Batteries in vehicle cabin

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

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

Hi ants, my chart is for working out the State of Charge of a battery under use, and is based on a battery with a load no greater than 5% of the battery’s Ah, to keep the error factor to a minimum.

For example, for the chart to be accurate when measuring a 100Ah battery, the current load must not be larger than 5 amps.

The higher the current load is over the 5% mark, the greater the error factor becomes.

What you are trying to do is unrelated to the SoC of a battery and is more closely related to determining the CCA of a battery.

[mackayvx Advanced Member Find all posts View Profile Close mackayvx]

This has been a great read as Im in the process of fitting an in cab battery in the trooper.

Leigh, Im half concerned about this fuse issue that you have discovered.
Is there any chance you could educate us any further in regards to the accuracy / reliability of the blade fuses that we commonly use?

I ask this as I installed my ARB CKMA12 in the trooper the other day and I happened to check the 40A mega fuse... guess what, one side of the fuse was melted into the housing, so Im wondering if the fuse is actually working correctly.

Do you have particular brands that you have tested or recommend?

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

Hi mackayvx, it does no matter if the fuse is from one manufacturer or another, if the fuse is rated incorrectly for the circuit it is being used in, it will still melt.

In the case of a Bolt-In type fuse, if it is not secured properly, this can cause it to melt and the very same thing happens with Push-in type fuses, like automotive blade fuses.

It the fuse terminals are note a tight fit, because of the heat a high resistance that can produce, and this then causes fuse can melt.

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

mackayvx,

Below is the typical rating of the fuses I was testing @25C:

Rated current X 100% 4 hours min
Rated current X 200% 5 seconds max
Rated current X 300% 0.2 seconds max

Of the 30 high precision fuses tested only two ruptured in the specified
time frame at the specified current.

At 200% rating the fuses that failed continued to conduct for several minutes
with no sign of rupture the fuse element, most of these fuses were ceramic
types therefore they didn't melt but became hot enough to burn the laminate material
they were resting on and would have certainly melted any fuse holder not made
of a ceramic material.

It took a minimum of 250% of the rated current for most of the fuses to rupture
within 20 seconds.

I have not tested any automotive blade types but visual observation of the
quality of some would lead me to believe they would fair no better than the high
precision fuses I tested. When I get time I'll grab a fuse at random and see what
it tests like.

The problem with fuses is with regards to vehicle wiring the common belief is you
fuse to protect the cable, most do just this with little regard as to the equipment
connected to it.

The fuse selected by the manufacturer would have been selected with two things in
mind, the rating of the cable and the calculated current draw of the equipment connected.

Specifying the rating of the fuse to protect the cable under short circuit conditions is
fairly easy as even very light cable of a short length will carry a 100 amps or so.

The problem that arises is what if it is not a dead short circuit, the insulation has
rubbed through and only a small section of the cable is grounded?

The other problem is overloading, if you don't calculate the anticipated demand on
the circuit, it is very important that the fuse be rated to carry at least the calculated
demand, once the demand exceeds the fuse rating the fuse will start to heat up.

Another point to consider is the fuses ability to break the current flow, no good using
a 10A fuse to protect a 100A cable if the fuse can only interrupt 50A, it will arc over.

The other issue with batteries and fuses is the demand is dynamic, a battery may draw
60 amps or more when discharged, but only a couple when lightly discharged, some will
say I have protected the cabling with 50A fuses and they have never blown, they may
not have but as in your case has it overheated?

The other issue is the quality of the fuse holder, the contacts can lose tension due to
poor materials or again due to excessive heat from the fuse, once the terminals lose
tension they become resistive, this causes them to heat further and eventually you'll
get arcing even if the fuse is carrying less than its rated current.

Not easy is it, basically you need to rated the fuse for the max demand, once this
is done then check that the fuse will also protect the cable, if it is to high a rating for
the cable then bigger cable is required.

Probably also better to use good quality circuit breakers if you can as the tolerances should
be better, but then if made in China?

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

I have just tested some auto types and to say the least I was a little
shocked!

A typical 7.5A micro blade actually carried 14 amps continuosly, there was
a voltage drop of 1V across the fuse so around 14W being generated and
no doubt would have got quite warm after awhile. It ruptured at 15A so
was in specs.

On the other hand I tested to different manufactures 5A micro blades,
one looked to be a high quality unit, the other a cheaper product.

Both these units did not rupture till the current reached 20 amps,
both fuses melted as would be expected as both fuses had a voltage
drop of around 2V across them at 18A, ie 36 watts!

A bit frightening to say the least, all the fuses tested could cause an
electrical fire when overloaded but not to the point that the fuse will
rupture.

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

Hi Leigh and sorry mate a 10 amp fuse in a 100 amp circuit, and remember we are talking about automotive circuits, so no voltage levels over about 30v, but a 10 amp fuse will not ARC.

What can happen, theoretically, is that the short circuit current draw can be so high that the fuse vaporises and covers the inside lining of the holder and the circuit is still partially intact.

But if the short circuit even is still there, what remains of the vaporised material, will itself, be further vaporised.

The fuse may take a split second longer to go fully open circuit, but it will still happen.

As for precision fuses and automotive use. They are not used in automotive circuits because there are to many devices in vehicle that will cause these type of fuses to blow in normal use.

As I posted earlier, there is an international standard for how fuses must react in given situations, in automotive use.

