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SJ-Chris's avatar
SJ-Chris
Explorer II
Mar 08, 2021

Battery short circuit protection?

I've been working on a solar system and 2000w inverter install. Lots of wiring. Lots of heavy duty cables (inverter to batteries). I've also got a battery monitor set up which uses heavy duty cabling. It got me to thinking....What is the potential danger of shorting the 12v battery(s) and how can it be prevented. In the back of my mind, I'm always thinking about the potential bad scenario of accidentally touching Pos to Neg (or someone else accidentally doing this somehow) and creating a disasterous short circuit. I try to be extra careful. So far, no issues.

It made me wonder if I should install a large fuse at the battery terminal for each of my 2-3 coach batteries. At least this way, if me (or someone else) accidentally shorted across the battery at the inverter or anywhere else there would be some protection.

So I was trying to think about how large of a fuse I would want/need. During a 12v lead acid battery short. Does anyone know approximately how many amps are flowing in that sort of scenario?

On one of my RVs I have 3 batteries (installing the 3rd now). I was thinking about installing something like this on each battery:
https://www.amazon.com/TopstrongGear-Solid-Brass-300Amp-Holder/dp/B07XRLN8JY/ref=sr_1_1_sspa?dchild=1&keywords=battery+terminal+breaker+400+amp&qid=1615193884&sr=8-1-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUExU09CVjRBNDJUVjJCJmVuY3J5cHRlZElkPUEwMTM1NzA4M1cyNzZFRkxKRlVUQyZlbmNyeXB0ZWRBZElkPUEwODgzMDI4MlU1RjRLNlIzVlBMNSZ3aWRnZXROYW1lPXNwX2F0ZiZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=

My RV (2015 Thor Magestic 28a) does have an Emergency Start switch (in case the chassis battery dies/etc) so it is possible that the coach batteries would be used in that scenario to start the engine. Does anyone have any idea how many cranking amps in general the v10 Ford e450 uses?

What are your thoughts? I try to be overprotected and put fuses anywhere that has the potential to be dangerous. I'm always on the lookout for ways to avoid potential problems.

Let me know your thoughts.

Happy (and safe) camping!
Chris
  • pianotuna's avatar
    pianotuna
    Nomad III
    Mar 09, 2021
    I do have catastrophic failure fuses on the positive post of each of my twin battery banks. I have had one of them blow. Mine are 350 amps each.
  • CA_Traveler's avatar
    CA_Traveler
    Explorer III
    Mar 08, 2021
    No one should be in a position to short the main cables because the negative battery terminal should be disconnected first - Standard safety measure. But accidents happen.

    My 2Kw inverter has a 200A fuse. I have a 300A house disconnect switch on the house batteries and 3000A for the chassis batteries.
  • DrewE's avatar
    DrewE
    Explorer II
    Mar 08, 2021
    x2 or whatever on the fuse size being determined by the wires you're using it to protect. A lead acid battery can produce a very high short circuit current, likely hundreds or thousands of amps.

    Peak current when using the emergency start could be well over a hundred amps, if the wiring between the batteries is sufficiently low impedance. The current required for starting a generator is also somewhere in the vicinity of a hundred amps, very very roughly speaking. I think the main battery fuse in my class C might possibly be 125A, but my memory could also be wrong and it's not convenient to run out and check at the moment. Note that a fuse, since it responds to its insides heating up, can withstand a current above its rated current for a limited amount of time without blowing. How long that is depends on how much current is flowing, perhaps among other factors. Starting an engine is, or at least should be, a relatively brief draw.
  • time2roll's avatar
    time2roll
    Nomad
    Mar 08, 2021
    Yes a fuse is good. Although if the wire is fairly short, neat and protected I would skip it. The engine start system has no fuse.

    Shunt will probably open like a fuse in a real disaster. (don't ask how I know ;) )
  • Old-Biscuit's avatar
    Old-Biscuit
    Explorer III
    Mar 08, 2021
    POS Cable from battery bank to the RV should already have a DC Fuse or DC Circuit Breaker close to the batteries

    That will trip if battery AMPs exceed the fuse/CB rating

    Individual on each battery not necessary
  • 2oldman's avatar
    2oldman
    Explorer II
    Mar 08, 2021
    Shorter link so all your post can be read without scrolling.

    Not sure someone would hook up a battery using cables as a dead short, but dropping a wrench can be a disaster.
  • ndrorder's avatar
    ndrorder
    Explorer
    Mar 08, 2021
    Fuses are selected based on the wire size and expected loads, whichever is less. Any short circuit will result in higher amperage through the cables and will blow the fuse.

    For example, you mentioned a 2000W inverter. However, most inverters will tolerate some level of surge. If 2000W is the surge capacity, divide by 12V and get 167A. So a 175A or 200A fuse would be about right. If there are more loads on the cables, add that amperage to the 167A and find the next higher fuse.

    Now take the fuse size along with the cable run distance to one of the many online cable size calculators to determine the appropriate cable size.

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