Smaller packs or 1 large battery?

I am looking at a single 72V 150A battery at a minimum. I am worried about having that much money invested in one battery. I would like to have smaller packs in parallel, with individual BMS, landed on a bus bar. If I lose one smaller pack I will be out less money and it will be easier to replace and repair since it is modular. From what I have read thus far, the stacked BMS's could cause a failure. Can someone provide more detail in either direction?

I think the logic that buying more than one BMS is cheaper than buying one is flawed somehow.

However, there is nothing wrong with your approach from a technical point of view. If you want to connect the packs in parallel, it is adviceable to have a shottky diode incorporated in your battery wires, but other than that it should be no problem.

With packs in parallel you can see what I call a cascading failure. If the BMS on one pack shuts down under load the remaining packs try to pick up the load causing a domino effect. Each BMS is hit with a sudden and increasingly higher current demand. The first BMS to go is probably resettable, but the ones that follow are likely toast.

Since this and related questions have come up in multiple threads recently,

created a central one for that on specific issue

https://endless-sphere.com/forums/viewtopic.php?p=1561188#p1561188

To discuss your specific design, please respond here

leave that one for the "general science" question using that specific 1P6S example please

In my case, I did it for packaging. I paralleled two small batteries because I could put them on either side of the rack on my wife's ebike. Two BMS of course. I went with a single port BMS design so I could leave them in parallel and charge in parallel.

Used a PVC drain pipe as a battery holder. One of these days, I will learn how to 3D-print.

looks great!

docw009 said:

Two BMS of course.

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Well to me, the only reason "of course" would be because of the physical separation between the two sub-packs, and cross-wiring each cell/group together would be more awkward.


> I went with a single port BMS design so I could leave them in parallel and charge in parallel.

Yes I've seen that mentioned, to avoid separate port BMSs.

Could you (or anyone) explain why, ELI5?

After reading some threads elsewhere today I am still at a loss as to why there is any concern at all with parallel packs and individual BMS. Is there real world proof of catastrophic parallel failures or is this all hypothetical? I can see how re-gen may cause issue with poor programing.

Series packs understandably could be problematic.

Unless you buy a cheap POS battery (or poorly construct your own), there's no intrinsic reason for it to fail "early", and no reason in general why multiple small packs would be better, wrt reliability.

Actually, the opposite is true IMO. Multiple small packs have drawbacks that outweigh any "risk" of a single pack failure:
- More BMSs = more systems/components to fail. You're significantly more likely to have a failure with multiple smaller packs, statistically speaking. And if one does fail, there's also a chance that that failure will cause damage to the other packs. It has been reported on this forum that BMSs have not protected packs when a short circuit has occurred. For example, shorted wiring, blown controller, etc, then the BMS mosfets also just blow closed-circuit, instead of providing protection by switching off "open-circuit". If one BMS does this, very likely that it will cause all the others to pop too, if they're all the same hardware with same rated components.
- Higher cost for multiple smaller BMS/packs.
- More wiring (to fail).

kewlnamebro said:

After reading some threads elsewhere today I am still at a loss as to why there is any concern at all with parallel packs and individual BMS

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Assuming a well built pack of quality cells in healthy condition,

a BMS becomes the most likely **cause** of an early death.

That risk goes up exponentially with each one added.

And just, why use more than one, when one will do the job?

Many choose to go without a BMS completely, call them a "battery murdering system" or

just use known good OTS components for specific functions rather than try to get everything in one unit.

I appreciate the feedback from everyone. Spending $1,500+ (1,000+ cells) on one battery, if it fails, will require a lot of effort to repair. Modular replacement makes more monetary sense.

I don't have the time or resources to build my own pack at this point in time. I have found a few individuals that build custom packs. Realistically there will be zero accountability if it fails; it could be reduced to an expensive, dangerous paper weight.

If anyone has a reliable source for a battery builder I would be very grateful.

“Is there real world proof of catastrophic parallel failures or is this all hypothetical?”
Not hypothetical in my case. Really cool long range battery that I had put together for a trolling motor left me adrift in the middle of a lake. I had twice the range of a lead battery but as soon as it got low enough for the first BMS to shut off they all went in succession. I took it apart and reconfigured it without the BMS’s and it works fine.

That sounds like the BMS were doing what they were configured to do, rather than malfunctioning.

Rather than a cutoff lower down, an alarm could go off earlier.

A wattmeter to let you know visually what DoD% you're at, even better.

It's also about knowing your battery and how you use it some of these things you can feel in the seat of your pants. But if no BMS you must monitor till you figure it out. And a C.A. Then still monitor daily
One big battery is the way to go. If asking one BMS.
Plus at what amp rate you using the battery ? Oh 150 amps you must be using high-quality cells or a really big battery.