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Battery Capacity and Testing

rongawer

Well Known Member
I was reading a post in the RV-10 section and need to provide some correction to that post.

I've been dealing with large, grid-scale batteries for a very long time and have currently just installed a 200MWh Lithium chemistry based battery in the California grid; I assure you, that if that battery did not meet the discharge specs, I would not have signed off on the final test.

I'm not sure where the poster got the "PB equivalent" ratings from, but they are not in the National Electric Code, nor does it jive with either IEEE consensus on battery discharge rate nor the manufacturer specs for the batteries I've dealt with, which are many. There are suspect manufacturers that use a phony "pBeq rating", but it's just that, a made up rating to justify a poor quality battery performance.

In order to have an 18Ah rating, a battery must be able to discharge at 18A for one hour without dropping below minimum voltage. I pulled up EarthX's discharge curve for my battery, an ETX900 (attached below), to provide a reference.

The linked post states, "My 18Ah SLA battery will only give me about 10-11 amps for an hour and a PB equivilancy is 1/3 of that." This is a good example of a failed battery. Amp-hour ratings are based regardless of the battery chemistry. Note that if the battery does not pass the annual test, the battery is failed and should be replaced.

Test your batteries annually folks...if you don't have a manual for your battery, I've provided a good test to use below that mirrors most reputable battery vendor tests.

This is an excerpt from EarthX's manual 2020 edition designated 111017_Y

Battery Inspection and Testing
The ETX hundred series battery is a maintenance free battery. Charging is only required as needed (see charging section in this manual). Inspection or testing is not needed for 24 months after purchase, and thereafter the following is recommended annually:
• Visually inspect the battery for signs of damage; plastic case is warped or swollen.
• Test the fault indicator: to test, touch the fault wire output of the battery to ground –
the internal battery LED should on as well as the cockpit indicator.
• Ensure the terminal screws are tight (properly torqued)
For electrically dependent aircraft the following annual battery capacity testis recommended: To test the battery capacity (should be done at near room temperature, 23DegC):
a. Fully charge the battery with an appropriate charger
b. Turn on all electrical loads for flight operation and start a timer.
c. Measure and record the battery’s discharge amps using a DC clamp-on
current meter at the positive terminal of the battery.
d. Using the measured amps in the previous step and the battery’s nameplate
rated capacity (in Ah), calculate the time to discharge the battery to 80%. Time to discharge 80% (Hours) = ���������� ���������������� ���� ��h ∗ .8
For Example (16 Ah Rated Capacity, 5 amp measured discharge rate) Time to discharge 80% = 16 ∗ .8= 2.56 hours
5
31
���������������� ��������h�������� ��������

ETX SERIES LITHIUM BATTERIES
e. Terminate the test after the number of hours calculated in the previous step has expired or if the battery is over-discharged (shuts off discharge current). If the battery is still supplying power at the termination of the test, then the battery’s capacity is greater than 80%. If the battery’s capacity is greater than 80% of it rated or capable of supporting the aircraft’s emergency load for the required amount of time, then the battery has passed the test.
f. Fully charge the battery with an appropriate charger.
 

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LiFePO batteries, at least in the sizes we use, apparently have different discharge characteristics than AGM Pb batteries. Below are the Odyssey-published discharge curves of the typical PC680 and PC925 we use. The PC680 is rated "16Ah" but can only provide that much capacity if the discharge load is below 3.0A. Above that rate the battery capacity is substantially less.
In order to have an 18Ah rating, a battery must be able to discharge at 18Ah for one hour without dropping below minimum voltage.
I think you meant "18A", not 18Ah, but the charts below show that our AGM batteries won't support that.


batteries.JPG
 
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LiFePO batteries, at least in the sizes we use, apparently have different discharge characteristics than AGM Pb batteries. Below are the Odyssey-published discharge curves of the typical PC680 and PC925 we use. The PC680 is rated "16Ah" but can only provide that much capacity if the discharge load is below 3.0A. Above that rate the battery capacity is substantially less.
I think you meant "18A", not 18Ah, but the charts below show that our AGM batteries won't support that.


View attachment 31283

Standby, I am sure Ron will com along and call you crazy as he did with me. The above is for lead acid. Most Lithium providers, Earth x excluded, list their Ah ratings as PB equivalent and will be 1/3 of this discharge rating. They do this because their cold cranking capacity is almost three times that of a lead acid battery with the same Ah rating. Not saying it is right only explaining their rationale for doing it. I have no desire to argue with you Ron. You can do your own research and post the data here to prove me wrong, though Earth X is the exception to the rule so will not accept that as proof that I am wrong.

