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Old 22-09-2018, 10:20   #1
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FLA 20hr Test to 10.5v - Flooded Lead Acid

20 Hour Capacity Test

Really the only true test of state of health, for cycling batteries, is to test them by doing a 20 hour capacity test. How to conduct a 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. Create a DC load = to Ah Capacity ÷ 20 (small light bulbs and resistors can work) EG: 225 ah / 20 hours =11.25amps
  4. Start DC load and a stop watch at the same time
  5. As the voltage drops during discharge adjust the DC load to maintain as close to C/20 rate as is humanly possible.
  6. Stop the discharge test when battery terminal voltage hits 10.499V 7 Note the hours and minutes of run time and figure your percentage of 20 hours that it ran.
  7. This is your batteries Ah capacity or state of health as a percentage of as new rated capacity.
  8. For example if a 100Ah battery ran for 16 hours it is at 80% of its original rated capacity. Lead acid batteries are generally considered end of life EOL when they can no longer deliver 80% or more of their rating but can sometimes continue to "work" down into the 70% range before failures start to occur..
  9. Recharge the battery immediately..
  10. Follow this up with equalization level voltage and measure specific gravity until all cells match. (not for non-Lifeline AGM's or GEL's).
A long slow recharge can have a slight reforming effect on flooded batteries and can actually recover some lost capacity.

Is 10.5V bad for my batteries?

A once yearly discharge test, done correctly, is actually less damaging than taking a battery to 50% SOC and leaving it in that range for a day or two, or the PSOC cycling (partial state of charge) the average boater thinks nothing of.

Regular PSOC cycling is more damaging than a once or twice yearly Ah capacity test. A capacity test, done correctly, simply counts as another deep cycle.

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 loaded voltage should ideally not dip below 12.1V or better yet 12.15V - 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....
-------------

Thank you John61ct Mainsail and others for this list.


Now for the load on the batteries during that 20 hours or less.
Assuming a Bank of 225ah / 20hr = 11.25amps are needed.

What resistance is needed?
Voltage= I * R = amps * ohms
12.8v at rest/11.25a= 1.14 ohms
12v/11.25a= 1.07 ohms
11.5v/11.25a= 1.02 ohms
11v/11.25a=0.98 ohms
10.5v/11/25=0.93 ohms
Simple ohms law calculator

So we have to vary the resistance (ohms) during the entire 20 hours.
For this example we would need a variable resistor between 1.14-0.93 ohms to do that easily. This is a load of between 144 watts and 118 watts.

Would this work and get down to 0.93 ohms?
Uxcell a11111000ux0042 100W 5 Ohm Ceramic Wirewound Potentiometer Variable Resistor

I suppose I should test the two 6v batteries separately, so the resistance range should be 1/2 of the above calcs.
but I haven't found a variable resistor (rheostat, potentiometer) in that range yet.



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Old 22-09-2018, 10:43   #2
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Re: FLA 20hr Test to 10.5v - Flooded Lead Acid

This one is just rated for 10a but maybe it will work over most of the process.
100W 1 OHM High Power Wirewound Potentiometer, Rheostat, Variable Resistor

but it is $17. I guess I'll try some old 60w-25w candelabra bulbs.
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Old 22-09-2018, 10:46   #3
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Re: FLA 20hr Test to 10.5v - Flooded Lead Acid

I suppose these 0.2 ohm resistors are the best way to do it.
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Old 22-09-2018, 10:50   #4
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FLA 20hr Test to 10.5v - Flooded Lead Acid

I used to test aircraft batteries for reserve capacity with light bulbs.
It gets you close enough.
You could of course stack light bulbs, and only have a variable resistor on one, that would keep it small.
To do this “right”is beyond the equipment capabilities of a cruiser, in my opinion. Probably great test, just not realistic.

My opinion is once they hit 10.5 V, an immediate recharge at the highest rate that your chargers can perform is in order, watching battery temp of course if one gets hot, then maybe discontinue charge.
Hold absorption voltage until they are well and fully charged, this may be eight or more hours.
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Old 22-09-2018, 11:26   #5
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Re: FLA 20hr Test to 10.5v - Flooded Lead Acid

That's excellent information, and corresponds to what I've always been taught. Do you have a source for this, though?
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Old 22-09-2018, 11:30   #6
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Re: FLA 20hr Test to 10.5v - Flooded Lead Acid

Maine Sail's adjusting BM article at his HowTo site

Search for 20 hour capacity test here https://marinehowto.com/programming-a-battery-monitor/
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Old 22-09-2018, 11:32   #7
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Re: FLA 20hr Test to 10.5v - Flooded Lead Acid

I'm saving up for an iCharger 4010-Duo for this and other maintenance tasks.

Has set-level amps discharging built in.

Can also be used as a very flexible DCDC charger in daily use.

Probably not built as robust as Sterling's though.
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Old 22-09-2018, 11:35   #8
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Re: FLA 20hr Test to 10.5v - Flooded Lead Acid

OP I would suggest hard returns between bullet points for readability of the protocol.

My standard blurb:

_____
There is only one way to measure State of Health without thousands of dollars worth of lab grade equipment.

20 Hour Capacity Test:

An accurate AH counter can be used, but not necessary, just an ammeter and a stop watch.

#1 Benchmarking to 100% Full is straightforward, hold Absorb until trailing current drops to endAmps. If not mfg spec'd I use .005C, .5A per 100AH.

