Ah vs Wh rating, what should be used to make comparisons? - Page 1
Forum section: electronics / beginners

author: juanfermed
author_rank: Regular Contributor
timestamp: January 18, 2015, 10:56:23 pm
content: Hi everyone,In one circuit I am in the design phase, I wanted to compare using Lithium Ion batteries and Lead Acid batteries. Just from the energy capacity point of view, a 12V 12Ah lead acid battery has NOT the same capacity/energy stored as a 3.7V 12Ah Lithium Ion battery right? They have the "same" Ah capacity but for sure not truly the same capacity, right?I was thinking that the Ah is misleading or at least be treated carefully when making comparison, as it is not a real measure of the energy stored in the battery, whereas the Wh capacity truly is. Being the energy measured in Joules in the international system, only the Wh -watt hour- rating is truly telling the energy a battery can store:Wh = [J/s]*hour *(3600s/hour) = 3600J. (In contrast, you cannot get to Joules using Ah without using the battery voltage, which would then result in the Wh rating)So, for comparing both batteries, I am 99% sure I should compare them using their Wh capacity:Lead Acid: 12V * 12Ah = 144WhLithium Ion: 3.7V * 12Ah = 44.4WhSo, is it correct to say the definitely the Lithium Ion battery has a true capacity 100Ah lower (44.4Wh) than the Lead Acid one (144Wh) even if the have the same Ah rating? I basically would just like to confirm this is the correct way of comparing them.Thanks!

author: Skimask
author_rank: Super Contributor
timestamp: January 18, 2015, 11:22:24 pm
content: Apples and oranges...

author: luigi_xp
author_rank: Newbie
timestamp: January 18, 2015, 11:52:12 pm
content: Well, Wh is the total power stored on a battery, so for comparing them like this, Wh is the correct;But normally, you're going to compare batteries of the same voltage.

author: Yansi
author_rank: Super Contributor
timestamp: January 19, 2015, 12:12:53 am
content: not power, but energy. Wh is energy. Wh = 3600J (Joule).

author: Circlotron
author_rank: Super Contributor
timestamp: January 19, 2015, 10:51:30 am
content: Quote from: juanfermed on January 18, 2015, 10:56:23 pm100Ah lower -> 100Wh lower.If that is what you meant then yes you are right.

author: tautech
author_rank: Super Contributor
timestamp: January 19, 2015, 11:00:32 am
content: The only way to compare capacity between 2 batteries ofdifferentvoltages is W/Hr's.

author: eas
author_rank: Frequent Contributor
timestamp: January 20, 2015, 01:20:07 am
content: Yes, Wh is usually the way to go, provided your circuit makes similarly efficient use of power delivered at dramatically different voltages.

author: retrolefty
author_rank: Super Contributor
timestamp: January 20, 2015, 01:29:31 am
content: Just keep in mind that there are also other important specs that can be used when comparing batteries that depend on the application.Energy density: Important for weight and space sensitive applicationsFor rechargeable: Number of rated discharge/recharge cyclesMaximum discharge/recharge current ratingPrice: Always a trade-off for application marketetc

author: juanfermed
author_rank: Regular Contributor
timestamp: January 20, 2015, 03:49:19 am
content: @Yansi yes, energy, thanks.@Circlotron, thanks for the correction, 100Wh lower.@ tautech, Ok thanks for the confirmation. Just for the sake of correctness, it would be W*hr, not W/hr.@ eas Yes thanks. Actually all the details of batteries I think we would need to take into account how voltage varies with capacity. But for right now I am talking just about "nominal" voltages for comparison.@ retrolefty. Thanks so much for pointing that out. Yes, we are taking energy density (Wh/cm3) and specific density (Wh/kg), the life cycles, discharge current, under/over discharge limits, price (which is higher for Lithium Ion).Thanks to all for your replies!

author: tautech
author_rank: Super Contributor
timestamp: January 20, 2015, 04:04:55 am
content: If one looks at a SLA capacity, it is spec'd as A/h so using Watts instead it should be W/h.

author: IanB
author_rank: Super Contributor
timestamp: January 20, 2015, 04:12:43 am
content: Quote from: tautech on January 20, 2015, 04:04:55 amNo, it's A*h, not A/h.This is easy to see if you think about it. A 10 Ah battery can theoretically deliver 10 A for 1 h, or 5 A for 2 h, or 1 A for 10 h. In every case, A * h = 10:10 A * 1 h = 10 Ah5 A * 2 h = 10 Ah1 A * 10 h = 10 Ah

author: juanfermed
author_rank: Regular Contributor
timestamp: January 23, 2015, 01:37:14 am
content: Yes, A*h, not A/h.

author: Fagear
author_rank: Regular Contributor
timestamp: January 24, 2015, 12:34:59 pm
content: Quote from: juanfermed on January 18, 2015, 10:56:23 pmTake a look at this video: