Battery Bank Fuse

[guruji]

Hi guys how much should the fuse be done in amperage with a 12v battery bank?

I have a 10amp and when trying to start the washing machine on a 1000w inverter it's blowing the battery bank fuse.

Any help please?

Thanks.

[wdyasq]

The proper application of 'Ohm's Law' will give you an answer.

"The mathematical equation that describes this relationship is:

    V = IR

where V is the potential difference between two points of interest in volts, I is the current in amperes, and R is a circuit parameter, measured in ohms (which is equivalent to volts per ampere), and is called the resistance."

Since we are lazy we go to an Ohm's law calculator site:

http://www.onlineconversion.com/ohms_law.htm

We plug in our 12V and 1000W to get amperage needed at 12V and find we need 83.3A of power.

HTH,

Ron

[frackers]

For any appliance that has a motor in it expect to see a current surge at startup of about 6 times the normal current. If the washing machine motor is rated at say 300watts (I think mine is actually over 1500 but lets stick with this example) then the peak power will be 300 * 6 = 1800 watts. Now the current to give 1800watts at 12volts is 1800 / 12 which gives 150amps. That would be the fuse rating.

This gives rise to another question though - can your inverter supply the surge that the motor startup will require. That depends entirely on its specs and of course the rating of the motor in the washing machine in the first place!!

If your inverter is really limited to 1000watts then the fuse should be spec'ed to suit it i.e. 1000 / 12 = 83.3 amps. Rounding up to the nearest likely value (since a fuse should be protecting only a fault condition) then you should be good to go with 100amp fuse.

[guruji]

THanks wdyask and Frackers if I do a 100amp fuse would it be dangerous for the system if a short happens?

Thanks again.

[Jeff]

That's the general idea of the fuse: to protest the battery bank if you get a short. If you want to use the full capacity of your 1000 watt inverter, that's what it'll take. Hopefully, you have heavy enouch guage wire going from your battery(s) to your inverter, at least 8 guage, and preferably 4. If you have smaller wire than that, you stand the chance of a fire before it blows a 100 amp fuse.

I used the fuse block out of my old electric furnace. It has two 50 amp, and one 30 amp fuse, but I'm only using the two 50 amp fuses in parallel.

Be safe!

[RUFUS]

To the code guru's out there

correct me if i'm wrong,

Overcurrent protection should be

125% of rated capicity,

assuming all of the wiring will hack it

                            Rufus

[electronbaby]

125% of rated capacity of the WIRE. The over current protection is there to protect the wire. Make sure you use a DC rated fuse.

[Jeff]

Very important point by electronbaby!

What I used was for a 220v AC furnace. Definitely NOT the ideal fusing for what we're talking about! It's the only thing I had "on-hand", couldn't afford diddly-squat, but better than nothing in a pinch. My peak draw from my inverter is 3200 watts. Divided by 12.7 volts equals 252 amps. The rated 1200watts/12.7 volts is 94 amps. That is why I did not include the 30 amp fuse in paralell with the two 50 amp ones. I can only hope I'm lucky enough these will blow before a major hazzard happens! Since these are the only fuses I have at this rating, maybe I could get away with only one 50amp fuse, but I would be without one if they blew. Anyone know what the conversion factor/difference would be? 50amp-220v fuse vs. what the same fuse would blow at on 12v?

I'm skating on thin ice here, but any input from the "guru's" would be greatly appreciated. I have thick skin, and not offended easily, and it may help the other newbie to keep from having a major home BBQ.

[ghurd]

Fuses are rated (mostly) in amps.

Pretty much, amps is amps.  I believe most fuses have enough slop to make up for voltage.

But I fuse items lower than some people.

If my inverter has 2 parallel 40A fuses, I add an inline 50A or 60A fuse.

So far, so good.

If the inline fuse is blowing, the inverter is undersized for the load.

No worries about "No user servicable parts inside.  Return to authorized service center to change a fuse".

Breakers are different animals.  I would Not use that concept with breakers.

YMMY,

G-

[RogerAS]

Isn't it proper to fuse each battery in a bank independently? Stops dead short batteries from draining entire bank?

Thought I read that here long ago.

[wpowokal]

Roger you are correct although not too many people want to know, it would cost them more.

And while we are about it fuses and switches have AC & or DC ratings because DC is harder to interupt than AC.

Ac swings between positive and negative each cycle passing through zero volts, which makes breaking an arc more achievable. DC has no zero crossing point and the arc may contiue across a gap if the fault is severe enough.

allan down under

[Flux]

This is a grey area.

There is no harm in fusing each parallel string individually but in real life it may not do any good.

You will not likely see the condition of a dead shorted parallel string, at best all you will see is one cell failure in a string and that will not blow the fuse. The rest of the cells in the sick string will just go up to gassing volts, the circulating current will not blow the fuse and you will still have the other strings drained. Regular inspection and checking is the only way to pick up the failure and the first sign of trouble may be acid spray from gassing on the good cells of the faulty string.

If you have significant lengths of cable between parallel banks all the same length to equalise the currents then individual fuses may be needed to protect the cables. With heavy bus bars then individual fusing probably doesn't make sense.

Flux

[Ungrounded Lightning Rod]

Battery wiring should be oversized anyhow:  Currents far below what would burn up the wiring will cost you a lot of power heating it up.  So a fuse sized for safety of the wiring will similarly be far above the max current of your alternator and should only blow on a short.  (It WILL blow on a short:  Lead-acid batteries have internal resistances measured in miliohms.)

Because of this a properly-sized main battery fuse will not usually blow unless something disastrous happens and it must.  (Example:  Your wrench slips while tightening the bolt on the DC in terminal on your inverter, lands across both terminals, and welds to them.)  So (as with automobiles) there's no particular reason to have it be a separate device in a socket.  Instead it can be inline in the power cabling.

Such a device is a "fusible link".  You can get them in various sizes, rated for service with lead-acid batteries in harsh environments, at your local automotive supply shop.  (Or for even harsher environments - with a correspondingly harsher price tag - at a marine supply.)

[guruji]

So would it be a good idea that one makes a setup of two fuses.Changing from one to the other depending on what load  one is going to use for good safety?

Thanks guys for sharing.

[Ungrounded Lightning Rod]

No I wouldn't be switching fuses according to load - or at all.

Instead I'd do it this way:

 - Fusible link at the battery end of the hot wire, sized to go if the battery wiring gets shorted.  (One at each bank if multiple paralleled banks are paralleled at a junction block.)  This is more the fuse at the pole transformer (which SOMETIMES keeps it from exploding when overloaded), rather than the mains fuse of a house drop (which doubles as a cutoff for the house).

 - Hook the inverter directly to the junction block (with wiring big enough that IT won't catch fire if pulling the sum of the fusible-link ratings on the paralleled banks) and depend on the inverter's internal protection to cover it (unless the inverter manufacturer calls for an external fuse.)  If the inverter requires a fuse or the wires to it or its terminals can't handle the fusible link ratings, put the appropriate protection at the junction block end.

 - The rest of the minor load circuits get a bunch of fuses in a distribution panel, just like the fuses or circut breakers for the distinct circuits in a mains-fed house.  Again they're sized to prevent fires from overheated wiring due to shorts or overloading, more than to protect the load devices themselves (which normally have their own fuses or other protection if they have an issue with hazardous overloading on failure.)  And again a fusible link near the junction block end if the wiring from the junction block to the distribution panel isn't up to the sum of the batteries' fusible link ratings.

[Ungrounded Lightning Rod]

Or at lesat that's my thoughts.  But before doing it I'd check the DC part of the electrical code to see what it requires and do it in a way that is within the code requirements.