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I'm wondering if it's possible to fit a 24VDC thermal protection fuse for a 48VDC battery system. I have a 60A solar charge controller connected to a 48V battery bank, and want to connect an inline fuse between them. By what factor should the amp rating of the 24V fuse be multiplied by in order to compensate for a doubling in voltage (24V to 48V)? Or is it not as simple as I imagine?

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    \$\begingroup\$ Current rating should be multiplied by 1. However if it's rated to break the circuit at 24VDC it may not be capable of breaking the circuit at 48V. leaving an arc where the fuse used to be, melting things and starting fire. Need I say it? I will anyway. Don't. \$\endgroup\$
    – user16324
    Commented Jan 17, 2021 at 22:37
  • \$\begingroup\$ Be sure to verify that your controller does not need battery present and connected before any S/A panel is connected! It is quite possible that it relies on battery for voltage stabilisation and with S/A power present but not battery it can be damaged. Also, fuse voltage rating should match maximum possible open circuit voltage, not battery nominal voltage. \$\endgroup\$
    – Martin
    Commented Jan 17, 2021 at 23:13
  • \$\begingroup\$ Actually, why do you want to fit a thermal fuse just between controller and battery? Which failure mode are you afraid of and want to prevent? If the controller is safety-certified, the best think to do, except you really know what are you doing, is to follow manufacturer/installation instructions exactly. (Then, if any fuse is recommended, exact specification should be followed.) \$\endgroup\$
    – Martin
    Commented Jan 17, 2021 at 23:22
  • \$\begingroup\$ I want to prevent a fire if there is a short in the controller. \$\endgroup\$
    – st4rgut
    Commented Jan 17, 2021 at 23:28

2 Answers 2

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There is always good choice for a fuse for any sort of impedance or voltage. The Charger ought to have some current protection, but this may be a good candidate in the fast-acting JLS series. enter image description here

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  • \$\begingroup\$ Littelfuse is a good brand. I think I'll go with their clamp down fuses, though ideally I'd like something resettable \$\endgroup\$
    – st4rgut
    Commented Jan 17, 2021 at 23:35
  • \$\begingroup\$ @st4rgut: If you want something resetable, then you want a circuit breaker, rather than a fuse. The circuit breaker must be rated for more than the normal circuit voltage (or more than the maximum voltage that will be across it when it is open). \$\endgroup\$ Commented Jan 18, 2021 at 0:02
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I want to prevent a fire if there is a short in the controller.

There are at least three "basic" failure modes which could under some circumstances lead to fire:

  1. Battery charging regulation in controller stops to react properly and battery gets overcharged by available solar power.
  2. Short circuit or other failure in the controller results in solar power being dissipated inside the controller as a heat. Depending on solar panel size, this can be significant amount of heat.
  3. Failure in controller leads to power from battery flowing back into the controller and significant heat is dissipated in controller, battery or both.

(There is likely few more possible failure modes, but let's stay with these.)

From these three, the suggested fuse placement would protect only against #3, and only if failure is short circuit "hard" enough to pull many times nominal current (which does not need to be the case while still dangerous amount of heat is produced). On the other hand in case of hard short circuit, the fuse needs to have interrupting current capacity higher than battery bank short-circuit current (so likely big, thick fuses as other answer recommends).

Because controller will have some diode-like reverse current blocking circuit at output, you would need failure of at least two components inside to create such short-circuit. So presumably it is not the most probable failure.

Situation #1 could be prevented by internal battery protection if present, but otherwise you are relying on controller to be safe enough. Why do you trust it here and not for #3? (Which could be prevented by internal battery overcurrent protection too by the way). For failure #2 again, you rely on proper controller design.

Do you have a specific reason to trust safety of the controller for all failures expect just short circuit at the output?

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  • \$\begingroup\$ I guess you are right. I contacted the controller manufacturer (MakeSkyBlue) who said the fuse's role is to protect the wire. I guess the controller has enough safeguards to protect against the situations you mentioned. \$\endgroup\$
    – st4rgut
    Commented Jan 19, 2021 at 18:11

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