Maximising battery life overnight.

[cuthound]

4 hours ago, dmr said:

Its the off load voltage that gives the state of charge, so its ok to see a lower loaded voltage as long as this "bounces back" to a good unloaded voltage, but then again its possible that the whole 50% thing is a huge myth and misunderstanding and then nothing goes wrong if you take the batteries below 50%. I think I'm in a minority with this thinking and even the batteries suppliers say 50%, but then thats in their interest.

 

If taking mine down to 40%, or even a bit less, gets me another day without running the engine then Im ok with that.

 

BT used to take their wet lead acids down to 0% every two years to assess actual capacity. They used to last 25 years, but were good quality batteries that spent most of their time on float.

 

It is more important for the battery to be either discharging or charging. They very quickly be sulphated with hard (irreversible) sulphate when left partially discharged for any length of time.

[Ronaldo47]

During the discharge cycle of a lead-acid battery,  at the lead oxide plate, sulphate ions from the acid replace oxide ions in the plate, converting some of the acid to water, and some of the lead oxide to lead sulphate. The water dilutes the acid in contact with the plate. For low discharge rates, the water has time to diffuse into the acid between the plates and be replaced by fresh concentrated acid,  so the voltage dioes not fall significantly  but for high discharge rates, the acid at the plate surface remains dilute, resulting in a fall of voltage. When the load is removed, water will have time to be replaced by fresh acid, so the voltage will rise again. 

 

This is the gist of the explanation of fall of voltage on heavy discharge behaviour from a 1940's book on accumulator charging, but I think the present lead acid batteries still work on the same principle.

 

According to the book, the critical voltage was 1.8V per cell. Discharge below this voltage resulted in the creation of hard crystalline lead sulphate rather the the amorphous type, and it was the crystalline form that was resistant to being converted back to lead oxide during the subsequent charge cycle. That was the figure  for 1940's technology, and might be different  for the formulations and acid concentrations of modern accumuators but I presume the principle would be the same.

Edited March 8, 2023 by Ronaldo47
typos