PB2A16S-20P — Do I need a separate charge controller? Plus some noob questions!

[micalk]

Preliminary design, EE but noob solar designer. Looking for clues…

Project: Barn interior / exterior area lighting, maybe about 700W LEDs; maybe some fans in summer, low heat / freeze protection in winter (future expansion for an insulated tack room); and intermittent 115V loads. Off-grid, but potential later to connect (much later). Southern Oregon, dreary winter this year.

Panels: ?
Battery: 24v 8s ?Ah cells LiFeOP
BMS: PB2A16S-20P
Inverter: ?
MPPT: Here’s the deal: the pb2a16s bms can limit overcharging, but is it a mppt charge controller or something more limited? JK website is unclear. Would I be better off with a separate MPPT like a Victron 250-70 or 150-35?

I know the trade-off for cost (don’t buy bigger than you need) but that’s difficult to determine. So here’s kinda what I’m thinking.

3x 500w 48V panels and I’ll be able to get good southern exposure and angle.
8x 260Ah 3.2V cells
3KVA Inverter

Is that enough panel to get me through winter days with lots of cloud cover or will I need additional panel(s)?

I could go with higher Ah batteries for minimal extra cost, but I’m not sure which is better - more storage capacity than charging capacity or vice versa? I’m thinking more charging capacity is better to maximize battery charge. At the same time it would be nice to have a portable fridge out there (for horse meds and stuff) so more battery storage is not a bad thing, just need enough juice to keep it charged. Is it okay to never or only occasionally fully charge the batteries in the winter? It dips into freezing here but it’s usually above the mark during the day.

Enough! I need answers so I can ask better questions.

 

[MisterSandals]

MPPT: Here’s the deal: the pb2a16s bms can limit overcharging, but is it a mppt charge controller or something more limited? JK website is unclear. Would I be better off with a separate MPPT like a Victron 250-70 or 150-35?

A BMS is a safety cutoff switch.

A charge controller, er uh, controls charging. As different as airbags and brakes.

 

[WrenchLight]

The BMS is necessary for all lithium batteries. It's considered part of the battery. So yes, you'll need an MPPT charge controller if you are doing separate components instead of an all-in-one aka hybrid inverter.

 

[MisterSandals]

3x 500w 48V panels

What is Voc? Assuming connecting in series?

8x 260Ah 3.2V cells

So a 24V battery? (8S)

500W x 3 = 1500W / 28V charging = 53.6A charging max

35A charger too small (150/35)

Your answer to the first question will determine input voltage model of SCC.

 

[micalk]

Thanks so much! You provided thought provoking answers.

The JK BMS says it can limit battery current anywhere from 10 to 200 amps, but that doesn’t mean it’s seeking the most efficient charging, which is what the MPPT will do. I thought that was charging amps, but maybe that’s discharge? Not that it really matters. I’ll add a 250/70 MPPT.

The questions about solar panels were great! Made me look and find out some stuff.

Here’s a design philosophy question: for an off grid system and all things being equal, is it better to have more PV so batteries are topped off more often, or to have more battery storage so all the available PV energy is stored but batteries are topped less often?

 

[MisterSandals]

The JK BMS says it can limit battery current anywhere from 10 to 200 amps,

The BMS is a safety switch. On or off.

I’ll add a 250/70 MPPT.

Are you just randomly picking equipment?

is it better to have more PV so batteries are topped off more often, or to have more battery storage so all the available PV energy is stored but batteries are topped less often?

Thats like asking if you should fill you cars gas tank weekly or daily.

Solar needs to produce at least your daily usage. Batteries need to store at least as much energy as you want to use when the sun isn't shining.

You're asking for advice on what to do without saying what you want to do. Are you running out of power at night?
Are you having difficulty keeping your batteries sufficiently charged? Is everything working great but you want to change something just because you can?

 

[micalk]

Thanks for your patience. And the generosity of your time. Not randomly picking; you pointed out one of my selections was inadequate and the other was needed. I agreed. Once I understand better what all these products do, how they relate and are connected, etc., I’ll be fine. I’ve been looking at diagrams of various designs and sussing out the parts, connections, and current flows, and it’s slowly starting to gel. There’s a lot that i don’t even know that I don’t know, I’m sure!

I’ve got plenty of experience with 110/220 household wiring which is helpful, and dc / analog wiring (in-service engineer for navy computers and displays, retired) but those are known source of supply voltages and capacities. Solar is much more dynamic on the supply side, and I don’t have the experience to allow for that. Yet. I’ve got all the tools I need to crimp lugs on cables of virtually any size, I can solder. I can layout and mount and dress wires neatly. I can do the work, just need to know what I’m working with. Last home, I added a 60A subpanel in a barn, about 100 feet from home. Trenched, installed panel, lights and switches, all hard conduit throughout, inspected and approved so I’m not without skills, just without knowledge.

I’m trying to bound my system. Everything is on paper. The 36x36 barn has no power, but needs lights in the center aisleway, tack and feed rooms, a couple floods outside, and some 115 for light usage and the occasional heavy draw (vet X-ray, a quick weld or two). Once there’s electricity i’m sure a small fridge will be wanted. So my loads and usage are all guesses - I don’t have real numbers. I’m planning on 24v LED lighting for the interior (24v LED strip lighting?), but LED floods are going to be 115. It would be good to have something like an oil filled heater in the winter to keep the chill out of the tack room (it’s insulated). So I want to be able to expand smartly, or have some built in extra capacity for unknowns. And that just adds to the dynamics here. Bottom line, looking at maybe 600 watts of lights for about three hours a day in winter, plus portable fridge, radio and other minor draws. Summer means more fridge current, clippers and maybe some fans (haven’t calculated for those yet), but less lights, no heater.

Barn has a monitor roof that is raised 30” facing due south, so I have a very convenient mounting location with an open back and ability to get a good angle if panels are mounted lengthwise. But the 500w panels are huge. I could fit four lengthwise, was looking at three to start. but they’re huge. The roof isn’t very high overall, but even so those panels are huge! So I have more thinking to do about panels.

That’s enough for now. Thanks again.

 

[MisterSandals]

I could fit four lengthwise, was looking at three to start. but they’re huge.

Starting with 4 would simplify the voltage since you could run 2S2P and get by with a 150V SCC.

2000W / 28V charging = 71A charging

Minimum

Amazon.com

Future proof for 6 panels 2S3P

https://www.amazon.com/Victron-SmartSolar-100-Tr-Solar-Controller/dp/B07FCRZ3F6/

 

[micalk]

thanks. Was looking at your energy audit sheet. nice piece of work. Gonna play with that a bit and look at numbers. It will be interesting to see how far off my gut is once some numbers are on paper.