Battery Voltage vs. State of Charge

Feb 6, 1998
11,454
Canadian Sailcraft 36T Casco Bay, ME
"
The answer is B. In the marine market all charge sources we use operate on CC/CV charging so bulk ends when the battery becomes voltage limited by controller, regulator or charger."


So your charging source is looking at charge current and supplying the needed voltage so the current stays at a constant value during bulk charging????
Until the charge source regulator sees the target voltage the charge source is putting out all it can, simple stuff. It is called constant current charging. If the charge soruce is only capable of supplying 5A then it will supply 5A until the battery voltage, or its "sensed voltage" tells it the "limit" voltage has been attained. At that point we begin "limiting voltage" to the pre-set limit point. Once voltage is held steady, constant voltage, the current begins to decline as the battery can only accept XX current at XX voltage and XX SOC....


Pretty sure this is not how it works electrically. My understanding is the charge controler "does magical things" and determines that it needs to bulk charge and sets the voltage at the predetermined bulk charge voltage (probably 14.4).
It is shocking to me how much the marketing mavens have made folks over-think something so darned simple. There is no magic. These devices simply work on CC/CV. Constant current until the absorption voltage is attained then CV. These are voltage regulated devices that is it.

If not at target voltage you are in BULK or CC
If at target voltage you are in ABSORPTION or CV (FLOAT and EQ are also CV stages)

Smart or so called "smart chargers" then try to determine the optimal absorption cycle based on either a super dumb egg timer or an algorithm that looks voltage rise and time.

Egg timer chargers work by starting a simple egg timer the minute the regulation sees the target voltage. Some are set at 1 hour, 2 hours 4 hours etc.. These are far from optimal and usually result in premature float.

Smarter charge regulation may use:

A Timer algorithm looking at;
Time to attain target voltage
*Sometimes % of field
*Sometimes power supply % current

*This can be tricked by domestic loads so charge sources may have other safeguards built in to not over absorb and some dumb ones can be tricked into remaining in absorption dock side, under domestic loads, which can be bad..

With these parameters they calculate the absorption time before dropping to float..

Keep in mind these are only used to determine how long to ABSORB the batteries. Ideally a battery would stay in absorption until the battery is full but using voltage as our reference this rarely happens and the chargers usually drop to float too early in favor of "safety".

That in no way insures that charge current is constant. My experience is that the bank rapidly becomes (12 minutes) voltage limited and the current falls of mightly.
Again bulk is not voltage limited despite some marketing departments insinuating it is to confuse people into thinking they are getting more than they really are.. If you are voltage limited you are in CV not CC/BULK....


The charge controler stops charging momentairly and does more "magical things" (like measure the "resting" voltage) and determines what charge regime to do for the next time period.
If you are witnessing a pause in current this is the charge controller dropping down to float or a second absorption voltage level. It is not thinking or processing. It is simply waiting for the battery voltage to decay naturally to the next voltage limit level, say from a 14.4V absorption to 13.6V float...

If the battery voltage is ever above the regulation set point the power supply or field is shut down until voltage comes back to the voltage limiting range where it will maintain the voltage level.


It is really a puso resting voltage assessment of the SOC.
No it is not.

If it really was CC charging then the regulator would need to know what the charging current was going into the batteries would it not?
No constant current is constant current it does not care about anything until the battery terminal voltage reaches the "limiting voltage" then it begins holding the voltage to that point.


They are using fixed voltage points when not charging to assess SOC and then determing which regime to do for the next time period.
No they are not. Chargers know nothing about SOC. They know at voltage or not at voltage. It is actually quite sad that someone with your quite decent level of electrical expertise has even succumbed to this marketing bamboozlement. If they can foll a guy like you it must be horrible for everyone else..

