well balanced, **and** don't need any backup LVC function, then you by definition don't need any protective BMS.
 
shabtronic said:
I should mark the batteries in their existing parallel groups - and try to keep them to same groups when I make my 6p strings. I'm thinking that these would have been matched for capacity/characteristics by the folk who made the 48v packs (maybe this is wishful assumptions :) ).
Yes, most likely the cells will vary widely by now from uneven wear, just atomize and capacity test each cell, sort them into like bins, discard too-low outliers, maybe buy some new if needed.

Then assemble into your 6P groups so each group is pretty closely matched to each other.

Matching internal resistance of cells also would be great, but that would be a much finickier job.

Note buying Grade-A brand new ones would make all that extra work less necessary, personally I don't mess with second-hand cells unless basically unused and sold by a very trusted vendor.


 
john61ct said:
shabtronic said:
I should mark the batteries in their existing parallel groups - and try to keep them to same groups when I make my 6p strings. I'm thinking that these would have been matched for capacity/characteristics by the folk who made the 48v packs (maybe this is wishful assumptions :) ).
Yes, most likely the cells will vary widely by now from uneven wear, just atomize and capacity test each cell, sort them into like bins, discard too-low outliers, maybe buy some new if needed.

Then assemble into your 6P groups so each group is pretty closely matched to each other.

Matching internal resistance of cells also would be great, but that would be a much finickier job.

Note buying Grade-A brand new ones would make all that extra work less necessary, personally I don't mess with second-hand cells unless basically unused and sold by a very trusted vendor.

Some great help and info from you there - thank you.

I'm just learning about all of this - so this is really great help.

I'll probably just use the spares I have for the moment - zero ££££ from my point of view - and I can go thru that learning curve on
battery construction.
 
I realised last night, how dumb I've been with this - I can simply tap the existing batteries at 36v and connect them in series.
yeah it means I lose 3s - but it's far far simpler - no need to weld or new bms e.t.c.
 
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