Understanding amp hours and lithium batteries

[Rando]

I am not sure how any of these are arguments in favor of Ah over Wh. I, and most power system designers, actually want to know what the energy efficiency of a storage system is, not what the 'current efficiency' is. For that you need to know how much energy (Wh) you put into the battery while charging, and then how much energy you can get out of the battery (Wh) while discharging. If you are actually trying to make an accurate energy budget (which we often need to do) you have to take into account that you typically charge a battery at ~14.0V, then discharge it at 13.2V - even if you get 99.5% of the Ah back out, you are only getting 94% of the energy back out. For a carefully designed system, this matters.

You are correct that as the battery voltage rises while charging the energy efficiency of the system is going down - using Ah you are ignoring this loss. I don't see this as an advantage. As a follow on to this, as batteries age, their internal resistance increases, which means it takes more energy to charge the battery and there is less energy available from the battery. Using Ah only, you are ignoring this, even though it is a measure of battery performance with time.

Ah are a simplification based on a legacy from when everyone used the same chemistry/voltage, power was hard to measure, and we primarily used linear regulators that were constant current devices. None of these are true anymore.

With good reason! Not just "how its done" or easy calculations.

Amp hours relate to the basic chemical reaction of the battery whereas Watt hours are much more affected by state of charge when charging and discharging and by rate of charge and discharge.

Taking a LFP battery as an example, when new the CURRENT charge to discharge efficiency is about 99.5%. As the battery ages this efficiency INCREASES! i.e. almost all the amps × hours put in can be taken out. BUT the Watt hours put in and Watt hours taken out depend where in the cycle they are put in and how fast they are out in. Watt hours in the early part of the cycle are reasonably efficient but decrease in efficiency as voltage rises.

TL;DR Coulomb counters show amps and amp hours natively because that is more accurate.

It is Watt hours that are derived.

But Wh are good for comparing energy usage over time, between say mains AC load devices and 12Vdc

or the two different sides of an inverter.

 

[john61ct]

I am talking about In Real Life practicalities

accuracies in measuring storage capacity and energy in vs out, usage vs replenishing.

Use Ah when it's one system all in the same nominal voltage range.

Use Wh when comparing unlike systems.

Boom, done, that is all.

 

[Rando]

I am well aware that I am not going to change anyones practices here. I was pointing out that the prevalence of Ah for batteries is not some superior system, but is an anachronism. With the increasing use of battery storage for serious applications (EVs, home power etc) and different chemistries, the industry is slowly switching to Wh. We have already seen this in the RV/camper market with packaged systems like Goal Zero, and others will follow suit.

 

[john61ct]

The market is so full of scammers

capacity claims are so subject to exaggeration since few consumers are set up to verify

almost as suspect as claimed cycles lifespan, power capacity and ESIR.

If you do cap test - important for tracking SoH% over time - then use Ah.

 

[mechengrsteve]

While planning my electric power usage in a truck camper, I'm trying to wrap my head around how the tech specs translate to real world use. For example, the lithium battery in the camper will be a 100 ah 12v LiFePO4 deep cycle battery. When I compare the amp hours to the battery i use to charge my phone when I travel, (Anker Powercore 20,100mah) the specs don't seem that impressive. The Anker costs about $50 and I get 20 ah of power. If I was to add a second battle born battery to my camper, it would cost about $900 for 100 ah of power. Why is there such a price discrepancy? And why not supplement your camper battery with several Ankor Powercores for your portable devices for more cost effective power storage?

Thank you!

1) Chinese manufacturers of small devices frequently lie egregiously on their ratings but for now, let's stick to what I suspect; The power bank is likely made from 18650 cells, likely in the 2200mAh to 2800mAh range (because that is what is most cost-effective). Assuming it contains, ~8 cells (@3.7v nominal), 8 x 2500mAh(avg) = 20,000mAh (@ 3.7V) = 74Wh. At 5V output, 74Wh/5v = 14.8Ah
2) You cannot compare battery amp hours if the battery nominal voltage is not the same. BB's are 12.6V nominal/ 100Ah or 12.6x100 = 1,260Wh.
3) Getting away from the confusing Ah conversion and going with Wh which are directly comparable, the power bank has 74Wh for $50 ($.68/Wh), vs the BB 1,260Wh/$900 ($.71/wh). As you can see, the cost per Wh is pretty similar when you get the units correct. Also, the BB has a sophisticated BMS which manages the battery to achieve 2-3k charge/discharge cycles while the power bank does not and likely only achieves several hundred life cycles. The power bank has Lithium Cobalt Oxide chemistry which has a specific power of 7 so is lighter per unit power than the BB which uses Lithium Iron Phosphate which has a specific power of 3. Power banks of similar capacity would weigh about half what the battle born weighs.