Acid specific gravity variation affect lead acid battery weight? - Page 1
Forum section: electronics / general

author: Circlotron
author_rank: Super Contributor
timestamp: February 06, 2024, 06:44:20 am
content: So we charge up our wet cell lead acid battery and when it is fully charged the specific gravity of the acid as measured by our trusty old school hydrometer has increased from a previous lower value. Does this mean that the overall weight of the battery has increased, or has this extra weight / mass been transferred from the lead plates into the acid?

author: Ian.M
author_rank: Super Contributor
timestamp: February 06, 2024, 07:02:49 am
content: E = m c2You haven't supplied enough energy to significantly (or even measurably*) increase the total mass so where do you think the extra electrolyte density comes from?Hint: consider the existence of SLAs, which obviously rule out significant gas exchange with the atmosphere then ask your self; What parts of the battery are in contact with the electrolyte and change during charging and discharging?* Work it out and you'll find fully charging a 200AH 12V lead acid battery theoretically makes it about 100 picogrammes heavier. That's several orders of magnitude below detectability on the best laboratory balances in the world!Edit: Equation corrected.

author: ebastler
author_rank: Super Contributor
timestamp: February 06, 2024, 07:07:19 am
content: Quote from: Ian.M on February 06, 2024, 07:02:49 amUmm... I think Circlotron was referring to a conventional battery, not a fusion reactor.

author: IanB
author_rank: Super Contributor
timestamp: February 06, 2024, 07:10:22 am
content: Quote from: ebastler on February 06, 2024, 07:07:19 amNo. You charge a battery, you store energy inside it. By mass-energy equivalence, the stored energy will increase the mass of the battery by an undetectable amount, but the physics is still a real thing.

author: mendip_discovery
author_rank: Super Contributor
timestamp: February 06, 2024, 07:22:24 am
content: I remeber there was a thing about AA batteries etc having extra weight when charged that you could notice a difference. Never tried it myself but it does make sense that it would heavier but not sure if it is enough to easily measure.

author: Andy Chee
author_rank: Super Contributor
timestamp: February 06, 2024, 07:36:49 am
content: Quote from: ebastler on February 06, 2024, 07:07:19 amAlthough it's commonly associated with the strong force that bonds the nucleus together and is involved with fission & fusion reactions, the formula E=mc2also applies to the electroweak force that bonds electrons to its atom (i.e. battery chemical reactions, static electricity on carpet)Of course, the electroweak force is 100 times weaker than the strong force, hence why its effects are more noticeable when a nuclear bomb explodes versus a Tesla EV exploding

author: ejeffrey
author_rank: Super Contributor
timestamp: February 06, 2024, 07:58:53 am
content: Quote from: Circlotron on February 06, 2024, 06:44:20 amThere is an unmeasurably slight change due to E=MC^2, but other than that no. It's just the sulphate moving from the plates into solution. The plates get lighter by the same amount as the electrolyte gets heavier.

author: Ian.M
author_rank: Super Contributor
timestamp: February 06, 2024, 08:05:14 am
content: Thanks to IanB and Andy Chee, for backing up my assertation that the battery cannot get measurably heavier during charging, and as interaction with the atmosphere is ruled out, even without any knowledge of the electrochemistry*, it is obvious the mass needed for the electrolyte density to increase must come from the plates.* The electrochemistry is easy to grok if you still remember a little school chemistry! Googlelead acid battery redox reactionand study any of the tutorial level academic results.Thanks again to Andy for politely correcting the famous equation I mis-stated. Case is significant in science and engineering and I should have done better!

author: ebastler
author_rank: Super Contributor
timestamp: February 06, 2024, 09:59:31 am
content: Thanks all. Agree, there is an E = m c² contribution; but as you all state it is very much negligible. I took the OP's question to ask whether one is actually "depositing" someting (matter, i.e. electrons, ions) into the battery during charging -- which would be a much larger and potentially measurable effect, but is of course not the case.

author: David Hess
author_rank: Super Contributor
timestamp: February 06, 2024, 12:41:48 pm
content: Quote from: Circlotron on February 06, 2024, 06:44:20 amThe charging process converts lead sulfate on the plates to sulfuric acid in the electrolyte, so the mass does not change from that. (1) Some water is lost through electrolysis to oxygen and hydrogen, especially in a flooded cell, and the mass-energy equivalence means that some mass is added because of the extra energy stored in the reconfigured chemical bonds.(1) This works because lead sulfate is conductive. One of the things distinguishing rechargeable from non-rechargeable chemistries is that all of the products have to be conductive.