Larry
 
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In order to have an 18Ah rating, a battery must be able to discharge at 18Ah for one hour without dropping below minimum voltage.

Sorry, but no one selling to the retail sector, excluding Earth X, markets their batteries that way. They list an Ah rating at a specific discharge rate and that rate is almost NEVER the Ah rating. Typically it is done at the 10 or 20 hour reserve capacity rating. According to Odyssey's own chart, a brand new 16 Ah battery will NOT provide anywhere close to 16 amps for an hour. Good luck arguing that one with them on a warranty claim, regardless of your expectations or what the NEC says.

Welcome to Retail Marketing, where nothing matches the expected.
 
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Sorry, but no one selling to the retail sector, excluding Earth X, markets their batteries that way. They list an Ah rating at a specific discharge rate and that rate is almost NEVER the Ah rating. Typically it is done at the 10 or 20 hour reserve capacity rating. According to Odyssey's own chart, a brand new 16 Ah battery will NOT provide anywhere close to 16 amps for an hour.

Yup, agree completely
 
The lead-acid guys all use the 20 hour rate for their A-h rating.
It's all about internal resistance. P=I^2*R, so the more amps you pull out, the energy lost to internal resistance goes up with the square of the current. i.e. heat. So, for marketing reasons, you get the 20 hour number. You can figure R out from the tables, even if they don't tell you.

You always have to look at the A-h rating and discharge curves with your application and load in mind. For the most part, our batteries are for cranking the engine. A low total system resistance will ensure getting the most energy to the starter.
 
Larry, for the record, I did call not you crazy and I'm sorry you took it that way. This isn't personal - it's not even subjective; it's engineering and data provided by quality manufacturers based on design criteria from a consensus standard - always, always, go back to the source reference. These forums can be fun and helpful, but referring to design standards and technical manuals needs to be done to reset your knowledge.

Lead-acid batteries DO have a decreasing curve in voltage at the end of discharge that accelerates as voltage decreases, where Lithium chemistry batteries, notably LiFePO, maintain voltage much better voltage to the end of capacity - as shown in in discharge curves in the attachment of my previous post, this contributes to the perception of a different capacity rating, hence the misleading "equivalent" concept - a battery rating is always the "rating"...not all batteries are built to fulfill those ratings.

LiFePO batteries, at least in the sizes we use, apparently have different discharge characteristics than AGM Pb batteries. Below are the Odyssey-published discharge curves of the typical PC680 and PC925 we use. The PC680 is rated "16Ah" but can only provide that much capacity if the discharge load is below 3.0A. Above that rate the battery capacity is substantially less.
I think you meant "18A", not 18Ah, but the charts below show that our AGM batteries won't support that.

The Odyssey charts you provided demonstrate time using the rates listed, which is not the same as the capacity, or stated rating, for the affected battery.

You are correct. Thank you for pointing out my typo, I did mean 18A - that is the amperage discharge rate for one hour that a 18Ah battery will provide - hence the rating.

Since you referenced Odyssey, I referred to their Technical Manual, which states the following regarding tested capacity of the battery:

RECOMMENDATION: Testing should be completed on a clean/main battery terminal surface, not a steel stud. Testing batteries individually in multiple battery situations is best. At minimum, each battery must be disconnected at one terminal (the same polarity).
1. Capacity Testing: This method tests the performance of the battery based on its Reserve Capacity (RC) rating, which means the test may be more time consuming, however it is the preferred test method for a state of health check. The equipment needed to perform this type of testing is called a discharger tester. The battery should be fully charged before using this test method. Discharge testers are designed to apply a constant current load to a fully- charged battery until the battery voltage reaches 1.75 volts per cell (10.5 volts per battery) or other appropriate end point voltage inline with published performance tables, which is 100% discharged. The length of time the discharge tester runs until 1.75 volts per cell is reached should be compared to the battery’s rated RC. Batteries which do not provide at least 80% of their rated runtime are considered failed.


Note that they also state that not providing at least the 80% rating as a failed battery. This test protocol is typical of what credible manufactures will list - mostly because it's based on the IEEE standard for battery rating and testing (IEEE 450-2002 if you have access and some time on your hands...)

Sorry, but no one selling to the retail sector, excluding Earth X, markets their batteries that way. They list an Ah rating at a specific discharge rate and that rate is almost NEVER the Ah rating. Typically it is done at the 10 or 20 hour reserve capacity rating. According to Odyssey's own chart, a brand new 16 Ah battery will NOT provide anywhere close to 16 amps for an hour. Good luck arguing that one with them on a warranty claim, regardless of your expectations or what the NEC says.

