Might be more than the blog owner wants to spend for what is just a fun hobby project.
There are some 3U rackmount size nominal 48vdc 50Ah lifepo4 (approx 4.8kWh capacity) with built in bms and LVD you can get from china now.
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.
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.
> 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!
For instance, telling a brand name cell from a fake cell can be extremely hard, but your vendor knows exactly where they bought them. I buy cells from nkon.nl, who are a high rep local vendor and I've yet to be disappointed but buying a pack that is sealed from a low rep vendor may well incur a lot of risk. You'd almost have to split open the shrink wrap to make sure you got what you paid for. One quick and dirty way to check is to weigh the pack and to compare the weight to the manufacturers spec for the cells. It's obviously not perfect but many fake cells have weights that are wildly different from the cells they pretend to copy (usually due to much lower actual capacity).
If your has equalization, try to discharge & equalize charge it 2-3 times first.
If you can actually get access to the cells, measuring cell resistance is not expensive. Anomolies will show themselves very quickly. Everything should be pretty clumped together in internal-cell-resistance. If you have a pack equalizer, it can be used to help you read this out. My <$100 charger has a cell resistance test mode.
Yes, that can't be stressed enough. And depending on the fault and how the BMS handles it a more dangerous condition could easily develop. For instance an undetected short would cause the other batteries to be overcharged to the point where they are much more likely to die themselves, and some of those may do so in pretty violent ways.
Cheap / missing BMS are a serious problem with cheap Lithium-Ion packs, they are the first line of defense and bad cells can even happen when supplied by good brands (but the chances are a lot higher with no-name stuff).
Another good test: charge the pack, balance it, measure the pack voltage and the individual cell voltages, then let it rest for a day or two and measure again. Any kind of deviation between cells is a sure sign something is not working well and is the earliest warning sign for trouble you can get. The rate of self discharge should be the same for all cells and it should be very low, too low to measure (and make sure you disable the balancer after the initial charge!). After a month or so it would be fine to see the pack drop a little bit, but not a lot.
Yet another quick test: use an IR camera to look at the pack both during charging and at rest. Any cells that are warmer than those around them are suspect.