Rant about Ah and Wh...
- Thread starter upnorthandpersonal
- Start date
Yes, ZKE has. Those fan based ones and CBA IV and others don't.
Texas-Mark
Solar Addict
- Joined
- Aug 4, 2021
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- 1,280
IMO it’s all about what you are trying to validate. To be honest I never look at Wh. My shunts measure Ah in/out and that is the only metric I really care about. Been doing solar in one form or another for over 20 years.
I was not really questioning the premise of the thread just clarifying for me so I understand better.
Been using a Turnigy power meter for 16 years to measure solar current in my RV. Never gave accuracy a second thought. That is, until I started measuring solar current with Cal's DIY BMS. Even though the two measuring devices displayed equal currents the Turnigy Ah measurement is constantly 7% higher than what my BMS outputs. Using a precision constant current power supply as the source for the two measurement devices, I determined my BMS is spot on while Turnigy Ah measurement is 7% high.
BTW, the rant should be:
When measuring cell/battery Ah capacity, why are people measuring Wh and then trying to convert to Ah?
Last edited:
timselectric
If I can do it, you can do it.
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wattmatters
Solar Wizard
What does it say when it's at 40 V (2.5 V/cell)?
Lt.Dan
Solar Wizard
Honestly I haven't used the smart shunt in so long because I removed it, I don't even remember. I don't think it ever got below 44-45v though because a cell would hit LVD.
Daddy Tanuki
Solar Wizard
The aside to this is that the various vendors from Cheena will play this to up their sales game
Daddy Tanuki
Solar Wizard
Or rest your sweaty arm across multiple terminals!
Daddy Tanuki
Solar Wizard
You Do not understand because you are so far up the curve in knowledge that it automatically computes to you. Not every one of the new users are so lucky. Hence this post (I think).
Daddy Tanuki
Solar Wizard
Especially the shady vendors
Daddy Tanuki
Solar Wizard
Most people can't even get kW versus kWh right and you have high hopes for Ah?
My pet peeve is people making statements like:
"I was putting 10 amps an hour into the battery".
"Those panels produce 5kW a day mate!"
Ah is a pretty useless unit on it's own and as I think the OP is frustrated by it's very loose and frivolous definitions or rather the variety of.
The main issue, with Ah especially, is it requires other information to interrupt as "energy". People however treat it as an energy reading, but it's not, it's only accumulative current over time. I can clock up 100Ah from a 10Ah battery using a buck converter.
Try this. On a small panel -> mppt -> battery system, look at the panel amps and the battery amps. Accumulate their Ah's ... what gives? Buck converter.
My pet peeve is people making statements like:
"I was putting 10 amps an hour into the battery".
"Those panels produce 5kW a day mate!"
Ah is a pretty useless unit on it's own and as I think the OP is frustrated by it's very loose and frivolous definitions or rather the variety of.
The main issue, with Ah especially, is it requires other information to interrupt as "energy". People however treat it as an energy reading, but it's not, it's only accumulative current over time. I can clock up 100Ah from a 10Ah battery using a buck converter.
Try this. On a small panel -> mppt -> battery system, look at the panel amps and the battery amps. Accumulate their Ah's ... what gives? Buck converter.
Steve_S
Offgrid Cabineer, N.E. Ontario, Canada
Allowable Voltage Range being 2.500-3.650, the "Center Voltage" would be 3.075.
Working Voltage Range being 3.000-3.400 places the Center Voltage @ 3.200. The source of debate BUT is supported by Manufacturer specs & docs. 3.200 Volts per cell "Nominal" is a manufacturer default for 50% SOC of the working range, NOT the 50% Mark between 2.50-3.65V.
Allowable is the safe range where no harm comes to the cells, above 3.650 or below 2.500 can & will cause harm. *Such is consistently documented by the Manufacturers for "Standard LFP Chemistry".
Too many vendors will do Capacity Tests by fully charging to 3.650 and immediately start to discharge down to 2.500. This effectively squeezes the WH ratings. This is not proper, as that is within the "Allowable" range and not the working range they are designed for. This is also NOT What Manufacturers test, when looking at Manufacturer Test Datasheets, it shows the "working range" is what is tested, as this is the "Deliverable AH zone" where it is supposed to be capable of delivering the specified AH/WH of the cells.
To Paraphrase.... (loosy Goosy numbers for the example)
This is no different than selling a car that has 400KM range, with a full tank, but topping the tank (extra 2 L to filler neck) to fudge the range test result... That 2L represents the minimal amount of extra power (mileage range) that is not spec'd.
