Safe to relly on LifePO4's BMS to protect the wire from battery to switch?

[Trimaran Ken]

Originally Posted by CarlF

But a bigger problem is that your new lithium battery may not be able to support the loads. Check the battery specs for the maximum continuous discharge (which is usually less than when the BMS cuts out). Routinely exceeding that will void your battery warranty and damage them.

In my experience, using a built in BMS, the BMS cuts off the power just fine when needed to protect the battery.


I rely on the BMS for protections listed in the BMS specs. Yes a BMS can fail, but so can any other component (like a circuit breaker). External BMSs are another story. There you can introduce user error, and incompatibilities.

[Sailingcal21]

All good discussions, but I'm having a hard time reconciling the difference between a BMS protecting the LFP, but not the wiring. Since the LFP is the source of energy, and the BMS protects the LFP by disconnecting its output, doesn't that also protect the wiring?

[mitiempo]

Originally Posted by Sailingcal21

All good discussions, but I'm having a hard time reconciling the difference between a BMS protecting the LFP, but not the wiring. Since the LFP is the source of energy, and the BMS protects the LFP by disconnecting its output, doesn't that also protect the wiring?

A BMS protects a battery from over and under voltage as well as over and under temperature. It does not prevent current output nor I think would it survive a dead short as the current would have to pass through it. Even if it did survive I doubt it would prevent the massive current that would flow.

I agree with others that so called "drop-in" lithium batteries have no place on a boat. The BMS has to be accessible and have the ability to control loads and charge sources directly or through relays rather than just closing a weak door as a built-in BMS effectively does.

[Sailingcal21]

Originally Posted by mitiempo

A BMS protects a battery from over and under voltage as well as over and under temperature. It does not prevent current output nor I think would it survive a dead short as the current would have to pass through it. Even if it did survive I doubt it would prevent the massive current that would flow.

I agree with others that so called "drop-in" lithium batteries have no place on a boat. The BMS has to be accessible and have the ability to control loads and charge sources directly or through relays rather than just closing a weak door as a built-in BMS effectively does.

My experience is that BMS's do have the ability to disconnect outputs during periods of high current both charging and discharging. The OP indicated that his LFP is current limited by the BMS to 200A and then clarifies that it's an Ampere Time 300AH with a BMS that is limited to 200A, so you are incorrect in your statement that BMS's do not limit current.

I do agree that the term "drop-in" is misleading and can lead to improper applications, however when properly engineered into a system can be an appropriate alternative.

[mitiempo]

Originally Posted by Sailingcal21

My experience is that BMS's do have the ability to disconnect outputs during periods of high current both charging and discharging. The OP indicated that his LFP is current limited by the BMS to 200A and then clarifies that it's an Ampere Time 300AH with a BMS that is limited to 200A, so you are incorrect in your statement that BMS's do not limit current.

The class T fuse that should be placed just after the positive battery post is not there for normal use, even if a bit more than the BMS can handle. It is there to prevent a catastrophic event. Drop a wrench across the positive and negative posts and see how the BMS handles the thousands of instantaneous amps flowing. I doubt it would survive. Yes a BMS will limit current - until it fries.

As an aside my personal belief is that the BMS in a large system - as typically used for a house bank on a cruising boat - should not have to handle any high current as most BMS do.

[Sailingcal21]

The BMS does not limit current, it disconnects the cells during periods of high discharge or charge current.

I’m not advocating not using appropriate protective devices, but the OP has raised a good question.

[ItDepends]

There is a lot to look at here... A BMS might be well designed to disconnect the battery from the system when the current flow exceeds the rated capacity by 10% or even 50%. BUT...

A dead short across the battery can generate currents of 1000, 5000, or even 10,000 amps. What do you think will happen to your BMS (even if it is relay based!) if it tries to interrupt a current of 5000 Amps? If it is a relay, odds are it will weld itself closed and NOT interrupt at all. If it is a MOSFET in a "drop in" battery it might just explode...

Your BMS is NOT designed as a VERY high amperage interrupt device that would be needed in the event of a catastrophic failure. That's what a proper fuse does.

So, NO a BMS does NOT replace a Class T fuse in a Li battery bank.

[NPCampbell]

Originally Posted by ItDepends

There is a lot to look at here... A BMS might be well designed to disconnect the battery from the system when the current flow exceeds the rated capacity by 10% or even 50%. BUT...

A dead short across the battery can generate currents of 1000, 5000, or even 10,000 amps. What do you think will happen to your BMS (even if it is relay based!) if it tries to interrupt a current of 5000 Amps? If it is a relay, odds are it will weld itself closed and NOT interrupt at all. If it is a MOSFET in a "drop in" battery it might just explode...

Your BMS is NOT designed as a VERY high amperage interrupt device that would be needed in the event of a catastrophic failure. That's what a proper fuse does.

So, NO a BMS does NOT replace a Class T fuse in a Li battery bank.

The internal resistance of the battery is in series with the load. This caps the max current that the battery can provide. If the BMS is designed to handle this level of current for the minimum internal resistance then there should never be an issue with the BMS.

That being said, I would never rely on the BMS to provide protection. I'd use explicit protection with a known current limit for my down stream devices. If a battery charger was in parallel with the battery and the charger malfunctioned the BMS wouldn't do anything to protect my downstream gear.

[mitiempo]

Originally Posted by NPCampbell

The internal resistance of the battery is in series with the load. This caps the max current that the battery can provide. If the BMS is designed to handle this level of current for the minimum internal resistance then there should never be an issue with the BMS.

That being said, I would never rely on the BMS to provide protection. I'd use explicit protection with a known current limit for my down stream devices. If a battery charger was in parallel with the battery and the charger malfunctioned the BMS wouldn't do anything to protect my downstream gear.

