BigBattery Reputation?

[aaron_c]

I see Will Prowse linked some BigBattery battery banks from his website...and damn if the price doesn't beat the ebay mysterymeat price for LiFePO4 batteries, even were I to DIY the whole thing myself.

What do people think? Worth taking a chance on?

Specifically, I'm looking at their 24 V 123 AH LiFePO4 batteries with some kind of BMS. They're selling for $900 including shipping.

 

[curiouscarbon]

Be aware that the word on the block is that those LFP power banks were used heavily in a past life, so expect them to be noticeably less dense storage wise compared to new cells. That said, they seem to be originally 5kWh and cycled down to 3kWh usable capacity.


If the drawback of it taking up more space and being heavier than a new battery is
not a problem, perhaps continue.

 

[aaron_c]

Be aware that the word on the block is that those LFP power banks were used heavily in a past life, so expect them to be noticeably less dense storage wise compared to new cells. That said, they seem to be originally 5kWh and cycled down to 3kWh usable capacity.


If the drawback of it taking up more space and being heavier than a new battery is
not a problem, perhaps continue.

Oh, that's really interesting. If they are cycled down to 3kWh usable capacity, should I expect them to have as many cycles as a "new" 3kWh LiFePO4 battery?

 

[curiouscarbon]

Oh, that's really interesting. If they are cycled down to 3kWh usable capacity, should I expect them to have as many cycles as a "new" 3kWh LiFePO4 battery?

Uncertain, but some people are operating with that hope in mind. The capacity loss curve might become sharper the further you go, but I can’t comment which way it is. That’s not clearly established as far as I know. Happy for others to chime in.

Assuming 5->3 that’s 40% capacity lost as a starting point. Many people consider 20% capacity time to replace, but it’s somewhat of a line to draw in the sand. If it still holds X amount of charge and that’s valuable, then it’s treasure!

 

[curiouscarbon]

btw i ended up getting frey/overkillsolar 100Ah cells and doing overkillsolar bms. it’s more labor. the bigbattery LFP packs are super appealing from a cost per storage point of view, but not from space. i’m limited by space so this is the optimization variable.

 

[gnubie]

Swiped from battery university so be aware that their information can be out of date and of course there are many tweaks to cell construction so at best this is a rough guide. 80% is typically where lithium batteries cycle life is stated as ending but there's a lot more capability to go.

 

[curiouscarbon]

⚠WARNING EXTRAPOLATION ALERT⚠


⚠WARNING EXTRAPOLATION ALERT⚠

Perhaps they have seen 9000 or more cycles. I’m deep in the hazy mist of extrapolated data, though.

 

[aaron_c]

Well I want to join the extrapolation fun!

Here's a chart from batterblog.ca, which I've never heard of before (not that I would have heard of anything...I'm pretty new at this):



But if this holds true then the capacity does indeed fall of a cliff (or at least roll down an increasingly steep hill) starting around 1000 cycles or 30-40% capacity loss. Which is where we think the cells in the Big Battery uh, battery are.

 

[aaron_c]

On the other hand this offering from Big Battery says it's using NEW cells. Makes a big deal about that. Do we know anything about Model 3 2170 cells?

 

[Picasso]

Model 3 2170 cells sold by Big Battery are rejects that are not up to the correct capacity. Never formed into packs. He will run out soon.

 

[aaron_c]

Model 3 2170 cells sold by Big Battery are rejects that are not up to the correct capacity. Never formed into packs. He will run out soon.

Oh man, there is always a catch with bigbattery, isn't there?

What's the downside to using these cells? Will it not charge properly (or waste power doing so)? I've read about problems buying cells that aren't sized quite right, but I'm not clear on exactly what the problem with it is, other than the need for some kind of balancing prior to setting up a battery bank.

 

[Picasso]

Only downside would be balancing.

ANYONE with these cells check internal resistance.

 

[aaron_c]

Only downside would be balancing.

Sorry for this really basic question but...what does that mean? I understand something about balancing the cells--but I'm hoping that, coming from BIgBattery, they've been balanced at least once when they were put together. But I'm not clear on what the long-term implications are. Or why internal resistance needs to be checked. Or how often it would need to be checked. Or what one should do about it if there were a problem.

 

[gnubie]

But if this holds true then the capacity does indeed fall of a cliff (or at least roll down an increasingly steep hill) starting around 1000 cycles or 30-40% capacity loss. Which is where we think the cells in the Big Battery uh, battery are.

This isn't for lifepo4 cell though, so I'm not sure if this curve would apply. With the cycle life time ratio of these cells to lifepo4 cells it might be possible to multiply the cycle life scale by 3 but that's only a guess as what happens after the nominal EOL for a lifepo4 is all new to me.

 

[Picasso]

Good paper link here. https://batteryuniversity.com/learn/article/rising_internal_resistance

 

[aaron_c]

Good paper link here. https://batteryuniversity.com/learn/article/rising_internal_resistance

Thanks Picasso.

So if I understand you correctly, some of the cells would have a higher internal resistance than they should, and the heat resulting from this would drop the voltage of that cell (and possibly nearby cells). And although this might not happen throughout the whole battery, it would...would the other cells try to equalize the voltage and lose charge that way? So it would result in a loss of efficiency because whenever you used the battery--and particularly when it was used hard, it would lose charge more quickly than a better quality battery would.

It would also be less efficient in terms of charging it, both because that would also trigger this leaking of heat and because the cells that filled up more quickly (because they weren't "leaking") would then have to be bled off by the BMS so that the charger could continue to charge the shitty cells. Either that, or the maximum charge voltage would have to be lowered to that of the shitty cells, effectively lowering the capacity of the cells to the lowest common denominator.

Did I get that right? And if so...what kind of loss of capacity would we be talking if all the cells were dropped to the lowest common denominator? 10%? 20%?

 

[aaron_c]

ANYONE with these cells check internal resistance.

I'm watching a youtube video about a similar bigbattery battery and they note that the enclosure is closed using rivets...so they're not exactly encouraging this kind of testing, unfortunately... Unless it's valuable to check it for the battery as a whole?

 

[Ampster]

Oh, that's really interesting. If they are cycled down to 3kWh usable capacity, should I expect them to have as many cycles as a "new" 3kWh LiFePO4 battery?

At that price 3kWh is $300 per kWh vs. $125 for Alibaba batteries that many here have had success with. Why would anyone take a chance on used when you can get new for less?

 

[Picasso]

@aaron_c I'd guess under 2% to start with on a single cell. These cells all just have higher vs spec IR so out of the gate Id think of it as an aged cell. The way this DIY powerwall groups uses them taking them to 100% is gonna age them faster vs the higher IR i bet.

 

[aaron_c]

At that price 3kWh is $300 per kWh vs. $125 for Alibaba batteries that many here have had success with. Why would anyone take a chance on used when you can get new for less?

Oh gosh, maybe I was looking in the wrong places. I looked for LiFePO4 cells on ebay but not alibaba. Are you talking about something like this? And if so, what's your opinion on these CALB cells? What risks would I face ordering from Alibaba?