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Old 19-01-2015, 22:53   #1
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Easy battery load tester?

I need to do a C20 test on an 860AH battery bank.

I am thinking to use my electric range cooktops that are powered through an inverter. Since the cooktop burners have rheostat variable temperature control, I am hoping to easily set and maintain the 43A DC required load throughout the test. As long as the bare burners (with no pots on them) will keep a constant current draw and not cycle on and off under some kind of protective thermostat control.

Anybody else have a good means of establishing a fixed but variably adjustable resistive load centered around 500W for such a test?

I am surprised I cannot find a simple 400-500W variable power load tester.

Many thanks for any ideas.




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Old 20-01-2015, 00:11   #2
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Re: Easy battery load tester?

try a plug in heater set to low. I'd think the stove would turn off and on as the plates got hot.


adjust current by adding lights and other static loads. turn fridge and other on / off stuff off.
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Old 20-01-2015, 01:15   #3
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Re: Easy battery load tester?

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Originally Posted by Maine Sail View Post
What are you using for a DC constant load while discharging?


cccccccccc............. cc


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A Meanwell Train supply, 12 to 12volt DC CC to a load bank of switched halogen bulbs. While it isn't a true load/time tester, it comes about as close as I need.


Lloyd



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try a plug in heater set to low. I'd think the stove would turn off and on as the plates got hot.


adjust current by adding lights and other static loads. turn fridge and other on / off stuff off.
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Old 20-01-2015, 01:24   #4
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Re: Easy battery load tester?

fixed but variably adjustable resistive load !!!!
Not exactly resistive if its through an inverter.....
Strange things people ask for !
Stick a 500w floodlight on your inverter ..
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Old 20-01-2015, 01:48   #5
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Re: Easy battery load tester?

A 500 watt flood light will draw a variably KVA though an inverter, based on temp of the tungsten fillimant, and the resultant temp of the inverter.

Any variable, will not be repeatable.

Lloyd

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fixed but variably adjustable resistive load !!!!
Not exactly resistive if its through an inverter.....
Strange things people ask for !
Stick a 500w floodlight on your inverter ..
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Old 20-01-2015, 13:00   #6
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Re: Easy battery load tester?

Putting a 500W lamp on the output establishes only the nominal load, I need the means to adjust the load down incrementally as the batteries under test move down from 12.8V to 10.5V over the 20 hours of the test to maintain a constant current load. Hence, the cooktop rheostat temperature control knob idea.

Emmalina, are you saying a resistive load on the output of an inverter does not effectively establish a resistive load across the terminals of the battery feeding it? That's not what I am observing. Please explain.

I like the heater on low idea best, if it has a continuously variable output heat control knob to allowing me to easily tweak it during the test to maintain a constant current.

But I cannot find a heater that like that does not have interference from a thermostat feature. Does anyone know of one?

Thanks
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Old 21-01-2015, 04:18   #7
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Re: Easy battery load tester?

I think you should do the C20 test on each battery individually to see if they are all the same. That way the loads are much more controllable with lights and even maybe the inverter on small AC loads.

I'd be interested in the results as I don't know anybody who has managed a full 20 hours monitoring the loads. If a battery has lost a lot of capacity then it may reach 10.5v after only 10 hours. If that happens I assume you have to do the test again with a lower C20 current based on a reduced estimate of the actual capacity of the battery. If it still doesn't last 20 hours then you do it again with an even lower load.
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Old 22-01-2015, 05:53   #8
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Re: Easy battery load tester?

Bought a triple mode 200W/500W/700W Husky halogen work light today at Home Depot and found a 600W halogen lamp dimmer in the basement (took it apart to verify it uses a Triac). With the light set to the 500W brightness mode, I should be able to easily maintain the C20 constant amperage of 43A from 12.8V down to 10.5V as I test the true capacity of my nominal 860Ah bank. I will just glance at my boats ammeter panel gauge occasionally throughout the test and adjust the dimmer slider with my toe as necessary as I hang out on the boat. And if I want to test the 4 batteries that make up the bank individually, the same set up but with the light in 200W mode should work the same way.

Total BOM $27

Flying down to tropical Mexico Saturday morning to give it a try.
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Old 22-01-2015, 06:21   #9
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Re: Easy battery load tester?

I am not sure this will be of any great value to you. To understand why check out Peukert's equation on the web. The basic problem is the more current you draw the less caspacity is available, to do a C20 capacity I understand you have to keep current at 5 amps or less. Check it out.
John
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Old 22-01-2015, 06:26   #10
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Re: Easy battery load tester?

