First top balance, OFFgrid caravan PV MPPT 8s280Ah LFP BMS 24V-220V Sine

[MrSparkle]

First time PV system and bat.-pack builder. EU, mid-south Sweden.

Main question:
1 - Can I use the Victron SmartSolar 150/45 MPPT with 2s500W PV (probably tiltable at stationary) and JKBMS B2A8S20P-HC to top-balance new EVE 8s280Ah first time (no load/inverter connected), when they arrive? If possible. Is there a guide for this specific balancing-configuration?

[EDIT-Answered] Secondary thought:
2 - I am a little bit confused about the fact that both SCC and BMS have specific settings for voltages, etc. Bulk, absorb (and float, Pb legacy). Could this not lead to conflicts of any kind? Or is it the BMS that dictating to SCC "I now want this specific Voltage and you give me exact what I want" and the SCC complies as an obedient slave and serves to the BMS exact what BMS desires?

[EDIT-Answered] 3 - Is the internal resistance in batteries the most important factor to match when building a bank from new cells? Does responsible suppliers of cells match both voltage and resistance? Example Luyuan? What is generally considered good span of resistance-differencies on new, delivered, matched cells from a reputable, responsible supplier?

Preferably. Single assembly (no re-assembly), balance with PV+MPPT+BMS (no load connected), production with load connected. If possible.

Prefer calm, conservative, patient way. I want things get done correct. Aiming max 0,5C charge/discharge. Not planing cycle 100% capacity. 90% -ish OK. Best would be to seldom go lower than say 40% SoC?

I am quite reluctant to invest in (or build/modify) special chargers just for first balance and after that the equipment would be sitting for ages with no need of use, collecting dust. Especially since JKBMS have active balance function. Even if such a charger could cost only 50$ or so. Just seems wastefull. Have been trying to read up as much as possible, mostly on this forum, about what needs to be done, how, why, to achieve desired end result. I have now some understanding that there are more than one path, how to balance a bat.-pack. I am also some what unwilling to reconfigure s-p-s just because of first balance, pre-production.

Circumstances:
At the moment total renovation of the caravan. All wood rotten. Alu skin good, keep. Hopefully I finish renovation and OFFgrid build to get at least some weeks of warm days aug-sept before autumn kicks in.

System:
PV 48V
Bat. 24V (7,1kWh)
Consumers 220V
100W (450?W surge) small house-fridge
800W rated simple microwave
45W laptop
Handfull small lamps
Estimated consumption 1-2 kWh/day

Bought - PV 2s500W STC 44,22 Voc - STC 14,04 Isc
To Buy - Victron SmartSolar 150/45
To Buy - EVE 8s280Ah double-threaded terminals
To Buy - JKBMS B2A8S20P-HC (heating function, cold climate winter)
Bought - Nova NV-P1500 1,5kW (3kW surge) 24V-220V pure sine inverter

The inverter is simple, no com-port of any kind.

Not studying/buying any cables/breakers/shunts/etc at the moment. Now all focus renovate caravan, study OFFgrid electronics and buying major components. Purchase cables/breakers/shunts/etc and final assembly, as last step.

Aside of main question about balancing. I am also gladly opened to any critics of the system design or choice of main components.

Considering to buy from Luyuan since people seem to have very positive experience with them (Amy?) and if I understand correct, they have an EU-stock, meaning no extra customs, VAT, etc and short shipping times?

 

[sunshine_eggo]

First time PV system and bat.-pack builder. EU, mid-south Sweden.

Main question:
1 - Can I use the Victron SmartSolar 150/45 MPPT with 2s500W PV (probably tiltable at stationary) and JKBMS B2A8S20P-HC to top-balance new EVE 8s280Ah first time (no load/inverter connected), when they arrive? If possible. Is there a guide for this specific balancing-configuration?

Yes, but it will be limited. You would set to 3.45V/cell absorption AND float. Set JK to balance above 3.40V. Once all cells are 3.45V, you're very very close to true top balance. If you want, you can drive incrementally higher to 3.55 and up to 3.60V/cell.

Secondary thought:
2 - I am a little bit confused about the fact that both SCC and BMS have specific settings for voltages, etc. Bulk, absorb (and float, Pb legacy). Could this not lead to conflicts of any kind? Or is it the BMS that dictating to SCC "I now want this specific Voltage and you give me exact what I want" and the SCC complies as an obedient slave and serves to the BMS exact what BMS desires?

They are unrelated in all ways. You set the BMS values to the limits needed to protect the cells. You set chargers and loads to operate inside those limits.

3 - Is the internal resistance in batteries the most important factor to match when building a bank from new cells? Does responsible suppliers of cells match both voltage and resistance? Example Luyuan? What is generally considered good span of resistance-differencies on new, delivered, matched cells from a reputable, responsible supplier?

IMHO, self-discharge rate is the most important as this is the most important property in maintaining balance. IR second. Capacity third. Luyuan cells have always been acceptably matched for me.

Note that 800W rated microwave likely uses 1200-1300W.

 

[MrSparkle]

Thanks @sunshine_eggo for great replies!

1 - I guess differences between highest-lowest cells would be minimal if compared when balancing to 3.450 - 3.600 around 2% maybe even less? Just a hunch, since charge curve exponential at bottom and top.

