Battery short circuit protection?

[SJ-Chris]

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

[SJ-Chris]

Chris,

Unless I am misreading your link, max wire size for those fuses is 1/0.

That certainly would not handle high amp start loads or high amp inverter use if that is what you contemplate.

Regarding wiring gauge for "Emergency Start" situations (jumpstarting the engine with the coach battery(s) when the chassis battery is dead):

When I google "what is standard car battery cable gauge" the answer that comes back is 4 AWG wire.

I do find this interesting because when you think about the engine starting and the Cold Cranking Amps for example being in the 500-1000amp range on most batteries. It seems like 4 AWG cables (in the engine compartment connecting to the chassis battery for startup) aren't thick enough. So it must be that the DURATION of the draw is very low such that the wires can handle it, right?

If I look at my 2015 Thor Majestic 28a, and I look at the cable that goes from the battery isolator solenoid in the engine compartment back to the house batteries (so that the chassis alternator can charge the batteries while driving), I see that it looks like it is a standard looking battery cable (probably ~4 AWG...not really sure, but it doesn't look particularly thick). This would be fine for the alternator charging the house batteries since that should probably max out at about 75amps (maybe 100amps at the most). BUT...most Class C RVs also use this battery isolator solenoid with an "Emergency Start" switch that allows you to temporarily connect the chassis battery to the house batteries in the event your chassis battery is dead and you need to jump start the engine with the house batteries. In that case, there could be 500-1000 Cranking amps coming from the house battery(s) to the engine starter during engine startup. If that wire wasn't large enough to handle that, I don't think Thor (or any other Class C manufacturer) would use that thickness of wire. So it must be okay. Again, it must have to do with the short duration of that heavy draw.

What do you think? (I'm no expert, but I'm expecting the RV manufacturers to be smart enough to get it right).

Regarding wiring gauge for inverter use:
I have 3 batteries in my battery bank. So the power draw from each will be roughly 1/3 of the total. They are hooked up "Method 3" style (although with varying lengths which was pretty much unavoidable). From the point where all the positive (and negative) cables come together, there are 2 parallel sets of cables that are 2/0 in size running to my inverter. So let's assume my inverter is drawing 100amps from the 12v batteries. Each battery will be supplying roughly 33amps (through a size 2/0 cable) to a common point, and then 100amps will be flowing to the inverter through two 2/0 cables (2 for positive and 2 for negative...so roughly 50amps per wire).

According to a voltage drop calculator, the 33amps running 10' on a 2/0 cable will have a .4% drop (about 0.05v of 12.6v) which is very good I think. The 50amps running ~8' through each of the wires to the inverter will have a drop of ~0.56% (about 0.07v of 12.6v). So my total voltage drop during the inverter drawing 100amps will only be ~0.12v. That's good right? I felt like doubling up on the wiring from my inverter to the batteries was being pretty conservative and would be helpful.

Let me know your thoughts.

-Chris

[wolfe10]

Chris,

Unless I am misreading your link, max wire size for those fuses is 1/0.

That certainly would not handle high amp start loads or high amp inverter use if that is what you contemplate.

[SJ-Chris]

I've got 3 coach batteries. Do you see anything wrong with something like these (but at 350 amps each):

https://www.amazon.com/dp/B082NK8QVF/ref=sspa_dk_detail_1?psc=1&pd_rd_i=B082NK8QVF&pd_rd_w=81Z5o&pf_rd_p=4269e1a0-a218-4fbd-9748-1cd337d2f2a5&pd_rd_wg=FPLSN&pf_rd_r=RTZPRBVZR311ZN532255&pd_rd_r=a3a69d18-d068-4c4c-94ef-f2da5f9c8f13&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEyWTlJQkVUODY4MzA0JmVuY3J5cHRlZElkPUEwNzQ0ODM3MUxTVFJVWEowQUZZRSZlbmNyeXB0ZWRBZElkPUEwNTA1OTk4REo3QUpEV1lKMlBKJndpZGdldE5hbWU9c3BfZGV0YWlsJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==

I figure with 350 amps each, if I need to use the Emergency Start switch to jump start the engine (if the chassis battery is dead), the draw will come roughly (except for some imbalance in wire lengths) 1/3 from each of my coach batteries. 350x3 = 1050 cranking amps....should be more than enough I'm hoping. I'll disconnect my chassis battery and try starting the engine and see if any of them blow and report back.

-Chris

[ktmrfs]

for fusing a battery you need to consider two things
1) max draw that you want to allow (when the fuse will blow or breaker trip)
2) max circuit interupt the fuse/breaker can handle. you want a fuse that will blow at say 200A but will not sustain the 1000A or so the battery can supply via a continuous arc across the open fuse.

Fuses that can handle that type of load are usually larger than the typical fuse and sand filled to extinquish the arc. And some have fuseholder that can be mounted directly to battery terminals.

[CA_Traveler]

Typo 950CCA above. Interesting that I can't edit or quote on this thread - another adventure...

[CA_Traveler]

Kinda mistated - Ratings on the switch label: 32VDC, 2,750A @ 10 Sec, 1750A @ 30 Sec, 1,325A @ 1 Min.

2x chassis batteries rated 950miCA each.

[afidel]

pianotuna wrote:

CA Traveler wrote:
My 2Kw inverter has a 200A fuse. I have a 300A house disconnect switch on the house batteries and 3000A for the chassis batteries.

3000 amps--is this a typo??

Probably not, gas truck batteries can be >1,200 cold cranking amps, I imagine the battery to turn over a big diesel pusher is significantly larger.

[afidel]

You might also want to get slow blow/time delay fuses, that way if there's a temporary spike during starting you don't keep trashing fuses.

[SJ-Chris]

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.

Thanks Pianotuna for letting me know I'm not alone in my thinking! I think I will put a 300 amp fuse on each of my 3 coach batteries.

Then, as a test, I'll disconnect my chassis battery and use my Emergency Switch (to start the engine with my coach batteries) and verify that none of them blow. If the draw from each battery is equal, then I'm guessing the cranking amps they would see would be probably in the 500-800 total (~200-280 amp) range. Hmmm...maybe I'll look for 350 amp fuses just to give myself a little extra margin.

[pianotuna]

CA Traveler wrote:
My 2Kw inverter has a 200A fuse. I have a 300A house disconnect switch on the house batteries and 3000A for the chassis batteries.

3000 amps--is this a typo??

[pianotuna]

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]

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]

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]

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 😉 )