Batteries in vehicle cabin

[404pug Avid PP Poster! Find all posts View Profile Close 404pug]

All of the Aussie Prados have the 2 tanks so no space there. Get an AGM battery and it wont be an issue..

[Guest Close Guest]

I'm wondering if I have chosen the correct fuse (and this might be a silly question for the electrical gurus).

I am using a 50A auto-reset fuse at the starter and at the aux batteries. Both batteries are connected by ~3m of twin sheathed 6B&S via 50A Anderson plugs. When the VSR is connecting both batteries while driving, only the fridge will be running (50L National Luna). Will the current be greater than 50A?

If the aux is depleted to say less than 50% SOC and I run the engine at idle with no electrical items (headlights, stereo, etc) being used but the fridge is connected and possibly strip lighting on the awning (night time scenario), will the current draw be greater than 50A?

Will the fuse be continuously tripping?

[AJ120 Out of control poster! Find all posts View Profile Close AJ120]

I have been running 50amp fuses at both end of mine for years and never blown a fuse.

Cheers Andrew

[mjrandom Out of control poster! Find all posts View Profile Close mjrandom]

I think 50a should be ok. What does the specification sheet on the VSR say it pulls?

[Guest Close Guest]

Thanks guys. The dual sense VSR is rated to 120A.

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

I have been doing a lot of fuse testing lately, general the specs show 200% current for around 20 secs to rupture the fuse, ie a 50A fuse should be able to handle 50A for around 20 secs.

I was surprised to find that out of the 30 fuses tested only 1 tripped in the specified time, the others all carried 200% for over 3 minutes at which time I terminated the test. This poses two issues, the fuses didn't trip as per the design spec and secondly at those currents the fuses get very hot ie hot enough to melt the fuse holder. The fuses where sourced from several different manufactures but mostly made in China.

The only fuse that ruptured as per its specs was a couple of units made
in Israel of all places.

So bottom line is you may not be as well protected as you thought.

As for the 50A fuses, they should be able to handle 100A for 20 seconds or there abouts, a typical lead acid battery discharged to 50% SOC will pull around 65 amps for a few seconds through 13mm2 cable within a minute that will be down to around 47 amps.

Cheers
Leigh

[Guest Close Guest]

That's a bit of a worry.

Is there a chart that gives an indication between SOC vs current draw? Is it dependent on the Ah of the battery?

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

But the above may help, it gives a rough idea as to the SOC of a battery
that has been off charge for several hours and has a small load applied.

This chart is found easily enough on the web.

[Guest Close Guest]

OK, I was thinking of something like this with an extra column for current draw. Below is just something I made up. It is not factual.

SOC	Battery V	Current draw
100%	12.70	0.00
90%	12.50	14.55
80%	12.42	20.36
70%	12.32	27.64
60%	12.20	36.36
50%	12.06	46.55
40%	11.90	58.18
30%	11.75	69.09
20%	11.58	81.45
10%	11.31	101.09
0%	10.50	160.00

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

The problem with current is the faster you draw from the battery the lower
the real capacity will be.

[AJ120 Out of control poster! Find all posts View Profile Close AJ120]

Thats interesting Leigh, I had a fuse holder melt on my Compressor last year on our trip, fortunately it was on the very last air up on a 6 week 11000km trip so I just direct wired it for that one time. I now have the same 30 amp fuse size but in a Maxi fuse holder instead of the standard size.

Seems like we should be very conservative with the fuse sizes we choose.

Personally I buy Narva fuses, no idea where they are manufactured though. Did you do any testing with that brand?

Cheers Andrew

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

Personally I buy Narva fuses, no idea where they are manufactured though. Did you do any testing with that brand?

Cheers Andrew

No I was testing high precision glass tube and chip fuses, bit of a worry
when they don't meet the quoted compliance standards.

Car fuses, that's another can of worms altogether, I certainly wouldn't class
them as high precision and it will get worse depending on where they are manufactured.

I would assume unless stated otherwise they are all made in China these days, some are good quality and some are pretty doubtful, I have seen 5A/7.5A/10A fuse with what looks like the same fusing element in them all, I have also seen 5A fuses with what appears to be a heavier fusing element that I doubt would blow at 30 amps, due to the very poor stamping quality.

I would treat any automotive fuse rating as a very ball park figure these days, the most worrying aspect is most think if you have a 10A fuse and it doesn't blow all is well. A poorly manufactured fuse is not good, if a 10A fuse for instance doesn't blow at 20A one thing you can fairly sure about is it will get dam hot!

Cheers
Leigh

[Guest Close Guest]

The problem with current is the faster you draw from the battery the lower
the real capacity will be.

I was thinking of the opposite in terms of charging a battery, i.e. at xx% SOC, the battery will draw max of yyA from the alternator. Is there such a chart?

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

No, the actual current draw is dependent on many things, the battery itself,
the battery temperature, age of the battery, charging voltage, connecting cables to name
a few.

[drivesafe Senior Member Find all posts View Profile Close drivesafe]

The international standard for the rating of automotive fuses and circuit breakers is the marked rating of the device must be able to carry that current rate for 4 hours.

To achieve this, the same device must carry 135% of the current rate for around 30 minutes and much MUCH higher rates for shorter periods of time.

The are a number of fuse makers who list the standards graph for automotive fuses and circuit breakers on their web sites.

The amount of current these devices can carry for short periods of time is quite high and is done so deliberately, so that the device does not go open circuit when a high current start-up device is turned on.

Most electric motors can cause a near short current load when they first start and this is the reason for the short period of high current tolerance.

For example, I use 50 automatic resetting circuit breakers in my dual battery systems and on many occasions, have jump started other vehicle and providing the other vehicle has not needed to be cranked for more than a few seconds, the circuit breaker has not tripped.

Hi AJ120, and it is common for the fuses used in air compressors to melt. This problem is caused by a number of factors but it is usually because the fuse was not big enough for the current being drawn through it.

Standard rule of thumb is to use a fuse with a marked current rate of at least 25% greater than the maximum CONTINUOS current load of the circuit. This reduces the fuse’s resistance in the circuit, which in turn stops the fuse from over heating and melting.

SPECIAL NOTE, you also have to make sure the thinnest wire/cable in the same circuit, must be able to handle the higher current and a again, the the fuse or circuit breaker must not be larger than 75% of the wiring’s maximum CONTINUOS current load ( 60% in a dual battery circuit )