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cell distribution vs just range of capacity
#1
I apparently need to post once before I can attach a file. So I'll try on my reply.

I’ve been reading up on building a solar/lithium system for my RV. We plan to tour the country totally without shore power. This forum has been a great help. I’ve salvaged around 700 cells right now with access to many more if I need them. I built a few practice packs with my battery spot welder from Malectrics (works great). I’d like to check in and see what some of you think about my plan. Here’s what I have so far:


I have a 90 Watt 120VAC mini fridge. So at ~30% duty it should use ~720 Wh/day. Our microwave is around 900 W continuous for a few minutes per day and I heard they can startup above 1500 W. My wife’s hair dryer is 1500 Watts for a few minutes per day. All-in-all I see us needing no more than 1500 Wh/day and a max occasional draw of about 1500 Watts. I’ll most likely buy a 2500 Watt inverter.  

My pack
The capacity of my pack will range from 2400Wh - 2800Wh depending on the test. From what I’ve read here I should limit myself to a max current of 2A on each cell. I would like to build these cells into a 7s34p pack. My chart shows the distribution for 240 cells which is about how many I need to keep it under 2A max draw. So with 238 cells, 2000 Watts is 2.0A/cell. Should this be enough headroom for my occasional 1500W draws? My main concern though is the distribution of the cell capacity. Is it too spread out? As you can see the bulk of them have a range around 300 - 400 mAh (on the 800 mA test). Will the outliers at the low or high end cause a problem? Do I need to start salvaging more cells to get a smaller range? I’m selling my lower capacity cells on eBay to offset the cost of buying used packs.
Thanks for any help
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#2
(01-17-2019, 05:42 PM)ppavone Wrote: I have a 90 Watt 120VAC mini fridge. So at ~30% duty it should use ~720 Wh/day. Our microwave is around 900 W continuous for a few minutes per day and I heard they can startup above 1500 W. My wife’s hair dryer is 1500 Watts for a few minutes per day. All-in-all I see us needing no more than 1500 Wh/day and a max occasional draw of about 1500 Watts. I’ll most likely buy a 2500 Watt inverter.

My pack
The capacity of my pack will range from 2400Wh - 2800Wh depending on the test. From what I’ve read here I should limit myself to a max current of 2A on each cell. I would like to build these cells into a 7s34p pack. My chart shows the distribution for 240 cells which is about how many I need to keep it under 2A max draw. So with 238 cells, 2000 Watts is 2.0A/cell. Should this be enough headroom for my occasional 1500W draws? My main concern though is the distribution of the cell capacity. Is it too spread out? As you can see the bulk of them have a range around 300 - 400 mAh (on the 800 mA test). Will the outliers at the low or high end cause a problem? Do I need to start salvaging more cells to get a smaller range? I’m selling my lower capacity cells on eBay to offset the cost of buying used packs.

The fridge is going to pull about 3x-4x that when it starts. So it's pretty sure that it'll be closer 300W during startup.

If you figure 900W for the microwave, 100W fridge, hair dryer 1500W. This comes closer to 2000W of max usage (with your current list). Since the fridge can have a surge of about 300W, then figure about 2500W. So going with a 3000W inverter this should be covered and you'll not have any issues. Sure, you could go with a 1500W inverter with a 3000W surge, but then you won't be able to add any more devices to the system (think maybe you add a toaster, coffee machine, TV, computer, sound system, etc, etc, etc). It's better to go higher than you'd currently use so you don't overload yourself later.

2A per cell is a wee high, unless you are headed towards LiFePO4 cells. If you're using recycled LiCo's (laptop batteries, ebike batteries, etc), then 500mA (0.5A) is generally the max recommended. Normal run current is usually around 250mA on most setups. This will give you the longest life of the cells.

You are also negating the ineffeciencies that will be introduced in to your system. There's always losses. So figure about 15-20% losses. This is a margin of safety. So, you say 1500W is all you'd need. But if we calculate the 20% losses, then we come to 1800W. We're getting close to that 2000W limit.

If we go with close to 500mA per cell, and also go with a 2000W standard load, then you'd need to have about 130p.
You can negate this by going higher voltage. So instead of 7s, you went with 14s, then you'd only need about 80p. However, the overall cell count is the same. You can't change one without the other and reduce overall counts.

I think that's it. Other's will correct my math if I'm off a bit. Wink
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#3
Sounds like I need to get back to work harvesting cells. More fun thanks!
I understand the more cells (80p, 100p etc) the less important the range of capacity should be. So going back to my concern about range. Would I be fine with say a 14s80p with a 2200 -3000 mAh range in capacity?
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#4
Capacity per cell doesn't matter as much as having packs of equal capacity. You can use 1200mAh or lower if you want. Just all packs need to be as close as possible to each other. Otherwise you'll be fighting imbalance issues.

However, depending on the cells original capacity dictates to a degree if you use them. If a cell originally was 3000mAh and now is 1200, you obviously wouldn't put it in the pack as its close to end of life
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at Discord Chat (channels: general, 3d-printing, linux&coding, 18650, humor, ...)
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#5
I've already taken out the cells that are way below new capacity. Taking your advice and keeping all my good cells >2200mAh I now have about 400. So i'll be going to buy another 50 laptop packs tomorrow.
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#6
For such projects I prefer using cells recycled from power tools, not notebooks.
The power tools cells are at least Sony V3s or Sanyos that can do 10A discharge.

The logic is simple: if a cell was created to hold at least 10A discharge, it will have a lot less trouble handling 2A (in your case) compared cells that have not been designed to exceed 1A.
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