Welcome to Retail Marketing, where nothing matches the expected.

Please refer to Odyssey's own Technical Manual, which I posted a link to above and provided their testing criteria.

I can name three manufacturers that provide full rated capacities for their batteries off the top of my head, in addition to EarthX, since I just purchased significant quantities from all three: BYD, Samsung and Tesla. You can do your own research on this, but it will reinforce what I've been telling you.

Unfortunately there's a lot of myth being propogaged here, I suspect based on bad experiences. And yes, there are unscrupulous vendors that sell cheap products that don't peform - but that does not justify a deviation from an international design consensus standard for batteries.

To summarize, an Amp-Hour rating identifies that amperage (that's a rate of current flow) for a period of time summed to an hour such that a typical RV battery of 16Ah must provide 16 amps for one hour to have that rating, other examples are 4A for four hours, or 8A for two hours. To state a rating and knowingly sell a product that doesn't perform to that standard is false advertising. "Welcome to marketing?" :rolleyes: more like "buyer beware".

Lastly, it your airplane do as you see fit - but it is your airplane; I would not go cheap on the battery. Especially if you have an EFII system for your engine.
 
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1. Capacity Testing: This method tests the performance of the battery based on its Reserve Capacity (RC) rating, which means the test may be more time consuming, however it is the preferred test method for a state of health check. The equipment needed to perform this type of testing is called a discharger tester. The battery should be fully charged before using this test method. Discharge testers are designed to apply a constant current load to a fully- charged battery until the battery voltage reaches 1.75 volts per cell (10.5 volts per battery) or other appropriate end point voltage inline with published performance tables, which is 100% discharged. The length of time the discharge tester runs until 1.75 volts per cell is reached should be compared to the battery’s rated RC. Batteries which do not provide at least 80% of their rated runtime are considered failed.
[/I]

Clearly we are interpreting the manuals differently. THe quotation above refrers to reserve capactiy, which is what the battery will deliver over a 20 hour period and the test proves that 80% of THE RESERVE CAPACITY can be met. That means that the battery will deliver 80% of the 16 amp hour rating when discharged at a .8A rate. This is NOT the same as discharging the battery at 16 amps for an hour. The odyssey chart clearly shows that at a 12A disharge rate, the voltage WILL drop below the critical level at ~1 hour. The chart further shows that at a 16A discharge rate, critical voltage is reached at around ~40 minutes, not one hour. Not really sure how to make it any clearer - Odyssey is telling you directly in their performance data that the battery WILL NOT deliver 16 amps for an hour.

FYI, BYC and Tesla are not classic retail marketers and would not expect them to use misleading data to market their products as most retail marketers do routinely. Retail Marketers are motivated by sales and sadly retail customers do not hold these practices against them, as commercial customers do, and therefore their strategies meet their objective. Caveat Emptor applies to most anything purchased at the retail level.

All of that said, Lead Acid batteries all have variable capacities based upon the discharge rate, so assessing capacity is never as simple as just looking at the Ah rating. You must always reference a chart and find the time based upon the discharge rate to determine available Amp Hours at that rate. Lithium based batteries are very different and cannot be compared directly to Lead Acid in the discussion of performance characteristics.

Larry
 
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As some have noticed, different manufacturers have different ratings and methods for reaching them. The only rating that really matters is the one that is done by the aircraft owner - and the result is pass/fail based on the real-world operating needs, regardless of the manufacturers wishes or marketing.

I follow the ETX instructions as listed in the OP's first post, modified for a target of minimum 80% capacity AND minimum of 45 minutes flight time supporting my shedded-load electrical scenario. My EFIS backup batteries are checked for minimum 45 minutes on their own in the event of alternator failure - the ships battery has to take up the rest of the required minimum load for at least that long as well.

Don't depend on the paperwork with the device - test it in your installation, for the way you will use it. That's the only way to be certain.
 
Clearly we are interpreting the manuals differently. THe quotation above refrers to reserve capactiy, which is what the battery will deliver over a 20 hour period and the test proves that 80% of THE RESERVE CAPACITY can be met. That means that the battery will deliver 80% of the 16 amp hour rating when discharged at a .8A rate. This is NOT the same as discharging the battery at 16 amps for an hour. The odyssey chart clearly shows that at a 12A disharge rate, the voltage WILL drop below the critical level at ~1 hour. The chart further shows that at a 16A discharge rate, critical voltage is reached at around ~40 minutes, not one hour. Not really sure how to make it any clearer.

