single wire low RPM cut in. Suggestions...

[BruceM]

If you have to go 4 batteries for capacity, you'll get better service life and capacity out of them by going 24V.  Half the current draw so lower losses in the battery and wiring to inverter, and less equalization charging and water added.  If you're sticking to 12V for other reasons...then find some larger capacity batteries or individual 2v cells and avoid parallel batteries.

[glort]

I'd also go 24V.

If you have to run any length of DC wire it can be half the gauge to start with.  You are going to be looking at near 200A with 2500W load with 12V.  Switches and connectors are pricey and hard to get at that sort of rating. 100A is much cheaper and easier to get.

Other thing is seeing you are going whole hog on this, Why not add some solar  panels to the batteries?  Normal used roof panels can be had easy and cheap where as 12V Panels here at least are firstly relatively low wattage and secondly stupid expensive on a watt for watt basis with even new roof type panels.
I run a cheap 30 A PWM controller with a bunch of domestic rooftop panels for months and it worked fine with 24V even though the load voltage of the panels as supposed to be 34 and open circuit was 42.
When I tried putting the " 24V" panels through the controller to a 12V battery, all the magic smoke came out as well as a lot of unpleasant sounds.

A few solar panels could mean the only time you ever have to fire the genny was for cooking or larger loads. Few solar panels would be great for those light loads and might supply them completely depending on how many you have which would prevent any cycling of your batteries and extend their life significantly. Would only take 4x 250W panels to supply 1/3rd of your generators capacity and you could probably run the coffee maker and the Microwave individually without the genny and then let it bring the battery's back up to full charge  in between uses.

Also be aware that a genny will have to run a LONG time to fully, 100% charge your batteries because the more charge, the greater the charging fall off. With with the solar it would float them at 100% till you used them and then work to bring them right back to 100% and keep them there automatically, silently and fuel free.  They may also be working one topping the night drain up in the morning before you are out of the house to fire the genny.  Quicker you get that power back in  the batteries the better!.
                                                               

[BruceM]

Good point by Glort on PV to finish charge the batteries and save a lot of generator time and noise, plus a big boost in battery life.

For those reasons I always let my PV system finish charging the batteries even on dark winter days...the generator does only the bulk charge.

[RJ]

Thanks for the input fellas.

I was doing some digging around for some ideas and came across this post in a solar forum that I thought was interesting.

http://forum.solar-electric.com/discussion/352587/battery-advice-and-recommendation

This fellow had the same idea I was thinking of doing but he did it 8 years ago. Never used his batteries and now they're dead. In this thread there was a reply that I thought was pretty reasonable:

"Here's a suggestion, get yourself a grid supported inverter charger which has load shaving capabilities, run critical loads off the  inverter, during the day use  your excess solar production to support or partially support the loads since the inverter would have AC pass through capabilities. In the evening when peak billing occurs, program the inverter to allow battery power to support the grid, when peak billing terminats, grid power will recharge the battery at lower rates. In such a configuration you would actually be getting some payback from the batteries rather than them just sitting there waiting for an outage, of course they will still be available in such an event. Having a strictly backup system has little return and the expenditure for the batteries goes, for the most part, waisted, which is a shame, expanding your array would save you energy during the day, by offsetting grid consumption. Here is an example of a unit which will work for such an application, others are available, Magnum, Outback come to mind but I'm not personally familiar with them, although I understand they're excellent products. https://www.solar-electric.com/schneider-electric-sw-conext-inverter-2524-120.html

Of course I'm making the assumption there is peak billing by including load shaving, but advantage can still be achieved without, in the form of grid support, during the day. Some ideas to ponder.

One could go further by selling back to the grid during the day, if loads are light, but that's a little more involved and of course if it is permitted by the utility."

The idea of load shedding has merit and something that I might consider in the future. My utility has very low off peak rates of .047/kwh , peak is significantly higher. Right now I'm renting a house, but will buy/build within the next 12 months.

However I can start building a system now.

Reason I went with the Aims inverter was it did what I wanted at a reasonable cost ~$500 without a lot of extra bells and whistles. It got my feet wet in the world of inverters and batteries maint. As soon as I started looking at 24v the price went 2x or 2x quickly.  That being said I certainly didn't get as far into building the system on paper to include wires and ends so thanks for that tidbit glort.

