Using Ah rating of a lead acid/car/leisure battery for any useful calculations - Page 1
Forum section: electronics / general

author: ziplock9000
author_rank: Regular Contributor
timestamp: June 06, 2020, 01:33:34 pm
content: Long story short, a load I attached to a battery was disconnected by the charger controller due to a low battery before it should have according to the figures and calculations I'm aware of. This got me thinking:If the Ah capacity rating of a battery applies only when it is FULLY drained, how can that value be used for any calculations regarding useful capacity which only allows a minimum voltage drain of 11v?*How can it be used to calculate what loads you can attach to it and for how long which just about every website does?Can somebody confirm my assumptions here are correct and secondly if true, what metriccanI use to calculate how long different loads can last?I understand there are discharge graphs that manufacturers provide, but I don't know how to use them to see how many Ah should be available over a certain voltage range.Also those graphs apply to new and ideal batteries, not the used ones I have.So another question what is a reliable way to calculate the current Ah capacity of a battery over it's useful voltage range which works for older batteries?* The exact minimum voltage may vary on system to system and how much you want to protect your battery, but the exact value is irrelevant here for the point made.

author: tunk
author_rank: Super Contributor
timestamp: June 06, 2020, 01:47:54 pm
content: Capacity depends on the load, e.g.:https://lygte-info.dk/review/batteries2012/Yucel%20Y12-6L%206V%2012Ah%20UK.htmlAlso, batteries lose capacity as they age.

author: helius
author_rank: Super Contributor
timestamp: June 06, 2020, 02:33:33 pm
content: Flooded batteries lose capacity by shedding material that then settles to the bottom of the housing. Sealed batteries release gas if overcharged, which cannot be replaced and so the electrolyte gets depleted. Both types experience sulfation, a reduction in usable plate surface area from the formation of tenacious crystals. Sulfation is increased by not storing the battery fully charged.11v (1.83v per cell) is so low that no meaningful energy remains to be used. Remember that as the battery discharges and reaches a lower voltage, its internal resistance goes up and it can supply much less current. This is unlike a capacitor which may explain the difficulty. To calculate energy in a battery you need to use its SOC (state of charge), a figure that reflects the coloumbic input and output during charge and discharge cycles. Best advice for lead-acid batteries that are not "deep discharge" types is to only discharge to 50% SOC because shedding increases below this point, shortening the life of the battery.You might think that the energy available is related to the voltage and so the output from 100% to 99% SOC when the voltage is 12.5V will be more than the output from 1% to 0% SOC when the voltage is 11V. While that is true to an extent, the extra energy is generally just wasted heat because whatever uses the power needs to be designed to function at both extremes. In some cases you might get a higher brightness from a lamp or something like that, but usually it doesn't help. Because it simplifies things most calculations just use the SOC (amp hours).Calculating the SOC is done using a load and amp- and volt-meters. It is also a function of battery analyzers, which includes some advanced chargers. There are certain conditions for full and low charge (temperature, dV/dA, open-circuit voltage, internal resistance) and the analyzer uses them to mark the endpoints of a discharge test.By the way, an "amp hour" is a non-SI unit. The proper unit for SOC is coulombs, with 1 Ah = 3600 C.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 06, 2020, 03:25:54 pm
content: Lead acid batteries are just very crappy. The Peukert effect is very real, you only get the rated Ah capacity at very low discharge rates (typically defined at C/20, i.e., a 10Ah battery must be discharged at 0.5A for it to last 20 hours). At C/2, which would be relevant on many applications include e-mobility, UPS, solar storage, etc., the effective Ah rating is already approximately halved. Compare to li-ion, where the capacity is typicallydefinedat C/2, and even at C*2, you get like 90-100% of the capacity (depending on the exact definition).Additionally, with lead acid, we often tend to define "end-of-life" as a capacity drop below 50-60% or so, so combined with the stupid Peukert, if you want to design for a C/2 system which uses the battery until end-of-life, you need to derate the capacity by 4! Meaning, if you need 5A for 2 hours, you need, not a 10Ah, but a 40Ah lead-acid battery.Given the price increase of lead (bare metal) during the last few decades, the fact that lead acid batteries always seems to perform under the specifications (or, more properly, they perform to specifications, but you need to understand the caveats and read the specs carefully), combined with the price drops in lithium ion, lead acid is not anymore a "cheap" alternative, but actuallya lotmore expensive than li-ion; the only reason to use it is lack of design experience/resources to jump into 2010's technology. This is very understandable, though, and I always feel almost physical pain when I have to recommend someone just to buy a lead-acid battery for a certain application because that's the only way to just get a simple 12V battery block that Just Works.

