C rating changes as multiple 12v batteries are used in Parallel question. ...and way more ;-)

Terrapin

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Jan 4, 2021
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315
Location
New England, USA
My situatiion is a small travel trailer that will remain all 12v. I have PD converter to install with lithium capable charging at bluk of 45amp. I plan to build the battery system my self. It will be a 4S 240ah single battery

This is the BMS en route:
4s BMS 120a LifePo4 Battery Management System for 12v DIY Batteries, Programmable, Bluetooth Included. - 8 gauge x 12" (wires were upgraded)
this is the one referred by Will's site. It is on back order but I am still snow covered so the build is set for early spring.

The (rare use) largest single intermittent load (maybe for 10 min. max) I want the system to power is a Dometic 800w microwave that is rated at 1300w max usage with the LP gas refrigerator fuel solenoid operating (with no internal frig. fans but a .5a one on a thermostat in the vent stack) and a few led ceiling lights. this will be through the Shore Power hookup and an inverter.

TV and radio will be on a separate closed loop system as "nonessential"*.

I am researching the transfer switch for the inverter loop so those questions will come later.

The inverter I have ordered hoping to do this is:
GoWISE Power 1500W Pure Sine Wave Power Inverter 12V DC to 120 V AC with 3 AC Outlets, 1 5V USB Port, 2 Battery Cables, and Remote Switch (3000W Peak) PS1005 (Brand Name/Packaging May Vary)
Link: https://www.amazon.com/gp/product/B01COA0UTE/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1
This is also one referred by Will's site.

#1 Before I ask my questions... am I on track so far? is this math working at all????

#2 what is my weakest link thus far.

#3 What is the calculation needed to determine the C rating of the cells I need to buy for the battery build?

#4 Or, is that a typical 12v 4S prismatic battery build C rate limitation a factor of the BMS used?

#5 If multiple 12V batteries are used in parallel to be able to tune their bandwidth for longer life (say, +/-35-85& charge) and still have decent capacity, then will the effective C rate of the bank system change or there an electrical bottleneck somehow?

#6 If it does rise what is the change calculation, does it simply double assuming adequate wiring?

#7 is it possible to simply charge the *"nonessential" battery at the same time as the rest but not have it's output shared?

# If that's possible must it be manually switched or can it be isolated for output only automatically?

#7 Is there anyone here who draws schematics for this stuff for noobs? Those would include a few peripherals like some external solar hookups to augment charging with suitcase panels (a single 120w folder is on hand so far...but others may come if the trees here don't kill the dream).

Well that's where I am at. ... just catching on enough to be dangerous...

Thanks to everyone who has helped so far. It was suggested I number my questions, I hope that didn't come off as cold. Any help is greatly appreciated.

I wish we had more direct sun so it was worthwhile to go with 100% full time roof top solar but we tend to seek the trees a lot. I hope we see some free juice from this first panel set though.
 

snoobler

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Jul 10, 2020
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HBR, AZ
1) Looks that way.
2) Most components have little safety margin, but should be fine.
3) It's on the datasheet. 1C for a 240Ah cell means 240A, but the BMS will be the limiting factor at 120A.
4) Typically.
5) If each is added with its own BMS, yes. 120A for each 12V 240Ah battery in parallel. You want to leave yourself a little margin as getting the loads shared between the batteries is rarely perfect. "Wiring" in link #2 in signature.
6) Yes, but wise to leave maybe a 20% margin of safety.
7) I personally wouldn't separate them. The amount of power on TV/radio is single digit amps.

Thank you for numbering your questions. Seriously.
 

chrisski

Solar Boondocker
Joined
Aug 14, 2020
Messages
2,687
I have a microwave in my trailer that draws 155 amps at 12 volts. I also need a 2000 watt inverter to power it. I don’t know the rating, but it’s probably a 1000 watt microwave, which s cooking watts, but draws close to 1800 watts to power it. Some do this with 2/0 wiring, but I went with 4/0 and a 200 amp ANL fuse.

That’s really pushing the limits on a 12 volt system. I actually turn the generator on for the microwave. We have two 10 minute periods a day we use it. One reason I turn the generator on is is 3 minutes of the microwave on equals one normal hour of power.

I’ve put some thought into what it would take for me to run the microwave, and I’m coming up with going from 4 golf cart batteries to 6 golf cart batteries, and making my system 24 volts. Of course I can’t wire 6 golf cart batteries to make 24 volts, so that means I buy 8 or switch to a 12 volt 100 ah battery and get 6 of those. The equivalent of two golf cart batteries gets me the energy I need to power the microwave to include cloudy days, and the 24 volts lets the amperage drop to half to where I am comfortable with the wiring.
 

