Discharging AGM batteries lower than 50%?

[JackConnick]

Another thread mentioned that discharging AGM batteries lower than 50% doesn't impact their life much, as long as they are charged back up quickly after.

I have 4x L14 6v Lifelines for 600ah in my house bank. If I stay on the hook for a 2-3 days we have to run our 5.5kw genset twice a day - gets old. We added 3x180 watts of solar which in the summer helps out during the day quite a bit. But another 100ah of usage would help us quite a bit. As these are fairly new, I'm not ready to switch to LiPO4 and no room to add more capacity.

This article author shows that going down to 70-80% doesn't hurt the batteries that much if at all.

https://tab-rv.vanillacommunity.com/discussion/9940/agm-battery-depth-of-discharge-myth-busted

I'm curious what others have experienced??

 

[tiltrider1]

I view batteries as a consumable. I had 8xL16 FLA and I never hesitated to take them down to 12.0v. What usually motivates me to replace a bank like this is recharge time. At the point it was taking me 8 hours to recharge I decided to replace. They were 8 years old at the time of replacement. I had planned to just replace them with 8 more FLA when LFP prices fell through the floor. At that time LFP were 10% cheaper but the weight was the motivation. Now it’s no contest at 40% cheaper.

Still, I understand not rushing while something is working. I say, use them to your convenience and then replace when they are no longer convenient.

 

[Frosty]

I'd be interested to see what happens with your voltage. When I first set up my AGM bank (2014) I read that running them below 50% reduced service life. But, I reasoned (because I only had room for 3 batteries) that, okay, I'll basically use the money I saved from a hypothetical 4th battery, to chip on on new batteries.

In other words, I'd trade a shorter life for buying fewer batteries, thus saving the extra money for new batteries when needed (and I couldn't fit more than 3 anyway)

That sounded totally logical.

But what I found is that by going much lower than 40% SOC the voltage got low enough for me to worry about things like my refrigerator control unit (this even with thick enough wires for low voltage drop). So even if I'd been fine with a reduced lifespan, I couldn't just go to say 20% SOC, due to the voltage.

I looked at the link. I wonder how they were able to go down to 20% SOC and still have decent voltage output?

I don't know, maybe it was just me and my battery bank. That said, I kept them above 50% SOC and did get ten years out of them before I decided to give LFP a whirl. Just..... my "logical" idea of taking them much lower and exchanging some service life for less weight and space taken up didn't really pan out due to the battery bank output voltage dropping.

That is one nice thing about the LFP: Voltage stays so chipper throughout the SOC percentages.

 

[twistedtree]

I'd be interested to see what happens with your voltage. When I first set up my AGM bank (2014) I read that running them below 50% reduced service life. But, I reasoned (because I only had room for 3 batteries) that, okay, I'll basically use the money I saved from a hypothetical 4th battery, to chip on on new batteries.

In other words, I'd trade a shorter life for buying fewer batteries, thus saving the extra money for new batteries when needed (and I couldn't fit more than 3 anyway)

That sounded totally logical.

But what I found is that by going much lower than 40% SOC the voltage got low enough for me to worry about things like my refrigerator control unit (this even with thick enough wires for low voltage drop). So even if I'd been fine with a reduced lifespan, I couldn't just go to say 20% SOC, due to the voltage.

I looked at the link. I wonder how they were able to go down to 20% SOC and still have decent voltage output?

I don't know, maybe it was just me and my battery bank. That said, I kept them above 50% SOC and did get ten years out of them before I decided to give LFP a whirl. Just..... my "logical" idea of taking them much lower and exchanging some service life for less weight and space taken up didn't really pan out due to the battery bank output voltage dropping.

That is one nice thing about the LFP: Voltage stays so chipper throughout the SOC percentages.

Good point about voltage. But setting that aside, battery life is based on the number of stored/recovered amp-hours. If you discharge to 50%SOC you are half the Ah capacity and will get a certain number of cycles. If you discharge to 70% SOC you will get corresponding more cycles, and if you discharge to 30% SOC you will get correspondingly fewer cycles. Well documented batteries have SOC vs lifetime Cycle charts.

And FWIW, LFP is the same.

 

[rslifkin]

Pulling them below 50% isn't the end of the world by any means. You'll get less cycles out of them before they're worn out the lower you draw them down on each cycle. However you won't necessarily get any less total power in/out of them over their lifespan. The only other thing to watch out for is that as you draw the batteries down lower you may find that voltage sag under heavy loads becomes excessive. Depending on the size of your bank vs your biggest loads that may or may not be a concern.

 

[Frosty]

@twistedtree
Right. For me, I thought okay, sweet, I'll buy fewer batteries (had space constraints anyway), carry around less weight, and though the life (may) be reduced, I've already saved the cost/space/weight of one battery (the hypothetical 4th battery I considered cramming in).

