SOLVED:Fuse monitoring lines beefy stack of batteries? SMT fuse interrupt rating - Page 1
Forum section: electronics / projects

author: incf
author_rank: Frequent Contributor
timestamp: January 31, 2025, 04:42:20 pm
content: I've got a space constrained commercial application that involves balancing a low voltage (<48V) series stack of 12V batteries.The batteries theoretically could put out several hundred amps if something went wrong.Each series battery has a line coming from it for balancing. I would like to protect from short circuits between batteries and to ground with some sort of fuse. However, space constraints make full size fuses with >500A interrupt rating impractical.I don't want to depend on copper traces vaporizing for fusing.Surface mount fuses are attractive, but they are typically rated to interrupt less than 50 amps. Which I think the batteries can easily exceed (at least temporarily) even on a thin copper trace.I am limited to 10 ohms of series resistance. As far as I can tell, adding a regular <=10 ohm series SMT resistor doesn't help, since their behavior is undefined when subject to ~30 volts (hundreds of watts - if failed short it could easily just burn a hole through to the ground plane).Maybe use a fusible SMT resistor in series with a SMT fuse to try to get below the interrupt rating? But even fusible SMT resistors seem quite disappointing when reading their specs, not entirely confident that they would "behave" under the circumstances (but maybe my intuition is wrong).

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 31, 2025, 04:53:46 pm
content: Polyfuses.

author: incf
author_rank: Frequent Contributor
timestamp: January 31, 2025, 05:01:17 pm
content: I've seen at least one model of polyfuse fail short when subject to a high current short circuit. (it caused some rather catastrophic problems on a system I was involved with)If I understand correctly, most polyfuses are glorified PTC thermistors. (Although a few are FETs, maybe those might be different)Is there a particular polyfuse construction that is reliable when potentially subjected to >500 amps?I read their datasheets, and they paint a rosy picture of an idealized device, but the memory of a smoldering mass from one prior project makes me hesitant to believe. I simply don't know enough about their internal construction.

author: bdunham7
author_rank: Super Contributor
timestamp: January 31, 2025, 05:07:15 pm
content: What is the maximum current in the balancing line during normal operation? I'm presuming that you accept that the fuse is for fire protection and if it blows, the unit is deemed unrepairable, at least in the field?

author: incf
author_rank: Frequent Contributor
timestamp: January 31, 2025, 05:10:13 pm
content: Quote from: bdunham7 on January 31, 2025, 05:07:15 pmLess than 35mA under all conditions. There is impedance sensitive analog circuitry that I don't want to disturb with too large of a series resistor.

author: Gyro
author_rank: Super Contributor
timestamp: January 31, 2025, 05:18:58 pm
content: Surely the fuses should be situated at the individual battery terminals. Otherwise you have no protection against damage to the (presumably low current) lines going to your PCB. Those sound like a location where you have more space.

author: incf
author_rank: Frequent Contributor
timestamp: January 31, 2025, 05:31:06 pm
content: Quote from: Gyro on January 31, 2025, 05:18:58 pmThat does make sense in retrospect.The length of the leads are short, and they are well protected. Perhaps we could jam a bunch of full size inline fuses into that space.We were intent on using the PCB as a "tie point" between series cells just for the sake of having a simple wiring harness. (common -> input_connector -> battery -> input_connector -> battery -> input_connector -> ... -> pcb fuse -> output connector)We only pass a few amps normally.

author: Gyro
author_rank: Super Contributor
timestamp: January 31, 2025, 05:36:46 pm
content: It is certainly the safest way. You can get in-line wire connectors for automotive blade fuses - I had assumed that these were low breaking capacity but I was looking at a Littlefuse datasheet for another thread the other day and, to my surprise, they were rated for 1kA @58V DC. Check for different body sizes though.

author: ConKbot
author_rank: Super Contributor
timestamp: January 31, 2025, 05:40:53 pm
content: The Littelfuse 448, 157, and 451 series all have higher interrupt ratings, with most having 300A@32 VDC ratings. More usable, but still not enough to handle some low impedance batteries or supercaps. Since you can handle 10 ohms it resistance, a resistor to limit the fault current to sane (but high enough to quickly clear the fuse) levels and the fuse to keep the resistor from bursting into flames may be the ticket?

author: incf
author_rank: Frequent Contributor
timestamp: January 31, 2025, 06:12:36 pm
content: Quote from: ConKbot on January 31, 2025, 05:40:53 pmI don't think the resistors would survive in a predicable fashion. I'd worry that once it burns up, it won't limit current anymore and might damage the PCB.I am marking thread as solved - we will go with inline fuses at each battery terminal to sidestep all of these issues.