Depth of Discharge, AGM

[Rabbit]

Can anyone tell me what is the "correct" normal depth of discharge limit on AGM batteries? Some supposedly-authoritative sources claim 50%, others say 70% with no damage. I've asked Renogy specifically about their brand, and after days still have no answer. I'm doing a major overhaul of both my systems (this won't be my only question tonight) and it'd be nice to know for bank-sizing reasons.

Thanks!

 

[jonyjoe303]

for all lead acid is 50 percent. You want to try not to go below 12.1 volts.

 

[Rabbit]

Thank you!

 

[John61CT]

50% but it is a guideline.

If you really need to, go a bit lower, but don't make a habit of it.

Shallower the bank lasts longer, deeper means you need to replace more often.

Assuming otherwise coddled, for example

getting to true 100% Full as per endAmps as often as possible is important, ideally at least most cycles.

 

[Rabbit]

Thank you!

 

[Ticklebellly]

Deep cycle batteries are designed to be deep cycled. Seems invalid to buy a battery labeled as Deep Cycle and not use the capability. Aiming to always recharge as often as practical can be argued as the way to minimise the times you might reduce battery life through deep discharge.

 

[highdesertranger]

the problem with the "Deep Cycle" label is a lot of them are not true deep cycle batteries.

remember it is always cheaper/easier to conserve power then it is to make it.

highdesertranger

 

[John61CT]

Any usage that regularly draws below say 10% is deep cycling.

As opposed to Starter batts, which rarely go below a fraction of 1%.

You can of course do as you like, choose to only get 200 cycles rather than 1200.

Same is true for lithium chemistries.

And with lead chemistries, discharging "too low" will do that, even if you get up to true 100% every cycle.

Abusing the bank in multiple ways will of course result in even shorter lifespans.

Saying "gee that's terrible" about these facts does not change the physics of the situation.

 

[Rabbit]

Thank you all!

There's currently a lot of misinformation on this subject on line, as I said in my original post. Apparently many believe that AGM can go deeper than FLA without incurring damage. This is what I needed to clear up, and i'm grateful for the help.

 

[John61CT]

That was AGMs marketing pitch when they were the new hotness, turned out to be "overly optimistic".

But true for Firefly Oasis to some extent, in the lab good for 3600 cycles at 50% discharge, drops to 1000 cycles at 80% discharge.

But they cost 5x what FLA does per AH, and much more easily damaged by high temps or overcharging.

IMO they're only worth that if you can't avoid chronic PSOC conditions.

And lab condition results are not matched IRL anyway.

 

[Ticklebellly]

from https://en.wikipedia.org/wiki/Deep-cycle_battery

"A deep-cycle battery is designed to discharge between 45% and 75% of its capacity, depending on the manufacturer and the construction of the battery. Although these batteries can be cycled down to 20% charge, the best lifespan vs cost method is to keep the average cycle at about 45% discharge."

In the FLA vs AGM discussion, theoretically, the AGM will better resist the micro physical damage that occurs at very low discharge states.

Over life, lots of small discharge cycles yield about the same total energy output as fewer, large discharge cycles. The occasional deep discharge does little in the bigger picture. I still argue that a focus on making sure any usage is replaced as soon as possible is more important than the small gains in comparing battery technologies. Getting three years out of a battery that cost $50 is cheaper than getting 5 years out of a battery that cost 5 times that.

 

[John61CT]

There are no reasonable sized true deep cycle batteries available at 12V for under a couple hundred.

Properly cared for even the cheapest can last 6 years, even double that of you're willing to push past the standard EoL specs.

But you won't find them as 12V "marine" batts in a big box outlet.

A good resource: https://marinehowto.com/what-is-a-deep-cycle-battery/

 

[John61CT]

A deep-cycle battery is **designed** to discharge between 45% and 75% of its capacity, depending on the manufacturer and the construction of the battery. Although these batteries can be cycled down to 20% charge, the best lifespan vs cost method is to keep the average cycle at about 45% discharge."

Note the writer is not clear enough here.

All numbers are SoC, not DoD

 

[Smitty716]

Go to your battery manufacturer website and see what they say . Would probably be better to call them

 

[Rabbit]

Thank you!

 

[John61CT]

Only the top-notch manufacturers make technically trained support staff available to be contacted by end user customers.

In fact that is a very good filtering criteria to use in selecting a vendor.

Same with your ability to get tech spec sheets.

In this case a chart showing #cycles lifetime vs average DoD%.

Here's one from US Battery

 

[Ticklebellly]

Note the writer is not clear enough here.

All numbers are SoC, not DoD

That quote from Wikipedia talks DoD.   I don’t understand the SoC bit.

This thread about comparing battery construction seems well off track now.

 

[John61CT]

No these are SoC, State of Charge numbers, from 0 empty - 100% Full.

"A deep-cycle battery is designed to discharge between 45% and 75% of its capacity, depending on the manufacturer and the construction of the battery. Although these batteries can be cycled down to 20% charge, the best lifespan vs cost method is to keep the average cycle at about 45% discharge."

To translate using DoD%.

A deep-cycle battery is designed to discharge between 25% (75% SoC) and 55% (45% SoC)

Although these batteries **can** be cycled down to 80% (20% SoC), the best lifespan vs cost method is to keep the average cycle at about 55% (45% SoC).

 

[Ticklebellly]

No these are SoC, State of Charge numbers, from 0 empty - 100% Full.

I remain confused.   I can talk SOC or DOD.   The Wikipedia reference is generic and a starting point for discussion with manufacturers of specific battery products.   How much faith to put in advertising put out by those selling the produce is most up for cynical perspectives.   Discharge depth against cycle life graphs are mostly for interest only as not many people have any kind of regularity in what depth of depth of discharge they subject their batteries to.
Getting different in the LiFePo4 world I am fiddling in now.   My circle now talks around rigid top of charge voltages followed by use down to set LV cut off.   That is, charge to some set full point then use all the way down to a set LV cut off.  Then the discussion is all about what charged voltage is 100% and what discharged voltage is considered a safe Zero voltage.

 

[John61CT]

If you ask specific questions the confusion clears.

Best to avoid preconceived ideas of how things "should" be, as in the comment I was addressing.

By reading hundreds or even thousands of past threads across multiple forums, you do get to know who is worth listening to when disagreements arise.

Soon you see the same few issues being discussed dozens of times every week, and realize you are on your way