FYI, BYC and Tesla are not classic retail marketers and would not expect them to use misleading data to market their products as most retail marketers do routinely. Retail Marketers are motivated by sales and sadly retail customers do not hold these practices against them, as commercial customers do, and therefore their strategies meet their objective. Caveat Emptor applies to most anything purchased at the retail level.

Larry

See your quote below.

You will find that most lithium battery sellers use PB equivalent ratings. That means that a 100 Ah battery can deliver 100 amps for about 5-10 minutes. Even Lead Acid batteries set the Ah rating at the 10 or 20 hour reserve level. My 18Ah SLA battery will only give me about 10-11 amps for an hour and a PB equivilancy is 1/3 of that. It is rare to find a battery that can actually deliver upon it Ah rating; The industry just doesn't work that way.

Larry

I read your words literally. You've stated that a 100Ah battery will only provide ~6% of its capacity. You also state that a 18Ah battery will only give 10-11 amps for an hour, which is much less than 80% of the 18Ah rating. Both examples define a failed battery, which caused me to start this thread to begin with. Please explain how that's misinterpreted.

Again, your words, "no one selling to the retail sector'. BYD, Samsung and Tesla are very large manufacturers of batteries that have a huge retail market. Possibly not a market you're familiar with, but there nonetheless - and you probably own some of their batteries and don't even know it...got an iPhone? How about an Android? Tear one open and see who made the battery. Ever heard of a Tesla Power Wall? Google it... retail sales.

I'm not sure what your goal is here, but mine is simple: an amp-hour rating is a battery's discharge-over-time rating and if it's rated at 18Ah (or 24,000Ah), and the battery does not produce that discharge within the prescribed time, it is a failed battery - allowing for the 80% capacity reduction over the life of the battery. People should buy a battery based on that rating and not be concerned what is the "equivalent" value or how much diminished capacity should they expect.

When I buy a new battery, the first thing I do with it is perform a full rated test of it; if it fails, I return it. if it fails miserably, I return it and remove that manufacturer from future purchases. And FYI, I no longer purchase Odyssey batteries. I do recommend Concord for Lead-Acid and EarthX for LiFePO for aircraft use. Buy your battery rated for your need, and then test it and know.

Back to your tack on sales and marketing - you will pay more for a quality battery.
 
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Battery Life

Always a good idea to test batteries (Especially) an electrical dependent engine. I test mine annually to insure they are at least 80% before the battery light comes on. (Also monitor voltage) So far after 3 years going strong. Just FYI you can purchase a cheap mA totalizer on Amazon. I know this is a good test for lead acid and I assume Earth X batteries but does anyone have any experience of failure mode for a EarthX battery? IE could it test out to 90% and fail a month later? By fail I mean much less than 80%. With 2 batteries I am not so worried about this but good to know.
 
EarthX Failure modes

I know this is a good test for lead acid and I assume Earth X batteries but does anyone have any experience of failure mode for a EarthX battery? IE could it test out to 90% and fail a month later? By fail I mean much less than 80%. With 2 batteries I am not so worried about this but good to know.

I've had a few EarthX batteries; I have replaced two of them at about the 8 year point. The indicator for me on one was that I starting to get a fault (5s on/5s off) that came in with voltage above 13.5V system voltage. It came in for a few minutes, and then went away. The second time it came in, I contacted EarthX for advice. They recommended that if the light goes out within 30 minutes, the battery is doing a balancing of cells - but this also is an indicator that the battery is aging. Their manual indicates this is a replacement criteria if the fault stays in more than 30 minutes.

On my second battery, I noticed that my Optimate charger provided a red light during a post charge test. (side topic: I highly recommend the Optimate charger. I have the TM275 and find it is excellent for maintaining Lithium batteries.).

The red test light on the charger indicated that the battery was dropping below 12.8V within 30 minutes during the post charge test, where the Optimate performs a minor discharge test, then recharges and reverts to maintaining voltage. Noting that the battery was at 8 years age and that EarthX indicates a normal service life is about 6 years, I replaced my battery on that fault.

I expect that if properly maintained, including not allowing the battery to stay in a discharged state below 13.28V, you would expect a normal life with end of life indications being similar to mine. That doesn't discount that the sucker could short circuit and fail immediately, but I would expect that is extremely rare having a Battery Management System built in. It does highlight the importance of having a discrete output from the BMS to an EFIS, or an idiot light if you prefer; this is a definite advantage over a lead-acid battery in that it is providing performance indications above simple voltage. If a lead-acid battery sulfates or experiences cell-reversal, the immediate indication is a loss of voltage with little to no warning. The normal failure mode of Lithium-Ion battery designs is the reduction in ability to raise cell voltage, driven mostly by over or under voltage conditions and high temperature charging (which is mitigated by EarthX by having a BMS that limits charging current in reference to cell temperature).

