What BMS should I pick for 30A BBSHD motor ?

[regan03]

Complete newbie here. I want to buy a 52V 21Ah battery for my BBSHD motor. I have two separate questions:

1. what BMS should I pick 30A or 40A?
2. in the attached photo is the BMS value 20A or is it 40A? I don't know if BMS refers to continuous or max discharge.

 

[john61ct]

You need to know how many battery amps you will be pulling.

Peak means just a few seconds.

Going much higher gives useful headroom, cheap BMSs can be "optimistically" rated

and less stress means more likely to last a while.

You can also use an external contactor +fuse for OCP, then the current rating does not metter

 

[regan03]

You need to know how many battery amps you will be pulling.

Peak means just a few seconds.

Going much higher gives useful headroom, cheap BMSs can be "optimistically" rated

and less stress means more likely to last a while.

You can also use an external contactor +fuse for OCP, then the current rating does not metter

BBSHD controller is rated at 30A. So you are saying I should go for a 40A BMS battery instead of 30 BMS? This would give the battery a little more longevity?

 

[Balmorhea]

All of those battery options are rated 20A continuous, which means that those batteries aren’t enough to supply the BBSHD’s rated power. The cells of some variations probably are capable enough, but something else— cell interconnects, plugs, wire gauge, and/or BMS— isn’t adequate to sustain full power for BBSHD.

 

[regan03]

All of those battery options are rated 20A continuous, which means that those batteries aren’t enough to supply the BBSHD’s rated power. The cells of some variations probably are capable enough, but something else— cell interconnects, plugs, wire gauge, and/or BMS— isn’t adequate to sustain full power for BBSHD.

I have two newbie questions about this:

1. So that BMS value we see on seller's batteries is "continuous discharge current"? Batteries have their BMS rated as 30A and 40A and never knew what those values represent.

2. What would it happen if I used a 20A battery on BBSHD? Already paid for the battery and I have minimal chances for a refund.

 

[electric_nz]

21000mAH of 3500maH Samsung cells (likely the 35E) suggests 6battery cells in each parallel group. The 35E are 8Amp Continuous current rated cells so the cells themselves should have no issues providing 30A continuous.

You could ask the seller if they can upgrade to a 45A BMS which would provide more headroom.

Otherwise it will probably work but the BMS May be a little underpowered for the load and could get warm or cut out at extended load.

 

[regan03]

21000mAH of 3500maH Samsung cells (likely the 35E) suggests 6battery cells in each parallel group. The 35E are 8Amp Continuous current rated cells so the cells themselves should have no issues providing 30A continuous.

You could ask the seller if they can upgrade to a 45A BMS which would provide more headroom.

Otherwise it will probably work but the BMS May be a little underpowered for the load and could get warm or cut out at extended load.

Unfortunately I can't ask the seller for an upgrade because I've already payed the battery.

Given that the cells are Samsung 35E do you think the seller made a "spelling error" when he said the BMS has 20A, and it actually has 30A? The seller is Chinese so "spelling errors" are not improbable.

 

[electric_nz]

dupe

 

[electric_nz]

Entirely possible - you might want to email them and ask what specific model BMS is used, or just wait for it to arrive and open it up

you can limit the BBSHD to 20A in software I understand, and you can gradually try ramping up the current to see if it works.

 

[regan03]

Entirely possible - you might want to email them and ask what specific model BMS is used, or just wait for it to arrive and open it up

you can limit the BBSHD to 20A in software I understand, and you can gradually try ramping up the current to see if it works.

If the battery is indeed 20A and if I use it with the 30A BBSHD (without limiting it in the software) it would damage the battery or it would damage the motor?

 

[Balmorhea]

If the battery is indeed 20A and if I use it with the 30A BBSHD (without limiting it in the software) it would damage the battery or it would damage the motor?

It won’t damage the motor. It might cause overheating of the BMS circuit, the power connector, some part of the wiring, or perhaps even steel strip interconnects. Or it might not. We don’t have enough information about your battery to know what part is the most current-restricted.

 

[regan03]

If the battery is indeed 20A and if I use it with the 30A BBSHD (without limiting it in the software) it would damage the battery or it would damage the motor?