author: ziplock9000
author_rank: Regular Contributor
timestamp: June 06, 2020, 05:09:43 pm
content: Thank you all for your responses but I'm still unclear.Many websites provide the following sort of calculation and conclusion when calculating how long a load can last on a battery (assume an ideal scenario):"A lead battery with a stated capacity of 1000wH will allow for a 100w load to last 10 hours." (They may use Ah instead)But surely this isonlytrue if the battery is allowed to drain to zero, which in the real world it would not to avoid damage to the battery.. Is this correct?In the real-world, the load would be disconnected around 11-12v...So the actual useful capacity is a less than 1000wH. Is this also correct?So the calculations they use are completely wrong?I'm, sorry this is very "back to basics", but I see so many websites use these sorts of calculations I wonder if I'm wrong.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 06, 2020, 05:47:17 pm
content: You need the following elements:* Peukert derating; look at the rated Ah (or Wh)at the actual current. For lead acid, at 1-2 hour discharge time, it's roughly half the rated 20h rating.* Derating for aging; this is, if you need to guarantee a certain capacity throughout the whole lifetime, then you need to only use part of the charge initially, so that once the battery loses capacity, youstillhave enough. For example, if the battery is rated to lose 40% of capacity (drop to 60% of original) after 500 cycles, and you want it to supply 100Ah after 500 cycles, it needs to beinitiallybe a 167Ah battery.* Any extra capacity that helps prolonging battery lifetime, or improve other specs (for example, if you need lower internal resistance, instead of looking for extra-low resistance special batteries, just increasing capacity may be easier to achieve the same)* Any extra capacity safety factor you think you need. For example, if you design an electric vehicle and need to commute 100 miles, you'd want to design the battery to have a range of at least maybe 130 miles;evenif you modeled all other battery parameters perfectly.Look at the exact ratings and definitions for each product, butobviously, that is, if they are not misleading you, when you buy a battery type which is supposed to be cycled (i.e., not a starter battery), the rated capacity is assuming discharging from 100% to 0%. Discharging to 0%isallowed. If it wasn't allowed (this is, if it was damaging to the battery), then saying you have x Ah when fully discharging (and damage the battery) would be misleading or outright lying. This normally isn't the case; yes you can discharge to 0% by definition, that's how theymeasuredthe capacity to begin with.But do note that lead-acid batteries, even those designed for cycling and full discharge, don't like being left partially charged, they will sulfate. Which is also why lead acid totally sucks for renewable energy storage, but works acceptably for UPS.Limiting the depth-of-discharge may, with some battery types, increase the lifetime, this is for you to decide, but youcandischarge a proper, properly specified battery fully. It's up to datasheet to specify what "discharging fully" means, it's usually a combination of reading a certain voltage under a certain current.

author: bdunham7
author_rank: Super Contributor
timestamp: June 07, 2020, 05:23:20 am
content: Quote from: ziplock9000 on June 06, 2020, 01:33:34 pmYou don't 'fully drain' the battery to 0V, mostly because there's so little energy in the last 10 volts or so as to not make it worth it. The battery doesn't like it either. The numbers vary with battery chemistry and so on, but for a bog-standard FLA deep-discharge battery, you can figure an SOC of 0% to be about a 1.75 to 1.85V/cell OCV and an electrolyte SG of 1.040 to 1.060. Or so.

author: ziplock9000
author_rank: Regular Contributor
timestamp: June 07, 2020, 05:29:44 am
content: Quote from: bdunham7 on June 07, 2020, 05:23:20 amI see, so because the last 10v are essentially almost nothing, the stated aH on the side of the battery is applicable down to that point too, not just down to 0v?

author: bdunham7
author_rank: Super Contributor
timestamp: June 07, 2020, 06:20:31 am
content: Quote from: ziplock9000 on June 07, 2020, 05:29:44 amIf the manufacturer is fully specifying an AH rating, they would have to define the start SOC and end SOC. So if you go to the Trojan or Rolls websites, they'll list a bunch of data regarding AH, cycle life vs DOD, etc. It's worth looking at if you are curious. In reality, I think most people don't pay much attention to those specifics because all FLAs are similar. However, if another manufacturer tried to gain a market advantage by testing their AH running the battery flat to 0V and not listing the cycle life, their advantage would be very slight. It's not like Li-ion where you can significantly increase the AH by changing the voltage cutoffs--in the case of Li-ion it's at the top end where 3.92 vs 4.15 vs 4.35 all give very different C ratings.Look at the two charts here and you can see that if you are setting up a system, you can use the info to select a low voltage cutoff according to how many cycles you would like the battery to last.https://rollsbattery.com/battery/2-ys-62p/

author: Siwastaja
author_rank: Super Contributor
timestamp: June 07, 2020, 07:11:33 am
content: Ah, now I see your confusion. Unlike with capacitors, batteries arenotdefined "fully drained" at 0V.What "fully drained" is, depends on battery chemistry and also the individual product, and must be defined in the battery datasheet. It's often a more complex condition than just voltage, for example, for a 3Ah lithium-ion cell the condition could be: hitting 2.5V while discharging at 1.5A.For 12V lead acid, it's a bit below 11V typically.Beyond that point, some extra energy may or may not be available, but if it is, draining it further will damage the battery.Ah rating is measured by the manufacturer from the highest allowed (safe) full condition to lowest allowed (safe) empty condition.

author: ziplock9000
author_rank: Regular Contributor
timestamp: June 07, 2020, 01:17:18 pm
content: Quote from: Siwastaja on June 07, 2020, 07:11:33 amThank you very much, you've confirmed what I was trying to elaborate