I currently have two ETX900 batteries, one is 5 years old and one is 1 year old. The 5 yo battery recently provided 92% full capacity and the 1 yo was 100% based on the annual test criteria.

Also, a discharge test can be very simple, such setting up a measured load on the ground (such as running a fuel pump, all the lights and some avionics to get a fixed load, preferably above 80% rated capacity of the battery) with a a system ammeter, or use a clamp-on meter (from Amazon no less) - and then just marking the time until voltage drops to 11V, and then doing the math (see the test in my first post) to determine the remaining capacity.

More "free" advice, is that you should set your voltage regulator at 14.2V for the ideal setpoint on lithium batteries, which is lower than the 14.4-14.6VDC common with most L-A setups. Above 14.2 will reduce the life of the battery - less than 14.2 unnecessarily limits the system capacity.
 
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<snip>I just purchased significant quantities from all three: BYD, Samsung and Tesla. <snip>

Ron, can you tell us what chemistry these batteries are and the specified float voltage range? Per cell is good for comparison.

Are these banks being typically managed to a mid DOD range?

If there are qualifying conditions of temperature and time (re float voltage) please so state.


Thanks.
 
Ron, can you tell us what chemistry these batteries are and the specified float voltage range? Per cell is good for comparison.

Are these banks being typically managed to a mid DOD range?

If there are qualifying conditions of temperature and time (re float voltage) please so state.


Thanks.

Bill, I'm interested in what you intend to do with information I provide - it comes across as a question from a lawyer for cross-examination rather than a quest for information. You can find most of this information with Google as well, just search on "BESS".

However, noting that my answer has little to do with a battery for an RV, I'll answer... the chemistry varies by installation and intended purpose, however the most recent one I did is by BYD and lithium-Ion chemistry. I've attached a photo of what our 600+MWh of storage looks like, just achieved COD back in March. The output voltage from the inverter bank is just over 13,000VAC, which is then stepped up to 230kV for transmission.

We don't actually monitor "per cell" as there are literally 300M+ cells in this pictured system...although they're essentially massive packs made up of 3.7V cells that look very similar to an 18650 cell you might have a for a flashlight. I haven't cut open an EarthX battery, but I suspect their internal construction is similar, albeit made of LiFePO.

There are many qualifying conditions with this Battery Energy Storage System (BESS) and it is highly instrumented with cooling, inverters, and charging systems, along with several other auxiliary controls and monitoring interfaces. This particular installation is a 100% DOD/SOC (Depth of Discharge and State of Charge) installation that is capable of daily full cycles for 20+ years, with many parameters of temperature and charge, although most of it is covered by an NDA and is not shareable on this forum.
 

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No lawyer question, just technical.

Thanks Ron, that is 7300 cycles for full charge discharge cycles, pretty impressive that they can do that. I suppose allowances have to be made for deterioration etc.

Based on the answer provided the application cycle gives a lot of insight into how these batteries are used. My experience was for mobile machinery where consistent power availability was important whether as full electric or hybrid, and near top SOC LiFePO would fall short of absorption and near zero SOC short of discharge power. Just a characteristic of the charge/discharge curves.

I suppose if one has full backup power capability then this is not needed and full range cycles are possible. In machinery it did not make sense to have full rated power for the prime mover and the energy storage just in case.

In regards to the float voltage, this application would have little time there or could drop the voltage as needed. Huge demands on silicon to buck/boost over the full range. Not at all the situation with a starting battery in a constant voltage system.

For readers the early, Prius hybrid only operated in a DOD range of about 5% to render long cycle life with a target in the mid SOC range. Lousy for a "plug in" hybrid.
 
They've definitely come along way with lithium based battery construction. Also price per kW is on track for about 16% reduction per year over the last 10 years from an industrial perspective. How that translates to a small retail battery is that most manufacturers are able to hold market pricing in spite of significant inflation rates. Case in-point is that EarthX ETX900 costs the same $449 today that it was in July 2017.

The design criteria is 10,000 full cycles, so yes, definitely a little extra in the bank for the life of the unit. However, unlike a mobile installation that might be weight or space limited, we can additional "packs" as needed to ensure 100% nameplate capacity over unit life.
 
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