Taking new aux 12v feeds
Discussion
Started thinking alound and answered most of own question, but would like sanity check and a way to find the ground 
please
With a Vauxhall Astra (2015), I'm trying to do several things. In priority -
Two problems, +ve and -ve...
I found add-a-circuit piggyback fuse holders like https://www.amazon.co.uk/gp/product/B07DNV67P5/ so +ve is sorted
though I'm not sure I trust them for more than 15A. I see (elsewhere) conflicting opinions on putting two fuses in parallel, hmm... question for another day.
I know -ve is dumped into the chassis, so how do I go about finding a bolt that already has the paint removed? Are there other precautions I need for ground?
pleaseWith a Vauxhall Astra (2015), I'm trying to do several things. In priority -
- unswitched 12v 10A feed for
computers/phone/dashcam whatever. Prefer not to add ugly cig-lighter mess, and anyway that is switched in this car. - unswitched 12v feed for small inverter. It could draw 90A surge, so would want its own fused cable run all the way back to the battery.
- if I don't go with (2), which seems likely, a separate 10 to 30A feed for the same inverter and I'll avoid heavy loads
- one day / later, might add a small domestic battery to supplement (1) for this kit. Or might get bored if it work well enough.
Two problems, +ve and -ve...
I found add-a-circuit piggyback fuse holders like https://www.amazon.co.uk/gp/product/B07DNV67P5/ so +ve is sorted
though I'm not sure I trust them for more than 15A. I see (elsewhere) conflicting opinions on putting two fuses in parallel, hmm... question for another day.I know -ve is dumped into the chassis, so how do I go about finding a bolt that already has the paint removed? Are there other precautions I need for ground?
It sounds like you'd be best running new cables from the battery. Maybe even a split charging system and leisure battery if you're serious.
10A is a lot to add to a circuit that probably wasn't designed for it. The wiring going to it may only be sized for the relevant fuse size. There will be some headroom, so adding a dashcam or something at a few hundred mA is unlikely to cause an issue, but adding 10A sounds risky.
How long are you likely to draw 90A for, or how often? The alternator is probably only 120A or so, so taking 90A will be more like starting the engine and will take energy from the battery (as opposed to it being supplied by the alternator).
There will be loads of earth points around the car already, so you can use one of those. Or just check continuity between the point you want to use and a known earth / battery negative.
10A is a lot to add to a circuit that probably wasn't designed for it. The wiring going to it may only be sized for the relevant fuse size. There will be some headroom, so adding a dashcam or something at a few hundred mA is unlikely to cause an issue, but adding 10A sounds risky.
How long are you likely to draw 90A for, or how often? The alternator is probably only 120A or so, so taking 90A will be more like starting the engine and will take energy from the battery (as opposed to it being supplied by the alternator).
There will be loads of earth points around the car already, so you can use one of those. Or just check continuity between the point you want to use and a known earth / battery negative.
By the time you undo any bolt that goes into metal, and do it back up with your ring terminal , it will have a good earth. The bolt thread will be making much more contact than the back face of the bolt ever would. The back face of the bolt will be making the contact with the ring terminal.
Inverters benefit from the shortest cables possible to the battery . DC loses voltage over distance badly. They also drain the battery even when off, so make sure you pull their main fuse when not in use. If you chassis earth your inverter make sure it's a good one, as you'll be passing a lot of current through it and any subsequent panel joints.
Nothing wrong with parallel fusing. Just fuse down a little.
Fuses at both ends if the cable run is long (to rear of car).
Inverters benefit from the shortest cables possible to the battery . DC loses voltage over distance badly. They also drain the battery even when off, so make sure you pull their main fuse when not in use. If you chassis earth your inverter make sure it's a good one, as you'll be passing a lot of current through it and any subsequent panel joints.
Nothing wrong with parallel fusing. Just fuse down a little.
Fuses at both ends if the cable run is long (to rear of car).
I doubt the existing wiring to the fuse box will like having an additional 90A/1000 watt peak load showing up, so I'd run a new fused cable from the battery, then have a feed from that wire fused to the inverter and then another smaller (<5 amp?) fused feed to the dashcam and other stuff.
Once you start looking behind the glove box you'll deffo find a suitable earth point.
You realise dashcams are best with switched power? That way they start recording automatically (unless you want motion activation?)
Once you start looking behind the glove box you'll deffo find a suitable earth point.
