AGM battery Depth of Discharge myth busted

rh5555

I have been wondering for some time if the "don't discharge your batteries beyond 50%" rule really applies to AGM batteries.  AGM batteries claim to be deep discharge, and 50% discharge doesn't seem very deep to me!  So I did a little research...

If you look at the data for Odyssey AGM batteries, you find that they are good for 630 discharges to 50% but only 400 discharges to 80%.  Seems obvious that the 50% depth of discharge (DoD) is better.  But hang on, if you flatten your batteries to 80% DoD, you won't have to do the recharging as often, so fewer cycles.  The important parameter is your lifetime capacity in Amp-hours, which you get by multiplying the battery capacity * DoD * number of discharges.  For a 225 Ah battery, your lifetime capacity is 71,000 Ah if discharged to 50%, but is 72,000 Ah if discharged to 80%.  Indeed, If you look at Odyssey's data for 100% DoD, you still get 72,000Ah of lifetime capacity!  For Odyssey batteries, there is absolutely no reason to worry about your DoD, you're not gaining anything by starting up that generator when your battery gets to 50%.

But we don't have Odyssey batteries, we have Harris batteries for which there is a dearth of information about its discharge cycles vs. DoD.   So instead I created an "Average" battery from Trojan, Concorde, Odyssey and Rolls AGM products (these were the only ones I could find with detailed DoD data). 

I've included the 3 data points for the Harris battery, and you can see that its number of discharges under-performs the average, but has much the same overall shape.  So now you can see how DoD relates to total capacity for our average AGM battery:

So going from 50% DoD to 80% DoD will just lose you 6% of your lifetime output from your battery.  Not really worth worrying about.

Take home message:  If you're using AGM batteries, don't get too fixated upon getting them recharged once you get to 50% depth of discharge.  Any time up to 80% is good.  More important is not to leave them discharged any longer than you have to.

TNOutback

Here is the Harris Battery lifecycle chart

rh5555

That's what I used for the 3 points at 30%, 50% and 100% DoD

JEB

This is very interesting.  Very well done.  To add to your database, here is some information from US Battery.  Trojan, Concorde and Odyssey are pretty high-end manufacturers, particularly Trojan and Concorde who are probably the very best at the AGM game. US Battery is more mainstream, in the same league with East Penn and Johnson Controls.  This data might help round out your findings and provide a closer parallel to Harris. 

rh5555

Thanks a lot @JEB.  More data are always good!  The US Battery data are in middle of the bunch regarding discharge cycles and when I added them to the analysis, they didn't change the averages significantly.  However, they are interesting in that they have the flattest total capacity of all the batteries I looked at:

It doesn't seem to matter at all how deeply you discharge them, they have 120,000 Ah of juice in them, and that's what you get!

JEB

I’ll say it again.  Very well done. This is top-notch analysis.  What I found the most interesting is that it seems to confirm than you can regularly hammer these batteries well below 50% DoD and not do too much damage over the long term.  But that wasn’t my concern. I was concerned about the occasional week-long boondocking trip where I wanted to draw down below 50% toward the end and was afraid to do so.  Your data seem to show that if you only beat on the batteries once or twice a year with a 70-80% draw, as I would, the impact is probably negligible.  

ecstatic

Does anyone have a chart that shows battery voltage values converted to relative battery percentage left for the 2 6V 400 batteries?

pthomas745

A state of charge chart?

rh5555

The numbers in @pthomas745 chart apply to batteries that have been disconnected for several hours.  Then they work well.  Any (recent) current draw (or charge) will make these numbers inaccurate.  Use with care.

DougWW

Very clear and very helpful. Thanks.

Sharon_is_SAM

@ecstatic, your two 6 volts are additive resulting in 12 v output, so the same chart applies for a single 12 v vs your set up.

ecstatic

^^ Thanks for that chart.  As far as I can tell those values correspond to the values under load.  Resting values will be higher:

ColoradoJon

rh5555

@ecstatic, here is a state of charge table for the Harris AGM golf-cart batteries in the 2020 TAB400 that you are looking for.  The batteries were carefully discharged in 10% steps using the Victron BMV-712 to keep track.  The batteries were rested after each step for about 2 hours with no current flowing by which time the voltage had stabilized.