As for the first paragraph, the exception is a short with thousands on amps flowing through the BMS.

As for the second paragraph some BMS (including the one I use) would shut off the charger based on cell voltage - eg first cell to hit the high point previously set.

All charge sources should be set to safe voltages for the battery bank of course. The BMS is a last resort but a good external BMS properly programmed is essential in my opinion.

[goboatingnow]

Originally Posted by ItDepends

There is a lot to look at here... A BMS might be well designed to disconnect the battery from the system when the current flow exceeds the rated capacity by 10% or even 50%. BUT...

A dead short across the battery can generate currents of 1000, 5000, or even 10,000 amps. What do you think will happen to your BMS (even if it is relay based!) if it tries to interrupt a current of 5000 Amps? If it is a relay, odds are it will weld itself closed and NOT interrupt at all. If it is a MOSFET in a "drop in" battery it might just explode...

Your BMS is NOT designed as a VERY high amperage interrupt device that would be needed in the event of a catastrophic failure. That's what a proper fuse does.

So, NO a BMS does NOT replace a Class T fuse in a Li battery bank.

Given the electronics will react way way faster then a fuse the fuse should never blow

Equally a fuse in the lead won’t protect a spanner across the terminals either

Fuses protect wires not devices. The BMS absolutely should handle the battery being shorted. That’s the whole point. The detection circuitry will react before the short circuit current peaks anyway. Mosfets can easily handle huge fast surges.

Again a fuse protects a wire, protection devices protect “ devices “

[NPCampbell]

Originally Posted by mitiempo

As for the first paragraph, the exception is a short with thousands on amps flowing through the BMS.

You will never get a short. You can only get thousands of amps (2000+) if the load on a 12V battery is less than 6 milliohms. This won't happen even if you drop a wrench against the terminals because the internal resistance of the battery will be higher than 6 milliohms. For instance, take a high current battery like Relion's HP series. The internal resistance will be around 30 milliohms at low SOC and 10-15 milliohms at around 90% SOC. The BMS should be designed to handle the current at the lowest internal resistance of the battery.

Originally Posted by mitiempo

As for the second paragraph some BMS (including the one I use) would shut off the charger based on cell voltage - eg first cell to hit the high point previously set.

All charge sources should be set to safe voltages for the battery bank of course. The BMS is a last resort but a good external BMS properly programmed is essential in my opinion.

A BMS is essential but it is designed to protect the battery.

The OP is relying on a "drop-in" Ampere Time battery. His battery has no comm port so it provides zero downstream protection. If an alternator, generator, battery charger or any other source generated over-voltages, the fuse/breaker would prevent against excessive current, the BMS wouldn't.

[mitiempo]

Originally Posted by NPCampbell

You will never get a short. You can only get thousands of amps (2000+) if the load on a 12V battery is less than 6 milliohms. This won't happen even if you drop a wrench against the terminals because the internal resistance of the battery will be higher than 6 milliohms. For instance, take a high current battery like Relion's HP series. The internal resistance will be around 30 milliohms at low SOC and 10-15 milliohms at around 90% SOC. The BMS should be designed to handle the current at the lowest internal resistance of the battery.


A BMS is essential but it is designed to protect the battery.

The OP is relying on a "drop-in" Ampere Time battery. His battery has no comm port so it provides zero downstream protection. If an alternator, generator, battery charger or any other source generated over-voltages, the fuse/breaker would prevent against excessive current, the BMS wouldn't.

I'm using Calb C180 cells. They have an internal resistance of less than .6 milli-ohms. 180 amp cells with a constant discharge of 360 amps (2C). Short term much more.

A BMS on a so-called drop-in battery is as you state designed only to protect the battery.

A BMS on a proper system - not with "drop-in" batteries - is designed for the entire system as mine is. High current does not pass through the BMS.

The BMS of a "drop-in" battery is inadequate for safety on a boat and therefore they should not be used on boats.

[hjohnson]

Originally Posted by NPCampbell

The internal resistance of the battery is in series with the load. This caps the max current that the battery can provide. If the BMS is designed to handle this level of current for the minimum internal resistance then there should never be an issue with the BMS.

That being said, I would never rely on the BMS to provide protection. I'd use explicit protection with a known current limit for my down stream devices. If a battery charger was in parallel with the battery and the charger malfunctioned the BMS wouldn't do anything to protect my downstream gear.

The internal resistance of a LiFePO4 cell is pretty close to nil. You drop a wrench across it, it will (very) briefly run current into the 5 figure range. With FET based BMSs, what you're banking on is the resistance of the FETs, and the ability of them not to melt before the BMS actually disconnects things.

Always put a Class T within 6" of the positive terminal. It's cheap insurance.

[Trimaran Ken]

Originally Posted by mitiempo

A BMS does not prevent current output nor I think would it survive a dead short as the current would have to pass through it. Even if it did survive I doubt it would prevent the massive current that would flow.

You are completely wrong! I have undersized LFP batteries in my golf cart and can easily get the BMS to shut down the battery by demanding more current than the BMS will allow on a continuous basis.


My BMS has has a maximum current which it will allow for 3 seconds and a continuous current maximum after that.

[Trimaran Ken]

Originally Posted by Sailingcal21

All good discussions, but I'm having a hard time reconciling the difference between a BMS protecting the LFP, but not the wiring. Since the LFP is the source of energy, and the BMS protects the LFP by disconnecting its output, doesn't that also protect the wiring?

You are completely right. Wire doesn't need protecting when there is no current, and the BMS will stop the current from getting too high. You just have to make sure that the wire has the capacity to take the maximum amount the BMS is willing to give.