When To Replace AGM Batteries - Recalibrating Battery Monitors

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When To Replace AGM Batteries - Recalibrating Battery Monitors

October 29, 2017, 03:34:22 pm

2004 rear bath. Original Trojans replaced with 300 amp (6-volt x 2) Lifeline AGMs about 7 years ago. Will hold a charge, but capacity seems to be diminishing recently (declining voltage), requiring more frequent use of generator to charge them. They are stored underneath rear-facing dinette bench, next to old slide-out battery compartment.

Is 7 years a reasonable lifetime expectancy?
Should I replace with Trojan Reliants or another AGM brand?
Is there a newer technology than AGMs that is worth considering?

FYI, we have 4 solar panels (340 watt) with upgraded controler, an upgraded 70-amp, multi-stage converter (per Larry W's upgrade several years ago), digital battery monitor, and whole-house 1800W modified sine wave inverter.

Thanks--Ted

Re: Advice needed on replacing old AGM batteries

Reply #1 – October 30, 2017, 01:58:38 am

Seven years is a good lifespan.
Trojan is a well known manufacturer of industrial grade batteries, they are my choice.
Lithium batteries are the next step up in battery technology and will cost several times more than AGMs.

If you have plenty of bucks, and have to own the state of the art battery, give AM Solar a call.
If you were satisfied with the performance of the existing battery, replace it with another set.

Larry.

Re: Advice needed on replacing old AGM batteries

Reply #2 – October 30, 2017, 04:57:00 am

"Is 7 years a reasonable lifetime expectancy?"

LD-TED

Apparently the answer is 'Yes'! I had Lifeline AGMs installed by AM Solar in 2010. Just this month I had the Mothership install a new set for the very reason you stated.

Like tires, they just timed out.

Re: Advice needed on replacing old AGM batteries

Reply #3 – October 30, 2017, 09:54:32 am

My experience also supports 7 years as a reasonable life for RV house batteries.

AGMs don’t seem to last any longer or shorter for me.

All batteries’ capacity declines over time, I guess about 3-7% per year. The Peukert effect The exact number is a deep technological debate. This can be a big deal. Here is an example I use from an expert:

Assume a 100Ah battery, and observe the rule that you don’t use more than 50% capacity before recharging, because sound so reduces the battery ability to fully recover and reduces capacity even faster. So you have 50 usable amps when batteries are new.

After 5 years of use, say you actually use 40 Ah of battery capacity between charges. Your batteries have 35 amps of usable capacity due to this ongoing Peukert decline. Yikes! You have been using more than 50% of battery capacity since year 4, and thus further accelerating their demise.

The upshot- adjust your battery monitor every year to reflect the Peukert effect so you know the actual capacity of your batteries. Discharge as low a percentage of battery capacity as possible, trying not to dip below 50% of actual (not initial) Ah capacity. If your amps draw is less than 20, you can get slightly more than the stated Ah capacity as well. In any case, 7 years is a good lifespan.

I just had to spout all my newly acquired battery knowledge.

Re: Advice needed on replacing old AGM batteries

Reply #4 – October 30, 2017, 11:32:50 am

Quote from: paul banbury - October 30, 2017, 09:54:32 am

The upshot- adjust your battery monitor every year to reflect the Peukert effect so you know the actual capacity of your batteries. Discharge as low a percentage of battery capacity as possible, trying not to dip below 50% of actual (not initial) Ah capacity. If your amps draw is less than 20, you can get slightly more than the stated Ah capacity as well. In any case, 7 years is a good lifespan.

I just had to spout all my newly acquired battery knowledge.

How does one adjust the monitor to accommodate these changes? I have a Link Pro. The manual seems to be written in Greek. 🤔 — Jon

Re: Advice needed on replacing old AGM batteries

Reply #5 – October 30, 2017, 11:51:11 am

Can’t help with a Link Pro. My Victron is accessed by pressing the Settings button. Sorry

Re: Advice needed on replacing old AGM batteries

Reply #6 – October 30, 2017, 12:20:41 pm

Quote from: Jon & Loni - October 30, 2017, 11:32:50 am

How does one adjust the monitor to accommodate these changes? I have a Link Pro. The manual seems to be written in Greek. 🤔 — Jon

As the battery ages, lower Link Pro's battery amp-hour stetting by 8-10%, per year, to estimate the decline in battery capacity.