It won’t damage the motor. It might cause overheating of the BMS circuit, the power connector, some part of the wiring, or perhaps even steel strip interconnects. Or it might not. We don’t have enough information about your battery to know what part is the most current-restricted.

I understand. But for sure the 20A BMS would limit the power of the BBSHD motor, right? The speed and torque won't be as powerful as it would be with a 30A+ BMS battery, right?

 

[john61ct]

I understand. But for sure the 20A BMS would limit the power of the BBSHD motor, right? The speed and torque won't be as powerful as it would be with a 30A+ BMS battery, right?

The BMS **does not limit** current.

The amps spec is like for a switch or fuse, you must buy one able to easily handle what you will use.

If you go over you burn it.

Or maybe best case it just shuts everything down, aka current **protection**

Which is not **current limiting** that would be lots more expensive.

BBSHD controller is rated at 30A. So you are saying I should go for a 40A BMS battery instead of 30 BMS? This would give the battery a little more longevity?

No, the BMS gets stressed by going right up to its rating.

Yes of course batteries last longer at a lower C-rate but completely separate topic.

The "rating" of the controller is not all that relevant.

First off phase / motor amps are not battery amps. Peak vs boost vs continuous issue as well.

Unless you just get a BMS rated much higher to be safe, or bypass it like I mentioned

you need to **know** how much battery amps you are going to run, and pay for a BMS that can handle lots more than that.

Your current limiting is done with your system design, or maybe your controller programming, or by adding a CAv3.

 

[john61ct]

Entirely possible - you might want to email them and ask what specific model BMS is used, or just wait for it to arrive and open it up

you can limit the BBSHD to 20A in software I understand, and you can gradually try ramping up the current to see if it works.

If the battery is indeed 20A and if I use it with the 30A BBSHD (without limiting it in the software) it would damage the battery or it would damage the motor?

You are definitely shortening the battery lifespan, and as voltage drops too low the BMS should cut off power

 

[john61ct]

Unfortunately I can't ask the seller for an upgrade because I've already payed the battery.

Given that the cells are Samsung 35E do you think the seller made a "spelling error" when he said the BMS has 20A, and it actually has 30A? The seller is Chinese so "spelling errors" are not improbable.

Why? It is very wise to buy batteries capable of 20C, but only run them pulling 5C.

Again you are supposed to know how many battery amps you need, for how many seconds at a time, vs continuous amps

and then make sure everything in your rig can handle much more than that.

Protecting the weak link from burning is only needed if you bought stuff that can't handle what you're actually using it for.

 

[regan03]

What battery should I choose for my BBSHD motor? The motor runs at 30A.

Some people say 30A BMS would be better others say 40A BMS would be better. This seems like a difficult topic especially for a complete confused newbie like me. What would be your pick?

 

[amberwolf]

I understand. But for sure the 20A BMS would limit the power of the BBSHD motor, right?

If the BMS has a current limit shutoff, then it would shutdown all power, completely disabling the bike, whenever you exceed the 20A limit.

If it does not have any current limit shutoff, then the 20A is the rating of the output FETs, so they would overheat whenever you exceed the 20A limit, and if you do it long enough, it would damage or destroy the FETs / BMS.


To limit the power of the motor, you must change the limit in the controller.

 

[amberwolf]

What battery should I choose for my BBSHD motor? The motor runs at 30A.

It depends on what you want to happen, and what actual battery you have the BMS attached to.

If the BMS you choose has a 30A limit that actually shuts the BMS output off above 30A, then as long as the BBSHD does not draw more than 30A, the BMS and system will oeprate normall. But if the BBSHD does draw more than 30A, the whole system will shut off.

If the BMS does not have a limit that actually shuts the BMS output off above 30A, then it's just that the FETs are rated for 30A, and above that the heat they generate begins to be a potential problem. As long as the BBSHD never draws above that (or not for very long), it wont' be an issue...and the ssystem wont' shut down.

Similarly, a 40A BMS with a shutoff will rpobably never shut off the system, as the BBSHD is unlikely to ever draw that much.