Also, if a wire rubs through on an earthed surface, the wire is grounded and the fuse will blow. You do not get a situation where it is partially grounded. If the circuit is grounded then it is grounded.

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

A bit frightening to say the least, all the fuses tested could cause an
electrical fire when overloaded but not to the point that the fuse will
rupture.

Hi again Leigh and automotive fuses are NOT designed for overload protection, they are designed for short circuit protection.

If a device is being fitted to a vehicle that has the potential to cause an overload situation, then the WHOLE CIRCUIT must be designed to cater for such an event, and this is the very reason Dual Battery Systems need to be designed properly, because your are more likely to have an overload event occur than you are likely to get a short circuit occurring.

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

Drivesafe,

I will disagree with you on these points, the fuses I was testing were all rated at
least 40V DCDC. The manufactures specifically states that the fuses max interruption
capability is 50A. It depends on the fuse package size, spacing of the terminals as to
its ability to extinguish an arc. If the manufactures states max voltage 50V, max interruption current 50A he has done so for a good reason, therefore members should ensure the protection device they are using are suitable for the intended purpose.

All the high precision fuses I tested are also covered by international compliance specifications, only two of 30 fuses tested meet the specification!

My point with regards the automotive fuses is the quality of them leaves a bit to be desired,
of the automotive fuses I tested none fused within less than 20 seconds for a 200%, only 1 out of 4 at 400% current flow, none therefore meet the specification. Also one would have to ask the question, could the cabling in a circuit normally fused at 5A handle 20A safely under a fault condition?

In even an automotive circuit the fuse has to be able to safely carry the circuit demand, pointless using 10A cable and a 5A fuse if the load on the circuit is 20A, therefore by default the fuse is also selected to handle the circuits anticipated demand as is the cable diameter. Example an engineer contemplating the air conditioning circuit, max draw by aircon is 20A, so I'll fuse it at 25A to allow a little overhead, cabling will be sized to adequately handle 25A over the cable run. Now one could argue the fuse was selected to protect the cable against a short circuit which it will but it was initially selected to handle the max demand.

As for partial shorts and overloads, I'm rather surprised you would state that, I have seen many instances of cable insulation rubbing through, the small contact area heating up and arcing etc yet the supply fuse has not blown, or the cable melted apart from at the contact area. The current through the cable will be proportional to the contact area.

A prime example of this is a typical lose screw clamp on a device, the terminal heats up due to poor contact area and melts the terminal block. Fuse doesn't blow, cable doesn't melt, terminal block does and device fails.

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

Hi Leigh and the international maximum design use is ( from memory ) 42v for automotive blade fuses, so those specs you got are inside the parameters expected. This does not alter the fact that we will be highly unlikely to see voltages much over 30v. ( as posted )

There are specialised blade fuses with 70 “something” voltage tolerance but these are not intended for automotive use.

As for the rest of the specs relating to automotive fuses, it is irrelevant whether you think they are good enough or not and it’s irrelevant what I might like to see.

The point is, these are the internationally accepted specs we in the industry have to work with, including any limitations they may present, so the industry has a set standard which governs what I or anybody else, working in this industry, must know and understand.

Here is an interesting link and if you snoop around the site, there is plenty of info there.

http://www.littelfuse.com/products/fuses.aspx

[mackayvx Advanced Member Find all posts View Profile Close mackayvx]

I have just tested some auto types and to say the least I was a little
shocked!

A typical 7.5A micro blade actually carried 14 amps continuosly, there was
a voltage drop of 1V across the fuse so around 14W being generated and
no doubt would have got quite warm after awhile. It ruptured at 15A so
was in specs.

On the other hand I tested to different manufactures 5A micro blades,
one looked to be a high quality unit, the other a cheaper product.

Both these units did not rupture till the current reached 20 amps,
both fuses melted, before that point as would be expected as both
fuses had a voltage drop of around 2V across them at 18A, ie
nearly 40 watts!

A bit frightening to say the least, all the fuses tested could cause an
electrical fire when overloaded but not to the point that the fuse will
rupture.

Leigh you are starting to worry me.

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

Wouldn't worry to much, as Tim pointed out generally a fault condition will be
a short and hopefully the fuse should rupture before any fire occurs.

In your case if the fuse has melted and can't be explained by a lose connection
arcing etc then check the current through your fuse under the worst case scenario,
and adjust fuse accordingly ensuring that fuse rating is also suitable for the cable, if
not then you'll need to increase the cable size.

You generally don't hear of many car electrical fires apart from land rovers or is that
Jeeps

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

Hi mackayvx, and while I agree with Leigh in that automotive fuses fall far short of the safety they could offer, they are all we have to work with.

But they also do their job and there was a make of vehicle, some time back that was susceptible to fires, but I am not sure what was the cause and I doubt that it was to do with fusing but more likely be poor wiring.

When the new VW beetle was released in the USA, they had a recall to fix wiring in the engine bay because of a fire hazard, nothing to do with fuses.

Anyway, there is no reason to worry, automotive fuses do have a proven track record for safety, not fires.

[goodone963 Spammyspamspammer Find all posts View Profile Close goodone963]

I am not an expert in this area, but you can find more fuses here.

[Pedro Advanced Member Find all posts View Profile Close Pedro]

This is mine which has been in back behind rear seat.
Also my solar installation