Terrapin

Solar Enthusiast
Joined
Jan 4, 2021
Messages
315
Location
New England, USA
1) Looks that way.
2) Most components have little safety margin, but should be fine.
3) It's on the datasheet. 1C for a 240Ah cell means 240A, but the BMS will be the limiting factor at 120A.
4) Typically.
5) If each is added with its own BMS, yes. 120A for each 12V 240Ah battery in parallel. You want to leave yourself a little margin as getting the loads shared between the batteries is rarely perfect. "Wiring" in link #2 in signature.
6) Yes, but wise to leave maybe a 20% margin of safety.
7) I personally wouldn't separate them. The amount of power on TV/radio is single digit amps.

Thank you for numbering your questions. Seriously.

So thanks for your input again Snoobs, I am trying to be smarter about what I ask and how I ask it. I was in remodeling design construction for 40 years. I used to beg my own costumers to do it...maybe 1 in 10 got the point. So when someone offers free help. ....Heck yeah it is coming back as requested. Thx a ton! so far so good.

Next big step is the transfer switch system.... Maybe I will keep it all together but I am dong this system the same time as i am building a modular "generator" that as going to power the nonessentials. (It's an old "one is none, two is one" school of though... or "there is nothing better in a system failure than a second system" mentality... I know me.
 

Terrapin

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Joined
Jan 4, 2021
Messages
315
Location
New England, USA
I have a microwave in my trailer that draws 155 amps at 12 volts. I also need a 2000 watt inverter to power it. I don’t know the rating, but it’s probably a 1000 watt microwave, which s cooking watts, but draws close to 1800 watts to power it. Some do this with 2/0 wiring, but I went with 4/0 and a 200 amp ANL fuse.
I feel a little lucky this is a tiny one. I am hoping against hope that it is on that reduces the actual power out put on lower setting rather than timing full juice on and off to cook at lower power levels. That may be the saving grace in a system so close to the edge.
That’s really pushing the limits on a 12 volt system. I actually turn the generator on for the microwave. We have two 10 minute periods a day we use it. One reason I turn the generator on is is 3 minutes of the microwave on equals one normal hour of power.
Yep.
I’ve put some thought into what it would take for me to run the microwave, and I’m coming up with going from 4 golf cart batteries to 6 golf cart batteries, and making my system 24 volts. Of course I can’t wire 6 golf cart batteries to make 24 volts, so that means I buy 8 or switch to a 12 volt 100 ah battery and get 6 of those. The equivalent of two golf cart batteries gets me the energy I need to power the microwave to include cloudy days, and the 24 volts lets the amperage drop to half to where I am comfortable with the wiring.
That's a lot of juice for that bigger nuke. ...I may never come back if I could store that much. LOL These affordable 270/280ah cells are really cool to see. I've only been watching 8-9 months or so. To see a jump in costs/capacity is crazy encouraging. My plan is to build pretty much the whole trailer upgrades and at the very last stage buy the actual cells, hoping for things to continue getting even better. At his rate I may be able to afford some new 3.2vs that put out that hold 400ah by then. I have room. Maybe two 270ah 4s batteries in Parallel as those prices drop. I have small couch I need to build and there is room under there.
 

mkaye

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Joined
Mar 23, 2020
Messages
237
i think you a very close to the max limit of the BMS for the uwave
take 1300W/13V = 100A DC current
not sure what the efficiency of the inverter is but it varies depending on the output
if we assume 80% that puts your battery current at ~125A
that's more than the DALY will allow
& as you mentioned that's not any other DC current required for lights/fridge/water pump etc
 

Terrapin

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Joined
Jan 4, 2021
Messages
315
Location
New England, USA
i think you a very close to the max limit of the BMS for the uwave
take 1300W/13V = 100A DC current
not sure what the efficiency of the inverter is but it varies depending on the output
if we assume 80% that puts your battery current at ~125A
that's more than the DALY will allow
& as you mentioned that's not any other DC current required for lights/fridge/water pump etc
Yikes.... see there.... I had not factored in the loss. That bms is 120a.... close indeed! Wouldn't it be terrible if I had to tell the Mrs I HAD do another battery so she could make her rainy night movie popcorn. Well at least I know it as a possibility. That rating of 1300w for a 800w output Mwave seemed a bit high, maybe we will get lucky...maybe not. But thanks a ton for that heads up!
 
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