Does Li-Ion Battery for Home Inverter Application Make Sense?

 Does Li-Ion Battery for Home Inverter Application Make Sense?

In my hometown in Southern India, many people don’t give much thought to their inverter systems. For instance, my grandfather could call a local vendor, and within an hour, an inverter system would be installed for around Rs. 20,000—no questions asked about battery capacity or brand. This is a typical scenario, and while I'm not here to focus on inverter sizing or brand selection, I’ll dive into whether Li-Ion batteries make sense for home and office spaces, particularly in terms of economics and implementation by doing it.

Techno-Commercial Review: Does the Cost Make Sense?

Having worked in the battery domain for nearly three years, I built a 24V, 90Ah LFP battery pack for this comparison. While the pack's size was influenced by packaging, MOQ, and budget constraints, the focus here is on understanding the cost-benefit tradeoff between Li-Ion and lead-acid batteries for home inverter or small office applications.

I here compare the battery pack I made with LFP cells with a standard go to lead acid battery in market. 

Table 1: Battery Specifications
	LFP battery pack	Lead acid
Make	DIY 25.6V, 90Ah Gotion LFP battery pack	Luminous LPTT12150H 150Ah Tall Tubular Solar Battery
Actual Parameters
Battery pack voltage (nominal)	25.6	12
Battery pack nominal energy (Wh)	2300	1800
Capacity @ C/3 (Ah)	86	119
Battery pack weight	16	45
Market Cost	Rs. 38540	Rs. 16800 (Amazon, 25% discounted price)
Relative Comparison (this makes sense for comparing two distinct product)
Capacity retention @ C/3 (%)	96	79
Lifecycle (for 90% DoD)	1000 cycles	270 cycles
Cost per kWh	Rs. 16960	Rs. 9340
Calendar degradation (80% capacity retention)	8 years	6 years
Self-discharge (per month)	<1%	3%
Maintenance	Zero maintenance	3-6 months
Energy Density (Wh/kg)	144	40
preferred DoD (%)	90	50
Wh efficiency (%)	Above 95% (5% lost as heat energy)	Above 85% (15% lost)
Charge time	2 hrs	6hrs to 15 hrs
data source	Tested data	http://www.iuc.res.in/tenders/Specification_150AH_30.pdf

 

The Lead acid battery specifications are very limited. I have considered the most optimistic values for both the cases (which are not available). You can comment on your views on the same for me to reconsider.

Understanding the Numbers in Practice:

Let's now apply these figures to a practical scenario where we experience:

• 20 full-day (9 AM to 5 PM) power cuts annually
• 50 two-hour power cuts annually

Average Power Consumption:

My average power consumption is 2 AC Induction Fan (consuming typically around 80W per fan, Note: a BLDC fan consumes 20-30 watts for the same speed which I recommend users) and 2 LED tubelights (20W X 2)

Table: Average power consumption
	Power consumption	Qty	Net Power
AC Fan	80W	2	160W
DC tubelight	20W	2	40W
	Net Power consumption		200W

 

With case 1, for 8hrs I need 200W*8hrs equivalent to 1600Wh of energy. I would like you to concentrate on the relative comparison numbers and with that I’m making a 2000Wh battery pack. (just to understand the case study better)

Lets break the problem, I’ll solve for case 1 and case 2 individually. All we are concerned about is the degradation in each case, the operational years to reach ~80% capacity retention and the cost spent.

I assume both the battery to be a 24V system, again all these calculations are for steady state conditions with nominal values considered/assumed in the most optimistic manner.

Assumed battery pack (Imaginary) for purpose of case study	Li-Ion	Lead Acid
Battery Nominal Voltage	24	24
Battery Nominal Capacity (C/20)	83	83
Wh Efficiency (%)	95	85
Inverter Efficiency (%)	90	90

The system efficiency (battery + Inverter) is considered with inverter efficiency being the same for both (so the study makes sense) and battery efficiency according to datasheet/relative comparison in Table 1.

	Li-Ion	Lead Acid	difference
system Efficiency (%) (battery + Inverter)	85.5	76.5	9% (Li-Ion better)

With this efficiency, to achieve a 200W of power, the current requirement is,

	Li-Ion	Lead Acid
Current requirements (for 200W)	9.7	10.9
Current requirement in terms of C-rate	0.1C or C/10	0.1C or C/10

For the required current, the deliverable capacity by battery is,

Battery Capacity (C/10)	81.34	74.7	difference
Usable battery Energy at required current (Wh)	1952.16	1792.8	8% (Li-Ion better)

The usable battery energy is battery capacity at required current, times the nominal voltage. Till here, a clear distinction between two systems can be seen. Out of 2000Wh, ~98% can be used in Li-Ion system whereas only 90% is usable in lead acid system.

