Estimate of quality AGM deep cycle battery if kept float charged and seldom cycled?

cdherman · December 2012

I have searching for a competent answer all over and cannot really find much.

My situation is that am grid tied and seldom lose power, but I have a single circuit with an old APC 1400 battery backup. Its dying and more than 10 years old. It powers my mission critical things in life, boiler pump, internet router, etc, but is too small for the fridge and freezer.

I am contemplating a replacement with a little more flexibility in some "what if" circumstances.

I have a nice little Yamaha inverter generator too, 2000w, for battery charging if the grid stays away too long.

Probably would get a Xantrex or triplite pure sine wave inverter/charger, 1000 or 2000w cont rated, and 100 to perhaps 400 AH of battery capacity.

This would give me backup over longer outages. Right now, assuming the world remains a stable place (I am not really a nut, but I think about possibilities) these batteries would remain fully charged in float mode from the inverter charger for extended periods of time, and seldom be discharged.

Now, as to why I am asking about battery life. If the AGM battery is going to degrade in the float mode say over 6-8 years, then I'd rather not have $1500 worth of batteries slowly cooking to death. I would just satisfy my need for backup wth a smaller battery, say 100 Ah and be done.

On the otherhand, if good quality deep cycle (not marine stuff, but proper deep cycle) batteries can be expected to last 15-20 years when keep floated and seldom or never discharged fully, then I might just invest in a nice matched set of 4 or so to give me some future proofing against all the contingencies.

Oh -- and I did read some that wet lead acid batteries might give longer life in these circumstances, but since I want the bats in my basement, that is a less optimal solution.... Not impossible though, if someone can convince me of it....

H2SO4_guy · December 2012

Re: Estimate of quality AGM deep cycle battery if kept float charged and seldom cycled?

I would consider used Telco batteries. I paid $635 for 12 2 volt cells which is 2 strings of 12 volts @ 600 AH each for a total of 1200 AH @ the 8 hr rate. These are 7 years old and are designed for a 20 year lifespan if kept in a float state for emergency backup, just like you want to do. I can sell them back for $387 if they don't work out so I am only $248 at risk, and they are working great for the last month. New these would have cost about $8400. There is a truckload due any day in Kansas City so you could pick up some yourself if you wish.

Good luck and consider other possibilities!

Skip

niel · December 2012

Re: Estimate of quality AGM deep cycle battery if kept float charged and seldom cycled?

it almost sounds like you would be interested in telecom batteries, but even they aren't meant to go 15-20yrs as lifespans that long aren't the norm.

i'm not quite sure what it is that is dying for you, the inverter, the battery, or both? if it is a case that what you have has been working then replace it with the same. for the extra loads then put another apc inline for those extra loads. you won't go for very long with the small batteries they utilize as this will only fill in the gap of a small short term outage. you aren't even close to the worst case scenario being handled, but you can run things with the inverter generator. if you put larger batteries on these inverters that they weren't designed for then you could have a case of insufficient charge to the batteries and it would kill them for the built in charger will not adjust to a larger battery for longer times of use.

BB. · December 2012

Re: Estimate of quality AGM deep cycle battery if kept float charged and seldom cycled?

cdherman wrote: »

Probably would get a Xantrex or triplite pure sine wave inverter/charger, 1000 or 2000w cont rated, and 100 to perhaps 400 AH of battery capacity.

You probably need around a 1,500 watt inverter minimum to power the typical Energy Star Fridge (motor starting current + defrost heaters). If you want to power a freezer too, you may need even a 2kW or larger inverter.

Now--you get into sizing the battery bank. If you want, for example, 3.3 kWH per day of battery powered devices, 2 days (1-3 days recommended) of "no sun" and 50% maximum discharge, a 24 volt battery bank would be around:

3,300 Watt*Hours * 1/0.85 inverter eff * 1/24 volt battery bank * 2 days no sun * 1/0.50 maximum discharge = 647 AH battery bank

And a 24 volt battery charger (for AC mains/generator backup--5% to ~13% or more current).

