Do modules tend to go out in pairs or singles? Also why does the ECU only check them in pairs?

Have you noticed/discovered that when you go in to fix a bad battery (replace a module), there will be a bad cell and the one next to it is also weak now, and has to be replaced as well? Is it ever just single modules?

Also why does the computer only track them in pairs, is there a specific reason for this, any thoughts?
Thank you

 

Thank you for the replies. I guess this explains why the red triangle of death shows up with 1V low on a block, maybe it's trying to detect a cell that has gone bad, out of 12? I need to confirm when the ECU would throw a code on a bad battery | PriusChat

 

For my most recent cell loss, initial voltage readings were

[ecu]
1. 7.54
2. 6.35 <- bad
3. 7.51 <- degraded
4 7.58
5 7.59
...

and a cell I lost the year before

...
11 7.43
12 7.45
13 6.0 <- bad
14 7.39 <- degraded
15 7.44
16 7.41
...

So it seems that the module "downstream" (not sure which direction but looks like "away" from the ecu) is degraded. However in my load tests they seemed to do OK.

So both times it happened to me. Here's another instance where it "may have" happened: Changing bad module on hybrid battery without balancing? Please help! | PriusChat

However there seem to be lots of instances where a single module goes out without affecting those around it: Gen II Prius Individual Battery Module Replacement | Page 57 | PriusChat and lots linked here Where do modules tend to fail, within the HV, for you? Any particular side? | PriusChat

I think I did a "recondition" on the neighbor in the first example and it helped, seems like normal now. In the second it didn't fail my load test so seems still strong, even without reconditioning. So I guess they don't get pummeled too badly?

So you may be OK just replacing "one". What you want to do is get one whose capacity either matches or exceeds your current batteries' capacities. Though it'll survive with a weaker one, it'll just be a little less efficient. Or just buy a random one and trust to luck

There is the chance, when you go to replace the bad one, that you'll run into another bad one while investigating the pack. Like one that "leaks" overnight. So you may want to replace that whlie you're at it. May be worth buying an extra module or two, to replace ones like that or ones you find weak.

An even more thorough job would involve load testing all the modules to make sure none are "very weak" but I haven't run into too many that have become relatively very weak, to date...An even more thorough job would be to "re order" the pack while you're at it, rumored to increase longevity and/or efficiency. Not sure best way yet...So basically you could do "more maintenace" while you're in there, which may give you better results long-term, or maybe it'll just work. For instance you could recondition every single module in there if you want. Then re-order them. (Or do a pack wide recondition after install, no re-order possible LOL).

When you get an error code, it means "bad block" (meaning 2 modules). But this doesn't necessarily mean 2 bad modules. Also you can use Dr. Hybrid to get a sense if other modules seem weak or not, though a load test on each one may be more accurate anyway.

 

I think I'm kinda getting what people are saying about "whack a mole" which is basically that if one module fails, possibly the "next worst" could fail soon after.

So when replacing a module, it's best to recondition the weak (or all) the other modules to try to prolong their life. And/or re-order the pack so that weaker ones are to the outside where they don't get as much heat, etc..makes sense. One way to recondition "all" the modules is a full-grid style charger like prolong, though that doesn't have reordering as an option.

I'm going to watch for this pattern to test if I see it myself. I did have a second module fail for me about a year later after replacing one, and I believe it was a weaker one...

 

The good news is this kind of satisfies my confusion about the computer tracking pairs: it's not because the pairs "act as a pair" and affect each other (since that would/could go to the next modulate after the pair anyway), it's just for convenience I guess. Even if sometimes they can affect their neighbor, it doesn't hurt them too bad so isn't the target...