You realise dashcams are best with switched power? That way they start recording automatically (unless you want motion activation?)
RogerDodger said:
Penelope Stopit said:
90 Amps?
Easily, depending on the inverter rating.90amps x 13v = 1.2kw - less than an average hair dryer.
Whether it could actually manage to pull that current, how long it would try before shutting down or releasing its magic smoke, and if I even have anything I want to run in the car which would draw that much... these are all fair questions!
blank said:
It sounds like you'd be best running new cables from the battery. Maybe even a split charging system and leisure battery if you're serious.
Part of the exercise is discovering how serious I actually am 
so I'll probably start with much lower fuse (2* 10A?) from glove box and then if I pop the fuses, I learned something - no harm done, not too much wasted effort.There is some vanning on my distant possible futures list so the car is a potential early experiment.
blank said:
10A is a lot to add to a circuit that probably wasn't designed for it. The wiring going to it may only be sized for the relevant fuse size. There will be some headroom, so adding a dashcam or something at a few hundred mA is unlikely to cause an issue, but adding 10A sounds risky.
I added up the fuses for the glovebox and they're 250A total. It seems to be served by a pair of red+blue (meaning unfused +ve?) which look about 6mm including jacket, so I'm guessing 1mm jacket => 4mm diameter copper => total of 25 sq.mm => 150A max by my "6A per sq.mm" rule of thumb (of dubious origin / for thinner stuff).The same box serves the passenger blower. I have the sense not to put a heavy load on both, and nobody else to worry about having a key to it.
blank said:
How long are you likely to draw 90A for, or how often? The alternator is probably only 120A or so, so taking 90A will be more like starting the engine and will take energy from the battery (as opposed to it being supplied by the alternator).
tbh it may be a fan heater in the car park, just to see what happens.Also fair warning about alternator rating... oh let's start another thread about that one, maybe there are lessons for all.
RogerDodger said:
Penelope Stopit said:
electronpusher said:
unswitched 12v feed for small inverter. It could draw 90A surge
90 Amps?90amps x 13v = 1.2kw - less than an average hair dryer.
A 1.2 KW Inverter is big
Mmmmm, you seem to have calculated the surge on the 240 volts AC side of the inverter using a 13 volts DC input voltage
Can't be done like that
You also seem to have calculated a surge current rating for a constant supply calculation
Can't be done like that
You also seem to have calculated using a 100% efficient DC to AC Inverter
Can't be done like that
electronpusher said:
RogerDodger said:
Penelope Stopit said:
90 Amps?
Easily, depending on the inverter rating.90amps x 13v = 1.2kw - less than an average hair dryer.
Whether it could actually manage to pull that current, how long it would try before shutting down or releasing its magic smoke, and if I even have anything I want to run in the car which would draw that much... these are all fair questions!
look here for current ratings, gives inputs and outputs, is very helpful
https://caravanchronicles.com/guides/understanding...
A third of the way down the page you will see the below image, bear in mind that the image shows calculation for 80% efficient inverters, your inverter will very likely better them
Penelope Stopit said:
OP Commented unswitched 12v feed for small inverter
A 1.2 KW Inverter is big
It would be a small inverter being momentarily abused. A 1.2 KW Inverter is big
Penelope Stopit said:
Mmmmm, you seem to have calculated the surge on the 240 volts AC side of the inverter using a 13 volts DC input voltage
Can't be done like that
You also seem to have calculated a surge current rating for a constant supply calculation
Can't be done like that
You also seem to have calculated using a 100% efficient DC to AC Inverter
Can't be done like that
Yes sorry, much inaccuracy. My figure of 11V (sent after you started writing your post?) was pulled from the thin air just above my chair, and I did forget to pull another figure for efficiency. Surge load isn't going to get the best efficiency... but really the only way to know is to try it and see, and the safety plan should consider up to 130A (70% efficiency).Can't be done like that
You also seem to have calculated a surge current rating for a constant supply calculation
Can't be done like that
You also seem to have calculated using a 100% efficient DC to AC Inverter
Can't be done like that
I have no experience of hardware for currents over 20A, mostly I push electrons round in smaller numbers.
electronpusher said:
Yes sorry, much inaccuracy. My figure of 11V (sent after you started writing your post?) was pulled from the thin air just above my chair, and I did forget to pull another figure for efficiency. Surge load isn't going to get the best efficiency... but really the only way to know is to try it and see, and the safety plan should consider up to 130A (70% efficiency).
