Car Inverter earth connection.

[Harry Bloomfield]

It does make a difference - use at least the same size as for the positive cable. As already said current carrying capacity, is not the main issue, volts drop is, due to the low initial voltage and long cable run.

 

[ipbr21054]

I’m happy that the positive side is now fine.

So I’m to use the same wire for the negative side but take it to the chassis.

 

[securespark]

Sure. On a negative-earthed vehicle, the bodywork is reliably connected to the negative terminal of the battery.

Some much older cars have a positive earth.

 

[ipbr21054]

Uk vehicle and negative earth.

 

[ericmark]

In a vehicle we do normally use the chassis as return, and often there is an earth braid linking parts, the worry is the bit of body you connect to will not have a thick enough braid linking it to rest of car, but that is unlikely.

The other worry is the effect on the battery, when a lead acid battery is discharged sulphur forms, and this can harden over time, so with a starter motor the time between discharge and recharge is very small, so the battery is OK, but when there is time between discharge and charge the charging takes longer, also most car batteries are designed for high current, and can be damaged if deep cycled, some batteries can stand it better than others, the leisure battery is a hybrid not a true traction battery or car starting battery, where a 60 Ah car battery is used, it would be replaced with a 90 Ah leisure battery with items like motor caravans, but normally we use a second battery and some form of split charging.

The main thing about split charging is the second battery is disconnected from prime mover battery during starting, so you don't draw starting current from the second battery, and second when ignition switched off, you don't discharge the prime mover battery.

With modern AGM batteries they can be left in a part discharged state for longer, but they are expensive. With milk wagons we installed Ni/Fe batteries as these could charge faster, so they could suck up the milk without engine running in farm yard and it would be recharged before they got to next farm, they were also used in buses so they could have inertia lights on what at terminus. Darn expensive things.

Using an inverter once in a blue moon is not problem, but using on a regular basis is, ask any canal boat owner. They try to once a month find a mooring with shore power, as the 6 hours the engine is running when continually cruising is not enough to fully recharge batteries, and over time they get slowly more and more sulphated. Charging lead acid takes time, does not matter how big the alternator.

Using a small smart charger 3 amp is ample over night once a week will likely save the battery for getting sulphated. But lead acid battery use is not simple maths, with things like canal boats they use stage chargers and pulse chargers, so when the engine is started it charges to 14.8 volt until the current drops to under 5 amp then goes to float charge at 13.8 volt, this means the bulk of the charge if put into the battery in the first hour, so it has 5 hours to put the last 10% back into the battery. Problem with road vehicles we tend not to drive them for 6 hours, and even 6 hours and canal boats still have a problem, today they end to use solar panels as well.

I used a second battery with many vehicles, some times so I could jump start wagons, some times to supply a caravan, as I said before in other post in your situation I would look at a jump start battery pack as the simplest way to run that machine, mine has a 300 VA inverter built in with a 12 Ah AGM (VRLA) battery, and can be re-charged from car or mains.

 

[ipbr21054]

I think members are overthinking this situation.
I appreciate the advice but not a understanding the true picture and just putting there comments here.

The key machine motor is 300w or less.
To cut a key will take 4 minutes or less.
Once the key is cut I will trip the in-line circuit breaker.
I then drive home however many miles it is
Let’s say 10 miles.
I will only do this say twice a week

So bear in mind I’ve driven to the customer.
Used the inverter 4 minutes.
Tripped the circuit breaker.
Driven home again.

The small amount of discharge whilst using the key machine will easily be replaced on the journey home.

There is no need for an additional battery or split charger. Voltage drop is going to be minimal.

I was asking if i could just connect the negative wire to the chassis.
By the sounds of it then yes.
I will use the same gauge wire of which is 4AWG.

 

[ericmark]

Yes OK, the problem would arise if sitting in a market stall doing same thing, so it was 6 hours latter the car engine is started.

I have not a clue about AWG, I am Welsh and I use British wire sizes not USA. So 4 mm² or 7/0.036 in old money.

 

[ipbr21054]

4AWG isn’t 4mm2
It’s 21mm2

 

[Harry Bloomfield]

Using an inverter once in a blue moon is not problem, but using on a regular basis is, ask any canal boat owner. They try to once a month find a mooring with shore power, as the 6 hours the engine is running when continually cruising is not enough to fully recharge batteries, and over time they get slowly more and more sulphated.

They tend to supplement the engine/alternator charge when on the move, with wind generators and photovoltaic panels when moored up.

 

[ericmark]

They tend to supplement the engine/alternator charge when on the move, with wind generators and photovoltaic panels when moored up.

Different type of inverter, yes narrow boats often have two alternators and the inverter combines there output, but this is not what is being talked about here.

 

[mattylad]

ipbr21054 Yes what your suggesting is fine.
If you have problems then you can always add a negative lead at a later date but the chassis should be adequate.
Good luck with the mobile key cutting business.

 

[Harry Bloomfield]

Different type of inverter, yes narrow boats often have two alternators and the inverter combines there output, but this is not what is being talked about here.

Doh! I was responding to the recharging when not on shore power.

 

[ericmark]

Doh! I was responding to the recharging when not on shore power.

So was I, a unit like this

makes the alternator give out power even when the battery voltage is above the normal charge voltage and it delivers pulses to the battery and measures the battery voltage decay time to determine state of charge, the whole idea is to push as much charge as one can into the battery without damaging the battery, and it has to step up the voltage to do this, so it is an inverter, at nearly £400 there is no way one would fit one of these to a car, for one thing looking at batteries totalling around 500 Ah not 50 Ah.

Now we know only one key to be cut at a time, then running engine, the problems I had foreseen are gone. There is no problem with what has been described, and I have not seen any earth braid which would not take that current.