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I think you should do the C20 test on each battery individually to see if they are all the same. That way the loads are much more controllable with lights and even maybe the inverter on small AC loads.

I'd be interested in the results as I don't know anybody who has managed a full 20 hours monitoring the loads. If a battery has lost a lot of capacity then it may reach 10.5v after only 10 hours. If that happens I assume you have to do the test again with a lower C20 current based on a reduced estimate of the actual capacity of the battery. If it still doesn't last 20 hours then you do it again with an even lower load.
Why??

Your battery capacity is based on the "as new" C20 rating. When it falls below 80% of capacity, by industry standards, it is dead. AGM makers claim 50% of new rating, but in my experience any time they dip below 70% of rated as new capacity they tend to fall off the cliff rather rapidly and become quite unpredictable. Same goes for good quality flooded batteries.

There is really no need to identify a new 20 hour rate on used batteries unless you are going to be doing some very specific use or testing with them. A C/20 test, at factory discharge rate, tells you all of what you need to know in terms of your batteries health and capacity.

I'll pass some batteries at 75% of new ah capacity, yet fail others. All this is simply based on my experience with a particular type of battery. Many batteries will continue to perform house duties below 80% of new but once they drop to 70% or so, in my experience, their predictability becomes poor and the batteries simply don't charge or discharge they way they should.

Of course all of this is dependent upon the particular application & use. I have some AGM banks out there that I let go another year as low as 65% of new capacity based on use, load, bank size etc. I also have some good quality deep cycle flooded banks I would let do the same. No real simple answer...

I let one customer talk me into letting him sail to Bermuda with AGM's that tested at 73% of new. He threw the "blah, blah, blah 50% is dead for AGM's" at me. I knew better for his intended use that this was a bad idea at 73%.. I tried hard to talk him out of it but in the end it was his boat and his choice.

Sadly it bit him in the a$$ pretty hard and his cost of replacement AGM's down there was astronomical compared to what I had just quoted him here in Maine, according to him more than double.. No simple answer.... I would have let a coastal cruiser keep those batts another season but for off-shore.... Oops....

While you could get very picky and try to identify the batteries new, used condition, 20 hour rate, as I have had to do for some testing I have conducted, you are going far beyond what is necessary for what you want to know.

If you really want to get that picky, for an Ah/Coulomb counter, then you'd also want to identify the new Peukert and capacity based on varying temp, unless you can control battery temp to 75-80F all the time.

I've yet to see an accurately calibrated Ah counter, that matched actual tested bank capacity, and don't expect to see one any time soon. Is this bad? Not really as I still find owners with battery monitors still get longer bank life, on average.

I always note the SOC and -Ah screens before removing batteries for Ah capacity testing. The closest I see is within 10% and I have seen them over 80% off.... One capacity test at factory C/20 is really quite adequate, for what you need, and will get an Ah counter far closer than it previously was. This is also far better than perhaps 99.8999% of what most boaters actually know about their banks current physical capacity..

Any battery that can't supply 50% of the original factory Ah capacity rating during a C/20 load test, at 77F (75-80F is fine), is really scrap lead...
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Old 22-01-2015, 06:35   #11
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Re: Easy battery load tester?

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Originally Posted by John Boor View Post
I am not sure this will be of any great value to you. To understand why check out Peukert's equation on the web. The basic problem is the more current you draw the less caspacity is available, to do a C20 capacity I understand you have to keep current at 5 amps or less. Check it out.
John

A C/20 test is:

Rated 20 hour Ah capacity 20

A 100 Ah battery 20 = 5A Load

A 200Ah battery 20 = 10A Load

A 225Ah battery 20 = 11.25A Load

A 105Ah battery 20 = 5.25A Load

The test begins on a fully charged battery at the C20 load. The load is held constant at a battery temperature of 77F (also held constant) and the test is terminated at 10.5V (some batteries such as certain telecom etc. may terminate at 10.2V)

You can count time or ampere hours or both to give you the current capacity. When you discharge at the factory 20 hour rate, for a capacity test, there is no Peukert correction needed as you are drawing the battery at the factory Peukert neutral rate...

If you were to discharge at less than the 20 hour rate, or more than the 20 hour rate, then there would be a Peukert factor. Best to just test at C20 at 77F to 10.5V, or 10.2V depending on how your batteries were originally rated, as this removes those variables......
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Old 22-01-2015, 06:37   #12
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Re: Easy battery load tester?

