Lead-acid battery: What does this mean? - Page 1
Forum section: electronics / beginners

author: PinheadBE
author_rank: Regular Contributor
timestamp: August 17, 2024, 03:10:50 pm
content: Hi,Dumb question, but what does that "20Hr" parameter mean?(See pictures)Thanks

author: wasedadoc
author_rank: Super Contributor
timestamp: August 17, 2024, 03:15:27 pm
content: It means 2.2Ah capacity if you discharge at the rate that takes 20 hours. ie 110mA.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 17, 2024, 03:18:45 pm
content: See attachment.

author: PinheadBE
author_rank: Regular Contributor
timestamp: August 17, 2024, 04:00:53 pm
content: Thanks to both of you. Very clear!

author: Stray Electron
author_rank: Super Contributor
timestamp: August 18, 2024, 01:06:32 am
content: It means that they discharged the battery at an extremely low rate so that it yielded themaximumnumber of amp hours for their test. It also means that in real world usage you're probably never going to get anywhere close to the rated number of Amp-hours.Look at the data sheet that PA0PBZ linked to and you will see what I mean.

author: J-R
author_rank: Super Contributor
timestamp: August 19, 2024, 06:10:43 am
content: Side note, the 20 hour rating is a standard in the industry, and the lower capacity at higher rates is an unavoidable aspect of lead acid batteries' chemistry. A proper lead acid system design will consider this, as well as avoiding a high depth of discharge.However, lead acid is cheap and common, so they've ended up in perhaps less-ideal applications such as computer UPS devices, where in some cases they only last a few minutes.Lithium ion chemistries don't have this characteristic of losing capacity at higher discharge rates, so there are applications where they might be advantageous, usually revolving around weight. For the price, in stationary applications, you can typically just build a larger lead acid bank for the same money and plan for say a 25% depth of discharge, with the minor advantage of now you do have a larger reserve capacity if needed in case of emergency.When designing my battery backup system, I planned for a full discharge to happen over 24-72 hours, depending on the outage situation. In case of a major storm which knocks out infrastructure, I go into conservation mode and only run critical loads, for example. I was able to get approximately 5x the capacity with lead acid as lithium ion for the same price.

author: PinheadBE
author_rank: Regular Contributor
timestamp: August 19, 2024, 02:09:14 pm
content: Thanks for the side note.In my case, it is a very simple use-case: we suffer more and more power outages, lasting from 1 to max 3 hours.I have already a commercialy built UPS for my NAS and routerBut I lack a system to be able to open my front door which has an electric opening system, commanded by a RF receiver. All of it is powered by 12 VDC.Therefore, I'll built a very simple UPS based on a lead acid battery and a charging module based on the CN3768 chip. Some diodes andvoila....The current drawn by the RF receiver is less than 2 mA in standby, so a simple 2.2 Ah battery should be sufficient.Thanks to all for your help