krby
Solar Enthusiast
Not sure if this is the right forum, but "DIY PowerWalll" didn't seem right ever. Moderators, please move this if needed, or let me know and I'll repost somewhere else.
I'm thinking about building a 24V 100Ah LFE pack for a "Goal Zero"-like portable power station. I would like at least 4kWh of usable capacity and 2kW of output. It's not going to cycle very often, so I'm sizing using 90% DoD. Part of this fun of this is the project, and I'll likely end up replacing the batteries in several years well before 1000 cycles as the technology improves.
So I'm looking at a battery pack, an inverter, and an AC LFE charger. No solar charging needed. I chose 24V to keep the current more reasonable, somewhere I've read that "12V for every 1000W". If I'm wrong about that, please let me know.
I would like to sketch out a plan here and get feedback from you folks.
LFE cells
I'm looking at the Sinopoly 12V 200Ah kit from Ruixu (https://www.ruixulithiumbattery.com/12v-2560wh-lifepo4-set-with-200ah-cells-without-bms). I would get two kits without BMS, for total of eight stacks. I would wire these all in series to give me a 24V 200Ah pack. I chose the "no BMS" option above because I plan on a single 8S BMS.
BMS
I'm a little confused on what exactly a BMS does. At a minimum, it's protection, but it feels like there are more advanced options? Without any other advice, I'll default to the Daly 24V 100 or 200A that Will suggests. I don't know enough about what a "more advanced" BMS will do, so I can't decide on whether this is a place to spend money. As part of the overall system, I will want some sort of small LCD display showing at least overall voltage and maybe current flow, not sure if the BMS is the right place for that or if I should just do this using a separate power meter
Balancing
Is this something a more advanced BMS will do? Or do I use an active balancer? Or, should I let the charger do this? In the R/C hobby, we use LiPos (no BMS on board for weight and cost savings) and then fairly programmable chargers that can be set to balance during the last (sometimes configurable) portion of the the CV phase. These chargers handle the "charge side" protections of a BMS. By default, some of them will refuse to charge if any of the cells are too low for the selected chemistry, or go into a "balance only" mode if cells are too far out of balance. It's probably overkill to use one of these chargers for this application, but I've got a smaller one to spare that can be configured for LFE batts. It would complicate wiring, as I would have to have balance leads to both the BMS and the charger. Does anyone make a single BMS/Balancer/Charger unit that is suitable for LFE?
Assuming I don't use the fancy charger, I assume I'll need some sort of balancing circuit unless a BMS will do this? I've read the term "active balancer" on one of Will's pages, but I don't know what that is. Is it just something like the ISDT BG-8S? (https://www.isdt.co/bg-8s.html?lang=en) I wonder if I could get something like that that would serve as my "status LCD", I guess it won't measure current though, no ammeter.
Inverter / Charger
I don't have the inverter picked out, but I'm looking for a 2000W 24V pure sine wave inverter. Suggestions are welcome? There are lots to choose from. One thing I am considering is an inverter/charger combo unit that people use in Powerwall-type house backup setups. I don't really need the auto-transfer feature, but it's appealing to have a single unit rather than two. My only concern with is that most of these seem built for 4-stage lead acid charge cycles, even if you can adjust the voltage for LFE. If I went this combo route, I wouldn't use the R/C computerized charger.
Other bits:
Of course a lot of these decisions are dependent on each other, which is sort of where I am looking for advice. I'm looking at Will's pages for DIY LFE packs and 24V system diagram trying to figure out which parts I don't need given my use case. I also get the impression the gear choice has changed since those pages were made, so I'd also like to hear what the latest is. If I could simplify with a more expensive thing that did BMS protection, balancing, and monitoring I'd be interested, but not if it drove the cost up $2k more than the separate components.
