BMS - On/off switch using thermistor connection?

[dilkes]

I am replacing the BMS on a 13s lithium pack. The replacement BMS does not have an on/off connection for a switch but does have a temperature sensor (thermistor). I measured the thermistor resistance at room temp at about 1200 ohms. I heated it up with my heat gun and saw the resistance dropped to zero. I assume the BMS logic detects this and would shut down to protect from overheating? So my question is - Could I replace the thermistor with an on/off switch such that when the switch is open (nearly infinite resistance), the BMS would see everything as OK and when closed (zero resistance) the BMS would detect "overheating" and shut down, this providing on/off capability.

I don't know a lot about this, but hopefully someone can advise me on whether this should work.

 

[harrisonpatm]

I am replacing the BMS on a 13s lithium pack. The replacement BMS does not have an on/off connection for a switch but does have a temperature sensor (thermistor). I measured the thermistor resistance at room temp at about 1200 ohms. I heated it up with my heat gun and saw the resistance dropped to zero. I assume the BMS logic detects this and would shut down to protect from overheating? So my question is - Could I replace the thermistor with an on/off switch such that when the switch is open (nearly infinite resistance), the BMS would see everything as OK and when closed (zero resistance) the BMS would detect "overheating" and shut down, this providing on/off capability.

I don't know a lot about this, but hopefully someone can advise me on whether this should work.

If you're trying to add an on/off connection, the simplest would be to splice in an appropriately-rated switch on the P+.

 

[dilkes]

Thanks for the suggestion. I assume that switch would need to be capable of carrying the full discharge amps load? 20-25 amps?

 

[harrisonpatm]

Thanks for the suggestion. I assume that switch would need to be capable of carrying the full discharge amps load? 20-25 amps?

Plus 20%? So you'll be thinking, that's a big switch, I don't have space, I'd rather use the thermistor. I should extrapolate from my earlier comment. It's just that while it's technically supposed to kill battery power in the event of overheat, it's not something you can totally rely on. You didn't make the BMS, so you don't actually know for sure the mechanism to cut power. Whereas if you add the switch, a very basic and simple element, you can be more sure that it'll function the way you want it to.

 

[amberwolf]

I'd second the master switch on the main P+, vs using the BMS to cut power. If the BMS FETs fail for any reason, it's typically a silent failure, meaning they are stuck "on", and can't shut off, so you don't even know this has happened...unless you have a situation that requires you to use the BMS to cut power and it doesn't.... At that point, your next option is to unplug the battery from the system to shut it off....

If you have a master switch, then it doesn't require unplugging, just turn the switch off.

As long as you are using the switch *only* when not under load, you can use one that is not rated for the full votlage of the system (as long as it is rated for higher current than you will ever draw). I use a cheapo automotive battery disconnect switch on SB Cruiser for this purpose--it's only rated for 12-24v (which, being automotive, means 13-30v, effectively), and the SBC is a 52v nominal (57v max) system, so it's not safe to switch off under load (while riding), as it might arc and weld the contacts closed and be unable to turn off, or continue to arc and heat and cause a fire. But it's safe enough to switch on or off when not under any load (parked).

BlueSea has been said to make better switches rated for higher voltages, if your budget permits.
https://www.bluesea.com/products/category/11/Manual_Battery_Switches
though I have not used them

The cheapo type switch is probably not much more than ten bucks, or less. Examples:
https://www.google.com/search?q=ebay+battery+cutoff+switch


That said....if your "thermistor" is actually one of these (either metal or ceramic):

Then it is not a temperature sensor, but rather a thermal switch that resets itself when cooled enough. These are either high resistance or open circuit when below the labelled cutoff temperature, but above that they trip and short together, or they are normally shorted and open when above cutoff temp. (easy to tell which if not labelled as NC or NO, by measuring).

The topic has come up before, so there's some more detailed explanations here in various threads about BMS switches.

 

[batteryGOLD]

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Could I replace the thermistor with an on/off switch such that when the switch is open (nearly infinite resistance), the BMS would , providing on/off capability.

Hi, why don't you test it? just solder some wires to BMS and switch.. than go testing battery, while riding veicule or at ebike ON state, just change switch position to get test results..

I've seen many bms with thermal switch pins available, but not connected to anything. Soo I think connecting a switch there, if switch is open, BMS stays ON, if the switch is close, than BMS maybe goes shutdown state. just sayin..

PS: the switch is simple for control only, not a main power switch! but You can use any kind of switch as long as it opens and closes :thumb:

The best way to know, is to test by yourself

 

[fechter]

If you put a switch across the thermostat, you could turn the BMS off, but some of them have a hard time turning back on after that. You typically need to disconnect the controller to get it to reset and turn on again. A 1k resistor from B- to P- might make it work if your controller doesn't draw any current when off.