Charging with CC/CV is really this simple:

Below Voltage - SUPPLY CONSTANT CURRENT
At Voltage - LIMIT/HOLD VOLTAGE TO PRE-SET LEVEL
Above Voltage -TURN OFF POWER SUPPLY, FIELD or PV and WAIT FOR VOLTAGE TO DROP


Which brings up the observation that since they are using resting voltage to assess SOC and that is highly dependant on the state of health of the batteries (with old batteries taking a lot longer to settel down to resting voltage), a lot of the problem is that the charge controller is set up to over charge older batteries by its very (limited) design.
There is no charger I know of that assesses resting SOC based on voltage.

when I notice the lights slightly dimming and then brightning every 15 minutes or so while using the engine and this coorisponding to the engine sounding like it is taking up a load when the lights brighten and loosing load when they dim.......I'm pretty sure the controler is checking the "resting" battery voltage to determine what it needs to do next.
Thoughts?
Sounds like you have a game of solar/alternator ping pong going on or a bad alternator regulator or a large load such as a fridge compressor coming on and adding load to the alt.. This is why solar should be set a .1V - .3V lower than the alt. If not we can get ping pong... When the alt comes on the current from that combined with solar is pushing the batteries into "absorption range" (limiting voltage) if the solar current, combined with alt current, pushes the voltage higher than the alt set point it shuts the alt down. Now with the alt shut down, we no longer have the current to maintain the voltage limit so the alt kicks back in and it ping pongs. These devices don't pause to check voltage it is all done on the fly. Some older really crappy MPPT controllers (and some cheap/sleazy Chinese controllers) did pause to check the max power point (unloaded voltage of array) I called this the old school "pause & observe" but these went the way of Dinosaur saddles many moons ago. Simple PWM charge sources, battery chargers, inverter/chargers, alternator regulators and solar controllers don't pause to check voltage this was only done very antiquated MPPT products as a way to track the max PV voltage.
 
Feb 6, 1998
11,454
Canadian Sailcraft 36T Casco Bay, ME
This video was made for my friend Ed but it explains and shows CC/CV and the relation ship of current to battery voltage. My bench top power supply's are no different than a typical marine battery charger except that I can control current and voltage manually.

 
Aug 3, 2014
68
CATALINA, BENETEAU OCEANIS 36, 400 Grosse Ile, Mi and Fajardo, PR
Good Morning,

I have enjoyed the thread and decided to try my own experiment in Culebra PR. Sorry for the formatting, I am using a Samsung.

I have 2 East Penn 8A4D AGM house batteries (400 amp hours) purchased 1/15. East Penn's web site shows the following:

SOC VOLTAGE UNDER CONSTANT DISCHARGE FOR DISCHARGES LONGER THAN 8 HOURS
V SOC
12.8 90%
12.7 80%
12.6 70%
12.4 60%
12.25 50%
Unfortunately, the website does not dislose the discharge amps.

The website gives a second method for measuring SOC:
RESTING VOLTAGE AFTER 24 HOURS

V SOC
12.8 100%
12.6 75%
12.3 50%
As a live aboard, my batteries never rest.

I charge with a 120 amp Electromaax alternator regulated by a Balmar MC 614 at factory settings.
I typically charge twice per day. Morning charge, 40 minutes generating about 28 amps. Evening charge, 30 minutes generating about 23 amps. I measured the amps by taking 4 readings with the amp clamp, averaging the declining readings, and knocking off a few amps since the rate of charging amps delines so fast (not real precise, I use to be in sales). In any event the morning amp clamp readings were:
Min AMPS V
START 89.7 14
14 42.9 14.3
27 29.4 14.3
43 23.7 14.1
Absorbtion voltage started about 27 minutes.
The solar panel generates about 20 amps, daily.

When I start the alternator battery voltage is about 12.4 to 12.45 with a 4 amp discharge. This does not seem to vary from day to day.

CONCLUSIONS
1.) Amp hours used per day 81
2.)Battery SOC before starting alternator 62.5+%.
3.)Battery SOC after running alternator 70%.
4.) Without a generator, large solar or shoreside charging, it is difficult to get batteries to a 100% SOC
5.) When the alternator reaches absorbtion, it takes about 3 minutes to generate an amp
6.) Back of an envolope, the alternator needs to run for 5 1/2 hours to reach 90 to 100% SOC.
 
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Feb 6, 1998
11,454
Canadian Sailcraft 36T Casco Bay, ME
Your absorption voltage is too low for East Penn AGM batteries. They like to see 14.6V measured AT THE BATTERY TERMINALS. I suspect your Balmar voltage sensing is also being measure in the wrong place. The customs setting in the MC-614 are critical as the factory settings are really just "lawyer safety settings" but not battery healthy settings....