If you have wet cells as your battery bank I would recharge the bank as normal and check the specific gravity (S.G) of all cells and if all are close then continue if not close then do an equalize cycle. When all are similar then load up the bank Like your stove for a reasonable time then check the S.G again if some cells are very different it indicates a bad cell then unfortunatly all batteries should be replaced since your bank is only as good as the weakest link (Cell).

John
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Old 22-01-2015, 07:04   #13
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Re: Easy battery load tester?

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Why??.....Your battery capacity is based on the "as new" C20 rating. When it falls below 80% of capacity, by industry standards, it is dead.
I only said "I assume" that you need to do the test again with a lower load if it reaches 10.5v after 10 hours!!!!!

The 80% rule I would "suggest" is the Standby and Telecom industry standard where reliability is important and not cost. For most of us the cost is probably more important - OK sometimes we might get our a$$ bitten, but I'm afraid that is the attitude of most of the long-term cruisers I meet.

This is why we all need to know how our batteries are behaving, especially at the beginning of a new season or long cruise. I'm afraid that most boat owners just don't have the knowledge or understanding to be able to asses this from daily observations.

A good reliable tester that doesn't take the battery down to 10.5v regularly would be magic!
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Old 22-01-2015, 07:26   #14
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Re: Easy battery load tester?

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I only said "I assume" that you need to do the test again with a lower load if it reaches 10.5v after 10 hours!!!!!

The 80% rule I would "suggest" is the Standby and Telecom industry standard where reliability is important and not cost. For most of us the cost is probably more important - OK sometimes we might get our a$$ bitten, but I'm afraid that is the attitude of most of the long-term cruisers I meet.

This is why we all need to know how our batteries are behaving, especially at the beginning of a new season or long cruise. I'm afraid that most boat owners just don't have the knowledge or understanding to be able to asses this from daily observations.
I saw that but did not want to OP to think he needed to do more than one 20 hour test..

The 80% rule is BCI for flooded batteries. The AGM makers have been able to convince BCI to allow for 50%. In all honesty I find both types tend to fall off the edge in the mid to low 70's.

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A good reliable tester that doesn't take the battery down to 10.5v regularly would be magic!
I have spent many hours on the phone with Midtronics etc. and unfortunately there simply is no such beast.

Taking the battery to 10.5V is not as harmful as the general boating public assumes, if recharged immediately there after. What is damaging is taking a battery to 10.5V and letting it sit there...

According to most engineers, at the battery manufacturers, it simply counts as 1 deep cycle. They also admit that partial state of charge use is actually more damaging than a properly done capacity test yet boaters PSOC cycle their banks all the time. When you weigh everything boaters do to murder batteries, a once yearly 20 hour test, done properly, is really the least of our worries.

Knowing bank capacity can actually be beneficial over the long haul because a boater who does not know his 100Ah battery is really only 80Ah's is now taking it to 70% DOD with each cycle, as opposed to the 50% they assumed.. One properly done cycle test to 10.5V can help correct for many cycles to 70% DOD vs. the assumed 50% DOD....
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Old 22-01-2015, 10:37   #15
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Re: Easy battery load tester?

Maine,

I suspect a 100Ah battery that has lost 10% of its capacity will be fully charged to its 90Ah capacity when the resting voltage shows the full charge voltage value as published by the manufacturer.

When the resting voltage indicates 50% of its capacity, the owner does not need to know about the reduced capacity in order to maintain the best practices discipline to recharge at 50% state of charge.

This is yet another reason why a simple voltmeter/ammeter remains the best way to monitor and maintain your batteries. The voltmeter will give you state of charge. The ammeter tells how your charging system is performing and what your load currents look like in real time.

Amp hour meters in most cases just confuse people and generally provide misinformation since they do not get recalibrated when needed by owners.

Using your voltmeter to indicate real time SOC is pretty simple once you have found its offset as a function of your boats typical load current during the day. Note the real time voltage at this current, disconnect the batteries and perform a 12-24 hour resting voltage test. Note the offset and apply it in the future when looking at the voltmeter while seeing your typical load current, to tell the SOC of the batteries.

We have done checks on the real time displayed voltage when operating at around 50% SOC against the resting voltage test several times, and find it quite accurate and repeatable. Even though at times during the day we may being pulling up to 165A from the bank when running our 16 KBtu air conditioning for hours at time, at say a 20% duty cycle.

The battery voltage at 165A may be reading 11.2V while the air conditioning is on, but once off and the batteries have had an hour to settle at our typical load current, we are confident once again in knowing where we are relative to 50% SOC. Which is really the only important number when discharging.
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