Bruce can you elaborate on the "big boost in battery life" by allowing the PV to do the finish absorption stage of charging.

Perhaps it's wise to not look for an inverter to do it all but piece it together? In general I like individual components when I build things, however all in one solutions like this Samlex are compelling yet again nealry 3x the price of the aims

https://www.invertersupply.com/index.php?main_page=index&cPath=1304_390_405

Specific samlex:

https://www.invertersupply.com/index.php?main_page=product_info&cPath=1304_390_405&products_id=16982

Appreciate the advice guys.

[mike90045]

to save $ by using expensive batteries for load shaving, requires awfully high electric rates.  Higher as in 2$ KWh, in order to break even on batteries.
But you do end up with the biggest UPS on the block.

do the math for your rates and battery costs/lifetime, before you buy.

[RJ]

to save $ by using expensive batteries for load shaving, requires awfully high electric rates.  Higher as in 2$ KWh, in order to break even on batteries.
But you do end up with the biggest UPS on the block.

do the math for your rates and battery costs/lifetime, before you buy.

No doubt.

The load shaving idea isn't intended to replace the battery cost, more to mitigate them. As in the link provided above, the gentleman built a UPS with 16 x Trojan T105 batteries and they never got used.

I'm in the SE now, and we lost power during Irma, although only for a few hours, and the year before that, Mathew. My thought is to possible kill two birds with one stone. I have not run the numbers yet but I certainly can for a purely economical standpoint.

For that matter a generator would suffice. The original purpose of the system was to run the basics in the house for a short period of time, say 6 hours.

Regarding my electric pricing structure, this is how it works - and it's rather unique as far as I can tell.

There are THREE main categories of cost incurred by our system by residential and commercial members:
1. Account charge     Residential    80¢/day         Commercial    $1.10/day
This recovers the cost of making service available to each member.

2. Energy charge       Residential    4.7¢/kWh       Commercial    5.7¢/kWh
This is the energy portion of the power cost.

3. On-Peak charge     Residential    $12/kW         Commercial    $14.75/kW
This is the rate for the highest ONE hour of electric use during the On-Peak time frame of the billing period.

[BruceM]

If you read up on lead acid battery service and lifespan, you'll see that sulfation is a common cause of early cell death.  It happens when batteries are  not  brought up to 100% of charge and held there long enough to break down the sulfation that always occurs.  The longer the battery sits at NOT full charge, the more sulfation you get.  Equalization charging (slightly elevated voltage for 4-5 hours) is used to combat this problem, but it uses power(generator or PV), water and eats the positive plates so you don't want to do it more than necessary.

With a PV charger added, the batteries will be kept fully charged at the float voltage most of the time, and thus can have a lifespan closer to the float service life (except when cycled at 50% or less for power outages) ...often twice the normal service life. 

[RJ]

Bruce. Yes I read up at battery university. Thank you for the clarification.

The gentleman that built his UPS said the following:

Code Select Expand

The batteries never got used or cycled as the whole system is only intend for back up power in the event of an natural disaster / earthquake or grid shut down etc.
I maintained the old batteries for the past 8 years, trickle charged them, equalized them when needed and filled them with distilled water when needed.
They still look like new .. but the batteries aren't taking or maintaining a charge anymore. So it's time to replace them.

That is why the thought of load shedding was considered he got zero use out of them, and appeared to maintain them correctly and never got a chance to use them. AGM batteries are an other option. Appear to be about twice the cost of FLA.

[glort]

3. On-Peak charge     Residential    $12/kW         Commercial    $14.75/kW
This is the rate for the highest ONE hour of electric use during the On-Peak time frame of the billing period.

[/quote]

$12 a KW???

That would have to be unique all right!
Is that a typo or something?
To me that says you are going to get charged $12 per day, anytime you use power in the peak period?

Can't be right, would you care to explain?

As for the rest of your rates, 4.7C, There is no possible way on this gods green earth you are going to save money on that rate! The cost of the equipment amortized over it's life wouldn't come near that alone.