I didn't discharge below 20%, because I value my batteries too much!  I would expect the open circuit voltage at 10% state of charge to be about 11.75V, as the voltage dropped a reliable 0.15V for each discharge step.

riooso

Thank you guys ever so much for these posts. I have also bought a "smart" battery minder and they make a huge difference in performance of my battery also soooo..... going forward I am not going to worry about all the "dire" consequences of having my battery go to 70%.

ChrisFix

@rh5555
Do you know why the voltage values are higher for your readings than the other state of charge charts that were provided just above yours?
Are the state of charge values specific to each battery manufacturer?
Thanks!

rh5555

I found it impossible to determine the conditions under which state of charge vs. battery voltage tables that are published on the web were compiled. I made my measurements after resting the battery under no load for a couple of hours, which seemed to me to be a reproducible condition. My guess would be that those other measurements were made under (unspecified) light load conditions which reduce the battery voltage.

ChrisFix

@rh5555
Got it...thanks for the reply.

Tundra57

I have been stating this fact frequently in many posts.

So yes I also agree its true.

80% is a good number. However please note that it is very important when considering battery life to recharge the batteries after being below 50% as soon as possible. Longer times at low discharge levels are more damaging than actually discharging them to that level.

Boomerb77

@rh5555 
Appreciate this excellent work! 

rhyno

Tundra57 said:

I have been stating this fact frequently in many posts.

So yes I also agree its true.

80% is a good number. However please note that it is very important when considering battery life to recharge the batteries after being below 50% as soon as possible. Longer times at low discharge levels are more damaging than actually discharging them to that level.

Is it safe to say that if you can't count on the sun being out the following day to have a generator at the ready if you plan discharge below 50%? 

Verna

@rhyno, most of us don’t plan on discharging our batteries to 50% when we’re boondocking. But, if after two or three days of the sun not giving enough solar gain to keep up with usage, you could use a generator. (I do not carry a generator.)

When this happened to me, I chose to connect to my truck and I ran the truck for three hours to get the battery charge high enough over 50% to make me feel better. Luckily the sun shone enough that day to bring the batteries up higher, with total recovery the following day. 

TNOutback

I recently had this issue while boondocking and 2 1/2 days of rain and clouds dropped my batteries down into the upper 60% range.  Annoyingly, hooking up my shore power cord to the generator would only result in 4-7 amps being sent to the batteries, running for hours only to gain a few percentage points.  I’ll be looking at another way to recharge the batteries directly instead of going through the onboard converter.

Dutch061

TNOutback said:

I recently had this issue while boondocking and 2 1/2 days of rain and clouds dropped my batteries down into the upper 60% range.  Annoyingly, hooking up my shore power cord to the generator would only result in 4-7 amps being sent to the batteries, running for hours only to gain a few percentage points.  I’ll be looking at another way to recharge the batteries directly instead of going through the onboard converter.

Yeah, it is a pain for sure. Watching with a BMV-712, the amperage starts out much higher but tapers fairly quickly. It can easily take 3 hours of generator time to get 20% depending on where you start at.

Brad

m_lewis

And that last 20% of charge takes longer still. 
A properly configured 3-stage charger will hold the bulk voltage to about 80% state of charge, then drop to the absorption voltage for the remaining 20%. Internal resistance goes up with SOC and charge efficiency goes down.
Most off-gridders, when faced with no sun/wind/water only use their generator to get to 80%, as fuel and run time gets wasted on that last 20%.  Only when renewables are back online will they go to 100% (or equalize, if using wet cells).
Temperature makes a big difference, too. If the charger is not temperature compensated, it can take a much longer time to get a colder battery up to 100% SOC. Rolls, a premium battery manufacturer, says to charge their 12 volt AGMs at 14.70 V at 77 degrees, but at 15.06 V at 50 degrees. Thirty-six hundredths of a volt may not seem like much, but it equates to several amps in the bulk stage.

