Electrical questions - Or how far out in left field am I?

[Gordie Allen]

Way back when I was new to this (I still have only three years under my belt) I installed a second alternator devoted to my house battery bank, but I've never hooked it up because of two questions in the back of my mind. I have a 120v apartment frig. and one or two 120v mini-splits running when on the road. The specs for the mini splits says a minimum ampacity of 12. They are each on their own circuit with 15 amp breakers and have never tripped the breakers. The 120v AC is provided by a 3000 watt Victron Multiplus.  I have four 250 amp deep cycle batteries wired in parallel. My two questions: One, is it OK to run the charging wire to any one of the four batteries? Two, is there any way the inverter might "see" or react to the current coming into the batteries from the alternator? My thinking is One, a single wire is OK. That's how it works with my two start batteries. Two, the inverter only "sees" a battery bank supplying 12v. In our two trips to California, we've never needed the mini splits for on-the-road air conditioning so I don't know how long 4000 amp hrs. would power the stuff. The math is beyond me when you factor in wire resistance, inverter efficiency, minimum battery voltage shut down built into the inverter, etc.
Sorry, I know there must be a shorter way to ask the questions.

[Oonrahnjay]

Way back when I was new to this (I still have only three years under my belt) I installed a second alternator devoted to my house battery bank, but I've never hooked it up because of two questions in the back of my mind. I have a 120v apartment frig. and one or two 120v mini-splits running when on the road. The specs for the mini splits says a minimum ampacity of 12. They are each on their own circuit with 15 amp breakers and have never tripped the breakers. The 120v AC is provided by a 3000 watt Victron Multiplus.  I have four 250 amp deep cycle batteries wired in parallel. My two questions: One, is it OK to run the charging wire to any one of the four batteries? Two, is there any way the inverter might "see" or react to the current coming into the batteries from the alternator? My thinking is One, a single wire is OK. That's how it works with my two start batteries. Two, the inverter only "sees" a battery bank supplying 12v. In our two trips to California, we've never needed the mini splits for on-the-road air conditioning so I don't know how long 4000 amp hrs. would power the stuff. The math is beyond me when you factor in wire resistance, inverter efficiency, minimum battery voltage shut down built into the inverter, etc.
Sorry, I know there must be a shorter way to ask the questions.

     Gordie, there is a way of running battery cabling (I think of it as "ladder" style) to keep resistances balanced between batteries.  There were some good posts describing it here but it has been a year or two since they were active and I have no idea how to find them.  But those will answer the best way to wire batteries in a bank and also where to attach inverter and engine generator/alternator 12V leads.  Maybe someone bookmarked those posts and can let us know where to find them.

[Iceni John]

SmartGauge's method 4 is the best way:  http://www.smartgauge.co.uk/batt_con.html   Those folk have some good advice on mobile electrical systems, and what may probably be the best battery monitor, better even than the Trimetrics and Victrons.   The experts at NAWS unanimously say that charging and load connections should be to the same points.

John

[Lin]

I just wanted to get some clarification.  Am I right in guessing tht you installed a second alternator for your house bank because it is 12v while your coach system is 24v?  If that is the case, and you want to run the 120v AC's off the house bank, the 12v alternator charging that bank would have to be putting out around 150 or so amps to run one unit.  What is the capacity of the alternator you installed?  The battery bank on its own can not run an AC for long.

[bevans6]

With four 250 AH batteries in parallel you have a 1,000 AH house bank.  With the Victron pulling roughly 140 amps from from the batteries for one mini-split it will take around 3.5 hours to deplete a fully, 100% charged bank to 50% if everything is perfect.  All current that is supplied from the alternator would add to the current from the batteries.  If your alternator can supply sufficient current you can run the mini-split indefinitely.  Most typical alternators are limited to about half their rated output on a continuous basis, so if you have a small alternator for charging the house batteries, it may suffer a bit from overheating.  FWIW and all that...  John's link for the battery connections is what I would use.

Brian

[TomC]

If running the mini-splits, just easier to run the generator. Creates far less heat. My three roof top A/C's are not even wired into the inverter. I think of the inverter as intermittent use-like for coffee maker, microwave, toaster oven, bathroom heater, even a water heater. Running A/C through the inverter is a continuous load that takes a huge battery bank and huge alternator to keep up the charge on the batteries. Diesel gensets are very reliable. Use the generator. Good Luck, TomC

[Gordie Allen]

Thanks for all the responses! The article on battery wiring was very informative, I will be rewiring them before we hit the road again. I have a 300 amp alternator made for heavy duty GM vehicles. At 150 amps nominal it should keep up. Lin, I have a 12v system throughout.

I'm also going to look closely at an adequate genset. Much simpler and less load on the 671.

So does anyone think the inverter cares whether the batteries are being charged by the alternator? Like I said, I think it's only seeing what's coming from the equalized bank, but I could be wrong there too.

[bobofthenorth]

So does anyone think the inverter cares whether the batteries are being charged by the alternator?

No.  It makes no difference to the inverter.

[Iceni John]

Just make sure the inverter's high-voltage cutoff isn't too low, otherwise the alternator may possibly cause it to shut off.   In cold weather alternators can produce enough voltage to bump up against some off-grid inverters' upper voltage limits  -  this is why some inverter manufacturers such as Cotek make "fleet" versions of their inverters, with voltage input ranges about one volt higher than off-grid versions of the same inverter.   Knurlgnar encountered this when testing a Go Power inverter:   https://www.youtube.com/watch?v=pqAKpiVgst0

John 

[Gordie Allen]

Thanks John. I'm pretty sure I can adjust the Victron.