question

loifs8 avatar image

Schematic wiring and setup

I’m setting up a small system for a shed using a mppt 100/20 and will use a small inverter 375w on a 24 volt li-ion 1.5kwh battery. Does my schematic look acceptable with fuses/breakers/battery protects/wire gage size? I will probably expand at a future date to larger components and more panels.

battery chargingBattery Protectwiring
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I should add I plan to use the 2 battery protects as a high and low voltage cutout along with the ve.direct inverter to limit low voltage. High cut will Be ~24 and low will be 19, inverter about 19.8-20v, similar setting for charge to 23.9-24v

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Paul B answered ·

As a suggestion I would consider putting the panels into series thus the PV voltage will raise to 88 volts, (much better charge early mornings, late evenings and heavy cloudy days etc) Do not go over the max voltage of 100v on the MPPT unit.

Re BMV Alarms for the whole battery, I would set the BMV to have low voltage pre waring of 25V and Hi Voltage Pre warning of 29.5V

if you are going to use the Time To Go function on the BMV then make sure you set the relay control setting for SOC to OFF or change it to 20% (current default is 50% ) as the TTG is based on this setting and bank capacity. otherwise TTG will always be reading LOW

sometimes I set my battery capacity to the usable level your capacity is about 60A/H so set this to say 50 A/H so that when the SOC reaches Zero you still have 10% Buffer left. if you do this then the Relay setting for SOC also change from 50% to 0 (or OFF)


you should have a cell level BMS for the batteries as well, This is highly recommended (plus as time goes on you will learn why they are so important and then as you expand you wont have expensive battery issues)


if the batteries are lithium phosphate then HV cutoff 30 volts

LV Cutoff 20 to 22


also if any one cell voltage reduces below 2.5v then cut off or above 3.8 to 4 volts also to cut off

the cell level control is quite important in my opinion.

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Thanks. The battery cells will have a low of 3 and high of 4.15 to get 18 to 24.9. I planned to charge to 24 and cutout at 19 to 19.5v, I will be balancing as needed manually as I don’t havea bms yet (Nissan leaf battery).

Panels in series would exceed 100v at low temps (-5 to -10) to get to 100.5 to 105 or so per the spreadsheet calculator

I could always switch the system down at low temps, especially since I shouldn’t charge below that anyway. Parallel should always be above battery voltage as well to allow the mppt operation

Thats Fine

Thats why I said Lithium Phosphate, as these cells are 3.2 volts

The batteries you are using will all have different settings to the LP batteries

keep the BMV low voltage wanings working and keep the manual balancing occurring once every few mths.


enjoy the learning curve


and as you learn more or even now please always contribute to the forum, even if you are not 100% sure everyone here is learning as well