Help with math, understanding, and componet selection.

I post on the FB page but this is my first post on here.

I need help checking my math, help understanding draw/charge/ bms and equipment recommendations.

My final PW will be between 14s92p and 14s120p. For the purposes of this I will use 14s120p.
14s= 51.8v Nominal
Cells 1300 LG 2600 2C above 90% (2,340 mAh min) & 360 Sony 2200 2C above 90% (1,980 mAh min)
I am going to call the average = 2,300 mAh or 2.3 Ah
120 cells X 2.3Ah = 276 Ah
51.8v X 276Ah = 14,296 kWh

Solar Panels
18 LG 305s
MPP Voltage (Vmpp)= 32.9v
?MPP Current (Impp)= 9.28a
Open Circuit Voltage (Voc)= 40.1
3s6p
3s= 98.7v Nominal, 120.3v open circuit max
6p= 55.68a
98.7v X 55.68a = 5,495w

I have purchased 2000 1A Axial Glass Fuses.

Scope:
1. I am building a Motorhome that has 300sq ft livable space.
2. I am building it with a 240v main service panel just like a house.
3. All 18 panels will be on the MH.
4. I have a 3500w whisper generator and could parallel a second one if needed.
5. Primarily off grid but I will be able to plug into Shore power 30A 120v or 50A 120/240v. I do not need to export to the grid but am okay with the ability too.
6. I have a Chevy Bolt w/ a 60 kWh battery. (I wish I could just use that battery)
7. Charging Level 1 is 120v at 8 or 12 amps (user defined), Charging Level 2 is 240v at 32 amps

Preferred System Capabilities:
1. I would like the Current consumption to be prioritized and excess sent to the batteries.
2. 240v ac output
3. Some minimal DC output
4. Charge the car at 240v level 2 but not often.
5. Prefer Hybrid Charge Controller/Inverter with generator capability

What I dont understand.
1. I was taught a charge controllers size was determined by dividing the total wattage of the panels by the voltage of the batteries so in my case 5495w/ 51.8v = 106A so you would need a controller to be rated above that. But after watching Peters videos he seemed a little worried to be charging at 60A and his batteries are much larger.
2. What is the formula I would use to determine how much amperage I am pulling from each individual cell?
3. BMS, I didnt plan on it but I think I see some fairly inexpensive options., well for PC. What about Mac?


Thank you

Generally I go with the formula .5A per cell MAX draw/Charge So for that many cells you could easily charge at 840A Now that is not realistic since the chargers would be massively expensive and you want to look at your usage and how fast you want it to recharge.

So yes you have 14kWh but actually when in use you will see more like 10kWh useable as you never fully discharge nor fully charge as it hurts the cycle life. So keeping in the same units for ease of understanding to charge 10kWh of batteries feed the RV with power at the same time you nee to figure our your usage per hour. I run my camper off 12V mostly and have a 7s (24V) power shed with buck converters for 12V. When everything I have is running I pull 1000Watts total or 1kw per hour. So a 10kwh bank would last me 10 Hours without any charging input.

So first you need to figure out your loads and what you want to run before you can think about the charging.

in regards to solar since they will be mounted on the roof flat your efficiency will drop and you might see 4500w in perfect conditions. realistically that will fully charge that bank in just over 2 hours of clear sun. Now take away your usage during the day and that time grows.

don't forget to factor in inverter/charger/wiring/panel soiling LOSSES if you want to get as close to actual numbers on your system... It helps with solar to do an insolation survey if you are parking the RV in one place to see actual hours of sun.

Efjay said:

I post on the FB page but this is my first post on here.

I need help checking my math, help understanding draw/charge/ bms and equipment recommendations.

My final PW will be between 14s92p and 14s120p. For the purposes of this I will use 14s120p.

Solar Panels
18 LG 305s
MPP Current (Impp)= 9.28a

What I dont understand.
1. I was taught a charge controllers size was determined by dividing the total wattage of the panels by the voltage of the batteries so in my case 5495w/ 51.8v = 106A so you would need a controller to be rated above that. But after watching Peters videos he seemed a little worried to be charging at 60A and his batteries are much larger.
2. What is the formula I would use to determine how much amperage I am pulling from each individual cell?
3. BMS, I didnt plan on it but I think I see some fairly inexpensive options., well for PC. What about Mac?


Thank you

Click to expand...

1. Your max amp is # of panels * Impp. 9.28*18 = 167.04. YOu don't have to have one charge controller. YOu can have multiple charge controllers. In fact if you have your panels mounted slightly different in certain areas, you will get better output if you have one controller per side. Example one facing the morning sun while another facing the dusk.

2. Per cell = Working Amps/No of cells per pack

Working amps = Watts/51.8V.
No of cells per pack = 120

So for example your car is charging at 240v*32A = 7680W. Your DC amps is 7680W/51.8V = 148.2A.

Each cell is therefore 148.2A/120pcs = 1.23A

But someone check my math please

Charge controllers seem to be rated on two things (apart from the basics iepanel Voc & nominalbatteryvoltage + type)
a) panel input power
b) charger output current
If you exceed either, most controllers will limit to their max rating.
Running at these limits regularly doesnot seem likely to help device life (ie due to worse temperature cycling & component stress, etc)!

With the charge currents (or discharge), be aware ofthe "C" rating, ie if each cell in say 2200mAHr, a "1 C" charge/discharge rate is 2.2A.
For longer pack life you should be keeping this number under approx 0.5C or lower eg 0.2C.
Can you charge or discharge at higher current than this, yes, but experience suggests this willshortencell life.

A BMS will help you see issues before packmajor failure & hopefully keep the pack balanced.
Be sure to usea BMSadequate balancing current for the pack size.

1) Charge controllers are rated by amps, but you need to make sure youre not exceeding the rating on the input OR the output. It also needs to be designed for lithium charging, otherwise itll be charging at the wrong current and/or voltage. Max current will be drawn when the batteries are fully discharged, and at that point the 14S battery will be at ~42V assuming a 3.0V cutoff voltage. At that point, a 5500W array would be providing 131A to the batteries.
2) ((Max Load (W))/(inverter efficiency)/(low voltage))/(number of cells in parallel)=max amp draw per cell
E.g. ((7680W/.8)/42V)/120 = 1.9A draw per cell to charge you car at level 2 when the batteries are almost flat - assuming your inverter is 80% efficient.
3) definitely need a BMS to extend cell life.