240 volt battery bank

[jkfoxfire174]

Hello: My solar off grid system began thirty years ago. I began with a twelve volt battery bank for a couple of years and found to be very inefficient. I then went to 24 volts, 36 volts and now I am at 48 volts. The system works well except during heavy snow and seven days of cloud cover. I thought of going to 240 volt battery bank. Not only can I see the danger of 240 volts but have also seen that others have not recommended it. All appliance in my house are standard house appliances. Nothing is for solar operation. My well pump is 240ac. Every thing works fine except I feel that my conversion from DC to AC would be better with a higher battery bank voltage and there would be less drain on the batteries. I have 40 Deka 6 volt 370 amp/h batteries set up for 48 volts. If not 240 volt battery bank, what about 96 or 120volts?  Joel

[hydrosun]

Outback breakers are rated to 150 vdc. To go higher the Midnite breakers are two ganged in series to make an effective 300 vdc. The arc when breaking the higher   voltage is quite long. Personally I would be really scared on any bank higher than 48 volts. There are too many exposed connections. I've worked on 550 volt solar strings but everything is insulated connectors. For the short distances from the battery to the inverter i don't see a huge advantage to going to a higher voltage. Using a mppt allows the higher voltage for longer runs to the solar panels.  There aren't many commercial inverters at     120 volts and none that I know of at 240.  It soulds like your main problem is storage capacity on cloudy days.  You could add more solar panes to get more on overcast days. You could go with larger commercial batteries. You could parallel battery strings to give higher storage capacity. There is a reason the rules change for dealling with voltages over 48 volts. Everything is more dangerous. I don't feel it is worth the little added efficiency to go to a higher voltage at the battery. I don't know if I have added to anything you already know. There was a home power article about a 120 volt battery bank and they were building a inverter for it.  Exeltech makes a 120 volt  1100 watt inverter.
I started out 30 years ago at 12 volt, changed to 24 volt and at times think about 48 volts but ther e are quite a few things that would have to be changed. I have some 24 volt pumps, lights, and 24 to 12 volt converters. Some of my panels only have two pairs. So I stick to 24 volts. But my system is smaller than yours but i've added more solar panels over the years to gain more on partial sunny days.
Chris

[boB]

What were you going to use for a 240V DC input inverter ?

[Frank S]

Going to a higher voltage than nominal 48v is stepping into an industrial usage environment.
  When one steps up to 240v inverters  then 3 phase would be the next step for a home owner 3 ph at least in the USA is rare.
 Trip-lite and Solar edge are 2 that manufacture the lager inverters for UPS, Sangster is another one that makes models to handle DC input voltages in the higher voltage ranges 
Univpo makes an 8000 watt split phase inverter that takes 92-132vDC input but it is only good for 12,000w surge power
 staying with 48v nominal the Xabtrex XW6048 is a workhorse many off griders have gone to it for good reasons
 

[Rob Beckers]

I thought of going to 240 volt battery bank.
...

Joel, possibly I'm missing something, but what would the correlation be between the battery bank Voltage and how long one can ride out those overcast days?

How long you can keep the lights on with nothing coming into the batteries depends on the capacity of the battery bank (and how fast you're draining them). How may kWh's of energy are stored in it. The Voltage has nothing to do with this. A 12V battery bank can store as much energy as a 240V battery bank, it's a matter of the size of those batteries that make up the bank.

Voltage comes into play when you need to move power/energy; at a higher Voltage it requires less current, and therefore thinner wires. Current limited charge controllers can move more power if they run at a higher Voltage, i.e. a 60A controller will do roughly 800 Watt at 12 Volt, while that same controller can handle a 3,200 Watt PV array at 48 Volt. That makes higher Voltage battery banks a little more cost-efficient for larger installations.

-RoB-

[thirteen]

If you go that big be sure to put some signs up telling people about the high voltage. Mainly for firemen or
EMTs. and power linemen. Or neighbors helping you. Just a safety thing. 13

[Bruce S]

I have to agree with what others are saying.
I worked on Industrial sized backup systems for a short while. They use 120Vdc on the back plane and even those  actually reached out to the meter I was using, so 240Vdc will a short looking to happen unless everything remotely close is insulated or non-metal.

Doing a little bit of math, tells me you have 1850Ah @ 48Vdc and this lasts about 6 days. 7 and you're looking at possible battery drain problems.
As others have said , possible more solar for the overcast days or ways to get the snow off the panels.
With a bank that lasts for about 6 days, have you looked into the possibility of a small genset?
THAT is no small setup you have there.
 

 

[Flux]

240 dc may be within the range of some VF drive link voltages, if you want to run motors that may work.

Other than that I fail to see any gain. In this day and age I suspect you will have trouble finding switchgear suitable for high fault level dc. The fault level from batteries is frighteningly high. For lower fault levels it may be no big deal.

I can't believe the system losses will be reduced significantly so like Rob Beckers i don't see where the battery situation is going to be any easier unless you can run some non critical loads directly from the solar without cycling the battery in sunny weather.

Flux