I got to say, it seems like you are waaaay getting carried away with this.  From what I understand you are wanting to have something to see you through occasional outages. You are now talking about all sorts of equipment that will cost big bux and net you a negative return with setting up to offset your power bill at that rate.
I may not be understanding everything correctly but but it seems to me you are going headlong into an expensive, complicated system that will require decent amounts of maintence to have any decent working life.
For your objectives, seems to me all you need is a generator worth a couple of grand and the rest you'd be better off putting into a solar panel system to offset your daily usage.

Batteries, chargers, inverters, panels, controllers, wiring, electrician fees..... It really does seem like you are building a monster that is going to cause you more grief than good.

If you want to go offgrid, then do it and properly. If you want a backup for the grid, seems to me you are going waay overboard and your money could be much better spent on things you and your family will get a lot greater benefit and pleasure out of. 

:0)

[RJ]

Glort, no not a typo. I copied pasted it directly from the COOP site. http://www.mcecoop.com/content/rate-structure


The way it works is the electric company take the highest ONE hour load you have during that peak hours for the month. So say you have peak usage of 5 kwh during that peak window in the month of December, you get billed at the rate $12/kwh or $60 plus you normal kwh rate. For simplicity sake say I have 500 kw used in a month. And my max one hour usage during peak hours is 5 kwh. ((500*.047) + (5*12)).  (23.50)+(60) = $83.50 for the raw electricity minus any fees. It certainly behooves one to cut usage during peak hours. As of now I have my water heater on a timer and turn off all my AC units.

I agree on complexity. I still want to build a UPS, however I think I will be best served to keep it simple, perhaps instead of 12v go to 24v as you and bruce suggested and run 4 golf cart batteries in series. Interstate 6v golf cart batteries at my local costco are $78 plus tax and $2 battery fee.

[BruceM]

That sure is a strange peak billing scheme. You only get billed extra for your highest hour's use...not extra for the total power you use during peak hours. 

[Tom Reed]

Billing for demand is common on commercial electric rate tarrifs.

[glort]

It still seems like it might actually be pretty cheap compared to what we pay here.

What is the peak period?

[BruceM]

It would certainly promote changing to gas hot water, clothes dryer, and stove. 
I like the sales pitch about how you have a choice...this is not really true for a working class family with both parents working and renting.  There aren't enough hours in the day for them to shift to!

[RJ]

Alright settled on a 24v system. with 4x6v golf cart batteries @225 AH. Was thinking 1500w pure sign inverter. Would like to get a good quality unit, and suggestions?

Looked at the Samlex 1500w here:

https://www.invertersupply.com/index.php?main_page=product_info&cPath=1304_390_405&products_id=2137

High efficiency
Can be hard wired
Temperature controlled cooling fan-reduces energy consumption
Low interference
Wide operating DC input range: 21.4 - 33.0 VDC
Commercial grade design suitable for heavy duty loads, long periods of continuous operation & for emergency back up
UPC-Universal Protection Circuit: low voltage, over voltage, over temperature, over load and short circuit
Low idle power draw
Dual GFCI protected AC outlets
Optional remote control (sold separately): Model RC-200
Safety certified to UL standards, FCC compliant

Price $460 ~3.26/w

Same inverter x 2000w is $705 ~2.83/w

At a point more money then that it might be worth looking into the combo units for future upgrade??

At some point I would like to tie solar into it, perhaps 3 or 4 250w panels. That will be added later, perhaps in the spring when there is more sun to deal with. Plus it will give me some time to find some panels locally on CL

Will need to select a 24v alternator.

The ones I have are over kill for the size of the battery bank.

Was thinking of getting perhaps a sterling alternator regulator to give me 4 stage charging (3)

http://www.sterling-power-usa.com/sterlingpoweradvancedalternatorregulators.aspx

Or perhaps pull the regulator out of the alternator and feed it into a solar MPPT charger such as this?

https://www.invertersupply.com/index.php?main_page=product_info&cPath=1304_258_260&products_id=1030

That way when I go solar I can dual purpose it, when the sun isn't shining I can disconnect the solar and hook up the car alternator to charge the bank.