Discharge via BMS or Bypass BMS? [SRO]

Let’s hear the pros and cons to both.

If you know specific models of BMS and whether they cut brakes on overcharge or not, please list the model numbers.

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I personally prefer bypassing the BMS and only using the BMS for charging, and using a loopkey.

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The same as b264 for me. I’d add that I prefer bypassing the BMS because it’s one less source of potential problem inside the enclosure while riding.

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can i add one question to this:
Does it makes sense or not to use a fuse on the main leads in additon to a charge only bms?

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High Discharge BMS
Advantage:

  • Over current protection
  • Possibilty to have a e-switch, so no need for a antispark switch
  • BMS will cut power when a group goes below the setted voltage cutt off

Disadvantage:

  • Bigger
  • Cost more
  • Some models have problems with braking with a fully charged bms. (I have never had this problem with a bestechpower bms, it is important that you set the over charge protection voltage at 4.25V/cell)

Charge only BMS

Advantage:

  • Cheaper
  • Take less place

Disadvantage:

  • No over current protection (You can shortcuit your battery pack, External fuse is highly recommended! )
  • No voltage cutt off a cell group
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How do you set overvoltage protection? I’ve got a HTX-D223V1

I wouldn’t list that as an advantage – I’d rather cause harm to my battery than my body :slight_smile:

Although I’d like to know about that… which bypassing does not accomplish the traditional way. Maybe if you ran your lights through the BMS or something you’d see your lights not turning on, even if the ESC[s] were bypassing it.

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Do you have an example of when this would be a problem? Something going terribly wrong in the VESC? I guess you could still get a short circuit if something goes terribly wrong in the BMS.

BI had a costumer who had shortcut his whole battery pack. I was happy that he had asked for a battery with high discharge BMS.

At this moment I deliver each charge only pack with a high discharge fuse. Because there are some stupid people in this world.

I have to check it. But almost 2 year ago I had some trouble with a cell group in my pack (10s). When I was around 32v one off the groups was around 3v (cut off voltage). You only feels your board powering off when you was accelerating hard. Then it turns off the output.

Because of this you don’t give gas and the sag that cells have will go away so you will go again to ±3v directly and the power and brakes are back.

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@pjotr47 If I understand correctly, you’re saying that there’s value in having the short circuit protection to protect against user error. That makes a lot of sense when you build battery packs for naive users. Probably not relevant for people who build their battery packs themselves and know what they are doing though.

I agree with @b264 regarding the cut-off to protect the battery. I would also rather protect myself than the battery. I see what you’re saying here:

I assume that what exactly happens in this scenario is very much dependant on particular BMS. With some, the risk of streetface maybe lower than with others. I’d still rather be in control than leaving it up to the BMS to decide whether it’s good time for cutting me off power.

Again, I’m with @b264 on this one. It would be much better if the BMS merely gave you some kind of warning that things are wrong. Having something else than the VESC connected to the BMS discharge pads (like the lights) is a great idea.

IMO, a small charge-only smart BMS is ideal.

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The batteries I build are most off the time for users. So I don’t know how people use them. But I can only suggest everybody who has a charge only BMS. Use a fucking fuse. It is only max 5 euro but can save your whole board/house.

How do people have car accidents? most of the time this is due to lack of attention. Same thing can happens when you are building. Just some wrong connection or idk what and :fire:

Same thing for your charge port just add a 5-10A fuse.

At this moment I am not afraid from cut offs. More from problems when your board go full speed or something like that.

A small smart BMS would be something awesome. At this moment I have a 60A and 100A smart BMS at home. I have just noticed a update. You can now set a delay in the app when the over voltage trigger must jump in.

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Also, The vesc fets switch battery power directly to motor phases. If the motor shorts, it could blow the fets which usually fail closed (connected). Mayhem could ensue. IMO fuse is mandatory on battery power leads.

+1

@b264 are you trying to get me off on my rant about monitoring vs BMS again? :slight_smile:

I bought one of these, pretty awesome!

I made an adapter and plugged in three 4s lipos, one of them 0.1v higher than the others. The balancer drained a high cell into a supercapacitor at 2A, then drained the supercap into a low cell at 2A. The current is adjustable, everything looked beautiful in the android app. Still testing but so far beautiful. Perfect device to charge to 4.1v/cell and periodically balance the pack. There’s a 1A ($90) and 10A ($500) version. Oh I see now a 5A version ($300).

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IMO charge only BMS does not give you that much protection, and the way it balances packs wears out cells prematurely (slightly overcharge all cells and drain them back down, repeatedly over a long period). It’s not much different from just using a balance board. Or IMO just charge to 4.1v/cell and check cell voltages periodically.

The smart bms is more useful for the monitoring than for the balancing and the protections. IMO if there was a tiny version that removed all the power switching and balancing circuits, and added a buzzer, wow that’d be awesome. I know I don’t know enough to make it happen, but I’m looking at this chip that’s on the smart bms, which is only like $4?

http://www.ti.com/product/BQ76940/technicaldocuments

https://www.arrow.com/en/products/search?cat=&q=BQ76940&r=true

And someone wrote alternate firmware for smart bms (for the xiaomi m365 scooter), so there’s a nice reference for HW and for SW.

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What about the Neptune Lite? It does exactly that - monitor only - and it’s really small. The only problem is it’s not exactly cheap.

There’s also the FlexiBMS Lite.

OK, that’s a good point. I think you convinced me to use a fuse on my next build. :slight_smile:

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Do you have any fuses you can drop links to, to make it easier for folks? Maybe an EU link and a US link, is that asking too much?

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30-200A :slight_smile:
https://nl.aliexpress.com/item/32946916978.html?spm=a2g0s.9042311.0.0.27424c4d8736zz

Charge fuse
https://nl.aliexpress.com/item/32621878368.html?spm=a2g0s.9042311.0.0.27424c4d8736zz

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This is my goto fuse for up-to-4A charging.

https://www.mouser.com/ProductDetail/576-099707.5WXN

This one for up-to-8A charging.

https://www.mouser.com/ProductDetail/Littelfuse/0997015WXN?qs=sGAEpiMZZMsh2y49K8ANrYB63nQN2nH57HxywdZNH7M%3D

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Thors are 32V fuses and they as well don’t provide a fusing chart (after which time at which current they will trigger)
Better to use 58V midi fuses from mouser for 12s

https://ru.mouser.com/Circuit-Protection/Fuses/Automotive-Fuses/_/N-ba8b5?P=1z0x8fmZ1ylo5uu

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I know I know, I have tested them and they are ok

It’s a personal thing, but if I use a fuse on the discharge side I don’t want to trigger it in the wrong moment. A blown fuse at full throttle is no fun at all, especially if strapped with bindings onto a 25kg board :sweat_smile: The extra 3$ are definitely worth to spend in my opinion.

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