What is a cycle and how many of micro-cycles can I get?

[Inq720]

I'm not looking for the obvious answer. So, please bear with me...

There is the easy (obvious) answer. A very large battery bank and a equally appropriate (huge) solar panel array in a stationary setting with clear view to the sky... say in Arizona. The batteries are deeply discharged through the night. The answer is obviously: one cycle per day.

Why Asking?
I'm reading the threads about improving battery life:

• https://diysolarforum.com/threads/best-practice-for-300kgf-‘fixture’-280ah-cells.16101/
• https://diysolarforum.com/threads/e...e-clamped-tight-or-spaced-for-expansion.7892/

In one post by Dzl, I find this:


The tests are very structured, very consistent and to 100% discharge and 1C.

My normal conditions are very different. Before I go to all the trouble of using threaded rod, springs, that I don't have room for, I'd like to get an understanding of what might constitute a cycle.

In my tiny system... a single 280 Ah, 4S battery and 200 watts of solar panels on a boat. My largest draw (ever) will be less than 0.5C. Most of the time, I'll be below 0.1C. Charging might be very irregular - shading of the panels, clouds, etc. Say I have small loads, and have good solar - thus, I'm charging. I go to a heavy load for thirty minutes or clouds go over - thus, I'm discharging. Is this a cycle? If so, I might have hundreds of cycles in a day.

What is the mechanism for the deterioration of capacity? I'm reading about LFP having memory (2/3rds down in this article). Does having many micro-cycles make things worse or better than the consistent, 100% discharge tests above?

Thanks.

 

[svetz]

Great question!

The memory effects in the paper you cited seemed fairly small and overcome by a full charge. Although this part was scary:

...batteries in house bank applications, because incomplete charge cycles are common when relying on renewable energy sources and shallow discharge cycles are also frequently experienced. These have the potential to render battery banks near unusable after as little as 2-3 years in regular service in the absence of memory-releasing cycles.

I found an article "Analysis of battery current microcycles in autonomous renewable energy systems", then found The Impact of Microcycles on Batteries in Different Applications. Sadly, both are for lead acid batteries, possibly some take-aways? Hopefully some of the more knowledgeable can point to a technical article on this.

 

[MattiFin]

In my tiny system... a single 280 Ah, 4S battery and 200 watts of solar panels on a boat. My largest draw (ever) will be less than 0.5C. Most of the time, I'll be below 0.1C. Charging might be very irregular - shading of the panels, clouds, etc. Say I have small loads, and have good solar - thus, I'm charging. I go to a heavy load for thirty minutes or clouds go over - thus, I'm discharging. Is this a cycle? If so, I might have hundreds of cycles in a day.

What is the mechanism for the deterioration of capacity? I'm reading about LFP having memory (2/3rds down in this article). Does having many micro-cycles make things worse or better than the consistent, 100% discharge tests above?

Thanks.

As a first approximation you can assume that you get constant total amp-hours out of the battery:
1000 full cycles at 280 Ah
2000 cycles discharged by 140 Ah
or 100000 2.8Ah cycles.

In reality the cycle life is even better than that with shallow dischages if you stay out of the SOC extremes (especially the top end over 3.55v)
But at some point the cells are going to age or run out of "calendar life" no matter how shallow discharges.

 

[Inq720]

...
or 100000 2.8Ah cycles.

In reality the cycle life is even better than that with shallow dischages if you stay out of the SOC extremes (especially the top end over 3.55v)
But at some point the cells are going to age or run out of "calendar life" no matter how shallow discharges.

I should live so long. Hell! my future grand-children should live so long.

I've been reading... if I know I don't really need the full 280 Ah... say even 100 Ah, it would be beneficial for me to set limits at 40% (3.2V) and 80% (3.4V). But warned that I should top balance more often since the BMS's balancing will never come into action.

 

[Mart Hale]

BU-808: How to Prolong Lithium-based Batteries

BU meta description needed...

batteryuniversity.com

This article has been one of the best I have seen for real test data on different levels of discharge. It seems the lighter the work on the battery the longer the life. It is like the oldest light bulb we have is low in wattage, but it still preforms....