For proper voltage sensing the Balmar MC-614 black/negative lead needs to be wired directly to the house bank negative terminal and the red volt-sense wire needs to be be direct wired to the house bank positive terminal.

Sensing voltage in any other place in the system can dramatically impact your charge times, especially on short duration charge cycles like we are used to on sailboats.. Charging at lower than optimal voltages will impact the cycle life of your battery bank. AGM batteries need to get back to 100% SOC as often as possible and in PSOC (partial state of charge use) use your really do need to use the highest safe absorption voltage, which for Deka/East Penn AGM batteries is 14.6V as measured at the battery terminals.

Alternators & Voltage Sensing - Why It's Important

In the recent Practical Sailor PSOC testing (May 2015 & August 2015 issues) the East Penn AGM faired rather poorly against the premium AGM batteries. For East Penn AGM's this makes it even more critical to use the highest safe absorption terminal voltage and get to 100% SOC as often as is humanly possible.

East Penn suggest that if using "return amps" as an indicator of 100% SOC that it be measured at 14.4V - 14.6V and 0.3% of Ah capacity in net accepted current. For a 100Ah EP AGM battery that is 14.4V-14.6V at the terminals and 0.3A before the battery can be considered 100% SOC....

IMPORTANT: Many boaters have unknowingly wired an Electromaax alternator so that the internal regulator is actually in control, not the external regulator. Make sure your internal regulator is correctly bypassed and that the MC-614 is actually doing the work....
 
Aug 3, 2014
68
CATALINA, BENETEAU OCEANIS 36, 400 Grosse Ile, Mi and Fajardo, PR
Thanks Maine,

I have made all the voltage sensing mistakes you mentioned in previous years. Voltage sense currently is ok. I believe a wire from the alternator D+ to the MC 14 #2 terminal (ignition input) causes the alternator to self regulate. This has been removed. I plan to move the absorbtion voltage to 14.6. Will post results in a few days.

QUESTION

When you return to your 21 year old boat, 3000 miles away, after 9 months and the fuel tank is empty, where do you think the fuel went?

A) Evaporated
B) Stolen
C) Bilge
D) Between hull and grid
E) Bilge pumped overboard into the yard

Nick
 
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Feb 6, 1998
11,454
Canadian Sailcraft 36T Casco Bay, ME
Thanks Maine,

I have made all the voltage sensing mistakes you mentioned in previous years. Voltage sense currently is ok. I believe a wire from the alternator D+ to the MC 14 #2 terminal (ignition input) causes the alternator to self regulate. This has been removed.
That's good! All that is left is to correct the low charging voltages... Be sure you are seeing 14.6V at the physical battery terminals when in absorption..
 
Jan 30, 2012
1,116
Nor'Sea 27 "Kiwanda" Portland/ Anacortes
[A] wire from the alternator D+ to the MC 14 #2 terminal (ignition input) causes the alternator to self regulate. This has been removed. I plan to move the absorbtion voltage to 14.6. Will post results in a few days. . . .
Nick

Double check before removing any of the connections at #1 - #4 on the MC 614 . #2 terminal (red) is 24 hour battery + (same as D+.) #3 terminal (brown) is ignition (switched battery +) input.

Charles
 
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May 17, 2004
3,545
Beneteau Oceanis 37 LE Havre de Grace
SOC VOLTAGE UNDER CONSTANT DISCHARGE FOR DISCHARGES LONGER THAN 8 HOURS
V SOC
12.8 90%
12.7 80%
12.6 70%
12.4 60%
12.25 50%
Unfortunately, the website does not dislose the discharge amps.

The website gives a second method for measuring SOC:
RESTING VOLTAGE AFTER 24 HOURS

V SOC
12.8 100%
12.6 75%
12.3 50%
Those numbers don't seem to make sense to me. Shouldn't the loaded voltages for a given SOC be lower than the resting?
 