The 100AH cell should in fact have roughly 105-107AH if going from 2.500-3.650, while being capable of delivering the specified 100AH from the 3.000-3.400Vpc zone. Anyone who watches their battery systems closely can & do observe that cells will generally stay close together while within the Working Ranges and will start to deviate outside of the working range. Depending on the Quality/Grade of the cells, the deviations may be quite small to significantly out from each other.
It should also be noted that Temperatures DO AFFECT charge capacities (some do not believe it does) and all tests should be performed at 25C/77F temp. I have done Temp Tests and @ 5C/41F the general loss was close to 12%, tested on Bulk, Grade-B & Grade-A Cells and this is fairly consistent. I did tests @ 30C/86F and found only about 2-3% loss of capacity from full. I've had to do so because I have a separate heated powerhouse which I do not heat to 25C but needed to determine the "sweet spot" for wintertime use which I found to be 15C/59F which is the best compromise with least effects.
It appears that far too many forget that ALL batteries have both an Allowable Range which does not cause harm to them and an optimal Working Range. This applies to all chemistries with relative variations.
Just for fun...
24V system:
24V/100AH (25.6V * 100AH = 2560Wh) (using 3.200 Nominal Formula)
24V/100AH (24.6V * 100AH = 2460Wh) (using 3.075 Centre of 2.50-3.65)
The saddest part of all this confusion, is that some folks set their BMS' to CUTOFF at 2.500 and when the BMS does cutoff for LVD it cannot recover enough to release to allow charge. (I think there is 2 threads where this happened to folks now) because people think they can... Fact is they shoudl be doing LVD @ no lower than 2.600 to allow for a BMS to recover and allow to charge without manual intervention...
Hope it helps & Good Luck in all your projects.
Working Voltage Range being 3.000-3.400 places the Center Voltage @ 3.200. The source of debate BUT is supported by Manufacturer specs & docs. 3.200 Volts per cell "Nominal" is a manufacturer default for 50% SOC of the working range, NOT the 50% Mark between 2.50-3.65V.
Allowable is the safe range where no harm comes to the cells, above 3.650 or below 2.500 can & will cause harm. *Such is consistently documented by the Manufacturers for "Standard LFP Chemistry".
Too many vendors will do Capacity Tests by fully charging to 3.650 and immediately start to discharge down to 2.500. This effectively squeezes the WH ratings. This is not proper, as that is within the "Allowable" range and not the working range they are designed for. This is also NOT What Manufacturers test, when looking at Manufacturer Test Datasheets, it shows the "working range" is what is tested, as this is the "Deliverable AH zone" where it is supposed to be capable of delivering the specified AH/WH of the cells.
To Paraphrase.... (loosy Goosy numbers for the example)
This is no different than selling a car that has 400KM range, with a full tank, but topping the tank (extra 2 L to filler neck) to fudge the range test result... That 2L represents the minimal amount of extra power (mileage range) that is not spec'd.
The 100AH cell should in fact have roughly 105-107AH if going from 2.500-3.650, while being capable of delivering the specified 100AH from the 3.000-3.400Vpc zone. Anyone who watches their battery systems closely can & do observe that cells will generally stay close together while within the Working Ranges and will start to deviate outside of the working range. Depending on the Quality/Grade of the cells, the deviations may be quite small to significantly out from each other.
It should also be noted that Temperatures DO AFFECT charge capacities (some do not believe it does) and all tests should be performed at 25C/77F temp. I have done Temp Tests and @ 5C/41F the general loss was close to 12%, tested on Bulk, Grade-B & Grade-A Cells and this is fairly consistent. I did tests @ 30C/86F and found only about 2-3% loss of capacity from full. I've had to do so because I have a separate heated powerhouse which I do not heat to 25C but needed to determine the "sweet spot" for wintertime use which I found to be 15C/59F which is the best compromise with least effects.
It appears that far too many forget that ALL batteries have both an Allowable Range which does not cause harm to them and an optimal Working Range. This applies to all chemistries with relative variations.
Just for fun...
24V system:
24V/100AH (25.6V * 100AH = 2560Wh) (using 3.200 Nominal Formula)
24V/100AH (24.6V * 100AH = 2460Wh) (using 3.075 Centre of 2.50-3.65)
The saddest part of all this confusion, is that some folks set their BMS' to CUTOFF at 2.500 and when the BMS does cutoff for LVD it cannot recover enough to release to allow charge. (I think there is 2 threads where this happened to folks now) because people think they can... Fact is they shoudl be doing LVD @ no lower than 2.600 to allow for a BMS to recover and allow to charge without manual intervention...