Then equalize if appropriate, and allow the batt to rest disconnected for at least 24 hours

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

#3 Apply a .05C load = 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 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 "rated" capacity. For example if a 100Ah battery ran for 16 hours it's testing 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 can still work beyond this point but are considered *less safe* and can be less predictable.

#9 Once you hit 10.499V recharge the battery immediately at the 20 hour rate. Once the battery is full, follow this up with equalization if appropriate

A long slow recharge (20 hour rate) can have a slight reforming effect on batteries and can actually serve to recover some lost capacity. It is not uncommon for a battery to have*more capacity after a 20 hour test than it had going into it.

(Credit to Maine Sail)

Any suggested edits welcome
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Old 23-09-2018, 12:37   #9
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Re: FLA 20hr Test to 10.5v - Flooded Lead Acid

Quote:
Originally Posted by john61ct View Post
I'm saving up for an iCharger 4010-Duo for this and other maintenance tasks.

Has set-level amps discharging built in.

Can also be used as a very flexible DCDC charger in daily use.

Probably not built as robust as Sterling's though.

I can understand why your are "saving up for it" looks like just the right thing, but >$300

https://www.amazon.com/iCharger-4010...harger+4010duo

https://www.icharger.co.nz/icharger-4010-duo


Incandescent bulbs are being replaced. We have very few of them, but I think we have enough 25w Candellabra Incandescent to provide the loads required. It clearly is not a very exact test doing it this way. ...More mucking around with batteries, then charging them up for long periods of time too.
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Old 23-09-2018, 14:58   #10
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FLA 20hr Test to 10.5v - Flooded Lead Acid

The bulb thing through an ammeter is as accurate as you make it. You can sit there for 20 hours and nail it, or check and adjust every hour and get very, very close.
My belief is that it’s not necessary to hold exact discharge current, that was done when all you had was known discharge rate and time, and from that you calculated capacity.
Now we almost all have AH counters, so you know exactly how many AH were removed, so start a bit high, so that you end up a bit low and I’ll bet the difference is insignificant.
Person who taught me Civilian test pilot “stuff” told me that exact is the enemy of good enough.
Military world if we could measure to three digits accurately, we strove to get three digit accuracy, many, many times when it wasn’t necessary, just cause the Engineers wanted it.

Does it matter if you calculate that you have exactly a 325 AH bank, or is knowing that you have 320 ish good enough?

Of course to really do it “right” you need to do at least five cap checks, throw out the high and low and average the middle ones right? One test is never enough.

Sometimes good enough, is good enough. What are you going to do with this data anyway that you need to know exactly?
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Old 23-09-2018, 16:16   #11
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Re: FLA 20hr Test to 10.5v - Flooded Lead Acid

I’m never going to do one of those 20 hour discharge. Sure it is the only way to know the true capacity of a battery. But to me it’s just so crazy to put the batteries through it. There’s just so man6 other ways to tell batteries general health and capacity. Does it reall matter if your batteries are are say 440ah instead of 469ah? What does it really matter if your battery monitor state of charge is wrong? Are you going not recharge beacause “hey we still have 5%).
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Old 23-09-2018, 16:53   #12
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Re: FLA 20hr Test to 10.5v - Flooded Lead Acid

Yes few go the trouble.

But not only for BM accuracy,

knowing the rate of capacity loss is faster than you expect can be an indicator you need to improve care,

also those that proactively replace the bank at a certain SoH, good to be able to plan ahead so next time you're in an auspicious civilized location.
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Old 23-09-2018, 17:38   #13
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FLA 20hr Test to 10.5v - Flooded Lead Acid

The people that will do a capacity test are the ones that over care for their banks now. Seriously doubt any casual user will.

I still think the Smart Gauge and an amp counter will tell me bank capacity, or it’s not as accurate as some who have tested say it is.
Let my bank discharge over time at my normal use rate to 50%, if it delivered 300 AH, then I have a 600 AH bank, at my normal use rate.
Takes a little over 24 hours for me to get there, with zero charging of course.
Do it again in a year, see if you still get 300 AH to 50%, if it’s less, then that is the drop in capacity.
I’m looking for trending, not absolute data, cause I don’t have the equipment, and to get good repeatable numbers would likely mean having the bank in temp controlled water bath as well as expensive gear, that I don’t have room to store and won’t use for anything else.

Way I see it, is if you really feel the need, you find someone like Maine Sail and pay them.
For some, it may be a hobby.
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Old 23-09-2018, 18:33   #14
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Re: FLA 20hr Test to 10.5v - Flooded Lead Acid

So much trouble, time, and worry for so little to be gained. In the real world very few cruisers are match the internet model of battery servants.

Far as this 20 hour discharge thing goes, doing these hard discharges in order to talk yourself into whether your batteries are at 96% rated instead of 92% is just imo crazy.

There’s just easier and non harmful ways to get an close enough estimate for real use.
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Old 23-09-2018, 19:01   #15
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Re: FLA 20hr Test to 10.5v - Flooded Lead Acid

There are good reasons to perform this test, bothersome as it may be.
1. Determine the unknown capacity of an older battery that has not been performing well. I have still have some 2 year old T105's that I've charged twice with low trailing end amps, and equalized. Now is the time to do a full discharge and know the capacity.
2. Determine the capacity of a new set of batteries after full charge and equalize, for future use.
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