Aug 3, 2014
68
CATALINA, BENETEAU OCEANIS 36, 400 Grosse Ile, Mi and Fajardo, PR
Thanks all,

Increased bulk and absorbtion voltage to 14.6. Increased absorbtion time to 3 hours. Previously, batteries went into float after 2 1/2 hours. I am getting better amp input results. I temperature sense off the start battery in the engine compartment. I suspect I should temp sense off the house bank? Batteries are combined with a Blue Sea 7610 ACR. Battery temp 32 C. Will post amp improvement results later.

Nick
 
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Feb 6, 1998
11,454
Canadian Sailcraft 36T Casco Bay, ME
Those numbers don't seem to make sense to me. Shouldn't the loaded voltages for a given SOC be lower than the resting?
And here lies the problem with trying to use a resting voltage for trying to determine SOC. The voltage cut off on this battery at the 20 hour "as new" discharge rate of 5.25A at 75F was 12.100V and that put us at 49.3% actual SOC.

When the voltage was allowed to rest and rebound it settled out at 12.32V which according to Lifeline Battery is exactly 60% SOC by resting voltage. Lifeline calls for a 50% SOC rested voltage of 12.2V and each 5% of capacity changes the resting OCV by .06V. So, 12.2V + .06V + .06V = 12.32V. This is why it's important to know how your batteries behave if trying to use voltage for SOC. If an owner was trying to use a resting voltage of 12.32V they would assume this battery was at 60% SOC when it in-fact was really at 49.3% SOC....
 
Oct 9, 2017
1
Na Na Houston
I have several questions about. 12v systems, and this seems to be the most comprehensive bit of information I have yet to discover on the web. First, I am not a boater, but I am using a similar power system. In fact, I use the same lifeline battery as featured here, the 31t, 105ah deep cycle and. I have 2 100 watt pv panels, and a basic 3 stage pwm cc. I have 5 usage months in the last year, and until recently, I'd been using the same voltage chart, and a voltmeter to monitor my battery. Knowing that this was an inaccurate method, I tried to never draw my battery down below 12.15. I rarely draw more than 5 amps. I recently installed a Bmv 700 battery monitor. My monitor, today, for example, indicates the following: 12.57 volts(resting about 30 min), in from solar-(hazy overcast) .13a, 2w, battery is down -11.6ah for the day. Soc 90.8%. Now, the ah usage is consistent with the percentage indicator. But the voltage is consistent with roughly 80% soc(I know it's not been resting long enough). My question. If the voltages 'bounce' they seem to bounce up. So if I draw down to 12.4, for example, and they bounce up to 12.55 after a good rest, is not my battery more charged than the chart indicates? Additional. If the new battery monitor is supppsed to 'learn' my battery's true capacity, is it, in fact, indicating that my battery still has nearly it's full amperage capacity intact, or is it still operating on the capacity that was programmed into it, upon setup? Will I need to regularly let the battery rest free of loads and solar charges for 4 hours per cycle, or more, to allow the monitor to 'learn' from testing voltages? I hope you folks can help. If I have trashed my battery, I am prepared to accept that, as a cost of learning, but I want to fully understand these concepts before I buy a new one. Thanks to the boaters from a modern nomad:)
 
Jan 4, 2010
993
Farr 30 San Francisco
The less rapidly you draw down the battery the higher the cutoff voltage needs to be. Somewhere earlier in this thread there was a SOC vs voltage in discharge. If you look at that graph at the 50% mark you see that the lower the discharge rate the higher the battery voltage will be at 50% SOC.
 

CarlN

.
Jan 4, 2009
578
Ketch 55 Bristol, RI
Thanks so much for looking at this "resting" vs under load voltage. Most manuals and discussions of batteries ignore this issue.

To that point, I am planning to do a capacity test of my four year old Lifelines as described in the Lifeline manual. This calls for discharging the batteries at a 25 hour rate until 10.5v is reached. Does that mean 10.5V resting or under load? I assume under load but the manual doesn't say.

In either case, I hate taking the battery voltage this low for fear of damaging the batteries. Is there a reliable test that doesn't take the voltage down so far?
 
Sep 28, 2008
922
Canadian Sailcraft CS27 Victoria B.C.
Carl
Should be the 20 hour rate, not the 25 hour rate.
I do not know of another way to find true remaining battery capacity.
As long as the batteries are recharged to full immediately after there won't be any damage.
 