With the usable capacity, the maximum achievable runtime in hrs for 200W of power requirement is computed.

	Li-Ion	Lead Acid
runtime (hrs)	10	9
Required DoD to run 8 hrs (%)	82	89

Also, the Depth of Discharge (%) for 8hr runtime is computed. DoD is a critical parameter in deciding the battery lifecycle.

For reference, as per datasheet,

At 100% DoD and standard operating conditions, Li-Ion cell linearly degrades from 100% to 80% capacity retention.

For Lead Acid,

Cycle count	Depth of Discharge (%)
1200 Cycles	30% DoD
500 Cycles	50% DoD
250 cycles	100% DoD

Its very clear that higher DoD is not preferred for lead-acid batteries, clearly stating its unsuitability for solar/off-grid applications.

	Li-Ion	Lead Acid
Lifecycle of battery pack at required DoD	1000	300
Degradation per year due to case 1	2%	7%

In similar way, case 2 is solved

	Li-Ion	Lead Acid
Required DoD to run 2 hrs (%)	20	22
Lifecycle of battery pack at required DoD	1500	1000
Degradation per year due to case 2	3.33%	4%

One key factor to consider is the calendar degradation.

	Li-Ion	Lead Acid
Calendar degradation per year	2.50%	3.33%
With case 1 and 2 happening each year along with calendar degradation, to reach 80% degradation, the years taken	2.55	1.44

It takes 2.55 years for a 2000Wh Li-Ion battery to reach 80% capacity retention for the specified case study. But the same happens in 1.44 years or 13 months early in lead-acid battery. When projected, we see a clear distinction.

Okay, let’s visualize yearly cost,

 

2kWh battery cost
	Li-Ion	Lead Acid
Cost spent in 3 years	Rs. 39839/-	Rs. 39031/-
Cost spent in 6 years	Rs. 79678/-	Rs. 78063/-
Cost spent in 10 years	Rs. 132796.8/-	Rs. 130106/-

The case study clearly states that the operational and long-term cost is similar for both meeting a 1:1 cost ratio. With Li-Ion there is an option to use solar and cycle it more efficiently that the lead acid batteries.

Now we see the cost is the same irrespective of battery type but why choose Li-Ion,

1.     Better lifecycle. Although I have chosen optimistic numbers (~1000 cycles for LFP), it usually have more lifecycles (>2000 cycles) based on the cell purchased.

2.     No maintenance, which is crucial especially for home applications where we usually put the UPS on the terrace and never think about it. In such cases there is basically no loss in cycles or performance for LFP battery but may not be the case for lead acid batteries.

3.     Higher DoD, this is a key requirement if you are using off-grid systems where batteries are cycled at higher DoD. Lead-acid battery lifecycle exponentially degrades with higher DoD.

4.     High currents, again a key parameter for off-grid systems where power requirements are not constant. Sudden surge in power demand is easily handled by LFP battery.

5.     Self Discharge and system Efficiency, which matters in long term as more energy is consumed/wasted in a lead-acid based system compared to Li-Ion.

For the same cost, you get better performance with LFP battery. The Li-Ion here represents the LFP battery which is well suited for grid applications.

Just a quick comparison on point 5:

	Li-Ion	Lead Acid
Self-discharge (per month)	<1%	<3%
Wh efficiency (%)	Above 95% (5% lost as heat energy)	Above 85% (15% lost)

If I cycle the battery daily,

	Li-Ion
Per day	5%*2000 = 100W	15%*2000 = 300W
Per Month	30*100 = 3000W or 3kW	30*300 = 9000W or 9kW
Energy cost (Rs.10 per unit)	Rs. 3 per month	Rs. 9 per month

Does not seem much but matters with overall EB tariff. 

The second half of the blog is the battery pack, Fabrication and Implementation

This half might be slightly boring for techno-commercial professionals... I have kept it short with more visual representations.

Battery Pack Specifications:

Parameter	Value
Battery Chemistry	Lithium Iron Phosphate (LFP)
Nominal Energy (Theoretical)	2300 Wh
Nominal Voltage	25.6V
Nominal Capacity	90Ah or 86Ah at C/3 (tested)
Battery Configuration	6p8s
Cycle Life	1000+ cycles
Operating Temperature Range	15°C to 50°C
Battery Pack dimension
Weight	~16 Kg
BMS	BMS4Academia Master and 6s slave X 2

 

1.     Cell

It all starts with the cell. LFP remains the best chemistry for grid applications as of today. While evolving, it is also widely used in EVs. I chose the Gotion 33140 LFP cell, which is used in the TATA Nexon EV. Gotion (also known as Guoxuan) is one of the leading cell manufacturers.

