Charging small back-up battery bank

[organic farmer]

I'm looking at putting together a small battery bank for back-up issues during a power outage.

How frequently does the grid go down in your town?

Looking at other posts, I get the idea that for some people they consider grid outages to be 'rare'. Seems odd to me, but that is how it sounds.

Howe many power outages do you normally expect per month? Per year?

For example, here in my town we usually see one or tow outages every month. We have lived here since 2005, so far we have not witnessed a single month without power outages. That is just normal PoCo incompetency. Then you have storms and disasters aside from normal outages. I generally expect 2 or 3 week-long outages every year, again unusual events like storms or disasters aside, these are just the normal outages.

To look for a power backup, we need to establish 'need'.

[schnitzeraffe]

Over the winter the outages are almost non-existent.

In the summer we get about one a week (spring and fall is less).

Lightning fries the transformers.

I already have a large whole house propane generator for those outages.

We really have no "need" to establish here.

It is a "want".

I "want" a small battery bank to power a few select items during an extended outage (hurricane) if and when the propane company can't make a refill delivery for what ever reason.

This is a back-up to my already configured back-up.

It will probably never "need" to be used.

But I "want" it.

And for what I "want", 750Ah of 12 volt battery capacity is required.

[Sunking]

So Sunking... question.

Will a 48 volt inverter with 188 Ah of battery capacity run the same length of time as a 12 volt inverter with 750 Ah battery capacity (assuming the same output of AC wattage)?

Yes on paper and in theory. A 12 volt X 750 AH = 9000 Watt Hour Capacity. A 48 volt x 188 AH = 9000 Watt Hour Capacity.They are exactly the same. In practice a 48 volt system has slightly more usable capacity because it is much more efficient with less wire losses as heat.

I'm finding it difficult to come up with the 48 volt 188 amp hour battery combo you gave cheaper than the 12 volt 750 amp hour battery combo I original listed.

In fact, it is considerably more expensive (what I found anyway).

That is because you do not know what you are looking for and not accounting for the huge saving in wiring, material, and hardware.

Battery cost should be roughly the same. Let's look at an example, a real one. Say you have two choices:

1 12 volt @ 900 AH
2. 48 volts @ 225 AH

Both batteries cost is the same of $116/Kwh battery.

You do your research and you find that a Trojan T-105RE is the best fit for you and a top rated battery. A T-105RE is a 6-Volt @ 225 AH battery. with a unit cost of $156. You will need 8 of them for the 12 volt 900 AH system, and 8 of them for the 48 volt 225 AH system. It is a No-Brainer decision on which way to go. You will save a couple of hundred dollars initial cost because the 48 volt system uses much less wire, much smaller wire, and only needs 1 Circuit Breaker or Fuse. But that is only the beginning of your savings. The 48 volt system is not only is less expensive to implement. but will last considerable longer and deliver more usable power because less is being burned off as heat. Lastly a lot less maintenance with only 1/4 the connections to keep tight and secure.

You would have to be in a 12 volt box fool to select 12 volts.

Does that shine some light on the subject.

[schnitzeraffe]

Does that shine some light on the subject.

Absolutely.

Thank you!

And I'm not set on 12 volt... it's just all I've ever known.

Are these flooded batteries the way to go?

I was planning on AGM so I wouldn't have to worry about maintenance.

And what size wire is acceptable with these 48 volt set-ups?

Do you have a recommendation for a 48 volt charger that won't take an eternity to recharge the batteries from 50% use?

[Sunking]

And I'm not set on 12 volt... it's just all I've ever known.

Yes I know, you are trapped inside a 12 volt box. Time to break out of it.

Are these flooded batteries the way to go?

I was planning on AGM so I wouldn't have to worry about maintenance.

You tell me. For a given capacity AGM will cost roughly 2 times that of FLA, and only last half as long as FLA. That makes using AGM 4 times as expensive. So you tell me; Is that a problem for you?

And what size wire is acceptable with these 48 volt set-ups?

You tell me, I am clueless. The gauge of the wire is dependent on the amount of current and how long the wire run is. Guess what you have not defined? No one can answer that question because you have not defined the parameters.

Do you have a recommendation for a 48 volt charger that won't take an eternity to recharge the batteries from 50% use?

That is easy peasy to answer. You want a charger that supplies a C/10 charge current where C = battery capacity in Amp Hours, and the number 10 is the hour charge rate. Example if you have a 100 AH battery you want a 10 amp charger. Assuming you discharge to 50% DOD or 50 AH recharge time is:

[50 AH / 10 A] x 1.2 = 6 hours.

[SunEagle]

Thank you.

Exactly the kind of helpful response I'm looking for.

So lower voltage - higher amperage batteries wired in series then?

Yes. The best way to build a battery system is to wire them in series.

Example; 6 x 2v 600Ah batteries wired in series will get you a 12volt 600Ah battery system.

Although a 12volt battery system is not going to be as good as a 48volt 150Ah system. Higher voltage means lower amps and smaller wire.

