Common pack design mistakes, how to avoid?

[john61ct]

The fact remains that every bank will eventually need robust balancing gear as it nears EoL, no matter how little is needed when new.

And some members can only afford packs built from sub-par cells or even second-hand.

Balancing gear that does a faster / better job than the average cheap-chinese protective BMS commonly used here is indeed A Good Topic

but perhaps deserves its own thread, not that germane to this one about pack wiring layout.

[litespeed]

This is my latest and greatest battery I built last year. 20s15p 30Q with 8 gauge battery wires and 8mm connectors.......it’s amazingly powerful and gives my bike 100 mile range at bicycle speeds.
Tom

Could you please explain your awesome 20S15P 30Q cell balance charging procedure. For example cell balance charging arrangement, how often and what balance charger you use/prefer?

Agree that there isn't a traditional BMS that comes close to outperforming balance charging; even moreso on your powerful DIY 200A battery pack

I use a MaxE controller with built in BMS.....As of right now still the best cutting edge controller that WAS available. I can charge at 30 amps if I wanted to. Now when I balance change To start with I use a Thunder Power TP820 Hobby charger.

Tom

[litespeed]

And didn't mean to not give photo credit on those nice packs Tom.

And I didn't say more buss bars are necessary. In a mass production pack, when saving a few cents on materials adds up over thousands of packs it might be a consideration, but I'll take overkill on conductivity any day... It sure isn't hurting anything by having more, aside from slightly higher costs, and weight I suppose.
And yes, in theory the positive and negative should come off opposite ends... But with the copper traces laid onto the nickel strips, it's so conductive it doesn't matter as much, and having both leads come out the same end is more convenient.

Thanks Buddy, I appreciate the kind words.

Tom

[john61ct]

I use a MaxE controller with built in BMS.....As of right now still the best cutting edge controller that WAS available

Link please?

[999zip999]

Oh I was trying to say the parallel group holds the same voltage and series makes higher voltage and the buss bars between the parallel to series holds the building ampage or heat. On demand of amps or power. I th4 this is closer. Or maybe I should give up.

[litespeed]

I use a MaxE controller with built in BMS.....As of right now still the best cutting edge controller that WAS available

Link please?

http://adaptto.com/

Tom

[transposon]

Yes... More buss bars are better... Some just skip the strips and do full sheets. The only downsides are more materials and time costs, but for a boutique battery designed to be abused, worth it.

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Voltron, where do you buy these nickel sheets?

[Haggan90]

I had an ide of building a pack like this, any comments?
The 8P should be more like 20P.

I was think about using Brass as the bus bar, as it seems impossible to spot weld nickel to copper..

Skärmklipp.jpg (226.94 KiB) Viewed 1462 times

[John in CR]

The fact remains that every bank will eventually need robust balancing gear as it nears EoL, no matter how little is needed when new.

While no doubt that's generally true, it definitely doesn't apply to "every [battery] bank". The Sony VTC1's used in Makita cordless tool packs over 10 years ago had no balancing circuity in the packs, because the cells self balanced to some extent due to an increased internal resistance as they neared the top of charge. A few of the packs I built in 2008 and 2009 are still in service. They're made up of strings of 5 X 4s2p blocks of cells (Makita had a robot welding machine that did weak welds on one cell at the end of many 5s2p toolpack batteries that quickly killed the other cell in parallel, and DoctorBass got the warranty returns for a portion of Canada. I made a number of packs from hundreds of those toolpack battery blocks, and the only ones still alive are made up of 20s2p strings mummified in plenty of duct tape with no way to check balance. I did make some packs with larger parallel connected structure including some with balance taps, but moisture/corrosion and/or mechanical failure of connections over the years killed all of those packs. The earliest pack failures were from a period when I got lazy about capacity matching when putting the 4s2p blocks into strings.

LFP told me the v1 Nissan Leaf modules I have are of the exact same chemistry as those old Sony cells. I still haven't put any of those modules in service to find out if they have the same propensity to self-balance.

[john61ct]

Yes, but I was shooting for "generally true", of course a mistake to use "every".

In the context of people saying "more robust balancing is only needed for packs built from old / salvaged cells"

What I've come to realize is, the fact that such cells are unmatched in their ESR and "voltage vs capacity mAh remaining", may be a greater factor than their absolute SoH% in requiring longer balancing times or higher balance current.

And since the best quality BMS (from a protective & reliability POV) do not seem to feature higher balance current anyway

I'm now leaning more toward accepting that factor for starting-new packs, and later on if greater/faster balancing becomes necessary, at that time disabling the BMS end-of-charge based feature and moving to either balance-chargers or dedicated (non protective, "active") balancers to do the job as a separate periodic maintenance routine.

[john61ct]

cells self balanced to some extent due to an increased internal resistance as they neared the top of charge
…
propensity to self-balance

I'd be wary of relying on that idiosyncrasy myself

especially since I prefer avoiding the top shoulder, charging only to 4.05 or maybe 4.10Vpc for longevity.

Apparently that alone may help prevent imbalances, or at least prevent any resulting negative consequences of imbalances from manifesting.

Of course that requires being careful to choose a balancing BMS that lets me adjust the start-balancing voltage setpoint to 3.9Vpc or even lower.

[John in CR]

In the context of people saying "more robust balancing is only needed for packs built from old / salvaged cells"

What I've come to realize is, the fact that such cells are unmatched in their ESR and "voltage vs capacity mAh remaining", may be a greater factor than their absolute SoH% in requiring longer balancing times or higher balance current.