 

Cell Parameters (as per datasheet)
Cell type	LFP
Model	Gotion 33140
Nominal Capacity	15 Ah (1C)
Nominal Voltage	3.2V
Energy	48Wh
Maximum charge voltage	3.65V
Discharge cut-off voltage	2.0V
Continuous Discharge current	15A
Maximum Discharge current
Cycle life	2000+ (80% capacity retention)
Weight	~268g
Energy Density	185 Wh/kg
Internal Resistance (1000Hz)	1.5 mohm <= Rint <= 3 mOhm
DC resistance	<8mOhm at 50% SoC, 25°C
Operating temperature (Charge)	-20°C to 55°C
Operating temperature (Discharge)	-30°C to 60°C
Calendar Life	8 years

Cell testing is performed to validate the datasheet,

Test conditions:

	Cell testing parameters
	Cell voltage upper limit	3.65V
	Cell voltage lower limit	2.0V
	Cell Charge Current	5A (C/3)
	Cell discharge Current	5A (C/3)
	Taper cutoff current	0.5A (C/10)
	Cell temperature	~24 – 27°C

 

Capacity Test Result:

Cycle No.	Chg. Cap (Ah)	DChg. Cap (Ah)	Coulombic Efficiency (%)	Cell surface Temp (max)	Cell surface Temp (min)
1	15.1048	15.0970	99.94%	31.4	27.6

With cell tested for its specifications as per datasheet, cells are sorted to form the battery pack. For cell sorting, all cells must be in similar voltage and temperature range. Impedance measurement is taken at 1000 Hz frequency, and the cells are arranged as per impedance data in ascending order.

Figure: Cell sorting data by 1kHz. Impedance measurement

Not all cells can be tested for capacity. Impedance is a quick way of ensuring cell status. As per datasheet all cells are within the 1kHz impedance limits. Also, Impedance testing at 1kHz takes only few minutes (for all cells) whereas a C/3 capacity test would take 7-8 hrs. for one cell. Out of 54 cells, 48 cells are chosen based on impedance data.

The lowest impedance cell starts at negative filling the parallel, then moves to the next series. While sorting there was a small blunder in cell numbering (cell at position 34 should be in position 11) which caused the anomaly in the above plot. This won’t affect the battery pack.

Figure: Cell arrangement based on Impedance data

Cells placed on cell holder as per impedance data	Complete arrangement as per impedance data

The cells are arranged in cell holder as per the impedance data.

During arrangement, thermistors are placed at appropriate positions on the cell.

Thermistor taped to cell surface using kapton tape	10K Thermistor

 

Once arranged, the cells are welded with busbar as per the required series and parallel combination (no separate image for busbar)

I went with a thick nickel busbar (0.3mm X 27mm width). The H busbar was made separately with nickel strips. Once done, they were spot welded onto the cell. For end busbar, the current requirements where high, so a nickel (0.3mm X 27mm width) + Aluminum (4mm thickness X 20mm width) combination was opted. The aluminum and nickel busbar were laser welded followed by spot welding onto the cell. In the whole project this was the most critical and time consuming part. 

 

Pack testing without BMS after complete welding process

Once the welding process is complete, the battery pack is tested to ensure proper welding of series and parallel connections.

The test conditions for pack testing were:

Parameter	Value
Battery pack Charge current	C/3 or 30A
Battery pack Taper cutoff current	C/10 or 3A
Battery pack discharge current	C/3 or 30A (cycle 1), 0.55C or 50A (cycle 2)
Battery pack lower voltage cutoff limit	23.2V
Battery pack upper voltage cutoff limit	28V
Cell lower voltage cutoff limit	2.9V
Cell upper voltage cutoff limit	3.5V

Two cycles were performed:

Cycle 1	0.3C CCCV CHG	0.3C CC DHG
Cycle 2	0.3C CCCV CHG	0.55C CC DHG

 

Cycle 1 (C/3 CCCV CHG and CC DHG) and Cycle 2 (C/3 CCCV CHG) test data

 

Cycle No.	C-Rate	Pack Charging Capacity (Ah)	Pack Discharging Capacity (Ah)	Pack Efficiency(%)	Pack Chg Energy	Pack Dischg Energy
1	0.3C Chg & Dischg	87.6349	86.1462	98.30	2396.65	2186.07
2	0.3C Chg	87.0443			2367.72
2	0.5C Dischg		86.1583			2170.95