I'm thinking about building a 24V 100Ah LFE pack for a "Goal Zero"-like portable power station. I would like at least 4kWh of usable capacity and 2kW of output. It's not going to cycle very often, so I'm sizing using 90% DoD. Part of this fun of this is the project, and I'll likely end up replacing the batteries in several years well before 1000 cycles as the technology improves.
So I'm looking at a battery pack, an inverter, and an AC LFE charger. No solar charging needed. I chose 24V to keep the current more reasonable, somewhere I've read that "12V for every 1000W". If I'm wrong about that, please let me know.
I would like to sketch out a plan here and get feedback from you folks.
LFE cells
I'm looking at the Sinopoly 12V 200Ah kit from Ruixu (https://www.ruixulithiumbattery.com/12v-2560wh-lifepo4-set-with-200ah-cells-without-bms). I would get two kits without BMS, for total of eight stacks. I would wire these all in series to give me a 24V 200Ah pack. I chose the "no BMS" option above because I plan on a single 8S BMS.
BMS
I'm a little confused on what exactly a BMS does. At a minimum, it's protection, but it feels like there are more advanced options? Without any other advice, I'll default to the Daly 24V 100 or 200A that Will suggests. I don't know enough about what a "more advanced" BMS will do, so I can't decide on whether this is a place to spend money. As part of the overall system, I will want some sort of small LCD display showing at least overall voltage and maybe current flow, not sure if the BMS is the right place for that or if I should just do this using a separate power meter
Balancing
Is this something a more advanced BMS will do? Or do I use an active balancer? Or, should I let the charger do this? In the R/C hobby, we use LiPos (no BMS on board for weight and cost savings) and then fairly programmable chargers that can be set to balance during the last (sometimes configurable) portion of the the CV phase. These chargers handle the "charge side" protections of a BMS. By default, some of them will refuse to charge if any of the cells are too low for the selected chemistry, or go into a "balance only" mode if cells are too far out of balance. It's probably overkill to use one of these chargers for this application, but I've got a smaller one to spare that can be configured for LFE batts. It would complicate wiring, as I would have to have balance leads to both the BMS and the charger. Does anyone make a single BMS/Balancer/Charger unit that is suitable for LFE?
Assuming I don't use the fancy charger, I assume I'll need some sort of balancing circuit unless a BMS will do this? I've read the term "active balancer" on one of Will's pages, but I don't know what that is. Is it just something like the ISDT BG-8S? (https://www.isdt.co/bg-8s.html?lang=en) I wonder if I could get something like that that would serve as my "status LCD", I guess it won't measure current though, no ammeter.
Inverter / Charger
I don't have the inverter picked out, but I'm looking for a 2000W 24V pure sine wave inverter. Suggestions are welcome? There are lots to choose from. One thing I am considering is an inverter/charger combo unit that people use in Powerwall-type house backup setups. I don't really need the auto-transfer feature, but it's appealing to have a single unit rather than two. My only concern with is that most of these seem built for 4-stage lead acid charge cycles, even if you can adjust the voltage for LFE. If I went this combo route, I wouldn't use the R/C computerized charger.
Other bits:
- Fuses, wiring (I'll use 2 or 4ga because I've got that on hand)
- Some sort of resistor circuit for the inverter to deal with the initial current rush. Any schematics or pointers to pre-built parts would be appreciated.
- Do I need a separate battery disconnect? Or will the BMS do this?
- Small display of at least overall voltage (bonus points for per-cell, and amps)
- For flexibility, I'll want a 24->12V step down feeding a panel with various other 12V DC-out connection, like banana posts, PowerPoles, and maybe a panel mount USB port.
Of course a lot of these decisions are dependent on each other, which is sort of where I am looking for advice. I'm looking at Will's pages for DIY LFE packs and 24V system diagram trying to figure out which parts I don't need given my use case. I also get the impression the gear choice has changed since those pages were made, so I'd also like to hear what the latest is. If I could simplify with a more expensive thing that did BMS protection, balancing, and monitoring I'd be interested, but not if it drove the cost up $2k more than the separate components.
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