12v Set-up

dj, this may not apply to all generators, but I understand the 12V outlet on many generators is not good enough to charge batteries. I only puts out about 12.6 to 13 V, which is great for running 12 V appliances, but not good enough to effectively charge batteries.

It would be OK for running the charger through an inverter though, if you don't mind the inbuilt inefficiency. The other advantage is that with a gennie you have 240V if you need it.

Gary, I charge the AGM in my camper trailer in parrallel with the two deep cycles in my vehicle and it works fine. Unless the caravan battery was at the rear of the van, I don't think the run would be much further than mine. I ran 25mm squared double insulated welding cable through the chassis of the vehicle and through the draw bar of the trailer. Using my digital voltmeter, at worst I get .01V drop. The reason I say 'at worst' is it sometimes measures as zero voltage drop.

I used 175Amp Anderson plugs as the cable was much too big for 50Amp plugs. Also to handle the possible current draw as I set up to carry 2 x 120 AH AGMs in the trailer and they will take a high charge current if they are run down and it is available.

"...at worst I get .01V drop..."

The devil is in the detail. How much current were you drawing at the caravan end when you measured your voltage drop???? Try flattening you van battery then meauring the voltage drop while the battery is charging (remember to measure both the positive wire and negative wire drops along the lengths of the cables). If you are not actually drawing any current along the wire you could get the same results from using a piece of wet string...

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To the original question.

A bit of maths (sorry) from the site I quoted

135 amps x .73 mOhm/metre x 10 metres = 1 volt loss with 25 mm^2 cable at the full 1500 watt output of the inverter or 15% of the power lost in simply getting from a to b. This will lead to an unbalanced discharge of the batteries (van first)

I agree with every thing said about voltage drop, length of wire etc.... but one point not added to this thread is that you may want to consider a 24volt inverter as they wire length verse voltage drop is halved at a 24volt rate (Approx) The real interesting thing is that the plugs them self can add to 1volt of drop when they get hot so try to direct connected where possible. I know its a pain to move appliances if they are hard coded but the benefits to us for example were about 7 hours additional running time on the fridge.

Solar is getting there with a 65 watt panel (65 / 12 = Amps) or 5.41 amps for about 650$ maybe an option, two will be the same price as a good generator (No Air-conditioning though but you can run the fridge and a few lights and never have to start the generator. If all goes pear shaped then at a pinch you can run the car, not perfect but only diesel's have an issue with that.

Anyway there is some great info here so hope my two bobs worth helps

Steve

Gary, I'm not going to challenge your maths. You are probably much better at it than me. Similarly I am far from an expert on 12 V systems.

In the interest of my continued learning, I'm prepared to take this discussion a touch further.

In real life nobody (that I know) constantly draws 135 Amps. I know I don't do it at all (other than cranking) as every fuse and circuit breaker I have is rated well below that level. I have never measured the current flow during charging, but I doubt the current going to my CT battery is often above 30 Amps and of course this drops as the battery gains charge. It can not be anything like 135 Amps as my alternator isn't capable of that (quite apart from the circuit breakers).

In discharge mode (which your calculation seems to be using), my fridge draws 5 or 6, add a couple of lights, occasionally the water pump, I suspect (but have not measured) that at most, I draw 10Amps, but more typically 5 or 6 (and 1 or 2 when the fridge is cycles off). Even using the limited data from the initial post, it will be only a fraction of 135 Amps.

Now to the resistance of the cable. The internet is notorious for the variation in answerers to questions, but my research shows that 25mm squared is close to 4AWG (4AWG is actually a bit smaller at 21.1mm sq) and it's resistance according to the site below is .000815 Ohms per metre, not .73 Ohms per metre. My run is around 8 metres, so by return length is 16m. Providing the site I have used is accurate, my cable resistance is less than .013 Ohms (my cable is thicker than 4AWG), not the .73 Ohms you have calculated,

www.bepmarine.com/PDF/Volt_drop.pdf

But more important than this to me is practical experience. When I go on an extended trip and return home, I put my batteries on my 3 stage charger. If I've had a long last day of driving, within minutes, the 3 stage charger goes to the 3rd stage. To me, this suggests they are at least 80% charged, which would be quite impossible with the voltage drop you suggest.

As always, this subject draws some rather diverse views. That's why, in the end I resort to real world experience. I'm happy that my system works far, far better than your calculation suggests is possible.

As far as the theory goes, I'm happy to continue learning so I can improve my system further over time.

"...that you may want to consider a 24volt inverter ..."