I have the opposite view of used batteries. As long as they still have the vast majority of their life remaining when I buy them, then they are better than new if they arrive in perfect balance, like all of my auto salvage packs except for one small hybrid pack that contained one 8s module with a weak out of balance cell. The reason I consider them better than new is because they are already proven to be perfectly matched, all with proven cell connections, and equal self-discharge characteristics.

FWIW, those old Sony cells have had a charge cutoff voltage of 4.1V/cell for the 12 years since I got them. Of course they fairly quickly self discharge back to 4V/cell, but the fact that they still have useful capacity with no hot spots in the packs, means they have to still be self-balancing the same way lead acid batteries. Those cells were known to self balance back when I made the packs. I wouldn't build a pack with more modern cells without a way to check for balance.

[john61ct]

Well if brand-new EV packs were available at a reasonable price per kWh, say only 50% more than used. . .

But they're not, used is all that's available to a non-wealthy hobbyist wanting an "EV grade" pack.

But collecting hundreds of cells from a large variety of salvage sources, as is more common among budget hobbyists

is a completely different story wrt the "need" for some solution for at least periodic checking if not in-use monitoring

and balancing when needed

even if no "Battery Murdering System" is used.

[DogDipstick]

Well if brand-new EV packs were available at a reasonable price per kWh, say only 50% more than used. . .

But they're not, used is all that's available to a non-wealthy hobbyist wanting an "EV grade" pack.

I can get tons. Brand new. Annybody can. I can have a shipment going to your door tomorrow. Why cant you? I have like 10 friends on Facebook offering new cells. Have held them in hand and offered to sell to those interested. Have sold some. I have tested EV cells, new. Reasonable price / Kwh.. and if 150$ is the price / Kwh to beat, I got you.... All day.

Linked instance: HERE are some NEW, POLYMER, POUCH cell EV GRADE well KNOWN batteries available to the consumer who doesn't ave to much money, for sale, from EU to Canada, shipped, for a GREAT price: ~80Wh and 11.59~$ each. That works out to about 144$/Kwh. Brand NEw.

>>>>>>>>>>>>>>>>>>>>>LINK TO CELLS FOR SALE<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
........................ https://endless-sphere.com/forums/viewt ... 1&t=102908
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Im not going to get into this discussion, I'll leave it to you guys, but... I did want to say that. You can find them, but it is a challenge to find and rely on the good sellers with this battery salesman world.

You are hitting on some good points. I love my ( lightly used) EV cells. You can absolutely be sure they are torture tested and came off the line serialized and sequentially went into the battery assy. together.. They test just as good as the brand new ones. Stay balanced. Something about the way they work with each other, the taper, and the termination current. Some advanced shit.

Carry on.

[fechter]

I had an ide of building a pack like this, any comments?
The 8P should be more like 20P.

I was think about using Brass as the bus bar, as it seems impossible to spot weld nickel to copper..

Skärmklipp.jpg

One way around that is to solder the nickel strips to the bus bar before spot welding. You could also use fasteners (screws, rivets, etc.). With both terminals on the same end of the module, the current sharing won't be perfect but if the bus bars are heavy enough it won't be a problem.

[Punx0r]

The advantage of buying little used automotive cells is you're taking advantage of the "bathtub" shaped failure curve of many products. You're eliminating the crib death part.

[AlexG]

Is it OK to build a pack from fully charged cells?

I once read in a BMS-innstall-instructions that "cells should be around 3,6 volts before connecting the BMS".

I have two smaller packs built with the Vruzend solderless kit and am considering to take these apart to build a lager pack.

Both of these have the same brand of BMS and fully charged the cells are around 4,16 volts. Both packs have been built from new cells and have only been used a few weeks.

This is the BMS from one of the packs that I will re-use in the new build.

https://vruzend.co.uk/collections/13s/p ... -uk-seller

I'm pretty new to this and do not have any tools to individually charge or discharge cells once the packs have been taken apart.

My thinking is that fully charging both packs should balance the cells nicely and they should all be within 0,01-0,03 volts.

Is this OK, or will I be adding to the "common pack design mistakes" by doing this?

[fechter]

Is it OK to build a pack from fully charged cells?

I don't see why not. That's what I did on several builds. I wanted the cells to be all at the same SOC before joining. You just don't want to leave them at full for a long period of time.

[AlexG]

Thanks for your reply, fechter.

The only reason I hesitated was if there was some issues with BMS, that it would misbehave or something if connected to fully charged cells.

Alex

[Pasha]

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Hi All, just wanted share my design to get your expert opinion as I am going to make 10S60P pack using LG MJ1 cells, which will be consisting of 5S60P X 2. As you can see in the pic for serial connections among parallel groups I am using 10AWG copper tinned silicon wire. Peak current draw would not be more than 3500Watts. Your thoughts?

[litespeed]

Very interesting design.

Look forward to hearing how it performs.

Tom

[Pasha]

Thanks litespeed for your reply. My question is that would it be better to use two 10AWG(ampacity ~120amps) copper for serial connections or pure nickel from each cell to to next cell in the parallel group.

[goatman]

im still learning, 10s60p is 42v@4.2v fully charged or 30v@3.0v.
3500 watts@30v is roughly 120 amps that's 2amps per cel. 60px2amps= 120 amps
you can go look at docs thread to see how the mj1 performs

isn't your picture 40p2s?

[hemo]

I only see a 2s wired battery pack, but guess this is just a trial design for the bigger one to come ?

10S60P is going to be mahoosive.

[Pasha]

Yes this is actually trial of 40P and the final would be 60P. For now I am going to make two modules of 5S40P connected in series. I am giving below the discharge chart of LG MJ1.

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