The Peukert number refers to how efficiently the battery uses power during charging and discharging.
A certain amount of the energy produced or used charging , by the battery, is wasted as heat, a byproduct of the chemical changes that occur .
Each battery has it's own Peukert number, determined by the manufacturer .
If you look in the Link Pro owner's manual, you will find a list of Peukert numbers for commonly used batteries, including your T-105s. The Peukert number is used during the programming of the battery capacity meter.

Larry

Re: Advice needed on replacing old AGM batteries

Reply #7 – October 30, 2017, 07:27:00 pm

Thanks to all. FYI, current AGMs installed by AM Solar when they added two solar panels to the factory set of two. They also installed the upgraded solar charger.

I particularly appreciate Larry's suggestion of reducing maximum capacity by 8-10% each year. I set my monitor at 300 7 years ago and have not changed it, not realizing that I should not discharge to 150 in perpetuity. It should be intuitively understood, but in my case, I never translated the concept to actual practice in succeeding years.

This reduced-capacity-over-time concept further reinforces the view that one should install the highest capacity batteries one can afford and fit in the compartment. In the absence of advice to the contrary, I will probably go with Trojan Reliant.

Thanks again for taking the time to respond.

Re: Advice needed on replacing old AGM batteries

Reply #8 – October 30, 2017, 08:35:41 pm

Do the battery's capacity decline even if you only use them a few months per year? We have two and a half year old batteries with nine months maximum use on them. What kind of life might be left? Probably have drained them low about 5 times. Thanks!

Re: Advice needed on replacing old AGM batteries

Reply #9 – October 30, 2017, 09:58:35 pm

Yes, the decline is inevitable. If you don’t store them on a good battery maintainer, and they self discharge completely, their decline is hastened. Batteries are amazing and complex chemistry, but they need love.

So really, we are all batteries.

Re: When To Replace AGM Batteries - Peukert's Law (decrease in battery capacity)

Reply #10 – November 01, 2017, 11:34:18 am

I hate to be pedantic, but Peukert's Law is not what we're talking about here--Peukert's Law isn't about battery lifespan or charging efficiency. In a nutshell, Peukert's Law says that with a lead-acid battery, the faster you discharge it, the less you can get out of it.

For example, if your 300 amp-hour battery bank is powering 10 amps worth of lights and whatnot, you'll (theoretically) get 30 hours from it (300 divided by 10). But if you run a coffeemaker or microwave oven, drawing 100 amps, you will NOT get 3 hours (300 divided by 100)--you'll get a lot less. That's Peukert in action.

But this has nothing to do with the batteries' lifespan in years. Assuming that you don't over-discharge them, once you bring them back up to 100%, they should be as good as ever.

And charge efficiency is a different parameter--it's a separate setting in amp counters such as the LinkPro or Victron BMV-700/702. In practical terms, it means that if you put, say, 110 amp-hours into a battery, you can only expect to get about 100 amp-hours out (at modest discharge rates). From what I've read, for the batteries we're using charge efficiency is typically 90%-95%, with Lifeline claiming the high end.

(Note that all of the above applies to lead-acid batteries--either flooded or AGM type. Lithium batteries have very different characteristics--for example, there's no Peukert effect as far as I know.)

Re: When To Replace AGM Batteries - Peukert's Law (decrease in battery capacity)

Reply #11 – November 01, 2017, 07:53:23 pm

Quote from: Andy Baird - November 01, 2017, 11:34:18 am

I hate to be pedantic, but Peukert's Law is not what we're talking about here--Peukert's Law isn't about battery lifespan or charging efficiency. In a nutshell, Peukert's Law says that with a lead-acid battery, the faster you discharge it, the less you can get out of it.