Feb 6, 1998
11,454
Canadian Sailcraft 36T Casco Bay, ME
I have several questions about. 12v systems, and this seems to be the most comprehensive bit of information I have yet to discover on the web. First, I am not a boater, but I am using a similar power system. In fact, I use the same lifeline battery as featured here, the 31t, 105ah deep cycle and. I have 2 100 watt pv panels, and a basic 3 stage pwm cc. I have 5 usage months in the last year, and until recently, I'd been using the same voltage chart, and a voltmeter to monitor my battery. Knowing that this was an inaccurate method, I tried to never draw my battery down below 12.15. I rarely draw more than 5 amps.
Do you have one battery, two or three? A regular use 5A load on a single 105Ah battery is a pretty high load and at that load a cut off of 12.15V is a pretty safe proximity to 50% DOD, on a healthy battery. Your recharge rate however is awfully low, especially with just a PWM controller, that is not taking advantage of any extra voltage an MPPT could convert to additional current.

For optimal health Lifeline recommends a bare minimum recharge rate of .2C or 20% of Ah capacity. You are nowhere near this with 2 X100W panels. Also, with that low a charge rate, you'll likely not get back to 100% SOC each day with such a small array especially if you are actually consuming energy while at the same time trying to charge.

Each day you do not return to 100% SOC your usable capacity begins "walking down". Do enough PSOC cycles back to back and you will have permanent capacity loss. If your PWM drops to float too early, and far too many do, then your time to 100% SOC is only extended and exceeding your sun-up hours.. Even at a .2C charge rate it takes about 5:45 to fully charge a Lifeline battery from 50% DOD. The last 7% of this takes about double the time it takes to replace the majority of the energy. If we assume you are drawing 5A or even 4A this only leaves you with about 4-6A of charging capability on a battery that really needs to see a minimum of 21A.


I recently installed a Bmv 700 battery monitor. My monitor, today, for example, indicates the following: 12.57 volts(resting about 30 min), in from solar-(hazy overcast) .13a, 2w, battery is down -11.6ah for the day. Soc 90.8%. Now, the ah usage is consistent with the percentage indicator. But the voltage is consistent with roughly 80% soc(I know it's not been resting long enough).
Please be very, very careful trusting your Ah counter for any resemblance of actual SOC. Your battery is full when the voltage is at 14.4V (NOT AT FLOAT VOLTAGE) and the net current flowing into the battery is below 0.525A. IF your system is meeting these two parameters then you should manually sync for 100%.

Ah counters offer a lot of good information but they are rarely accurate for SOC. You would be well served to read the article below for more information.

Making Your Battery Monitor More Accurate (LINK)


My question. If the voltages 'bounce' they seem to bounce up. So if I draw down to 12.4, for example, and they bounce up to 12.55 after a good rest, is not my battery more charged than the chart indicates?
No they may not be. This article (Under Load Battery Voltage vs. SOC Chart) is a prime example of why. The battery was discharged to 49.3% SOC yet the resting voltage indicated a resting voltage of 70% SOC. If you are discharging based on a 70% SOC resting voltage you are really discharging to closer to 80% DOD/20% SOC. Lifeline is pretty adamant that Ah's removed indicates true SOC not a resting voltage.


Additional. If the new battery monitor is supppsed to 'learn' my battery's true capacity, is it, in fact, indicating that my battery still has nearly it's full amperage capacity intact, or is it still operating on the capacity that was programmed into it, upon setup?

Your battery monitor does not such thing. It learns nothing. You as the manager of it need to program it correctly and if you don't it can lead to short battery life. I would set your battery monitor to alarm at 12.2V..


Will I need to regularly let the battery rest free of loads and solar charges for 4 hours per cycle, or more, to allow the monitor to 'learn' from testing voltages?
Again, Ah counters do not "learn" anything. They are bone simple calculators. You need to program them correctly in order to get the most accuracy they can deliver. The article above will clarify all this. It may take two or three reads until you get it but it explains it all. The only learning SOC meter I know of, that is actually rather accurate, is the SmartGauge. Until then stop discharging, under your average loads, at 12.15 - 12.2V and that is your bottom.

