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Is this BMS legit?


#1

I am wondering if this bms will actually work for a 10S4P battery pack. Seems a little cheap considering other BMS boards for a 10S setup can be up to $50. http://www.aliexpress.com/store/product/New-Battery-Protection-BMS-PCB-Board-for-10-Packs-10s-36V-Li-ion-Cell-max-30A/939205_32574042165.html


#2

Output current is a bit low, might not work well when current demands are high.


#3

I don’t see any reason why you couldn’t charge with a cheap low current BMS and discharge straight off the batteries using the VESC for low voltage protection.
all these people paying big bux for high current BMS, I don’t think its even necessary.


#4

I would guess it’s just for another layer of protection to the batteries for better peace of mind, but I agree.


#5

I don’t even like the BMS low voltage cut off… it cuts so hard you SYF.
I don’t even use BMS at all.


#6

@lowGuido I bought my BMS before fully understanding how LVCO works with BMS. Fearing a SYF scenario, I bought a 80a continuous discharge BMS (I think it’s 240a burst) and now I’m thinking that it’s overkill (and it’s huge) and heavy!

Does using the BMS for discharge help to assure consistent battery drain across the groups? Will the BMS detect the voltage of any of the cell groups and keep the voltages in spec across the pack? If not, I agree that bypassing BMS for discharge would be better, and it let’s you buy a cheaper BMS that only comes into play when charging.


#7

using the BMS for discharge generally just switches off a FET when the voltage sags to a particular level to stop you for accidentally over discharging.
the actual balancing of the cells is continuous so long as the BMS is connected. so even if you are not discharging through the BMS its still balancing them.


#8

Thanks @lowGuido

My understanding was that balancing only occurs during charging, but you’re saying that balancing happens during discharge as well, even if the BMS is bypassed during discharge. Have I got that right?


#9

the BMS is essentialy a series of bleed resistors connected to each cell via a bunch of transistors. when one cell is higher than the rest its transistor is turned on and the excess V’s bled out until it matches the rest. the charging and discharging circuits are then very basic hi and lo voltage cutoffs.
I mean you only have to look at the BMS board to see there’s nothing too smart on it.


#10

If you’re running an inline fuse it should be fine, yeah. If the VESC fails for some reason and shit starts shorting like what happened on one of the raptors i have in my garage at the moment, the pack could over discharge and set your toes on fire if you don’t at least have a fuse to kill the power in lieu of the BMS.

I’ve never seen a VESC short out the way these two did. It was so bad it blew the fuse in front of the space cell and took out the receiver and the other VESC with it just prior. Just a whole pile of dead shit in that board. Mind blown. but the space cell was fine because of the fuse.

I’m thinking about doing this though, just using a small BMS to charge and directly discharging from the pack and then using a 60Amp automotive fuse to protect the pack which is rated at 80 continuous.


#11

if this thing happened without a fuse the BMS would still feed it whatever its max current was until shit burns. so fuse is always a good idea.


#12

Awesome @longhairedboy and @lowGuido bIg thanks!

Summary: It’s better to use VESC’s LVCO, an inline fuse to between the battery and VESC, and a smaller, lower-cost BMS that is bypassed during discharge. So my next BMS will be a smaller 60A that supports 10A.

@longhairedboy do you think the built-in power switches on the Bestech BMSs will still work if we bypass the BMS for discharge? I’d think not, since the cells would be wired right to the VESC with nothing in between.


#13

no they wont work. the rely on the output FET’s to switch on and off.

thats one downside I guess… although I have never been a fan of soft FET switching…


#14

I have never used a BMS before, and I am just getting ready to build my own 18650 battery, should I be using one, what’s the difference in charging for BMS vs no BMS? You still need to balance charge it right?


#15

you should definitely use a BMS, don’t think that I’m spreading some anti BMS propaganda or something.
if you are uncertain about anything at all then use the BMS exactly as directed.

but, if you are 100% certain you know exactly what you are doing and are super careful you can get away without one.


#16

– So a bms is basically just a way of balancing the cells without using a balance charger? What do you use to charge your batteries then? Laptop power supply?


#17

I use a balance charger.
if you have a BMS then you can use a laptop style charger


#18

Ok, I get it, although I could do it without one, being able to use a simpler charger is the deal breaker for me… I’m gonna pick up this bms: http://www.batterysupports.com/36v-37v-42v-10s-60a-10x-36v-lithium-ion-lipolymer-battery-bms-p-267.html

And a laptop power supply. If I want to use a laptop power supply and I have a 10S4P battery, do I need a 36v 10A power supply? Or could I use something higher than 10A or higher/lower than 36v?


#19

yeah I think the main reason people use a BMS is for the simple 1 plug charger and power switch.
which are great features.


#20

So what do you think, this bms: http://www.batterysupports.com/36v-37v-42v-10s-60a-10x-36v-lithium-ion-lipolymer-battery-bms-p-267.html

with this charger: http://www.batterysupports.com/36v-42v-5a-lithium-ion-battery-charger-10s-10x-36v-lion-lipo-p-166.html

The only thing that bothers me about the charger is that it’s MASSIVE! Feel like I am almost better off using a balance charger.