Understanding battery self discharge

[markz]

https://www.mcgill.ca/oss/article/general-science-you-asked/concrete-truth-about-batteries-stored-concrete

The batteries of today, however, are encased in plastic or hard rubber, which significantly reduces this energy waste. Batteries do naturally self-discharge over time, however, but this is due to an internal process, not the type of material on which the battery is sitting.

A compound called lead sulphate naturally forms during battery use and it can crystallize onto the negative plate of the battery, especially when the battery rests for a long time. These crystals end up impeding battery performance over time.

Heat is bad for batteries. According to manufacturer Pacific Power Batteries, a car battery in a hot climate will last, on average, only two thirds as long as it would in a cold climate. For example, a battery that lasts 4 years in a cold environment would only last 2 years and 8 months in a hot climate. Keeping the battery on a slab of concrete, which can act as a heat sink, might actually extend its life by absorbing some of that heat.

And remember that batteries, as a rule, last longer when they’re cool.

https://gelcoservices.com.au/docs/Lead%20Acid%20Battery%20Self-Discharge%20Paper.pdf

The graph shows the fall in specific gravity per day of a new fully charged battery with
6% antimonial lead grids. Self-discharge can thus be minimised by storing batteries at
temperatures between 5 and 15C

 

[john61ct]

Lead batteries last longer if kept at 100% SoC, even 99% could mean greatly shortened life.

The self discharge rate varies greatly by type, even model. And age.

Really best to determine for **your** battery by actually measuring.

But rule of thumb for me is top up at least monthly, every 2-3 weeks is better.

With the right gear, user-custom adjustable setpoints you could just leave it on a float voltage, very low current available.

But then you are really trusting that gear, safest is to leave the bank 100% isolated when not cycling.

Note LFP has no need to be kept full, in fact should be stored at low SoC, so needs much less checking so long as 100% isolated from any circuitry, including BMS

 

[eMark]

Is it a 12V 105Ah Marine battery ... https://batteryclerk.com/products/ajc-12v-105ah-deep-cycle-marine-and-boat-battery ... or a regular automotive battery.

Depends on quality, how many discharge cycles and overall condition relative to it's expected life.

Run some tests by applying a "leisure" load after fully charging and record any voltage variance, then repeat and record the resting voltage and variance of the leisure load voltage one day later, then repeat and record variance in leisure load as well as its resting load after three days, then after one week, then after two weeks. This will give you a pretty good idea of the battery's overall condition and how many days after fully charged (for leisure use) to leave it on the boat.

"If the battery is left in a partial state of discharge for as short as 3 days, the lead sulfate material will begin to harden and crystallize, forming a permanent insulating barrier. As this barrier becomes thicker and thicker, the battery's ability to accept a charge or deliver energy is diminished, resulting in the perception that the battery is no longer usable." ... https://www.emarineinc.com/Marine-Batteries-Maintenance-101