I hope you folks can help. If I have trashed my battery, I am prepared to accept that, as a cost of learning, but I want to fully understand these concepts before I buy a new one. Thanks to the boaters from a modern nomad:)
There is a lot to getting the most from your battery bank and with Lifeline batteries a .2C charge rate, regular equalization's, proper charge and float voltages, proper absorption duration, regularly recharging to a known 100% SOC (14.4V and less than .525A flowing into the battery) as well as temp compensated charging are all necessary to get the most out of the bank.
 
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jviss

.
Feb 5, 2004
4,627
Tartan 3800 Westport, MA
RC, a question: when you say 0.525A at 14.4V is "full," is this, indeed, 0.5%C, or 0.005C? I always thought that when you got to form 3 to 6% C at acceptance charge you were done.

Thanks,

jv
 

CarlN

.
Jan 4, 2009
578
Ketch 55 Bristol, RI
Sorry, typo on my part. The Lifeline manual says to discharge at 25 amps not hours. Still don't know whether 10.5v is measured under load.

Screen Shot 2017-10-11 at 9.39.50 PM.png
 
Feb 6, 1998
11,454
Canadian Sailcraft 36T Casco Bay, ME
RC, a question: when you say 0.525A at 14.4V is "full," is this, indeed, 0.5%C, or 0.005C?

Thanks,

jv

It is .005C not .5C, because .5C would be 52.5A.......

It is a 105Ah Lifeline AGM battery thus 0.5% of Ah capacity in tail current, at 14.4V (NOT FLOAT VOLTAGE), means the battery can be considered to be at 100% SOC. Odyssey and Deka/East Penn want to see 0.3% of Ah capacity at absorption voltage for a "full battery" using return-amps (tail-current) at absorption voltage.



I always thought that when you got to form 3 to 6% C at acceptance charge you were done.
A tail-current of 3% to 6% is not even close to full. Regardless of battery type I have yet to see any lead acid battery, flooded, GEL or AGM, deliver it's rated capacity if you stop charging at absorption voltage before dropping below 1% of Ah capacity, in tail-current, at absorption voltage. I have 20 hour capacity tested hundreds and hundreds of batteries, likely over 1000 capacity tests at this point. I have two 20 hour tests running right now, as I type this. The 3% - 6% number we hear bantered around was initiated by battery monitor manufacturers who recognized most boaters will not run charging equipment long enough to allow current to taper to the proper tail-current level. They also recognize that Ah counters get themselves way, way out of sync when they PSOC cycle so they simply raise the tail-current to try and correct for the inadequacies of Ah counting for SOC. This is only "cruiser full" in order to reset the Ah counter but this is not full and will lead to shorter battery life.

Folks all too often fail to realize that the tail-current to determine 100% SOC needs to be measured at absorption voltage and not at a float voltage.
 
Feb 6, 1998
11,454
Canadian Sailcraft 36T Casco Bay, ME
Sorry, typo on my part. The Lifeline manual says to discharge at 25 amps not hours. Still don't know whether 10.5v is measured under load.

View attachment 141781
You can run a 25A RC test on a Lifeline battery but I think you'll be disappointed with the results you get. When the battery is brand new, & cycled for break in, an RC test (reserve capacity or reserve minutes) and a 20 hour Ah capcity test will yield comparable results.

I have found that as a battery ages the results from an RC test do not yield the same SOH results as the lower 20 hours rate discharge test. Seeing as you use your bank closer to the 20 hour discharge rate the 20 hour test is significantly more representative of the SOH (state of health) for your actual use. it also gives you a capacity to program into your battery monitor that is actually a lot more accurate than an RC test on a used battery.

Both tests discharge to 10.5V as the cut off voltage. The common fallacy that a properly done discharge capacity test will harm the battery is just that, a fallacy. In most cases, well over 90% of them, the battery yields better capacity results after a 20 hour test than it did before the test. In layman's terms this is essentially because internal pathways through the plates, that are infrequently used, get re-opened and become more active contributors.

The 10.5V cut off or termination, for any capacity test, is an "under load" cut off.

How do I conduct an accurate 20 hour capacity test?