But at least for me, I found below about 40% SOC, the voltage the batteries produced was lower than I was really happy with. So going down to a theoretical 20% (my original clever plan) didn't pan out.

But it sounds like maybe other people's AGM's keep their voltage up high enough even going that low. Interesting.

(It's moot for me now because I have gone to LFP; but the thread caught my eye because of how my clever plan didn't really work out. I was figuring based on "reduced life but hey I only bought 3 not 4 and they are easier to fit in"; but not thinking about voltage.) (Oops.)

 

[Bacchus]

Jack
I absolutely agree that greater than 50% DOD DOES NOT "DAMAGE" BATTERIES and does not significantly reduce the useful life AHs they are capable of delivering.

See posts #12 & 15 in this thread for an attached analysis I did using published DOD vs Cycle Life. It also included a Trojan recommendation that <80% DOD does not damage battys (and still provides a safety margin against damage).
If te 50% were a realistic limit the atty mfg cycle life curves would show a sharp decline in cycles for >50% DOD - but they do not show a sharp decline but rather a gradual decrease which is largely off set by the greater increase in AH / cycle.

New battery technology ?
Take a look at the attachment included in post #12.

I'm contemplating replacing my house AGMs this season as they are showing some weakness after 11 seasons (and 6 month long off seasons w/o charging ability).
No argument that 50% is not a bad design criteria for batty banks but occasional / frequent discharges >50% do not damage or significantly decrease their life cycle AH capacity... # cycles is a meaningless measure that many hang their hat on without considering MANY cycles at LOW AH don't produce any more usable energy than FEWER HIGH AH cycles!

 

[twistedtree]

@twistedtree
Right. For me, I thought okay, sweet, I'll buy fewer batteries (had space constraints anyway), carry around less weight, and though the life (may) be reduced, I've already saved the cost/space/weight of one battery (the hypothetical 4th battery I considered cramming in).

But at least for me, I found below about 40% SOC, the voltage the batteries produced was lower than I was really happy with. So going down to a theoretical 20% (my original clever plan) didn't pan out.

But it sounds like maybe other people's AGM's keep their voltage up high enough even going that low. Interesting.

(It's moot for me now because I have gone to LFP; but the thread caught my eye because of how my clever plan didn't really work out. I was figuring based on "reduced life but hey I only bought 3 not 4 and they are easier to fit in"; but not thinking about voltage.) (Oops.)

I had a similar clever plan back when I had lead, but never pursued it far enough to encounter the low voltage problem. Luck for me, and another reason to switch to LFP

 

[JackConnick]

Jack
I absolutely agree that greater than 50% DOD DOES NOT "DAMAGE" BATTERIES and does not significantly reduce the useful life AHs they are capable of delivering.

See posts #12 & 15 in this thread for an attached analysis I did using published DOD vs Cycle Life. It also included a Trojan recommendation that <80% DOD does not damage battys (and still provides a safety margin against damage).
If te 50% were a realistic limit the atty mfg cycle life curves would show a sharp decline in cycles for >50% DOD - but they do not show a sharp decline but rather a gradual decrease which is largely off set by the greater increase in AH / cycle.

New battery technology ?
Take a look at the attachment included in post #12.

I'm contemplating replacing my house AGMs this season as they are showing some weakness after 11 seasons (and 6 month long off seasons w/o charging ability).
No argument that 50% is not a bad design criteria for batty banks but occasional / frequent discharges >50% do not damage or significantly decrease their life cycle AH capacity... # cycles is a meaningless measure that many hang their hat on without considering MANY cycles at LOW AH don't produce any more usable energy than FEWER HIGH AH cycles!

Thanks. Much what the author I cited said.

One of the problems is the way the AGMs charge off my inverter/generator. They basically slow down to the point I'm only getting about 15-20ah into them at 85-90% and I turn off the genny. So that means I can't really can't replace that capacity on the hook. I only can access from 80ah to 300ah, which is silly small for the size of the bank.

For the few days I hang on the hook for 2-3 days I'll risk going to - 60-70% if I need to.

 

[rslifkin]

Thanks. Much what the author I cited said.

One of the problems is the way the AGMs charge off my inverter/generator. They basically slow down to the point I'm only getting about 15-20ah into them at 85-90% and I turn off the genny. So that means I can't really can't replace that capacity on the hook. I only can access from 80ah to 300ah, which is silly small for the size of the bank.

For the few days I hang on the hook for 2-3 days I'll risk going to - 60-70% if I need to.

