Wiring Among Battries

pebbledropper wrote: »

The load on bank wiring is something I've never learned well. Here's what I know:

ARRAY: 6 160W panels paired in series to 24V in 3 parallel strings. Voc per ea panel is 22.2V. Isc per each panel is 9.65A; but they don't add. So, the maximum Amps per string is 9.65. Three strings = 28.95A x 1.25 MPPT possible = 36.2A. I think this is the absolute most DC the bank would see from the controller; and that's before line loss of 1% and inefficiency of the controller.

The way I would estimate from the array to a 12 volt AGM battery bank (typical maximum, not worst case maximum):

• 6 * 160 Watt panels * 0.77 panel+controller derating * 1/14.25 volts charging = 51.9 amps typical maximum charging current
• 6 * 160 Watt panels * 1.25 panel+controller derating * 1/12.0 volts charging = 100 Amps typical worst case current (sub freezing weather, reflection off snow, MPPT controller)

CHARGER: Magnum MSH3012 125A DC, 87% efficiency rated

• 125 Amps + 51.9 Amps = 176.9 amps typical maximum
• 125 Amp + 100 Amps = 225 Amps typical worst case maximum

Of course, that will be your choice as to if you will be charging a "dead" battery bank both with Solar and Genset, and deep in a Canadian cold snap or in Texas/Louisiana stormy weather.

SHORE/GENERATOR: The Magnum hybrid has the capability to pass 60A of AC current; but I think my switchover and Progressive Industries protector would limit it to 50A. I'm pretty sure my diesel Onan can't do 50A.

Of course, the AC current limits do not really affect the AC inverter's DC charging current too much at these levels:

• 125 amps * 14.25 volts charging * 1/0.87 charger efficiency * 1/120 VAC nominal line = 17.1 amps typical AC charger input current

It would only become an issue if your AC loads (charger + 120 VAC "pass through" loads for the inverter) exceed your genset's limits.

I am not sure how the Switchover and PI Protector can limit AC current from the genset--Other than by burning out. Unless one or the other includes a fuse/breaker. (not sure what PI Protector/Switchover you have--So just guessing here).

DEMAND: I think the most DC load the bank would have to send is the total of DC appliances operating at one time before the DC main breaker trips. I don't know that number but will be very surprised if it exceeds 3,000W at any one time. The DC load demanded by the inverter for AC power I think is low, too. We're not talking 15k btu A/C use.

That is, of course, the $64 question... For the sake of worst case planning, assuming your loads are all AC appliances and a 3,000 watt inverter:

• 3,000 Watts * 1/0.85 inverter eff * 1/10.5 battery cutoff voltage = 336 Amps @ 12 volt DC battery bus
• 3,000 Watts * 1/0.85 inverter eff * 1/10.5 battery cutoff voltage * 1.25 NEC Breaker/Fuse derating = 420 Amp rated fuse/breaker

So, the next question is how do you "rate" the battery bank for current sharing. For 1 or 2 parallel battery strings, I would suggest 100% current rating per battery string (if one battery string has problems, the other would carry the full current).

If you have three or more parallel strings, I would suggest a maximum current derating per string of 50% (210 Amps maximum).

Then, figure out how much current a copper cable can carry. NEC is pretty conservative, Marine charts are pretty "liberal":

• https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm (4/0 is rated for ~195 to 260 amps maximum or less depending on lots of variables)
• http://www.boatus.com/boattech/articles/choosing-cables-and-terminals.asp (table 3; 4/0 ~387 to 445 amps)

The other question is cable length and current... For charging/accurate charge controller reading of battery bank voltage, I would suggest approximately 0.05 to 0.10 volt drop in the harness (at charging current levels).

For discharging currents, I would suggest 0.5 volt maximum drop from Inverter to battery bank (including battery bank interconnects).

I would be willing to put 3 DC breakers in, but why? Plus, at the cost of those things in marine grade with resettable levers, I think I could go ahead and pay for 4/0 all the way. Not sure, though.

This is one of those things that people seem to rarely think about. We are so used to a single ~80 amp @ 12 volt battery is a car with a heavy gauge cable to the starter. More or less, the idea is that the single 80 AH battery will go dead before the starter cable can heat up enough to start a fire.

Now, you have 300 AH AGM batteries--Three strings of them in parallel for a 900 AH @ 12 volt battery bank. Or roughly 10x the capacity of a "car battery", and with AGMs, another 5-10x the peak current capabilities.

Where a shorted car battery can output a few hundred amps into a short circuit... That AGM bank can put several thousands of amperes into a dead short.

The fusing current for a 4/0 copper cable is around 3,196 amps. So, if there is a short circuit in your 4/0 battery cabling (wrench dropped on battery bank, cable + to - short, etc.) is more than enough to get the 4/0 cable red hot and start a fire. That is why it is a good idea to have a single breaker/fuse for 1-2 battery strings, and if you have 3 or more parallel strings, you should have a fuse/breaker per string.

Fuses and breakers (and their associated wiring/hardware) is not cheap--Especially when you are looking at 100+ rated devices.

BlueSea makes a very nice fuse holder that takes good quality T type fuses:

https://www.bluesea.com/products/category/Fuse_Blocks/Terminal_Fuse_Blocks

They are not cheap, but very small/compact and can easily be bolted directly to many batteries (or to the battery bus bar).

Using Circuit Breakers are nice too--Gives you a handy on/off switch when working on / diagnosing your system.

Sorry for the long post--And I hope it is not too confusing. We try to be very clear about safety issues/design questions. You may choose to do less than I have documented above (many do)--But at least understand the risks/rewards. And where/how you may choose to install the battery bank (i.e., not under a bed or in a home closet--But something a little more fire resistant).

-Bill

pebbledropper wrote: »

Okay on the safety thing for individual parallel protection. I'm a fan of the DC circuit breakers with resetting levers. They act as a switch, as you said. Given the above numbers, what Amp rating do you recommend for each? I hope I can find a way to attach the breakers directly to the Pos battery bolts. I may have to use an open copper bar about 3" long. That seems pretty dangerous, though. Any ideas?

Bus bars are frequently the best way to make connections. You can make your own or purchase them (some with fuse/breaker integrated).

Take a look www.2manytoyz.com website... About 1/2 way down, he has lots of pictures and discussions as his small installation slowly grew into a large backup power system for his home.

For an MPPT controller for my 12V setup, I'm looking at the Midnight Solar Classic 150, the Outback FlexMax 80, and the Morningstar TS-60. Based on what you know of these 3 and considering my system and the above numbers, can you rule out any of them for me? Alternatively, given my situation, is there one that shines above the others? I want to be able to get data from my controller to play with in my computer.

I am probably not the best person to tell you what controllers are best for your needs...

I really like controller with passive heat sinking instead of fans. The MorningStar is passive. And the Midnite + Outback are active fans--Which can be quite loud from what I have read.

In any case--Where ever you install the charge controller will need good ventilation... Anything electronic, when it gets hot will fail sooner (roughly, for every 10C/18F increase in temperature, the unit will age 2x faster). Also thermal cycling (hot/cold/hot/cold cycling) can be hard on electronics too (actually pop IC and components off boards, crack solder joints, etc.).

Make sure it is the MorningStar TS 60 MPPT type controller (and not the TS 60 PWM type). The Midnite Classic both have Internet connectivity (small server built in). I believe the other controllers require additional hardware for computer logging.

Hopefully, some others here can give you better suggestions based on your needs.

-Bill