#1 Fully charge battery then allow it to rest, disconnected, for 24 hours

#2 Make certain battery temperature is between 75F & 80F

#3 Apply a DC load that = Ah Capacity ÷ 20 (small light bulbs and/or resistors can work)

#4 Connect an accurate digital volt meter to the positive and negative battery terminals

#5 Start the DC load and a stop-watch at the same time

#6 As battery voltage drops, during discharge, adjust the DC load to maintain as close to the C÷20 rate as is humanly possible

#7 Immediately stop the discharge test when battery terminal voltage hits 10.499V

#8 Note the hours and minutes of run time on the stop watch and figure your percentage of 20 hours that it ran. This is your batteries Ah capacity or state of health as a percentage of the as new rating. For example if a 100Ah battery ran for 16 hours it's at 80% of its original rated capacity. By industry standards lead acid batteries are considered “end of life” when they can no longer deliver 80% or more of their rating. They will work below this point at low discharge rates but your risk of failure increases rapidly as your SOH (state of health) drops.

#9 Recharge the battery immediately at the 20 hour rate. Follow this up with equalization level voltage and measure specific gravity until all cells match. (not for non-Lifeline AGM or GEL). A long slow recharge can have a slight reforming effect on flooded batteries, and AGM's, and can actually recover some lost capacity. For AGM batteries follow this up with a couple of discharges to 11.7V followed by full 100% recharges at the highest charge rate your equipment can deliver. AGM's like high charging current.

"Isn't 10.5V bad for my batteries?"

A once yearly discharge test, done correctly, is arguably less damaging than taking your battery to 50% SOC and leaving it in that range for a day or two or the continual PSOC cycling the average boater thinks nothing of. Regular PSOC cycling is more damaging than a once or twice yearly Ah capacity test done correctly correctly meaning 100% to 0% them immediately back to 100% with virtually no time spent at 0% SOC / 10.5V . A capacity test, done correctly, simply counts as another deep cycle. In most cases, in the order of 90% or more, I see batteries perform better after a 20 hour test than they did before.

WARNING: The only time your batteries should regularly be taken below *12.1V is during a capacity test. For regular house use, at your average house loads, your deepest under load voltage should ideally not dip below 12.1V or better yet 12.2V. Unless you are running short duration high load device such as an inverter, windlass, electric winches, thruster, water maker etc. don't let your bank voltage dip below 12.1V.

For certain situations, such as an off-shore passage or open ocean racing, discharging to 70-80% DOD is perfectly acceptable provided the batteries receive a proper 100% SOC re-charge as soon as you get to your destination. Discharging below 50% SOC, *on a regular basis in a PSOC environment, drastically shortens battery life when compared to 50%.

*Firefly Carbon Foam AGM & some GEL batteries would be an exception for regularly discharging below 50% SOC.

How can I conduct an approximate 20 hour capacity test?

#1 Allow battery to attain a steady 75-80F temperature

#2 Fully charge battery and equalize, if it's capable

#3 Let the battery rest for 24 hours

#4 Apply a DC load for 2 hours that = Ah Capacity ÷ 20 (small light bulbs and/or resistors can work)

#5 Allow the battery to rest for at least 24 hours at 75-80F (40 hours or more with AGM or GEL is more accurate)

#6 Check specific gravity or resting open circuit voltage and compare to the manufacturers resting voltage SOC tables

#7 Use basic math to determine the approximate Ah capacity. For example, a 100Ah rated battery has been discharged at 5A for 2 hours. This means so you removed 10Ah's of capacity. If the battery was in perfect health specific gravity readings or open circuit voltage readings should show the battery at 90% SOC. *If SG and OCV only show the battery at 60% SOC then the battery has lost approx 30% of it's Ah capacity.

IMPORTANT: This test is an approximation only and NOT an accurate Ah capacity test. Variances can be anywhere from 10-18% off an actual 20 hour capacity test depending upon your particular battery.
 
Jan 30, 2012
1,116
Nor'Sea 27 "Kiwanda" Portland/ Anacortes
#6 As battery voltage drops, during discharge, adjust the DC load to maintain as close to the C÷20 rate as is humanly possible.
Maine: For simple people like me when conducting the light bulb test: how does one know to add/subtract light bulbs load as the discharge hours transpire?

Charles
 
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