Discussions about LiFePO4 cells and Battery University

Interesting....please continue. I asked about the integrity of your cell logs data...let me ask it from a different angle. How do you know the balancers are coming on when they're supposed to and that they're drawing the amount of current they're supposed to? 2A on the Mott boards sounds like total max current for the whole board. This would equal 333mA per cell (Edit: D'oh! make that 666mA per cell). Do you collect any data directly through these control electronics that shows that the controller is seeing what you are seeing in CellLogs?

 

Ok, at least that is now clear to me. It's really unfortunate there is no way to service or even verify operation of the balancing electronics. This could be a real source of trouble, but it may also be really difficult to pinpoint something without performing what would presumably be destructive teardown of the whole assembly. Your cell replacement options are limited too. Do you have a schematic of the electronics? You appear to have instances of adjacent cells (6&7) diverging, which can be a symptom of erroneous Voltage readings in control electronics.

Looking forward to the next installation....

 

Dan,

Are the first four frames of data here Mott cells only, on four consecutive days? If so, I would say there is either a problem with the balancers or some fairly severe cell defects, or both. The balance is getting worse and worse for four days in a row--if the balance scheme is working, it should be getting better--sort of like what it happening in the final frame, but much more slowly. By the first day with the BMS (second frame), the imbalance is severe enough that I would want to manually pull down those top three cells at least. If these were A123 cells, I would say you are definitely doing some serious Li plating with all the time above 3.8, 3.9, and even 4 Volts. You do not want to expose any cells to that--over time they will become your weaklings unless you act quickly to stop the abuse. However, these are Mott Cells and I have no idea what the recommended Voltage range for those is. You may be fine. Still, the way balance appears to worsen on a daily basis under normal use tells me either the balance scheme needs refinement, the hardware is not working properly, some cells are defective, or some combination of those things. It sounds like you go from 15 A right to the float charge. I would be more comfortable seeing other current levels in between, taking hold when any cell hits 3.8 or maybe less--so that would be a refinement in the controls strategy which would help limit plating. At the same time, I have to question if you've got some balancers that are always stuck on or off, OR you've got several cells with serious self-discharge problems or impedance growth. You'd have to run several tests to get a handle on what parameters have changed over time. If you have daily data, or enought periodically collected data you might be able to see what the trends are. If cells that started off spending a lot of time at the high side of the balance spectrum are the same cells that are now fading the fastest, this would be a strong indicator for Li plating. Going too low doesn't appear to be a problem for you at all (but again, I'm thinking in A123 numbers, don't know what Mott or RealForce want to see). I would want to be sure whatever the permissible Voltage numbers are, that they were the same for both Mott and RealForce since they are presumably being charged by the same system using the same rules.

I'll stop there for now and see how I'm doing with staying in the right ballpark....

 

I am dubious about how well these CellLogs keep their accuracy over time and also taking the range of ambient temperature that they would normally be subjected to into account. May be worth checking each cell volts CellLog value against a 3.000 volts calibration cell or source at the highest ambient temperature expected during their use.

You might find them a bit inaccurate when they are working at an ambient temperature of 40 Deg.Celcius which may happen if the hot air given off by your charger increases the ambeint specially by the end of charge which is when they need to be at their most accurate.

 

What would be the mechanism causing such a loss of accuracy? This would surprise me, unless the design of the electronics is extraordinarily bad.

 

The more I hear about Enginer, the less I like the system. Seems like a lot of fumbling around in the dark. It's fine to develop a system that way, but to sell a system like that seems crazy to me. Anyway....I'm not sure if I've been of any help to you here or not. Has any of this conversation been useful for you? Any further questions or ideas you want to bounce off me? I'm enjoying the discussion...not sure about anyone else.

 

Dan,

I'm glad to help advance the cause and I appreciate the dialogue. 3 years ago I would have been right there trying this stuff with you if I could have afforded it.... About 5 years ago I was playing with WVO instead--a PITA, but do-able on a shoestring if you've got a couple simple things going for you. Anyway, I've only been at A123 for just under 2 years now, and while I've been dabbling in battery and solar power for many years (my WVO pumps at the house all ran on solar-charged LA batteries), I've had to dive MUCH deeper since arriving here. It's been a baptism in electrons--this is still a frontier industry and we are ALL learning, mostly the hard way. You've been at this longer than me, but I'm sure with a fraction of the resources--and my resources are nothing to write home about. Point is, I'm sure you could show me a thing or two because you've lived with a system for a long time now and have witnessed certain trajectories.

More to come....

 

Hi Dan,

I used the word "dubious" because I have never checked for accuracy as I have never used them for voltage protection on my PHEV kits so in my case there is not much point in checking them for accuracy. Maybe I am wrong but I think that they were never intended for PHEV kits even though they seem to be doing a good job for you and others. I only use them as a quick visual check at the end of a discharge. I just do not have the time and will to do all the data logging collection that you have done and I cannot say with any confidence that the Li plating (or what ever is causing your cells to lose capacity) of your RFE cells was caused by your Pacific EV BMS or whether the RFE batteries just do not last the distance as the RFE 20-ah cell Spec. sheets claim.
Anyhow this thread has been interesting so far.

 

The bit of extra capacity trick should work, but of course there are limits to how bad the problem can get before it makes more sense to just replace the whole cell.

Can you repost the "all data" charts for both packs with the weak cells (8 and 11) highlighted like what you did with the close-ups for the RealForce pack? I may be able to suggest some strategies for improving your performance and your understanding of what has gone wrong with these cells.

 

I strongly recommend finding a low impedance path to connect to cell group (CG) 11 in the Realforce pack. Current flowing in your sense lines causes all kinds of problems with monitoring, as you are seeing here. You'll definitely want to disconnect those extra cells during charging of the pack and charge them separately. Left connected, you'll get the same issues during charge, and mismanagement by the control system will be the result. In the longer term this can result in serious electrical abuse of the cells. Next to that, the problems you've been having might not look so bad anymore. There should be virtually zero current in your sense lines. Putting current flow (power) there is trouble, period. It's possible you are also defeating the design intent of the control electronics in other ways too.

This is a sure sign of impedance growth in your Realforce cells. As impedance grows, higher C-rate capacity falls, but at lower C-rates the total capacity will show less change.

Do Mott and Realforce provide any specs for the batteries they are selling?

 

Actually, you may be within the margin of error depending on how you arrived at your 1/2 kWh number. 6.6Ah * 3.0 V=19.8Wh. Cell 11 having this additional capacity enables the rest of the cells in the pack to contribute just as much (assuming nobody else bottoms out first): 19.8Wh * 16 cell groups = 316 Wh, or about 1/3 kWh. Does that seem close enough to explain your result?