I have no experience of hardware for currents over 20A, mostly I push electrons round in smaller numbers.
You've lost meI have no experience of hardware for currents over 20A, mostly I push electrons round in smaller numbers.
My reply was to RogerDodger
Then later posted you input output info above
RogerDodger said:
By the time you undo any bolt that goes into metal, and do it back up with your ring terminal , it will have a good earth. The bolt thread will be making much more contact than the back face of the bolt ever would. The back face of the bolt will be making the contact with the ring terminal.
Makes sense, thanks. I'm still dubious about this use of steel as the conductor, but it's too late to worry about that here.(wondering about an experiment: voltage sense on each earth point I can find, measure the difference... wish I had time for that)
RogerDodger said:
Inverters [...] also drain the battery even when off, so make sure you pull their main fuse when not in use.
I have a 30A relay which served it while on the ciggie lighter of previous car (unswitched, presumably 10A fused) and controlled from the dash lighting conveniently visible on the ciggie lighter.I know the relay may weld shut instead of breaking the circuit, if operated during a surge load.
I haven't checked the inverter's "off" current, but the switch is easy to hit accidentally anyway so it would need more than that.
RogerDodger said:
Nothing wrong with parallel fusing. Just fuse down a little.
Fuses at both ends if the cable run is long (to rear of car).
With the add-a-circuit / piggyback fuses, I think I'm fusing to the capacity of that fuse hardware. Two at 15A gets me near enough to continuous rated load (300W @ 12V 85%, best if engine is running), and I could see how warm they get. This at least is something I'm comfortable doing, and I'm moderately comfortable hanging that extra load off the glovebox supply.Fuses at both ends if the cable run is long (to rear of car).
Fuse at both ends... for convenience on failure? Big fuse near battery and smaller in the boot? I did not understand this.
But I'll probably strap it under the passenger seat this time.
(edit = format fix)
Edited by electronpusher on Wednesday 4th December 13:05
electronpusher said:
I added up the fuses for the glovebox and they're 250A total
Can't be done like that, circuits are fused higher than the consumers ratingsBest method for wiring an inverter is by connecting it directly to the battery
An ammeter will give current consumption of inverter when being pushed to its maximum
Anyway
Enjoy the job, it's all good fun
Edited by Penelope Stopit on Wednesday 4th December 13:03
Penelope Stopit said:
Nice article thanks.TFW said:
We decided that it wasn’t a mobile phone or laptop that we needed to charge, so what is it you want to run? Well, it could be anything… and the most common thing that people want to power is………… a hairdryer, closely followed by hair straighteners.
Yep, laptop has its own 12:19 boost jobbie (smaller, more efficient, no need to carry the laptop PSU). If it was somebody else's laptop with a different DC plug, maybe.Last few times it was running the soldering iron. My excuse to want a gas soldering iron
but not really sure I would use it much.No hair care hardware. Maybe an angle grinder.
electronpusher said:
RogerDodger said:
By the time you undo any bolt that goes into metal, and do it back up with your ring terminal , it will have a good earth. The bolt thread will be making much more contact than the back face of the bolt ever would. The back face of the bolt will be making the contact with the ring terminal.
Makes sense, thanks. I'm still dubious about this use of steel as the conductor, but it's too late to worry about that here.(wondering about an experiment: voltage sense on each earth point I can find, measure the difference... wish I had time for that)
RogerDodger said:
Inverters [...] also drain the battery even when off, so make sure you pull their main fuse when not in use.
I have a 30A relay which served it while on the ciggie lighter of previous car (unswitched, presumably 10A fused) and controlled from the dash lighting conveniently visible on the ciggie lighter.I know the relay may weld shut instead of breaking the circuit, if operated during a surge load.
I haven't checked the inverter's "off" current, but the switch is easy to hit accidentally anyway so it would need more than that.
RogerDodger said:
Nothing wrong with parallel fusing. Just fuse down a little.
Fuses at both ends if the cable run is long (to rear of car).
With the add-a-circuit / piggyback fuses, I think I'm fusing to the capacity of that fuse hardware. Two at 15A gets me near enough to continuous rated load (300W @ 12V 85%, best if engine is running), and I could see how warm they get. This at least is something I'm comfortable doing, and I'm moderately comfortable hanging that extra load off the glovebox supply.Fuses at both ends if the cable run is long (to rear of car).