[schnitzeraffe]

Thanks guys for putting up with the FNG.

You're all providing good information.

You tell me, I am clueless. The gauge of the wire is dependent on the amount of current and how long the wire run is. Guess what you have not defined? No one can answer that question because you have not defined the parameters.

Sunking, I'm planning on a 2000 watt inverter that will have a continuous draw of 250 watt AC, brief periods of 600 watt, and surges of 1200 watt.

Wire run will be 2 to 3 feet.

[Sunking]

Sunking, I'm planning on a 2000 watt inverter that will have a continuous draw of 250 watt AC, brief periods of 600 watt, and surges of 1200 watt.

Let's start with the Good News. The run is short so we do not have to worry about over sizing teh wiring.

OK now for the bad news. Remember when I said 12 volts gets real expensive real fast, now you get to learn that. With a 2000 watt Inverter it makes no difference what power you intend to run, it is a 2000 watt Inverter period end of story. Being 12 volts it will require a wire that can safely conduct 200 amps. Ever see on a large house with over head service conducters from the utility? Those are most like 200 amp Service Conductors. Utililities can use smaller guage wire. See where this is going? OK from the battery to Inverter if you use a 90 degree C rated cable insulation like THHN or THWN-2 will require #1 copper but you will never find any, or very difficult to find, and termination connectors wil be even harder to find. So regardless if you use 75 or 90 degree insulation you can use 1/0 AWG. 1/0 is real common, easily found along with termination hardware with 1 catch. Finding someone that will sell you small quantities you require. Normally they sell by a real and smallest is 100 feet @ $2.40/ft. Try Lowes or one of the box stores.

Another thing biting you in the butt is a 2000 watt Inverter on 12 volts. At 12 volts largest recommended is 1000 watts using #6 AWG copper. 2000 watt sis the end of 24 volt range crossing over to 48 volt where I would be. The real problem for you is you have no use for 2000 watts. Operating a high power Inverter at low power is very inefficient. That 250 watt gizmo may pull as much as 400 watts from the battery. If you have not purchased the Inverter I would suggest a 600 to 750 watt model. Do that now you can work with 8 AWG which is easy and cheap compared to 1/0 AWG. You are going to find 1/0 very difficult to work with and some expensive tooling required to terminate correctly.

So assuming you stick with a 2000 watt Inverter here is my advice, take it or leave it. Very doubtful you can terminate 1/0 AWG properly. Spend the few extra bucks and buy terminated cable. your life safety is worth it. Maybe not your life, but wifey, kids, and house. A few ways to do this. Electrical shops can do it, Golf Cart shops can do it, a few electricians can do it. Or just buy them on-line. In fact it might be less expensive if you are forced to buy a 50 to 100 feet roll.

Good Luck

[schnitzeraffe]

I was thinking with 2000 watt I would have a decent cushion to keep from overloading the inverter.

A 750 watt unit should have a surge capacity of around 1500, right?

That still seems awfully closed to my 1200 surge.

What do you think about just going with 1000 watt?

If I do that, do you still recommend 24 or 48 volts?

[Sunking]

I was thinking with 2000 watt I would have a decent cushion to keep from overloading the inverter.

A 750 watt unit should have a surge capacity of around 1500, right?

Correct

That still seems awfully closed to my 1200 surge.

Not a valid concern

What do you think about just going with 1000 watt?

No problem, it will require 6 AWG

If I do that, do you still recommend 24 or 48 volts?

That is the right question, good for you. As I stated earlier operate at as high of a voltage as you can. Here are the maximum reccomended limits.

12 volts up to 1000 watts
24 @ 2000
48 @ 4000.

Do not take away that you cannot use 48 volts @ 1000 watts, because you can and is a better choice if you can find a 1000 watt inverter at 48 volts. FWIW a 48 volt 1000 watt Inverter up to 15 feet one-way wire distance requires a cheap easy to work with #12 AWG wire. Exact same wire you use on every home receptacle. A 12 volt 1000 watt uses #6 AWG. Huge difference. Hint if your Inverter requires anything larger than #6 AWG, you battery voltage is too damn low.

Hope that helps.

[schnitzeraffe]

I was looking at this Cotek.

How about a recommendation on a 48 volt charger to replace the ~110Ah used daily (shortest charge time as possible)?

[Sunking]

Think Golf Cart and EV chargers. Keep in mind with a standard 120 VAC circuit you are limited to 1500 watts and at 48 volt battery is a max of 20 amps. Some notable brands:

Eagle Dual Pro
Iota
Lester
Japlar Schauer

If you want to save a few bucks drop drown to 24 volts which is a good match for you and you open up a lot more option by using the Marine Market chargers and other companies like Samlex. Another idea is to buy you an Inverter with a Battery Charger already built-In like those from Xantrex , Tripp-Lite, and Samlex

Go figure it out.

[schnitzeraffe]

THANK YOU!

[Sunking]

You are welcome.