For example, if your 300 amp-hour battery bank is powering 10 amps worth of lights and whatnot, you'll (theoretically) get 30 hours from it (300 divided by 10). But if you run a coffeemaker or microwave oven, drawing 100 amps, you will NOT get 3 hours (300 divided by 100)--you'll get a lot less. That's Peukert in action.

But this has nothing to do with the batteries' lifespan in years. Assuming that you don't over-discharge them, once you bring them back up to 100%, they should be as good as ever.

And charge efficiency is a different parameter--it's a separate setting in amp counters such as the LinkPro or Victron BMV-700/702. In practical terms, it means that if you put, say, 110 amp-hours into a battery, you can only expect to get about 100 amp-hours out (at modest discharge rates). From what I've read, for the batteries we're using charge efficiency is typically 90%-95%, with Lifeline claiming the high end.

(Note that all of the above applies to lead-acid batteries--either flooded or AGM type. Lithium batteries have very different characteristics--for example, there's no Peukert effect as far as I know.)

Well, now I’m more confused. Andy, are you just making a semantic point? Does one still need to recalibrate the battery monitor? 🤔 —Jon

Re: When To Replace AGM Batteries - Peukert's Law (decrease in battery capacity)

Reply #12 – November 01, 2017, 08:57:12 pm

Sorry if my explanation wasn't clear, Jon. Yes, if you have an amp-counting battery monitor such as a LinkPro or Victron BMV-700/702, you'll need to lower its "battery bank capacity" setting every year as Larry recommended, because otherwise it'll give more and more overly optimistic readings as time goes by and your batteries age.

My (semantic) point was that this has nothing to do with Peukert's Law, so the title of this thread is misleading.

If you want a battery monitor that doesn't need recalibrating to compensate for batteries' gradual loss of storage capacity over the years, there is one: Balmar's SmartGauge. It constantly monitors your battery bank's overall condition and adapts its readings accordingly--you could say that it constantly recalibrates itself, so you never have to. But it costs twice as much (about $280) as a simple amp counter. It's your choice.

Re: When To Replace AGM Batteries - Peukert's Law (decrease in battery capacity)

Reply #13 – November 01, 2017, 09:23:17 pm

I’ll let Andy or Larry or someone who has more expertise than I answer your question Jon. I do recalibrate because I suspect my batteries suffer some abuse each season, and their Amp capacity has diminished as a result, and I don’t want to exacerbate this abuse by overly optimistic readings of my batteries’ state of charge.
Here is a very informative treatise in the topic- Keeping Your Battery Monitor More Accurate Photo Gallery by Compass Marine...

Andy’s Point is not mere pedantry. If that is a term. He is right. And what’s great about that is that users like me who sip small amounts of Ah (we use 3-15 Amps per day) can, all things being equal, get more Ah from the same battery than those who like to use inverters to microwave, run servers, etc.

Re: When To Replace AGM Batteries - Peukert's Law (decrease in battery capacity)

Reply #14 – November 02, 2017, 02:31:16 pm

Quote from: paul banbury - November 01, 2017, 09:23:17 pm

I’ll let Andy or Larry or someone who has more expertise than I answer your question Jon. I do recalibrate because I suspect my batteries suffer some abuse each season, and their Amp capacity has diminished as a result, and I don’t want to exacerbate this abuse by overly optimistic readings of my batteries’ state of charge.
Here is a very informative treatise in the topic- Keeping Your Battery Monitor More Accurate Photo Gallery by Compass Marine...

Andy’s Point is not mere pedantry. If that is a term. He is right. And what’s great about that is that users like me who sip small amounts of Ah (we use 3-15 Amps per day) can, all things being equal, get more Ah from the same battery than those who like to use inverters to microwave, run servers, etc.

Thanks for the link. It’s does indeed explain the theories. Guess I’ll have to do some head scratching over the LinkPro manual to figure out the programming, but I fear it will still be Greek. On the bright side, we also are very low amps users, and have never discharged below (indicated, and now I realize probably wrong) 80% capacity. We use several Luci Lights for most of our evening lighting, much preferring their softer glow to either fluorescents or LEDs. Probably wouldn’t work for most, but does for us. — Jon

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