Not plausible unless you have a 24 volt alternator,(plus 24 volt solar panel?), and 24 volt battery, IN ADDITION to the 12 volt system he is currently running fo standard crank/running of vehicle, and existing 12 electrics in the van. Waaaaaaay too much hassle, better/cheaper/more convenient to carry a generator.

Best option is to use as many 12 volt appiances as possible (minimising losses throught the 12-240-12volt conversions), LED lighting wherever possible (very low power consumption - 3 watt led light = ~60-75 watt incandecent brightness), and minimizing 240 applications to those devices that have NO OTHER OPTION. This also means that you are not left high and dry if the (one and only) inverter packs up in the middle of nowhere. It also means you can get away with a smaller inverter, or even two smaller inverters for redundancy (better option than one big one anyway).

Yea, agreed. However, running that many amps in a parallel config is going to drain the closest battery first as suggested I was thinking of this option for the the inverter only. To run a series configuration, 24 volt would possible limit that drain and while it would take some additional cable I think it would do the job. The idea of the 25mil welding cable is best though, especially with the l;long cable run. Many get that wrong.

Good pick up.

Steve

PS. Two Batteries in parallel make 24 volts so two charged simultaneously at 12 volt is possible if you have enough amps in the alternator. Batteries running series make 24 volt. There are inverters that take that input.

PS you can weld with them as well if you cleaver, Looks a bit rough but I have done it (After removing the batteries from the truck though as hydrogen explosions weren't on the menu.

Again, the drain on a 24 volt system converting to 240 is more forgiving if cable length than 12 volt. It all depends on position, right ???

Generators stink, make noise and are banned in many National parks. Better to try and get around the need, Use Solar. Two 65 Watt panels @ 650AUD each are enough to run a lot of gear. You are camping after all, hopfully

Norm C

Please read again, 0.73 milli Ohm (mOhm) per metre was what I said for 25 mm^2 wire. I will use your example of 0.815 milli Ohm per metre.

Your maths was correct and I will use the 0.013 ohm total resistance

The original question posted was about running an inverter, potentially at 1500 watts max., drawing off two batteries as equally as possible (would have to design for that for safety, given the potential for excess heat to cause a fire). Inverter uses 130 amps-> half that is 65 amp

I will use your lengths of wire run as a working example (seems to be the direction the original post was taking).

V = I x R
= 65 x .013 = .845 volts lost in the 16 metre run to power ("half") a 1500 watt inverter at full power, assuming equal load sharing. working it out further that equates to about 55 watts power dissipated in the wire as heat, ie one standard low beam headlight!!!

You will charge your battery in the van, but at a slower rate and probably not to 100% (effectively current limited by the resistance of the cables. By looking at the above equation it is reasonably obvious that if you are charging the battery at about 30 amps at the normal charging voltage at one end, you are going to get about 0.4 volt (ie. about half the above value - straight maths...) below what you would like at the van battery's terminals for a full charge at the other end of the wires (90-95% charged???).

Steve Y,

Just wrong....

Two batteries paralleled up is still only 12 volts, but with the CURRENT CAPACITIES added ie 48 amp.hr and 60 amp.hr equalling a total of 108 amp.hr.

Paralleling the batteries is what you do when you jump start another vehicle, and you don't blow everything up by doing that....

One thing worse than bad advice is wrong advice.....

Steve Y,

Ignore last post, I misread yours...::embarassed::

The original post was talking about using the van and vehicle 2nd battery paralleled, not two under the bonnet...

Gary, You are right though and trying to set this all up is getting out of control. I wouldn't even try my own suggestion but it is real and I was just trying it relay that.

To every one else. Every one is right and it does depend on how you use your gear. I use a Gas fridge as a second fridge so my daily amp draw is low. We head out to Knowing for a week at a time with a drive here and there and it is all OK. The temps in Summery are 38c, the beer lives in the Waco as we all know that 3 Way fridges don't deal with that but you can get them low over night and limit opening and ------Well I'm out of breath.

PS I meant Series as well. To many beers to write this... Sorry I confused every one. me included :-)

Thanks for all the constructive criticism. Maybe a petrol battery charger isnt the way to go. I havent had anything to do with solar before thats why I didnt even consider it. It sounds as if it will do the same job for around the same price. I will do some more reseach and have a look. Thanks
Matt

We all have reading problems tonight Gary. Missed the m in mOhm. Sorry about that.
Anyway, my system works well for me, but might not for someone else who has high curent drawing devices at the back end.
Enough confusion for one night I think.