# A quick question about battery lifespan

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Solar Expert Posts: 300 ✭✭✭
I was wondering, and tried to research but didn't find much and didn't want to hijack the other current battery thread. I was wondering about the following theoretical lifespan comparison for a telcom battery vs. a deep cycle battery. The difference though is the telcom battery would be 120 ah, vs the deep cycle battery of 100 ah.

(cycled on a regular basis until one dies, at a discharge rate of 2 amps/hour, recharged at 2 amps/hour)
If 10 ah were drawn from each of them, which would last longer?
If 50 ah were drawn from each of them, which would last longer?
If 90 ah were drawn from each of them, which would last longer?

• Solar Expert Posts: 5,436 ✭✭✭✭
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Re: A quick question about battery lifespan

I think the following links have a section on cycles vs lifespan:

http://www.batteryfaq.org/

http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries

Tony
• Solar Expert Posts: 300 ✭✭✭
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Re: A quick question about battery lifespan

OK, that link to windsun put me on another path for the research. So it seems that for example a Trojan deep cycle battery discharged to 50% would have about 1600 cycles, where as the telcom battery, discharged to 40% to offset for the 20% increased capacity of the Trojan, would have about 1350 cycles. So as an estimate, the deep cycle battery, even with 20% less capacity, would still give about 20% more cycles than a telcom battery. I really thought it would be closer than that for the telcom battery.

OK, it is even more dramatic at shallower discharge. With the previous example, 20% DOD for the Trojan is 17% DOD for the telcom. The Trojan would give 4000 cycles at 20% DOD, the telcom would give about 3000 cycles at 17% DOD. Once again, a dramatic difference in favor of the deep cycle battery.
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Re: A quick question about battery lifespan

In general, Telecom batteries were designed to "float" for 40 years with the (few times a year) of fairly shallow discharge (some batteries, if I recall correctly, some long life telecom were designed for to discharge to only 85% or 80% for frequent cycling).

Telecom battery banks were not usually defined/installed in system for daily cycling (emergency use + genset).

From what I have seen and looking at the typical deep cycle battery specification... If you design one system to deep cycle 50% every day vs 25% every day--Yes, the 2x larger bank will last ~2.2x longer. So--in pure dollars out of pocket, a larger bank will cost about the same amount of money when spread out over the years. What you gain is the hassles of changing 2x your bank (maybe) every ~10 years vs your 1x bank every ~5 years (assuming deep cycle batteries designed for load and are properly recharged, etc.).

You can also go with Fork Lift type deep cycle batteries which may last 20 years of deep cycling--But you will probably need more solar panels (bank has higher self discharge, less efficient charging) and more water than a Tojan RE battery bank.

At this point--Darned if you do, and darned if you don't.

If this is a new installation--I tend towards recommending buy less expensive batteries and keep the battery bank a bit on the smaller side (i.e., 2 days of backup power to 50% discharge vs 3 days of backup/no sun to 50% discharge) and see how things work.

If the bank gets killed (something left on, kids really like playing XBox, etc.) you don't feel as bad replacing after 3 years. And, by that time, you will have a better idea of your load requirements--so you can adjust bank capacity/solar array/backup genset to match for the next 10+ year bank.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 5,436 ✭✭✭✭
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Re: A quick question about battery lifespan

When we built our new house a few years ago, we made the trade off between the price/longevity of T105s VS L16s. We decided that if the smaller batteries lasted 5 years, and the larger 8 we would win the economy game, life cycle cost per kwh. As I t turns out, in our real world, we only discharge the T105s about 10% on an averge day, so I expect them to beat the 5 year life.

That said, the L16s would certainly beat the 8 year as well, but they would suffer from limited charge current. Bottom line, size the batteries to the load as best you can, and design a charge regimen to be in the right current range.

Tony
• Solar Expert Posts: 2,490 ✭✭✭
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Re: A quick question about battery lifespan

Personally I wouldn't waste my money on Trojan t-105's. I have had a couple sets through the years and they never lasted any longer than the GC-2's ( Golf Cart ) from Sams or Cosco. ( \$80.00 ea ) Back in the day T-105's use to be \$50 and a Local Golf shop carried them. Now for \$125.00 and a shipping charge, your in the hole big time before you hook them up. If you get them on-line and you have a issue, you can forget the warranty. With Sam's you get a replacement, no questions asked, at the store. The cost of batteries today is related to the cost of Lead, the big guys negotiate big long term contracts and can produce a cheaper product.

One I would stay away from is a " Werker " Battery, Battery Plus carries, I'v had bad luck from them. Any Battery you buy that has been on the shelf over 30-90 days wet, I'd be leery of.
• Solar Expert Posts: 1,571 ✭✭
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Re: A quick question about battery lifespan
I really thought it would be closer than that for the telcom battery.

It might still be that close, it just depends which "telcom" battery you're talking about. AFAIK, there's no one single battery design that's classed as a telcom battery. Some are AGMs, some are GELs and some are even flooded. All used for telcom applications. For example, here's a page from just 1 manufacturer of 22 different types of telcom batteries: http://www.hoppecke.com/content/view/full/112

That same manufacturer's OPzV Gel Telcom batteries are rated at: "... have a live expectancy of up to 18 years and a cycling expectancy of up to 1200 cycles with 80% depth of discharge."