Solar Shed Part 19: Going Lithium with EG4-LLs

[Drizzt321]

@Syonyk

Or just call and ask, apparently they’re free with the information on it: AIMS LF Inverter RJ45 Protocol Information | Second Life Storage & Solar

That’s really handy, thanks.

@Syonyk

The benefit there is less air movement and more “Can radiate heat off.” In any case, I’m not one to complain too much about over-spec’ing things, but I don’t think you’re going to have wire temperature issues.

Ah, point. Yeah, I’m thinking actual heat dissipation entirely away, not just “is there enough free space to radiate well and thermal mass”.

And yeah, I don’t need to buy spools of wiring like you might as well. At least not for this.

[GoodeNerd]

@Syonyk

This is a “less potentially exciting” lithium chemistry. Energy density is lower than the standard 3.7V nominal/4.2V fully charged chemistries, though power density is quite good. Importantly, though, LiFePO4 is a much more stable chemistry - it’s not nearly as runaway prone as the other chemistries can be, which means it’s safer for something like “the corner of a shed.”

I’m finally setting up a DC coupled solar install on my garage this spring and my plan was to use a 2-3kwh bank of lead acid batteries that I have available until I give up on driving my LEAF, then part it out and replace the lead batteries with the LMO batteries. Your statement makes me reconsider using those modules in my garage, but I’ve been parking the car in the garage for the last 6 years and I only intend to charge them to 80ish%. For new stationary batteries LFP makes perfect sense, but how much more concerned should I be about a LMO pack that’s been well behaved for 11 years and an estimated 1500 cycles? LMO has a higher thermal runaway threshold than LMC, but not as high as LFP. Putting them in a metal container for various reasons would probably be worth while.

[Syonyk]

@GoodeNerd

Your statement makes me reconsider using those modules in my garage, but I’ve been parking the car in the garage for the last 6 years and I only intend to charge them to 80ish%.

It’s probably fine. Just be sure to have active external balancing and charge/discharge shutdown on them. You don’t want to repeatedly overcharge a weak cell because the bulk pack voltage is within bounds.

For new stationary batteries LFP makes perfect sense, but how much more concerned should I be about a LMO pack that’s been well behaved for 11 years and an estimated 1500 cycles?

I mean, we use spicy lithium chemistries (LiCoO2) in phones and stuff constantly, and they only rarely catch fire. Just treat them well and they’ll be fine. Don’t deep discharge them, don’t overcharge them, and don’t charge them below… oh, 45F would probably be my preference on those. Check the datasheet and add 5-10 degrees.

Putting them in a metal container for various reasons would probably be worth while.

I honestly don’t think it matters. They won’t vent, unless they’re undergoing thermal runaway, at which point “a metal container” doesn’t mean anything to 20kWh of lithium going off, and at that point, no offense, your garage is a smoldering pile of rubble. Unless you’re going to put rapid flooding systems and such on the metal enclosure.

The substation battery that burned up 5-6 miles from me was LFP. Didn’t stop water from getting somewhere it shouldn’t have been and lighting the thing off, though at least the fumes vented weren’t too bad (I saw the monitoring reports - it really wasn’t anything to be concerned about).

[GoodeNerd]

@Syonyk

It’s probably fine. Just be sure to have active external balancing and charge/discharge shutdown on them. You don’t want to repeatedly overcharge a weak cell because the bulk pack voltage is within bounds.

That’s a good point, a good BMS is even more important on elderly modules.

@Syonyk

I mean, we use spicy lithium chemistries (LiCoO2) in phones and stuff constantly, and they only rarely catch fire.

True, but a phone or laptop that catches fire will burn itself out in a few minutes where, like you said a few dozen kilograms of self oxidizing lithium isn’t going to leave anything more than a miniature crater.

@Syonyk

The substation battery that burned up 5-6 miles from me was LFP. Didn’t stop water from getting somewhere it shouldn’t have been and lighting the thing off

That was the biggest reason for a container, keeping things away that shouldn’t be around large batteries like water, children, and curious cats. I could just use plywood for those purposes though.

[Syonyk]

@GoodeNerd

That’s a good point, a good BMS is even more important on elderly modules.

It’s important with any module. The main purpose of them is to catch and manage the variance in cells that happens over time. If some of your cells are weaker, they’ll (for a given amp-hour charge/discharge through the pack) be “fuller” when full, and “emptier” when empty. That’s stressful, and can lead to “exciting” failures, if the cell is being driven to 4.5, 4.6V when full. You want to make sure that your BMS will terminate charging on any cell overvoltage, and terminate discharging on any cell undervoltage. Ideally, for a pack this big, you’d also have data export so you can see the state of the pack over time - a weak cell will stand out in a bar graph of voltage at a given state of charge. You’ll also want balancing capability that works at lower voltages, if you can find it - some BMS units won’t balance until full, and if you’re planning to charge to 80%, that’s never going to hit. Though you can always manually bring the pack voltage up every now and then to balance it out.

@GoodeNerd

True, but a phone or laptop that catches fire will burn itself out in a few minutes where, like you said a few dozen kilograms of self oxidizing lithium isn’t going to leave anything more than a miniature crater.

A few very energetic minutes, that are more than enough energetic to light surroundings on fire.

An iPhone 15 battery is 3.3Ah - so, using the rule of thumb, twice the electrical capacity in runaway energy (6.6Ah @ 3.7V = ~25Wh). If that goes off in a minute, which isn’t unreasonable, that’s about 1500W for a minute - which will light surroundings on fire. Also open flame, if it’s venting and catches.

@GoodeNerd

That was the biggest reason for a container, keeping things away that shouldn’t be around large batteries like water, children, and curious cats. I could just use plywood for those purposes though.

Yup. Keeping stuff off the batteries is fine, but it doesn’t need to be a runaway proof enclosure. Not that you can build one anyway at any reasonable cost.

[GoodeNerd]

@Syonyk

You’ll also want balancing capability that works at lower voltages, if you can find it - some BMS units won’t balance until full

I’ll keep an eye out for that feature when I’m shopping, but yeah worst manually should be fine. It’s going to have to be a pretty decent BMS anyway because there are 48 modules to track.

@Syonyk

Ideally, for a pack this big, you’d also have data export so you can see the state of the pack over time - a weak cell will stand out in a bar graph of voltage at a given state of charge.

I already had this feature on my must have list because I want to identify the weak modules and remove them from the pack to make an overall more health pack even if it makes the pack smaller.