Hope it helps & Good Luck in all your projects.
That's just not true. The goal of the folks in the rant is to accurately determine battery capacity. Battery capacity is measured in Ah. Why would anyone measure battery energy and then try to convert it accurately to capacity?
Don't understand your second line. What does "clock up" mean? A 10Ah battery has 10Ah of capacity. How can one get 100Ah out of this battery? You don't understand the definition of battery capacity. It is defined at a specific discharge rate.
Where? The maximum allowable is 4.20V. That is the chemistry limit.
The only reason manufacturers specify 3.650V is because there is no point going for the 4.20V for the final percentage or two and risking damage by going over 4.2V
Ah is not an energy unit. Nor it is a capacity unit. It's a accumulated current unit. It even says it in it's name. Ampere-Hours Amps times hours.
Using it on batteries is a legacy thing because batteries are consider current sources in many fields.
If I put 10 amps through a circuit for 10 hours it's 100 AmpHours, it doesn't matter if it came from a battery or a tree.
To get 100Ah out of a 10Ah battery you just step the voltage down 10:1. So if it's a 12V battery, draw it at 1.2V
The only way Ah makes sense is if you use the full test curve provided by the manufacturer. Using that you can add the magic ingredient to convert to instantaneous "power". From instantaneous power over time and calculus you get to Wh.
If you use a different curve and come up with a different capacity figure, it's wrong. Period.
Technically, yes. If you take a current cell like the EVE ones to 4.2V, they'll swell like a balloon though.
Using the Ah to do a capacity test against the rated Ah capacity of the cell is perfectly sound. One of the reasons this is done is because you take the losses out of the equation even with a cheap measurement device. You also only have to measure one thing over time, instead of two (voltage and current) if you want to measure energy.
No, your battery (before the load you put on, which happens to be a step down converter) will still give you 10Ah.
And that would be exactly my point. You don't do it the other way around. The manufacturer advertises the Ah rating at a certain C rate. You measure that to verify said capacity.
They are meant to swell. That is also in the datasheet. A fully charged cell is actually physically larger than an empty one.
I have taken cells to 4.2V. I can't confirm they are Eve cells, but if they are they are B-Grade rebranded as I didn't pay anything like Eve money for them
They do however look suspiciously identical in every tiny detail.I didn't intend to take them there and when I found that one cell there it was an emergency procedure to get it back down. Not because 4.20V is destructive, but because the charge controller hadn't finished yet and was still pushing 8 amps into the pack until I switched it off and used a 1ohm then 2ohm resistor to bring it back under 3.65V. I manually switched the system off at 14.00V until the BMS arrived.
The cell was not bulged, still cold and no signs of fool play. ... yet.
The ultimate reason why LFPs have the scary reputation they do and people "kid glove" them is nothing really to do with LFPs at all, it's their LiMCO cousins which were far, far more common with DIY hobbyists going 5 to 10 years back. Those things are touchy as hell with incredibly small margins. They have been and continue to be a source of house and workshop fires.
LFPs have got insanely large margins for error if you work by 3.650V per cell. However it's an interesting read following the overcharge test papers. They serve as a reminder that while they are robust and they have got margins allowing you to "beat on them" quite a lot... when you push them WAY too far, it happens very quickly and the amount of energy released in such a short space of time is insane.
The paper I read connected 1 C current with an unregulated voltage and stood well back. The voltage carried on rising and rising along with the temperature until (it was around 6V I recall) the cell internal temperature has got to the point the chemistry starts to self discharge, effectively shorting itself out internally, which immediately causes the release of a massive amount of heat which just accelerates the reaction further. The cell made it all the way up to 10V before finally dropping to 0V and 0A. The whole test was over in under 10 minutes. The total capacity of the battery was released in about 1 minute and the cell reached several hundred degrees celecius. The vent gas is far hotter and can ignite combustible items near by. Ideally battery banks should be in a metal box with room to swell if needed and NOT be pressure sealed, but vented via a metal pipe to the outside world to prevent explosion.
This is why you do not play near the "do not exceed" voltage without really knowing what you are doing. Because the "Kaboom" voltage is not that far away.
Yes, but not like a balloon. I can confirm this first hand from doing tests in my lab (yes, I do/did tests like this for customers professionally): a Winston cell can handle it, an EVE/CATL prismatic cell can't.
I think most people working with LFP on this site understand that the chemistry is safer and more forgiving.
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