Cycle No.	C-Rate	Cell Voltage(V)			Voltage difference
		Maximum	Minimum	Average	Maximum	Minimum	Average
1	0.3C Chg & Dischg	3.52V	2.91V	3.30V	76.8mV	1.10mV	13.74mV
2	0.3C Chg
2	0.5C Dischg	3.52V	2.91V	3.21V	66.60mV	7.90mV	20.71mV
CycleNo.	C-Rate	Temperature			Temperature Difference
		Maximum	Minimum	Average	Maximum	Minimum	Average
1	0.3C Chg & Dischg	43	24.5	33.8	8.7	0.3	3.6
2	0.3C Chg
2	0.5C Dischg	45.3	29.3	39.1	8.7	2.9	4.26

 

Once the battery pack is tested and validated for its welding and performance, further assembling is carried out.

Side delrin sheets are attached with long M4 bolts, Hall sensor mounted on negative busbar	BMS assembled (1X BMS4Academia MASTER & 2X 6s slave (4s used in each)). On negative busbar – 1X prepath relay, 1X prepath resistor, 1X mainpath contactor, 1X Hall sensor On positive busbar – 1X 150A fuse
Side view	Ropes added for easy removal from casing
Top view with all Electronics in place
Labelled

 

Finished Battery Pack
SBX75 Anderson connector	Power switch and WIFI antenna

 

The firmware for the battery pack was developed inhouse with current calibration done for the current sensor. 

The battery pack has WIFI capabilities and connected with Thingspeak.

The BOM:

S.No	Component	Description	Cost per piece	Total Qty	Net Cost	UoM
	Electrical parts
1	Li-Ion cell	Gotion 15Ah LFP cell	340	48	19257.6	Nos
2	Master BMS	BMS4Academia Master BMS	5440	1	5440	Nos
3	Slave BMS	BMS4Academia Slave BMS	2782	1	2782	Nos
4	Busbar	Nickel H busbar, 0.3mm x 27mm, 8 meters	240	8	1920	Mtrs
5	Current sensor	Electroohms HL050T05-CB10 50A hall effect current sensor	1800	1	2124	Nos
6	Contactor	Main path contactor,	1500	1	1770	Nos
7	precharge resistor	50 Ohm 10W wirewound resistor, through-hole	20	1	23.6	Nos
8	Relay	prepath relay, 12V coil, 230V 5 pin relay module	30	1	35.4	Nos
9	Fuse	150A, 32V MIDI Littlefuse	130	1	153.4	Nos
10	Thermistor	10K thermistor	2.5	1	2.95	Nos
11	End Busbar	Aluminium end busbar, positive and negative	160	2	377.6	Nos
12	DC-DC buck converter	LM2576 DC-DC buck converter	40	1	40	Nos
13	Wire + heatsleeve		150	1	150
14	Power switch	low voltage power switch	10	1	10	Nos
15	Wifi Antenna	MT76813DBI ESP8266 Serial WIFI wireless Gain Antenna	60	1	60	Nos
	Mechanical parts
16	Cell holder	33140 Cell holders 2P	4	96	384	Nos
17	Delrin sheet	4mm thick delrin sheet	N/A
18	TVS M4 long bolt	M4 TVS 160mm bolt	N/A
19	M4 Lock nut	M4 lock nut	N/A
20	M3 standoff		N/A
21	M3 brass insert	M3 X 6mm Brass heat set threaded insert	N/A
22	Mechanical Enclosure	Complete enclosure	4000	1	4000	Nos
			Net Cost		38530

 

The Battery pack is now operational at my home with a UTL GAMMA+ 2300 solar Inverter supported for Li-Ion battery.

This is not a one-man work; I thank the following people who volunteered this project:

1.     Ajith, Cell welding expert.

2.     Giri Kumar Ji, expert in battery pack fabrication and welding.

3.     Naresh, cell and battery pack testing and data analytics

4.     Parthasarathi anna, Facilitator

5.     Shubham Gupta, Battery pack design expert.

6.     Vinoth, cell and battery pack testing and data analytics

I thanks the centre for their support. 

Some references:

1.     https://github.com/FarhanAhamed007/BMS4Academia/tree/main

2.     https://www.amazon.in/Luminous-12150H-Solar-Battery-Multicolour/dp/B075SRCS7V?th=1

3.     https://gpowerups.com/batteries/exide-batteries/FE04-EP150-12-TDS-Rev-4.pdf

4.     http://www.iuc.res.in/tenders/Specification_150AH_30.pdf (used in this study)