All lead acid batteries charge slower as they get closer to full, it's the nature of the beast. Be aware that repeated partial cycles (especially for more than a couple of days at a time) do lead to sulfation and shorten the life of the batteries. Any lead acid battery (including AGM) wants to be charged to 100% as often as possible, although in reality it's not practical to do with a generator.

It might be a good time to look into adding some solar to the boat. Even if you can't add enough to keep up with all of your usage, it allows you to run the generator for a bit in the morning, then let the solar finish the rest of the charge throughout the day.

 

[JackConnick]

Yes I already have 3x 180 watts. Here in Seattle its a crap shoot how much I get out of them. Doesn't really charge much, but carries the boats load during the day.
I have 2x 90amp alternators and they do a good job charging things up if I go 2-3 hours, which is many days.

 

[Bacchus]

Thanks. Much what the author I cited said.

One of the problems is the way the AGMs charge off my inverter/generator. They basically slow down to the point I'm only getting about 15-20ah into them at 85-90% and I turn off the genny. So that means I can't really can't replace that capacity on the hook. I only can access from 80ah to 300ah, which is silly small for the size of the bank.

For the few days I hang on the hook for 2-3 days I'll risk going to - 60-70% if I need to.

That is exactly what I've concluded and how I've operated my AGM house when anchored or operating on gen alone. I've felt that a once / week full top off was adequate to avoid any permanent loss of capacity. My AGM house has lasted 11 seasons with very good performance... considering replacement this year due to some noticeable capacity degradation.

Stopping discharge at 50% DOD significantly limits usable capacity if operating on gen and only able to get to ~85% SOC. IMO That would force excessively large house banks or MUCH longer gen run times with low AH gain. I suppose those that do it often install solar (or LiFePO). I don't doubt the benefits of Li where it makes sense but I'm perfectly happy w AGM and won't be boating (maybe won't live) long enough to even justify the investment to switch.

 

[Coastal Cruiser]

Answers to all of your questions can be found in the Lifeline owners manual and in the other technical data available on their website.

I would trust that info more than internet opinions.

 

[Frosty]

Answers to all of your questions can be found in the Lifeline owners manual and in the other technical data available on their website.

I still miss that thorough manual! (Because I no longer have Lifelines.) Spoiled me for others where you only get a single thin sheet of paper for a "manual."

The chart of Discharge Curves at Various Rates (Appendix C) would have shown me that my clever idea was not going to work out all that well. But in the beginning there was so much in the manual that I missed it.

Luckily I had enough amp hours in the bank to usually stay above 60% SOC so all was well (turned out I didn't use as much power as I thought I would).

I would trust that info more than internet opinions.

I'd say both have their uses. At least for me. (e.g. it sounds like different AGM's might behave differently - Bacchus's bank seems to hold its voltage at lower SOC percentages than mine did as shown above) The Lifeline manual is super useful though.

In case anyone is interested:

https://lifelinebatteries.com/wp-content/uploads/2015/12/6-0101G_Lifeline_Technical_Manual.pdf

 

[SteveK]

I thought the rate of charge and the time in absorb gets batteries to 100%.

Unless GEN run time is stopped or Absorb time not long enough, why else would AGM batteries not reach 100%

 

[rslifkin]

I thought the rate of charge and the time in absorb gets batteries to 100%.

Unless GEN run time is stopped or Absorb time not long enough, why else would AGM batteries not reach 100%

Given enough absorb time they will. But it's not practical or efficient to do on generator power unless you need the generator running for something else. Late in the absorb phase the batteries will only be accepting a few amps. Getting that last bit from 90% to 100% can take a couple of hours. In some cases that last 10% takes almost as long as it did to get from 50% to 90%.

 

[SteveK]

Post 14 graph. How does a battery discharge more than 100%?

 

[DDW]

The main advantages of LFP (on a boat) are the lack of a charge tail, and the lack of voltage sag at low SOC. Light weight, capacity, and cycle life are secondary benefits. I removed Lifeline batteries from the sailboat at 14 years age, they still tested at 80% AH capacity. And you could charge them as hard as any normal LFP installation (0.6C commonly). But the internal resistance had gotten higher (and starts higher than LFP) and there is no avoiding the need for a 6 hour absorb cycle every few days.

 

[Frosty]

Post 14 graph. How does a battery discharge more than 100%?

I'm no expert, but I'm guessing it relates to the fact that the amp-hour capacity of a given Lifeline AGM is a "20-hour rating." In other words, that's the rating they use to say that a battery is, for example, a 125ah battery. But that doesn't mean it couldn't put out more ah in some situations, right?

 

[rslifkin]

Post 14 graph. How does a battery discharge more than 100%?

