Pack balancing & Fusing

S

steveeb

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Brand new to the forum, and starting to collect 18650's for a small solar powered system. I've watched what seems to be hundreds of 18650 videos on YT, but I'm still rather confused on this topic.

I see many DIYers test and sort cells that they deem worthy of their packs. The main criteria seems to be 1. the cell's ability to retain charge over time (i.e. low self discharge) and 2.the tested capacity of the cell. After weeding out the self-discharging cells, parallel packs are assembled in such a way as to equalize the capacity of the pack within the intended series of packs. So each pack could consist of individual cells with a wide range of capacities. The repacker website is what most people use do accomplish this. The question becomes, shouldn't there also be another criteria when selecting cells for packs, and that is the cells' max discharge rating? If the discharge ratings are all different in a cell pack, wouldn't that be a disaster waiting to happen?

For example: You have 100p pack. 99 of them have a max discharge of 10A, but one of them has a max discharge of only 5A. If the current drain on that pack exceeds 5A, that single cell will be pushed beyond it's limits. Now this brings up my second point of confusion and that is FUSING. A DIYer's response to my question might be "Well that's why I fuse each cell." And I would ask, What's the rating of your fuse wire? If the fuse wire is rated anything over 5 amps,that onecell is still notunprotected.

Can someone please clear this up for me. I'm no EE, but it seems obvious that the max discharge rate should be a top priority when choosing cells for packs and secondly when selectingfuse wire the amp ratingshould be chosen based on theweakest link.

TIA.
 
Fusing.
- Its true that if you have a 5a fuse on each cell in a pack, and you have 100cells in parallel, and you pull more than 500a (5a per cell) you'll start blowing fuses. However, a lot of DIY battery banks for solar off-grid applications - 14s (48v) - don't get anywhere near 500a. 500a @ 48v would be in the range of 24,000watt 240v@100a inverter! In my own case the max pull on my battery is 700ma / cell (e.g. less than 1a).

- The fuse protects the pack against a cell the goes into a dead-short - its not about protecting the individual cell.

- Bottom line - you want a fuse that will support the max draw *per cell* but small enough that if the whole pack starts shorting thru a bad cell it will blow. So a lot of solar battery banks do very well with 2a, 5a, and even 15a (remember its pack -> cell protection).

Mixture of capabilities
- Yes, if your pack has to deliver at least 5a per cell (500a for 100p) then your 'least' cell has to be 5a. Again, as in the example above, most battery banks of any size top out at 1a per cell... and most any cell can do 1a so its not so much of an issue. In my own case, I have a mix of 3a (least) and 10a (max) cells in my packs... and at 48v250a max pull (for 12,000 watt inverter) the 250a circuit breaker will ensure I don't go over 700ma per cell. 0.7a is way less than 3a (my least cell).

- If you have a mix of cells - e..g 2000mah, 2100mah, 2200mah, ... 3000mah - you typically want to evenly distribute them by 100mah increments + cell type. AND in deference to serious work by others, you want to keep general IR as close as you can as well.
So if you are building a 14s100p pack (e.g. 1,400cells) then you want to distribute these thru all 14 packs as evenly as you can - and each pack must be the same 'total ah' for it to stay balanced in 14s. That's why folks process the 1400 cells first... and the sort them out by mah / type / IR.... and then distribute them evenly among the 14 packs.

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I'm sure you'll get many other comments but I hope this helps you get started and think of your next set of question :)
 
Thanks for the quick reply! I'm going cross eyed reading and watching videos on this subject.

OffGridInTheCity said:
Fusing.
- Its true that if you have a 5a fuse on each cell in a pack, and you have 100cells in parallel, and you pull more than 500a (5a per cell) you'll start blowing fuses. However, a lot of DIY battery banks for solar off-grid applications - 14s (48v) - don't get anywhere near 500a. 500a @ 48v would be in the range of 24,000watt 240v@100a inverter! In my own case the max pull on my battery is 700ma / cell (e.g. less than 1a).
Click to expand...

Ok I get that.

OffGridInTheCity said:
- The fuse protects the pack against a cell the goes into a dead-short - its not about protecting the individual cell.

- Bottom line - you want a fuse that will support the max draw *per cell* but small enough that if the whole pack starts shorting thru a bad cell it will blow. So a lot of solar battery banks do very well with 2a, 5a, and even 15a (remember its pack -> cell protection).
Click to expand...

The only example of blown fuses I've seen are ones where a single cell has gone bad (AveRage Joe), not a whole pack of blown fuses.

I think I'm starting to get it...
Example
100p pack with a current draw of 7a. One cell goes bad, the fuse for that cell should blow hopefully.
Now we have 99p with the same 7a draw.
It will work fine as long as the other 99 cells can make up for the one dead cell.
As cells go bad and fuses blow, the other cells take up the slack until there are too few left and they all blow.
Eventually the pack dies completely.

Is this right?

But my question remains. If we're only drawing 7 amps form the pack, if a cell dies and you have 10 amp fuse, how will that fuse ever blow? Maybe the question I need answered is what happens when a cell dies. Does it always short out? I want to make sure that my house doesn't burn down if one rogue cell decides to do whatever it is bad/dead/broken/heated/punctured/dropped/crushed cell does.
 
The reason for individual cell fusing is for a cell going short circuit. If you have 100 cells in parallel and 1 goes short circuit, without protection the other 99 cells could dump 100s of amps into that cell, most likely causing a fire, if the cells built in protection fails.
 
Current protection for a pack is done by the main fuse and not cell level fiuses. Rest have been explained by the others :)
 
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