647 AH * 0.05 rate of charge = 33 amp minimum
647 AH * 0.10 rate of charge = 65 amp nominal
647 AH * 0.13 rate of charge = 84 amp healthy sized charger
647 AH * 0.25 rate of charge = 162 amp not to exceed

This would give me backup over longer outages. Right now, assuming the world remains a stable place (I am not really a nut, but I think about possibilities) these batteries would remain fully charged in float mode from the inverter charger for extended periods of time, and seldom be discharged.

Yep--A problem. Batteries designed for deep cycling are not great for floating... Batteries designed for long term floating are not great for deep cycling.

Now, as to why I am asking about battery life. If the AGM battery is going to degrade in the float mode say over 6-8 years, then I'd rather not have $1500 worth of batteries slowly cooking to death. I would just satisfy my need for backup wth a smaller battery, say 100 Ah and be done.

I am not sure that AGM's are long life batteries anyway... If you can deal with the monthly checking of electrolyte levels, "cheaper" flooded cell batteries (i.e., golf cart type) will last at least as long, if not longer, as the AGM's at 1/2 the cost (or less).

On the other-hand, if good quality deep cycle (not marine stuff, but proper deep cycle) batteries can be expected to last 15-20 years when keep floated and seldom or never discharged fully, then I might just invest in a nice matched set of 4 or so to give me some future proofing against all the contingencies.

The only batteries that will last that long are fork lift/traction batteries. Which (usually) have higher self discharge, use more distilled water, and should be cycled (anyone have suggestions--like a 25% discharge once a month??). So, towards the end of life of the forklift batteries you might be looking at ~1% to 2% self discharge per day:

647 AH * 27 volts floating * 0.01 self discharge = 175 WH per day = 0.2 kWH per day...

That is only ~$0.02 to $0.05 of power per day--So it is not a killer, but add costs of distilled water, your time for monitoring/maintenance--What is it worth to you?

Oh -- and I did read some that wet lead acid batteries might give longer life in these circumstances, but since I want the bats in my basement, that is a less optimal solution.... Not impossible though, if someone can convince me of it....

I can understand not wanting a large battery bank in the basement... Hydrogen gas, electrolyte mist, risk of fire (if shorted)--All add to the concern.

For folks living off grid--Battery bank in basement is done often. For an emergency system that may never get used very much--May not be worth the hassles.

A 100 AH battery bank--At what voltage (12/24/48 volts)? And what do you want to run, and for how long (how much power)? Are you planning on running solar array too?

A 647 AH battery bank @ 24 volts (or 324 AH at 48 volt) is not a huge battery bank--And at 3.3 kWH per day support, can give you a fairly comfortable off grid life (when supported with a solar array or backup genset).

But for the very rare storm/earthquake type outages (days to weeks), a genset (with a couple weeks of backup fuel) is probably going to be the more cost effective solution... But, that also assumes you don't use very much power and can use a smaller genset (your Yamaha 2kW unit, a Honda eu2000i 1.6kW unit, etc.) that will use 1-2 gallons of fuel per day to keep you "safe"... Store 20 gallons of gasoline and you have 10+ days of backup power. Longer than that, what then (city water, sewage, failures; government "ordered" evacuations, etc.)? A home on a 50x100 foot lot--You may be able to store 20 gallons of gasoline with fuel stabilizer safely (rotate through car once a year). If you have a large propane tank available--other options.

If you are out of the city and/or on a sizeable piece of property--Then larger capacity fuel storage or a battery shed/bunker may be a real option...

-Bill

mtdoc · December 2012

Re: Estimate of quality AGM deep cycle battery if kept float charged and seldom cycled?

There's a trade off to be made with flooded versus AGMs. More maintenance, more concern about corrosive, explosive gassing, lower cost versus no maintenance, no gassing, higher cost. Deep cycle flooded batteries will potentially give you more deep cycles and longer life if you go with forklift type traction batteries. But longer life only if you keep up with maintenance and as mentioned by Bill forklift batteries require regular cycling .

If you're willing to pay up - for your application, with desired long float life and rare deep cycling, have a look at Odyssey Batteries. Float life is rated at 8-12 yrs. and you get 700 cycles at 50% DOD. You can get the PC1800 FTs 12V 218AH for about $500 if you look around. I have these -and am happy with them - though I can't speak personally of the life span yet. Ask me again in 8 yrs.:roll:

H2SO4_guy · December 2012

Re: Estimate of quality AGM deep cycle battery if kept float charged and seldom cycled?