A 40A BMS without a shutoff will probably never overheat it's output FETs either.



Now...the battery itself.

If you have cells that aren't designed for the 30A the BBSHD may draw, then they may overheat or be damaged, if the current draw is enough higher than what they are designed for.

So if you buy a pack with a 20A BMS and the cells are also desgined for that kind of maximum output, then the cells *and* the BMS would be stressed or even damaged if the BBSHD keeps drawing above that for long enough periods (how long? depends on the cells and the BMS and how well each are made, and how much current is actually drawn, and how well the pack is able to cool itself).

So if you've already bought a pack only able to handle 20A, then you will want to change the current limit in the BBSHD to match that; rather than changing the BMS to allow more current than the pack was designed for.

(especially since many sellers of packs don't even use "real" cells, meaning not the actual ones they say they are putting into them, so they may or may not really even be capable of what the pack was advertised as rated for, much less half again as much.

The pack may actually be capable of quite a lot more than they advertised it as, but unless it's tested at the higher currents, you won't know, and the test may be destructive.

 

[john61ct]

So, it depends

Until you are in a position to actually measure, you can't go "minimum size" in your battery and BMS

without limiting the bike's performance perhaps way below what you need.

So for now, oversize them both, ideally close to the maximum your controller will allow.

Then the next packs you buy can save some weight (and maybe money) by going smaller, because by then you will know by experience what the minimum level of current your use case actually requires.

You always will have a restraining "weak link" in the system, and IMO that should be your controller, until your experience level maybe allows it to be your muscle control on the throttle.

Or a CAv3 if you don't have the willpower to restrain yourself

 

[fechter]

The continuous rating is how much you can run for long periods of time without overheating. The max rating is where it will trip and cutoff if you go over.

I would go for at least 30A continuous rating for a BBSHD. More would be OK. The BBSHD will limit the current to 30A unless you blow it up or have a short in the wiring.

 

[john61ct]

What do you think BBSHD would try to pull as a peak load through a very over-spec'd controller

standing start pointing up a hill

with say a 400lb total load?

Obviously with an over temp sensor protecting the motor, just trying to get an idea of the maximum peak amps for say 10-20sec

 

[fechter]

If there was no current limiting on the controller it might be able to pull 80A or so. I have not tried this with a BBSHD but that’s what I can pull on my A2B motor which is a similar size. The current becomes limited by the winding resistance if not by the controller.

 

[Matador]

What battery should I choose for my BBSHD motor? The motor runs at 30A.

Some people say 30A BMS would be better others say 40A BMS would be better. This seems like a difficult topic especially for a complete confused newbie like me. What would be your pick?

For my BBSHD i chose a BMS thats designed for 80A continuous and 200A peak at 14S. I think for a BBSHD rated for 30A, I would not chose a BMS bellow 35 or 40A continuous... Otherwise, under or close to 30A continuous, every time you pin the throttle, the BMS would cut off the power and enter protection mode which is a pain in the butt. Basically, every time you pin the throttle, the whole ebike system shuts off.

 

[Matador]

My logic... For example

If you got a battery thats rated for 80A for example a 14S4P of LG HG2 (20A cells each). And you got a BBSHD rated for 30A....
Then chose a BMS that can take as high a current as the battery can provide or a bit lower. Example 40A to 80A BMS. Not more though... This will ensure the BMS protects for overdischarge current... So if the 4P cell pack can take 80A without overheating (check cell rating), the BMS should not allow to pull 100A which is too much.

But make sure the BMS amp rating is higher than the BBSHD controller amp rating so that the controller is able to pull the amp load it needs (up to 30A continuous) without the battery shutting off while you ride (anoying) due to the BMS protection kicking it.

If you do the shunt mod to your BBSHD contoller in the future to pull 50A, your 80A BMS will still allow you to use your battery and the BMS will still protect it from overdischarge current higher than 80A ( let's say you try to use your battery on another bike with a 200A controllet, the BMS wont allow more than 80A thus protecting cells from overheating). But, typically BMS wont just limit to a steady flow of 80A. Actually, when the BMS hit 80A or more, it will shut the battery off xompletely.