Battery Bank Voltage Question...

machineman

I've been looking at the Xantrex XW Hybrid Inverter/charger specs and it says VDC input range 44 - 64v. Does that mean my battery bank needs to be a 48v or 60v system?

So if I need 2200ah and spec an AGM battery with 224ah at 6v then 10 batterys in series gives me the 60v but I need 10 parallel strings to get 2240ah. Thats 100 batteries!!!!

XW interter specs: http://www.xantrex.com/xw/pdf/DS20070618_XW-Hybrid.pdf

BB.

Re: Battery Bank Voltage Question...

44-64 VDC is the typical range of a storage battery (for 12 VDC, that would be 11-16 VDC). 11 VDC discharged, ~16 volts is just a bit over the 15-15.5 VDC used for equalization).

Wind-Sun has a larger 12 VDC battery.(PVX-2580L) (a bit over twice the Watt*Hour capacity of the 6 volt model)...

That would be 10 strings of 4 batteries (48 VDC).... 40 is better than 100--- :cool:

That seems to be about the largest AGM from Concorde...

Use a large Surrette battery... 8 VDC @ 1,100 AH, 2 strings of 8... 16 batteries... etc...

-Bill

But, you have to be able to manage the weight... The large AGM battery is ~165 lbs, the Surrette is approaching 500 lbs each...

machineman

Re: Battery Bank Voltage Question...

I checked several different battery combinations.. They all work out to be about the same price $25k - $27K. Way more than I anticpated. Looks like I need to cut back somewhere like lowering my backup days to 1. Aside from wieght and size, Is it better to have lots of smaller batteries or less bigger batteries?

BB.

Re: Battery Bank Voltage Question...

I can't think of any real reason to have more, small, batteries rather than a fewer large batteries other than for reasons of physical size...

The larger batteries can have upwards of 10 year warranty, the smaller batteries 1 year plus a couple of prorated years... Having fewer cells and interconnections can reduce the points of failure.

I personally like one large capacity string (with cells in series) as it is easier to diagnose problems (just put a volt meter across a set of contacts to find the bad cell--with parallel, you need a DC current probe and/or to disconnect a string to accurately measure cell voltages under load) plus, there are less issues with distributing current equally among series connected cells vs parallel connected cells (one place I read said that parallel banks don't last as long--all else being equal).

However, Wind-Sun has said before that they do not see any difference in battery life between series and well designed parallel battery banks (again--I am sure all else being equal).

The issue with reducing the size of your battery bank's capacity to one day is that you are now increasing the number of cycles (earlier wear out) and increasing the load (which reduces usable storage capacity).

For example, from the Sun Xtender chart (PVX-2580L):

1/2/4/8/24/48/72/120 Hour Rates
[SIZE=-1]165/[/SIZE][SIZE=-1]209/[/SIZE][SIZE=-1]214/[/SIZE][SIZE=-1]236/[/SIZE][SIZE=-1]258/[/SIZE][SIZE=-1]285/[/SIZE] [SIZE=-1]295/[/SIZE][SIZE=-1]305 amp*hours @ 12 VDC...

[/SIZE] If you reduce from a 3 day to 1 day of storage, the ampacity of the this example (largest 12 volt AGM from Concorde) would change from an 8 hour (1 day under load) to a 24 hour rate (3 days of load)--

236/258=91% or about 10% loss in usable energy under the higher discharge current for a smaller bank.

There are differences between the AGM and flooded cell batteries that can help save size/money (a bit anyway)... The AGM's are rated (and guaranteed) based on 80% discharge cycles instead of the 50% discharge that is normally recommended for a long life for a flooded cell battery.

And, you are correct, reducing your overall power use through conservation, insulation, and more efficient use of power (CLF Lamps, energy star appliances, ground sourced heat pumps, using gas/solar-thermal for heating instead of electricity, etc.) are all going to be cheaper to implement vs just "throwing money" at the problem through bigger and bigger solar panels / charge controllers / inverters / battery banks / etc.

Going solar, by itself, is not usually a good way of saving money. Without rebates, Grid Tied solar costs around (just very rough cost per kWhr pricing--not a quote) $0.25 per kWhr, off-grid probably costs around $1.00 per kWhour (including batteries, replacements, more components, backup generator fuel costs, etc.)...

If, you are going solar to save on your power bill / going green--and you have utility power (and the utility does allow net metering)... Generally, your best bet to start with is a grid tied system with a generator for backup power. You could also begin to experiment with a small solar/charge-controller/battery/inverter system on the side to better understand the issues (and save on generator noise/fuel costs if you have lots of power failures in your area).

-Bill

machineman

Re: Battery Bank Voltage Question...

Looks like the Crown flooded cell batteries cost about half as much as the Concord AGM and Surrette. My plan was to put them in an underground concrete basement with minimal ventalation, hence the need for AGM. My system is off-grid, nearest power pole is 2miles away.

machineman

Re: Battery Bank Voltage Question...

Good point about frequint cycling and lifetime. Its not a primary residence so discharge cycling should be considerbly less although 2days of battery back-up would be nice for cloudy days.

BB.

Re: Battery Bank Voltage Question...

You can also work the costs on installing utility lines into a rough $/kWhr price too...

Say, $50,000 to install the lines (share costs with neighbors?), $0.12 per kWhr price and you use 1,200 kWhrs per month.. And the cost for a 20 year "loan" would be 10% per year...

$50,000 * 10% / (12 months/yr * 1,200kWh/m) + $0.12 / kWh = $0.347 + $0.12 = $0.47 / kWh (after 20 years, the $0.347/kWhr term drops out)

Also, what do you see development like in the area for the next 20 years? Would having utility power to the property be a positive if you ever went to sell the property?

Using my (rather simple) example, it would be "cheaper" to install utility power than to install a pure off-grid system...

And, if you still wanted to offset your utility power, you could install grid tied solar and offset the $$/kWhr charges.

Of course, depending on the amount of power you actually use (i.e., if you really conserve down to 300 kWhr/month--your apparent cost per kWh goes up), and the utility charges, these numbers can change quite a bit...

-Bill

machineman

Re: Battery Bank Voltage Question...

I had my battery voltage incorrect in my calculations. So when I moved it to 48v my required ah went down to 680+. Thats 24 Crown 225ah 6v batteries. My total system cost is about is around $16K - 18K depending on what batteries.

I like your idea of having a grid tie so I could make some money when I'm not there but I have no nieghbors to I would have to foot the entire cost of running the power line.