I don't have the exact numbers but even then it's cheaper. The only issue is - can you trust yourself (or your planning) to ensure you don't go below 40%? Real life is hard to plan.

What happens if you do let it go below 40% (either as a brief dip to high 30s, or letting it go to 0)? Is it closer to a case of "oops you hit 39%, this battery is now worthless" or "oops you went down to 0%, better not do that every day as statistically the battery will last 0.001% less long"?

edit: I pasted my comment (prefaced with "context = lead-acid batteries") into ChatGPT and got this answer, maybe someone who knows could, instead of having to explain it all to me, just confirm GPT-4 got it right, or if not the correct it / add detail?

> If a lead-acid battery is allowed to go below 40% state of charge (SOC), it can cause some issues, but the effects are not necessarily immediate or catastrophic. The depth of discharge (DoD) and frequency of these deep discharges can impact the battery's overall lifespan and performance.

> 1. Brief dip to high 30s: If the battery's SOC occasionally dips into the high 30s, it may not cause significant harm. However, consistently discharging to that level can gradually decrease the battery's overall lifespan.

> 2. Discharge to 0%: If a lead-acid battery is allowed to discharge to 0%, it can cause more serious damage. Repeatedly discharging the battery to this level will dramatically shorten its lifespan and could potentially cause it to fail prematurely.

> The relationship between DoD and battery life is not linear. For example, discharging a battery to 50% DoD might give you around 1,000 to 1,200 cycles, while discharging it to 80% DoD might give you only 300 to 500 cycles. It is generally recommended to keep lead-acid batteries at a higher state of charge to maximize their lifespan and maintain optimal performance.

> In summary, allowing a lead-acid battery to occasionally dip below 40% SOC may not cause immediate damage, but consistently doing so can reduce its lifespan. Discharging the battery to 0% can cause even more significant harm, especially if done repeatedly. It is best to keep lead-acid batteries at a higher state of charge to ensure their longevity and maintain optimal performance.

ChatGPT is not wrong... but in my personal experience, once you go below 40% it's all downhill from there. At least if you've done it 2-3 times. Batteries are not always exactly the same, and temperature conditions vary as well. I've been nursing batteries for years, and this is just my anecdotal comments on that.

One thing that we can all agree on though is that you must never ever use car batteries. Lead acid need to be the deep cycle kind.

More deets from ChatGPT

> please provide an example data to show non-linar relationship between DoD and lifespan in 5% increments.

Depth of Discharge (DoD) Approximate Number of Cycles 05% 3,500 - 4,000 10% 3,000 - 3,500 15% 2,500 - 3,000 20% 2,000 - 2,500 25% 1,800 - 2,200 30% 1,600 - 2,000 35% 1,400 - 1,800 40% 1,200 - 1,600 45% 1,100 - 1,400 50% 1,000 - 1,200 55% 900 - 1,100 60% 800 - 1,000 65% 700 - 900 70% 600 - 800 75% 500 - 700 80% 300 - 500 85% 200 - 400 90% 100 - 300

> thinking through the following steps please create an asci bar chart for this table, dod given in 5% increments:

1. Calculate the number of cycles for the first increment 2. Extract the maximum number 3. Calculate the number of stars with 1 start = 500 cycles 4. Put number of starts into a table 5. iterate over the table

Once the table has been completed create the asci bar chart plotting DoD vs Cycles

Example: 1. 5% DoD = 3,500 - 4,000 2. 4000 3. 8 stars

DoD Cycles

05% ********

10% *******

15% ******

20% *****

25% ****

30% ****

35% ***

40% ***

45% **

50% **

55% **

60% **

65% *

70% *

75% *

80% *

85% *

90% *

Lol, the way this came out to sit perfectly with OP's intuition. Even down to the drop from 2-stars to 1-star!