BMS in Series for Failsafe - Possible?

BipedalPrimate · Mar 9, 2023

My naturally cautious nature leads me to ponder BMS failures because they are critical to any LiFePO4 battery build and are a single point of failure.

AFAIK Mosfet based BMS may/could fail with the Mosfets stuck in a charge or discharge state thereby leading to unpleasant outcomes.

Is this likely & what really happens when BMS fails?

If such a situation were to occur, would a second BMS (same make/model) in series with another BMS provide a level of safety/redundancy.

Both of the BMS would have their balance/cell voltage leads connected but one would have balancing disabled to avoid the balance circuits fighting each other. They would both monitor cell voltages. Should one BMS fail, the remaining BMS would provide cell voltage protection at least.

Am prepared for members to respond with 'what a stupid idea' but am throwing this out to the hive mind as a learning exercise.

fdom · Mar 9, 2023

I would not see why this should not be possible. You add another small voltage drop to your chain and you must see that the balancers do not work against each other.

However, if you have a well dimensioned with a Conservative charge profile and discharge profile I would more worry about availability than safety. Unless your cells get well out of balance there should be no reason why your BMS would need to act (if I disregard from the balance function).

Many posts about availability problems with the BMS and almost no posts regarding safety issues would support this.

rodrick · Mar 9, 2023

A spare bms an a proper fused system to me seems the most reasonable swapping out a bms only take a few minutes
just don’t use you bms as your only line of defense that’s what fuses, breakers and disconnects are for

Supervstech · Mar 10, 2023

BipedalPrimate said:
My naturally cautious nature leads me to ponder BMS failures because they are critical to any LiFePO4 battery build and are a single point of failure.

AFAIK Mosfet based BMS may/could fail with the Mosfets stuck in a charge or discharge state thereby leading to unpleasant outcomes.

Is this likely & what really happens when BMS fails?

If such a situation were to occur, would a second BMS (same make/model) in series with another BMS provide a level of safety/redundancy.

Both of the BMS would have their balance/cell voltage leads connected but one would have balancing disabled to avoid the balance circuits fighting each other. They would both monitor cell voltages. Should one BMS fail, the remaining BMS would provide cell voltage protection at least.

Am prepared for members to respond with 'what a stupid idea' but am throwing this out to the hive mind as a learning exercise.

I cannot imagine a scenario where this would be a needed feature.

The BMS really does a fine job of monitoring cells, and preventing problems at the cell level.

As long as your charging parameters are below the BMS limits, it shouldn’t be stressed unless a cell fails.

I agree, having a spare BMS to swap in is a far better plan.

BipedalPrimate · Mar 11, 2023

Thanks to all.

It seems I may be trying to solve a problem that should not happen - BMS failure allowing overcharge or overdischarge.

Agree that as long as charge & discharge parameters (in charge & discharge devices) are within cell specifications, a BMS failure should not be catastrophic.

Ampster · Mar 11, 2023

Normally the BMS would be the last line of defense but in a scenario where the FETs failed closed, the Low Voltage Disconnect and High Voltage Disconnect on the Inverter would be your protection from over discharge or over charge. BMS failure should not happen but FETs do fail closed, which is why I use a Contactor based BMS on my large pack.

BMS in Series for Failsafe - Possible?

BipedalPrimate

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fdom

LFP for marine application

rodrick

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Supervstech

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BipedalPrimate

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Ampster

Renewable Energy Hobbyist

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