Fuse at both ends... for convenience on failure? Big fuse near battery and smaller in the boot? I did not understand this.
But I'll probably strap it under the passenger seat this time.
(edit = format fix)
Edited by anonymous-user on Wednesday 4th December 13:05
- fuses at both ends - it's good practice with long runs - reduces the chance of a fire due to cable heat up in the event of a short. Not sure of the exact science but I know you are advised to do it by the experts. I sell 12v kit and double fuse if the cable run is over a few metres.
Edited by anonymous-user on Wednesday 4th December 13:47
Penelope Stopit said:
RogerDodger said:
Penelope Stopit said:
electronpusher said:
unswitched 12v feed for small inverter. It could draw 90A surge
90 Amps?90amps x 13v = 1.2kw - less than an average hair dryer.
A 1.2 KW Inverter is big
Mmmmm, you seem to have calculated the surge on the 240 volts AC side of the inverter using a 13 volts DC input voltage
Can't be done like that
You also seem to have calculated a surge current rating for a constant supply calculation
Can't be done like that
You also seem to have calculated using a 100% efficient DC to AC Inverter
Can't be done like that
I posted rough numbers as an example
"Mmmmm, you seem to have calculated the surge on the 240 volts AC side of the inverter using a 13 volts DC input voltage"
hmmmmm - I never mentioned surges, or quoted them
"You also seem to have calculated a surge current rating for a constant supply calculation"
Mmmmmmm - I never mentioned surges
"You also seem to have calculated a surge current rating for a constant supply calculation"
Mmmmmmm - I never mentioned surges.
"You also seem to have calculated using a 100% efficient DC to AC Inverter"
- I never calculated anything lol
ALL I mentioned is 90w is not an unusual figure in the world of inverters
Penelope Stopit said:
You've lost me
My reply was to RogerDodger
I realised your reply was to Roger but I took it as a fair criticism of my facts & calculations, resulting in that "90 amps" looking maybe it should be higher.My reply was to RogerDodger
RogerDodger said:
- fuses at both ends - it's good practice with long runs - reduces the chance of a fire due to cable heat up in the event of a short. Not sure of the exact science but I know you are advised to do it by the experts. I sell 12v kit and double fuse if the cable run is over a few metres.
Interesting... I can see why you want a fuse nearest the battery: you can't rely on a fuse in the boot to prevent a short-circuit causing a fire in the middle of the car. So I'm still thinking the rear fuse is for convenience after nuisance blowing.(edit +)
Anyway - kit is coming in a couple of days, and the weekend might still be free.
RogerDodger said:
Penelope Stopit said:
RogerDodger said:
Penelope Stopit said:
electronpusher said:
unswitched 12v feed for small inverter. It could draw 90A surge
90 Amps?90amps x 13v = 1.2kw - less than an average hair dryer.
A 1.2 KW Inverter is big
Mmmmm, you seem to have calculated the surge on the 240 volts AC side of the inverter using a 13 volts DC input voltage
Can't be done like that
You also seem to have calculated a surge current rating for a constant supply calculation
Can't be done like that
You also seem to have calculated using a 100% efficient DC to AC Inverter
Can't be done like that
I posted rough numbers as an example
"Mmmmm, you seem to have calculated the surge on the 240 volts AC side of the inverter using a 13 volts DC input voltage"
hmmmmm - I never mentioned surges, or quoted them
"You also seem to have calculated a surge current rating for a constant supply calculation"
Mmmmmmm - I never mentioned surges
"You also seem to have calculated a surge current rating for a constant supply calculation"
Mmmmmmm - I never mentioned surges.
"You also seem to have calculated using a 100% efficient DC to AC Inverter"
- I never calculated anything lol
ALL I mentioned is 90w is not an unusual figure in the world of inverters
Yes was trying to help the OP and your good self
The OP mentioned small inverter
I questioned 90 Amps as that is from one big inverter
Then you posted
RogerDodger said:
Easily, depending on the inverter rating.
90amps x 13v = 1.2kw - less than an average hair dryer.
Which I read as your good self questioning my input90amps x 13v = 1.2kw - less than an average hair dryer.
Thought it best for evolution and all that to point out what I took to be a misunderstanding by your good self of how inverters work and how their current draws and output wattage are calculated
How did you wish me to read it as?
Ok then, for some reason I had taken your above post to be a calculation "90amps x 13v = 1.2kw - less than an average hair dryer"
Have a good afternoon and keep the faith
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