It looks like the percentage is "discharge as a percentage of rated amp-hours at the 20-hour rate". Lead acid batteries have a noticeable Peukert effect, so the more slowly you draw power out of them the more total power you get before they're fully discharged. That's why the 20 hour discharge curve ends at the 100% mark, the faster curves end at lower percentages, and the really slow 120 hour discharge ends at almost 115%.

For example, a Fullriver DC400-6 battery (which I have in my house bank) is rated at 415 AH at a 20 hour rate. But at a 100 hour rate it's rated at 460 AH. Discharge it faster and the rating goes down. It's rated 374 AH at 10 hours and 340 AH at 5 hours.

 

[rslifkin]

To add to the above, this also means that assuming your power usage stays the same, with a lead-acid battery bank, doubling the size of the bank will actually somewhat more than double your usable capacity because you'll be discharging more slowly.

 

[SteveK]

Got it, thanks. Fully discharged is at 10.5V on all the curves.
Lower rate=longer time.

 

[Bacchus]

I still miss that thorough manual! (Because I no longer have Lifelines.) Spoiled me for others where you only get a single thin sheet of paper for a "manual."

The chart of Discharge Curves at Various Rates (Appendix C) would have shown me that my clever idea was not going to work out all that well. But in the beginning there was so much in the manual that I missed it.

Luckily I had enough amp hours in the bank to usually stay above 60% SOC so all was well (turned out I didn't use as much power as I thought I would).

View attachment 163181

I'd say both have their uses. At least for me. (e.g. it sounds like different AGM's might behave differently - Bacchus's bank seems to hold its voltage at lower SOC percentages than mine did as shown above) The Lifeline manual is super useful though.

In case anyone is interested:

https://lifelinebatteries.com/wp-content/uploads/2015/12/6-0101G_Lifeline_Technical_Manual.pdf

Even at c/20 discharge rate you would still be at 12V at 60 % DOD (40% SOC) I would think if you are discharging at a higher rate like C/5 you should have a larger bank or need to run gen much more often / longer.

 

[cafesport]

Lifeline actually recommends owners conduct regular 100% discharge tests to asses their batteries. When new it’s not unusual for the batteries to test a bit below spec but they also improve after a few cycles.For long periods of partial state of charge (@Anchor w/o full recharge) they suggest a monthly 10 hour absorb and then an hour or four of equalization regimen to increase cycle/calendar life and capacity.



It also seems ambient temps will reduce cycles to a greater degree than deep discharges. A 96 degree engine room install is according to their specs is as bad as habitually dischargeing below 20%SOC and that SOC reading is only accurate if you have reprogrammed your capacity settings settings. Best to stay above 60% SOC until you really know what are working with.

I usually do capacity tests at .1C to save time and to get an idea of the voltage sag vs remaining capacity. Normally I parallel at least two banks and don’t experience sag problems.

 

[twistedtree]

Thanks. Much what the author I cited said.

One of the problems is the way the AGMs charge off my inverter/generator. They basically slow down to the point I'm only getting about 15-20ah into them at 85-90% and I turn off the genny. So that means I can't really can't replace that capacity on the hook. I only can access from 80ah to 300ah, which is silly small for the size of the bank.

For the few days I hang on the hook for 2-3 days I'll risk going to - 60-70% if I need to.

And that’s what really leads to shortened life. When the batteries don’t get regularly back to full charge, they sulphate and you permanently lose some capacity. But like you, few people want to run a generator long enough to bring the batteries to full charge. So the reasons to switch to LFP just keep on adding up….

 

[Frosty]

Even at c/20 discharge rate you would still be at 12V at 60 % DOD (40% SOC) I would think if you are discharging at a higher rate like C/5 you should have a larger bank or need to run gen much more often / longer.

I was typically discharging at about 6 amps or less, so about .02C (if I am expressing that correctly - bank was 375ah total. So to say it another way my typical max load was about 2% of bank size). I came of age with kerosene lamps and iceboxes so I'm still "small potatoes" in my electrical usage. Daily total between say, 40-75ah (relecting cool or hot weather). No generator; no inverter (then). So basically no "Peukert" loads at all. I was able to charge up to 100% almost daily just with solar.

I think the factor for me was that I really didn't like to see battery voltage below about 12.2 volts. That did not square with going down to 20% SOC, which would have resulted in lower voltage.

May not have been correct to worry about, but I would think of things like my refrigerator control box and didn't want to stress them (see, I did modernize there - no more icebox).

Now that I have LFP, who knows what I'll start using in terms of appliances. Next thing you know I'll be blow drying smoothies while I make toast...

********

Anyway, all I was really trying to say was that -- depending on your tolerance for lower voltage, going down to 20% SOC with an AGM bank may involve more than just overall life of the batteries. There is also supplied votage to consider. Again though, it sounds like my intolerance for voltages below around 12.2v may be lower than that of other people.