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Discussion Starter · #1 ·
I am replacing my 70 ah AGM batteries with 100ah Lithium Ion and had a question about BMS boards. When a board is rated at 100 amps, is that the maximum charge/discharge rate is allows? Currently, (pun intended) I have a 400 amp controller on my motorcycle (a little overkill, admittedly).
 

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Not "allows", very few BMS have current limiting.

Maybe shuts down, as with a circuit breaker, overcurrent protection, latching style, but that is also rare.

More like "will burn if you allow" current to go higher - either your self-control manually, or your controller programming, or putting a fuse / breaker on the battery power wire.

Best practice is to purchase a BMS that can withstand 15-20% over the most you will ever draw from the pack.

Assuming the BMS is not too optimistically rated, may want to increase that %

or some rigs just use a low-current BMS

wired for voltage monitoring only, used to control the current itself with external contactors, the high amps do not flow through the BMS at all.
 

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Do you have an example of a 100 amp rated BMS that you are considering? The BMS products offered for small lithium-ion batteries (such as 4 LiFePO4 cells in series) typically pass all of the battery's current through them, and have a current limit - they shut down if current is excessive. A BMS for an EV doesn't carry the battery current, but can be used to turn off a contactor that the current goes through.
 

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If you could show or reference your hardware, it would be easier for us to help you. The continuous rating of a controller is typically 1/3-1/2 of the max rating. Your 400 A controller might be only good for 150-200 A in continuous use. This is probably a more useful capacity limit to work with.
 

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I'm with Brian - the BMS on an EV is a device to protect when charging - most do nothing when discharging

The only thing that a BMS could do is open the main contactors if one cell went too low - and THAT would be a bloody silly idea and would get the manufacturers sued for the accident when your car lost power on an overtaking maneuver

The amount of current handling capacity is all about CHARGING
 

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A good charger should handle all that HVC stuff properly.

BMS just a failsafe, preventing damage when the primary fails.

A monitor based on the cell level voltage LVC could give advanced notice to pull over before damaging your pack.

Also implementing a "limp home" mode at the most efficient Wh/mi speed.

Finally most try to also handling balancing, but not well in most cases
 

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Discussion Starter · #8 ·
That makes sense that a BMS wouldn't limit discharge. My controller is a 400 amp peak /150 nominal limit and limits discharge when voltage begins to get low and my charger prevents overcharging. I was looking at a Daly BMS such as this one.
122927
 

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This is one of the newer, cheaper lines of Chinese all-in-one BMSes. Some of their features are of course individual cell or cell group voltage monitoring, current limit(both charging and discharge) and shut-off based on cell voltage(hi and low) through mosfet strings, cell balancing(100mA max?), optional LCD screen, and Bluetooth(WiFi?) monitoring through a smart phone.The units I've seen also have some battery temp monitoring, hi and low. The Amp rating is a max figure, and like your controller, should be reduced by 2/3 - 1/2 for continuous use. All of this in a small, compact, inexpensive package. Auxiliary cooling may be needed in higher current applications

I've just started using this type of BMSes, and don't have a lot of experience with them. Has anybody else tried them out? I hope they are not in the to-good-to-be-true classification.
 

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The only thing that a BMS could do is open the main contactors if one cell went too low - and THAT would be a bloody silly idea and would get the manufacturers sued for the accident when your car lost power on an overtaking maneuver
The BMS can also tell the motor controller that the battery needs the current to be lower (due presumably to high temperature, or even very low temperature), managing rather than just brutally killing current.
 

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I've just started using this type of BMSes, and don't have a lot of experience with them. Has anybody else tried them out? I hope they are not in the to-good-to-be-true classification.
Cheaper variants seem to work fine in the 12 volt lead-acid replacement applications (in RVs) that I mentioned. I don't see why these higher voltages variants couldn't work well.
 
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