rh5555

and the stock WFCO converter in your TAB only charges using 13.6V.  No wonder it takes an age.

m_lewis

WFCO data sheets for the 8955 (55 Amp output) says that in an unusual condition, if the battery voltage is low enough, it will bulk charge at 14.4 V, after no more than 4 hours it will drop to normal at 13.6, then after 44 hours of no significant current draw changes, it will drop to float at 13.2 volts. All of these voltages are lower than the battery manufacturers recommend, but they are safer (less gassing, less chance of overcharge at higher temps). They do NOT monitor and adjust for battery temperature.
They also claim to have studies showing a fully discharged battery at 11.9 volts can be fully recharged in 3 hours while also powering a 20 amp lighting load. I find this claim somewhat dubious, certainly so with the 224 A/h AGM batteries in my T@B400BDL at a normal temperature (<80 degrees F). For example, with an imaginary charge efficiency of 100%, maintaining 55 - 20 = 35 Amps of charge, it would take over 6 hours at a bulk voltage. Real world, charging at 13.6 V will virtually never reach 100% SOC, as that takes 13.60 V at the battery, not at the convertor. Voltage drop along the battery cables, connectors, fuses, switches, etc guarantees a lower voltage at the battery.
Again, in their defense, given the myriad possibilities of battery size, chemistry, temperatures, camper loads, and allowing for safety tolerances, the product is fair for the price. They sell a bunch of them and they are reasonably reliable. It is not an optimum charger for for individual installations nor calibrated for maximum battery life or total amp hours of usage.

SamG340

Sorry to bump an old thread, Very interesting stuff

rh5555

With all the focus on moving to lithium, it's good to revisit AGM batteries.  They perform pretty much the same - tolerate deep discharge and can be recharged fast.

• Advantage of AGM - they are comparatively cheap and can be charged in the cold
• Disadvantage of AGM - they are heavier and bulkier.

For me, the additional weight is an advantage, helps reduce tongue weight as they are mounted at the back.

A common problem of both chemistries is that the provided WFCO chargers do not do an efficient job.  They never enter bulk charging mode due to their own internal design.  If you need to fast charge either chemistry, you'll need to upgrade your charging system to a Progressive Dynamics system (or better).  I'd suggest following this thread and mount the replacement converter really close to the batteries.  This issue is only of real concern to us boondockers who rely on a generator to fast-charge our depleted batteries.

 

The picture to the right is what I did:  Even with this setup (converter about a foot from the batteries, the upgraded WCFO charger still will not enter bulk mode, and it's nigh-on impossible to force it to to do so.  The Progressive Dynamics converters come with a dongle to allow you to force bulk charging mode.  At some point I'll swap them out...

SLJ

rh5555 said:

With all the focus on moving to lithium, it's good to revisit AGM batteries.  They perform pretty much the same - tolerate deep discharge and can be recharged fast.

• Advantage of AGM - they are comparatively cheap and can be charged in the cold
• Disadvantage of AGM - they are heavier and bulkier.
• Another disadvantage of AGM is slower charging than Lithium. 

AGM is no longer comparatively cheap. There have been a lot of changes and price drops since this post started. ~$300 now for a 100AH lithium which weighs half an AGM and has twice the usable power, recharges much faster, has no chance of accidently damaging it with over discharge pretty much makes it an easy choice for most. Even if you only charge it to 80% it still outperforms and outlasts an AGM and due to its longer usable life, it's cheaper to operate.

rh5555

I agree that Lithium batteries can be charged twice as fast as AGM, but they don't have twice the usable power.  That was the message in the original post.  You can safely discharge AGMs by 80% without compromising them.

If you discharge a Lithium battery of equivalent Ah rating to zero, you only get 25% more power.  If you're not concerned about weight, then AGM batteries are not as bad as people make out.  When my batteries get to end of life, I'll probably switch to lithium, but I'm in no hurry.