I am in the market for a 72-volt deep-cycle battery pack. That's six 12-volt batteries or twelve 6-volt batteries in series.

It's for a 3.6KVA UPS. It's rated at 87% efficiency, so that probably means that the batteries must discharge 57 amps at full load. Obviously, I want good run-time, but realize that the bigger batteries come at a very high cost.

Flooded cells are the cheapest, and I can install them in the basement where they are accessible for maintenance, so I would prefer them because of the lower cost. Marine batteries tend to be sealed or AGM and that comes with a premium cost.

Should I just go to Bass Pro and get six of their "cheap" flooded deep-cycle 12-volt batteries? Or is there a better path?

I did extensive research on this about 3 years ago. I wanted the best bang for the buck. My criteria was long life (years). I found out that this meant thick plates. The best deal I found was the US-2200 6V golf cart battery. I got mine from Interstate. They resell it as a similar part #. You can tell it is the same by the cap assembly. The Trojan T-105 is basically the same. I paid about 65.00 each for 4 of them 3 years ago. I keep mine on a desulfater. I had used 12V deep cycles from Costso before but when the stuff hit the fan the batteries bit the dust so I wanted the best for my second design. The plates are just too thin on those. I run a 12 V back up sump pump and a 300W pure sine inverter. Works nice. But you are looking at 800 bucks or so for 12 of these. I don't know of a better way though.

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

Well I have no technical knowledge, this is from experience.

I have a Basement Watchdog battery sump in my basement, and have been very pleased with their brand of batteries. And you can get them at HD.

I've had the unit 18+ years, and just last year went to my 3rd battery.

__________________
KLK, MCSE

1990 500SL

I was always taught to respect my elders.
I don't have to respect too many people anymore.

If space is not a primary concern then I also vote for the 6v golf cart batteries. I would have put them on my boat, but just couldn't find a way to fit them. I went with 5 'deep cycle' marine battery from battery warehouse, and they are kindof a joke, and don't seem to last longer than 3 years.

__________________
Cheers,
Robert

Cost per amp-hour and long life with few discharges are the primary concerns. We don't lose power often, but I would like to protect a few circuits with the UPS so that they can remain energized if I need to connect a generator (again).

I would consider several small APC UPC's, located as needed.
I have 4 of the small regular units, I don't like those integrated in a powerstrip, protecting and powering TVs, VCRs, even some clocks (Alarm).
In addition to several large units protecting my computer systems.

With units like this you can place them as best needed, and move them as things change.
AND they provide surge protection for the devices protected.

Figure 60 to 100 bucks each, much cheaper if u go to the APC store and / or consider rebuilts etc.

And I do have a manual generator setup, so these give me the hour or so to decide if I need it, hook it up and get it running.

__________________
KLK, MCSE

1990 500SL

I was always taught to respect my elders.
I don't have to respect too many people anymore.

3600 watts is quite bit. I should have asked:

What will the actual load be?
What run time do you want?

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

3.6KVA is not really 3600 Watts, but more like a bit less than 3000, depending on the power factor of the load. Electric motors are quite on the inductive side. But yes, it's a big unit. It weighs about 200 pounds without batteries and runs on 230V. It takes a balanced 230V (neutral is not used except when the unit is bypassed) and puts out two banks of 115V which need not be balanced. It also conditions the power, instantly correcting for high- or low-voltage.

I'd like to put all of the computer equipment on it, plus some lights. Probably not the refrigerator or furnace, but those sure are tempting with such capacity available. I'm just afraid that the large inductive loads will induce spikes.

As for run-time, as long as practical. That depends on what batteries I can get.

And as for small APC units, I have six or eight of them here now. I don't like the cheap ones, that run either on battery or mains, but not both. <$100 APC units are really not all that good. Once, I saw one switch to batteries and fry the load equipment. Lots of nice smoke. Good thing that it wasn't my equipment.

Well the US2200, U2200, T-105 are the best bang for the buck as I said. But you will have to decide if 12 of them will give you the back up time you need (hopefully it will). They are rated about 225AH, however you don't get to use all that. First of all they are usually charged to about 90% not 100%. And you shouldn't discharge below 20% and 30% is even better. So you just lost 30 to 40%. And the 225 AH is a 20 hour rating (about 11A). If you are drawing as high as you said then you have to derate based on that rate. Also you have to derate for the storage temp of the batteries. At 65 deg F subtract 5%. Take off another 5% if they are at 50F and another 5% if they are at 40F. Now apply your 87% efficiency to it. At the 57A you mentioned maybe they will last a couple of hours? But that would give you around the 3000 W you mentioned. At 1000W maybe 8 hours? If that is not enough you can use 24 batteries hooked series parallel but that is a lot of batteries. Otherwise you can use some L-16 type forklift batteries which have almost double the capacity but at 3x the cost. And the main thing to remember when making this kind of investment in batteries is don't cheap out on the charger. You will need to spend quite a bit to get good chargers to get the max life out of those batteries. They can last over 10 years if maintained properly, but much less if not. I would recommend a temperature compensated charger. You will probably use 2 36V chargers.

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

The UPS is designed to keep the batteries charged while in use. It is possible to disconnect the batteries with the power on, but without any backup or power conditioning.

What do these batteries typically cost? I have been searching online, and am having a very difficult time finding prices.

11A at 72V is 792 watts. That's 673 or so at the output (assuming good PF). It seems that derating would be a real issue.

One thing to look at is the size of battery pack the charge circuit was designed for. If it was designed for a much smaller pack, that is OK, but the charge time will obviously be longer. Another problem that can occur with an undersized charger is if the charger is "smart" then if it charges for a while and thinks "well the batteries should be all charged up by now", but the charge voltage still shows that they aren't, then it will throw an error code. It is not a big deal but you may have to restart it until it gets them all charged up.

As far as price, I thought more about what I paid and I think I paid 68.50 each for the U2200 3 years ago. Batteries are kind of a regional thing. They are heavy, so people buy what is made nearest to them. Some areas of the country sell more US Batteries, other areas you can get a better deal on Trojan or even Surrette. People often say "go to Sams Club" (or wherever) "they have X brand of battery". Well in some areas they do but in other areas they are selling a different brand. Same for Costco etc. I would call your local Interstate location and get a quote on the U2200. I would guess they should still be less than 75.00. I don't know for sure though. You could go to an EV (Electric Vehicle) forum and they know all the prices.

Let me clarify the discharge rate issue. The AH spec is Amps x Hours. So for example 10 amps for 20 hours = 200 AH. However, if you discharge at a higher current, the effective capacity goes down (and it goes up if you discharge at a slower rate). There are different ways to express this. For the U2200 it is like this:

Amps Hours Effective Capacity
11.5 20 230
25 8 200
75 2 150

Deep cycle batteries are often (but not always) specified at their "20 hour" rate.

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

I have noticed that they are rated at a 20-hour discharge cycle. The UPS itself came with six fairly small batteries, probably about 1/3 the size of a group-24 battery each, but it is also designed to be connected to an external battery pack in lieu of or in addition to the internal batteries.

I'll dig up the manual for the thing to see what it says about the external battery capacity. But one thing that I'm sure of is that I won't buy batteries from the UPS manufacturer.