If incremental above 3.450, how big steps would be suggested to say 3.600? Guessing due to exponential, bigger at 3.450 and smaller the closer to 3.600?

3 - Will probably aim for Luyuan then. And yes, read some where that a Micro can surge up to 3x at start plus inefficiency. Been thinking about that. Would not be surprised if fridge+laptop+micro would exceede 1,5kW or 3kW surge. Worst case scenario, unplug/turn off laptop and fridge, because coffee (instant) is the top most thing in the morning!

 

[sunshine_eggo]

Thanks @sunshine_eggo for great replies!

1 - I guess differences between highest-lowest cells would be minimal if compared when balancing to 3.450 - 3.600 around 2% maybe even less? Just a hunch, since charge curve exponential at bottom and top.

Could be even less. I've seen 280Ah batteries with cell SoC deviations of < 0.1% be sufficient to trigger protection and interfere with charging to 3.55V/cell.

If incremental above 3.450, how big steps would be suggested to say 3.600? Guessing due to exponential, bigger at 3.450 and smaller the closer to 3.600?

I'd shoot for 3.450V straight away. If that's too much, 3.40. Once equalized at 3.45, I'd shoot straight for 3.60. 3.55 if that's too much.

The issue is that the MPPT will spike voltage when charge is cut. If nothing else is connected, no problem. If you have the inverter connected, it will likely give an over-voltage alarm and shut down.

3 - Will probably aim for Luyuan then. And yes, read some where that a Micro can surge up to 3x at start plus inefficiency. Been thinking about that. Would not be surprised if fridge+laptop+micro would exceede 1,5kW or 3kW surge. Worst case scenario, unplug/turn off laptop and fridge, because coffee (instant) is the top most thing in the morning!

Micro surge is HUGE as the magnetron activates, but it's also SHORT - like the first half of the cycle, i.e., 1/120th to 1/60th of a second. It happens so fast, many inverters don't even register it happening. They just end up delivering less current for a slightly longer time and at lower voltage.

I had no issues using a cheap WZRELB 24V/2000W inverter to start a 1000W out/1600W in microwave.

Electric motors are the larger concern because they are about 5X the run current, and their surge time is typically measured in seconds, not milliseconds.

 

[MrSparkle]

@sunshine_eggo
The plan is to use SCC+BMS to initial balance due to lack of CC CV charger. Nothing else connected during balancing. Though good to know "if else" for better understanding.

So the 800W rated microwave on 1,5kW (3kW surge) would probably not be any concerns. The day is getting better and better.

Electric motors longer starting times, totally logical. Starting to think about the simple, "half-size" domestic compressor-fridge. Just measured with a simple watt-meter. About 450W for 2-3s when compressor starts and then 100W continues. I wonder how big is the surge-window of my simple inverter. Hmm, will dig through manuals. And back to calculator, again.

 

[sunshine_eggo]

@sunshine_eggo
The plan is to use SCC+BMS to initial balance due to lack of CC CV charger. Nothing else connected during balancing. Though good to know "if else" for better understanding.

So the 800W rated microwave on 1,5kW (3kW surge) would probably not be any concerns. The day is getting better and better.

Electric motors longer starting times, totally logical. Starting to think about the simple, "half-size" domestic compressor-fridge. Just measured with a simple watt-meter. About 450W for 2-3s when compressor starts and then 100W continues. I wonder how big is the surge-window of my simple inverter. Hmm, will dig through manuals. And back to calculator, again.

The lightweight inverters tend to claim a 2X surge rating, but those are for VERY short durations... the aforementioned 1/60th-ish of a second. Some actually give a time.

If I had to guess... If you're running the microwave, and your fridge cycles on, you might see an overload. If the fridge is already running at 100W, and you use the microwave, probably won't see an overload.

A compromise might be a small 500W inverter dedicated to the fridge if practical. I wouldn't implement that unless the fridge/microwave interaction is a problem.

 

[MrSparkle]

Yeah, I begin to get the feeling it would be going that way.

The "Mad Hatter" solution to such a problem would then be to wait for the fridge to kick in before starting the micro. That means standing there in the morning, newly awakened and 100% grumpy, waiting god knows for how long for the fridge to start, in order to get your coffee. Or dial back the fridge to bottom. Or turn off completely. Placing a switch for fridge outlet could work. I know, I know, should have done all the reading and math before purchase. Gees, how crazy of solutions to a problem can a bonkers and stubborn mind give birth to?

Could there be some cheap modification/add on, like a capacitor after the inverter that takes care of the 3-4 seconds compressor surge? I could guess one could cost as much as 2x strong inverter.

 

[sunshine_eggo]

Wouldn't worry about the coffee maker. They're resistive loads and should have no surge. Your typical filter coffee maker is about 900-1000W

 

[MrSparkle]

@sunshine_eggo
I realize now I made a mistake describing various aparatus. I use a microwave to heat up water in a cup and then to add instant coffee. Hence, my micro IS my "coffee-maker". ^^

As I said coffee is most important and needs to be made NOW. Hence, micro. 45 seconds, done. No way I am waiting god knows how long in the morning for a regular coffee-maker. I´d rather ditch the fridge and drink spoiled milk.

Although all the new insights about differences between types of household appliances are golden knowledge!