BaronVonChickenPants
Member
- Joined
- Dec 5, 2017
- Messages
- 59
Greetings all,
As a long time lurker I decided it's time to start a build thread. The build is still in the early stage of planing and parts collection but it's starting to gain momentum so I need to bounce some ideas off of those with some real world experience rather than just theory and planning.
At the moment we have an off-grid system running on an APC SmartUPS3000VA with 48v PbA pack ~9kWh, 12 x 250w Trina solar panels and a chinesium 40A MPPT charge controller.This was a temporary/testbed system assembled from existing and scrounged parts meant to put theory into practicewhile we assemble the proper system.
Proper System:
We have an APC SmartUPS7500VAwhichuses 32x12V5AH SLA'sfor a nominalpack voltage of 384Vand a charge voltage of 432V.
As the pack is high voltage, even under peak load the current draw should never reach 20A.
The trick is chargingsuch asystem from solar.and finding a BMS formanaging something like 4p120s cell arrangement.
I am looking at building packs from 10AH LiFePo4 cells, something like these,as the nominal and charge voltagenumbers all line up nicely and 4p should give reasonable redundancy if I have a cell failure.
I have seen MPPT solar charger that had a PV input voltage of 300V and a nominal battery voltage of 350V, I'm not sure if it was just a typo but as it was still ~100V under my needs I didn't look into it any further.
In theory, with the 12 solar panels rated at 384Vmp and 8.2Imp,it should be possible with MPPT maintaining at leasta 350v solar outputand a boost converter lifting thesolar output to charge voltage would be less than 25% increase so should be achievable with reasonable efficiency. As charge currents are all under 10A nothing needs to super heavyduty with the exception of warning labels and insulation/isolation.
Question time:
I notice most powerwall builds appear to build cells into parallel 3.2v battery packs then join packs in series to achieve the desired pack voltage eg: 48v = 10p14s.Each cell is connected to the bus via fuse wire incase an individual cell fails,as this will likelynot be detected by the BMS.
I assume this is to keep down the cost andnumber of BMS components required compared to 14s10p where a each cell would be individually managed.
How then do you inspect individual cells for voltage, IR, etc once assembled? Or do you just monitor fuse links and hope for the best?
Are there any readily available BMS system for higher voltage packs, I know they exist in the DIY Electric Vehicle world where 400V or more is not uncommon but these are typically designed around much larger winston/thundersky cells and MUCH larger current draw. Most of the BMS mentioned in the forum seem to peak around 48v
How viable is it to live without BMS, I realise this is a can of worms, but with LiFePo4 systems naturally matching much closer to PbA systems and using new cells which should match in spec's, how likely is it they would wander out of balance? How would you balance them if they did?
Would I be better off with larger 40Ahsingle cells instead of 4x10Ah packs?
Are the any available solar charge controllers that do not require an input voltage near double the nominal pack voltage, like most of the alibaba/ebay ones that appear to be PWM?
Are there any other suggestions/ideas to consider?
Cheers,
Jordan
As a long time lurker I decided it's time to start a build thread. The build is still in the early stage of planing and parts collection but it's starting to gain momentum so I need to bounce some ideas off of those with some real world experience rather than just theory and planning.
At the moment we have an off-grid system running on an APC SmartUPS3000VA with 48v PbA pack ~9kWh, 12 x 250w Trina solar panels and a chinesium 40A MPPT charge controller.This was a temporary/testbed system assembled from existing and scrounged parts meant to put theory into practicewhile we assemble the proper system.
Proper System:
We have an APC SmartUPS7500VAwhichuses 32x12V5AH SLA'sfor a nominalpack voltage of 384Vand a charge voltage of 432V.
As the pack is high voltage, even under peak load the current draw should never reach 20A.
The trick is chargingsuch asystem from solar.and finding a BMS formanaging something like 4p120s cell arrangement.
I am looking at building packs from 10AH LiFePo4 cells, something like these,as the nominal and charge voltagenumbers all line up nicely and 4p should give reasonable redundancy if I have a cell failure.
I have seen MPPT solar charger that had a PV input voltage of 300V and a nominal battery voltage of 350V, I'm not sure if it was just a typo but as it was still ~100V under my needs I didn't look into it any further.
In theory, with the 12 solar panels rated at 384Vmp and 8.2Imp,it should be possible with MPPT maintaining at leasta 350v solar outputand a boost converter lifting thesolar output to charge voltage would be less than 25% increase so should be achievable with reasonable efficiency. As charge currents are all under 10A nothing needs to super heavyduty with the exception of warning labels and insulation/isolation.
Question time:
I notice most powerwall builds appear to build cells into parallel 3.2v battery packs then join packs in series to achieve the desired pack voltage eg: 48v = 10p14s.Each cell is connected to the bus via fuse wire incase an individual cell fails,as this will likelynot be detected by the BMS.
I assume this is to keep down the cost andnumber of BMS components required compared to 14s10p where a each cell would be individually managed.
How then do you inspect individual cells for voltage, IR, etc once assembled? Or do you just monitor fuse links and hope for the best?
Are there any readily available BMS system for higher voltage packs, I know they exist in the DIY Electric Vehicle world where 400V or more is not uncommon but these are typically designed around much larger winston/thundersky cells and MUCH larger current draw. Most of the BMS mentioned in the forum seem to peak around 48v
How viable is it to live without BMS, I realise this is a can of worms, but with LiFePo4 systems naturally matching much closer to PbA systems and using new cells which should match in spec's, how likely is it they would wander out of balance? How would you balance them if they did?
Would I be better off with larger 40Ahsingle cells instead of 4x10Ah packs?
Are the any available solar charge controllers that do not require an input voltage near double the nominal pack voltage, like most of the alibaba/ebay ones that appear to be PWM?
Are there any other suggestions/ideas to consider?
Cheers,
Jordan