Real Life example.

In 2006 I purchased used Lucent AGM Telco batteries that were designed to float until needed, but not deep cycle. There is one 48 volt string @ 115 AH @ the 8 hr rate. These run my lights and sometimes other loads. Just the other day using a Kill-A-Watt meter hooked into an Exeltech XP-1100 inverter, I used over 2 KW not counting the inverter draw and was around 50% SOC. I paid $250 for these and can sell them back currently at $193. So they have cost me about $10 per year to run. How's that for cost effective?

Skip

cdherman · December 2012

Re: Estimate of quality AGM deep cycle battery if kept float charged and seldom cycled?

Wow, lots of great information. I did more reading this afternoon and evening too, and have came to the conclusion that while flooded lead acid bats would be superior in terms of longevity, it might not be worth the trouble to use them for a battery backup setup. As noted, the cumulative maintainance hassles cannot be directly equated to dollars and cents (or sense) but its real...

I assume the telco bats are flooded as well, right? If not, who is getting this truck load in Kansas City soon? I might be in the market....

My current APC 1400 has a couple 12v gel bats inside that I replaced about 30 months ago. Now, its starting down thermal breakdown again, just like 2.5 year ago, where the bats are requiring more and more charge to keep them at the target voltage that the APC unit wants. That causes the charge circuitry in the APC to heat up and kick on the fan, which runs constantly. To get the fan to shut up, I'd be looking at another $100 worth of proprietary sized gel batteries......

Or I could gerry rig two 12V AGM bats to the APC unit (guys do it all the time). The prospect of doing that, led me to research some of the more current hard wired inverter/charger offerings. Pretty neat circuitry what is out there, although amazingly, not THAT much more than the old APC is capable of....

I dunno -- I get on a kick and try and analyze things carefully before I make changes.

For ease of use, a single decent AGM battery that gets floated to death and needs replacing every 6-8 years might be the best solution, in terms of "maintainance free" solutions. But the APC 1400 wants 24 V, so I'd need two bats.

Anyhow, thanks a pile for all the very detailed responses. I really like this site. Used a lot of the input here to design my grid tie setup 3 years ago which has been a sucess. I should come back and give back more.....

cdherman · December 2012

Re: Estimate of quality AGM deep cycle battery if kept float charged and seldom cycled?

H2SO4_guy wrote: »

Real Life example.

In 2006 I purchased used Lucent AGM Telco batteries that were designed to float until needed, but not deep cycle. There is one 48 volt string @ 115 AH @ the 8 hr rate. These run my lights and sometimes other loads. Just the other day using a Kill-A-Watt meter hooked into an Exeltech XP-1100 inverter, I used over 2 KW not counting the inverter draw and was around 50% SOC. I paid $250 for these and can sell them back currently at $193. So they have cost me about $10 per year to run. How's that for cost effective?

Skip

Hmmm -- I was posting as you posted this up. So they are AGM bats? I'd need a boat load of them to get to 24v and I don't think my dinky APC 1400 could ever keep them charged. But they might make a good setup for a Xantrex Prosin 2000, hooked to 6 of 'em.

How does one find telco bats locally? You mention something in your first post....

H2SO4_guy · December 2012

Re: Estimate of quality AGM deep cycle battery if kept float charged and seldom cycled?

If you want to PM me, I'll give you my phone number and share the info with you.

Skip

niel · December 2012

Re: Estimate of quality AGM deep cycle battery if kept float charged and seldom cycled?

keep in mind that if you opt for larger capacity batteries on the apc that the charger may not keep proper pace with it so an external 3 stage charger would be warranted that can properly charge them at that voltage. for agm types you should be able to charge with as high as 20% of the ah capacity safely and this is being said as a blanket statement as some can go even higher. in any case try to at least shoot for a 10% rate. the internal charger of the apc will still send power to the batteries, but will be insufficient to give a bulk charge even though this may suffice to maintain the larger batteries when already fully charged.

Estimate of quality AGM deep cycle battery if kept float charged and seldom cycled?

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