DIY LiFePO4 Battery Pack

In the past few years, the cost of solar panels are decreasing drastically but the overall cost of the Off-Grid solar system is still significant. The cost of the traditionally used Lead-Acid battery and their limited lifespan compared to solar modules (25+ years) increases the total cost of the whole system. So, If you are planning to install new solar panels for your home or office, it is very important to select the right battery for your system. You need battery solutions that have greater capacity, a high power potential, a longer lifespan, are sustainable, safe, and fit into your needs.

Lithium-ion batteries have become a go-to option for energy storage in solar systems, but technology has advanced, a new winner in the race for energy storage solutions has emerged: lithium iron phosphate batteries (LiFePO4). There are many advantages of the LiFePo4 battery over traditional Lead-acid batteries which are described in detail in the next step.

In this Instructable, I will show you, how to make a LiFePO4 Battery Pack for applications like Off-Grid Solar System, Solar Generator, Electric Vehicle, Power wall, etc. The fundamental is very simple: Just to combined the number of LiFePo4 cells in series and parallel to make a bigger pack and finally to ensure safety by adding a BMS to it. The LiFePo4 cells come in a variety of sizes, but here I have used the 32650 type.

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Disclaimer:

I cannot be held responsible for any loss of property, damage, or loss of life if it comes to that. This tutorial was written for those who have knowledge of rechargeable lithium battery technology. Please do not attempt this if you are a novice. Stay Safe.

Materials Used:

1. LiFePo4 Cells ( Amazon )

2. Cell Holder ( Amazon )

3. Nickel Strips ( Amazon / Banggood )

4. BMS ( Banggood )

5. Cables ( Amazon / Banggood )

6. PVC Heatshrink Wrap ( Amazon / Banggood )

7. Heatshrink Tube ( Amazon / Banggood )

8. Barley Paper ( Aliexpress / Banggood )

9. Fiber Glass Tape ( Amazon )

10. Kapton Tape ( Amazon / Banggood )

11. Double Sided Tape ( Amazon / Banggood )

12. XT60 Connector ( Amazon / Banggood )

Tools Used:

1. Spot Welder ( Banggood )

2. Spot Welding Pen ( Banggood )

3. Soldering Iron ( Amazon )

4. Wire Cutter ( Amazon )

5. Wire Stripper ( Banggood )

6. Hot Air Blower ( Amazon )

7. Multimeter ( Amazon )

Constant Power Delivery:

A major difference between LiFePO4 batteries and lead-acid batteries is that the Lithium Iron Phosphate battery capacity is independent of the discharge rate. It can constantly deliver the same amount of power throughout its discharge cycle. However, for lead-acid batteries, the rated capacity decreases with an increase in discharge rate.

Life Cycle:

Lithium batteries also have a longer cycle life than lead-acid batteries. LiFePO4 batteries can also last a very long time. Good quality batteries are rated around 3000 cycles, at a full 100% charge/discharge cycle. If you did that every day it makes for over 8 years of cycling! They last even longer when used in less-than-100% cycles, in fact for simplicity you can use a linear relationship: 50% discharge cycles mean twice the cycles, 33% discharge cycles and you can reasonably expect three times the cycles.

Charging Time:

In the case of LiFePo4 batteries charging is four times faster than lead-acid batteries. Therefore less time to charge and more time for battery usage. The battery gets charged 100% in just 2-4 hours.

High / Cold Temperature Performance:

It also got superior high-temperature performance when compared to lead-acid batteries. Lithium batteries perform especially well at high temperatures than Lead-acid batteries. Lithium batteries also have a higher discharge capacity in cold temperatures as well.

Battery Installation:

LiFePo4 can be installed in any position as they don't have any chance of leakage. Whereas for Lead Acid battery's chances of leakage is high.

Weight:

A LiFePO4 battery also weighs less than 1/2 of a lead-acid battery of similar capacity

Batteries may be used in series and or parallel to achieve higher operating voltages and or capacities to achieve the desired requirement for a specific application.

Series Connection:

Connecting Cells in series add the voltage of the two batteries, but it keeps the same amperage rating (also known as Amp-Hours).

Example: Connecting two 3.2V / 6000mAh cells in series will produce 6.4V, but the total capacity remains the same ( 6000mAh ).

Parallel Connection:

Parallel connections will increase your current rating (Amp-Hours ), but the voltage will stay the same. Ultimately you will make a single cell with a higher capacity.

Example: Connecting two 3.2V / 6000mAh cells in parallel will produce 3.2V, but the total capacity will be increased to 12000mAh.

To make the battery pack, you have to first finalize the nominal voltage and capacity of the pack. Either it will be in terms of Volt, mAh/ Ah, or Wh. You have to connect the cells in parallel to reach the desired capacity (mAh ) and connect such parallel group in series to achieve the nominal voltage (Volt ).

For this project let the requirement is: 12.8 V and 42Ah Battery Pack

Specification of 32650 Cells Used: 3.2V and 6000 mAh

Capacity (mAh):

The desired capacity of the battery pack = 42AH or 42000 mAh.

The capacity of each cell = 6000 mAh

No of cells required for parallel connection = 42000 / 6000 = 7 nos

Commonly cells in parallel are abbreviated in terms of ‘P’, so this pack will be known as a “7P pack”.When 7 cells are connected in parallel, ultimately you made a single cell with higher capacity ( i.e 3.2V, 42000 mAh )

Voltage(Volt) :

The desired nominal voltage of the battery pack is 12.8V.

The nominal voltage of each cell = 3.2 V

No of cells required for series connection = 12.8 /3.2 = 4nos

Commonly cells in series are abbreviated in terms of ‘S’, so this pack will be known as a “4S pack”.

So we have to connect the 4 parallel groups (7 cells in each group ) in series to make the battery pack.

The final pack configuration is designated as a “4S7P pack” with a final specification of 12.8V,42AH.

Before connecting the cells in parallel, first, check the individual cell voltages. It is important to use the same battery model with equal voltage and never to mix batteries of different ages. For paralleling the cells, the voltage of each cell should be near to each other, otherwise, a high amount of current will flow from the cell with a higher voltage to the cell with a lower voltage. This can damage the cells and even result in fire on rare occasions.

If you are using brand new cells, the cell voltage is near 3.1 V to 3.2 V, you can join them together without worrying much. But if the cell voltages are different, then you have to charge the cells to the same voltage level by using a good Battery Charger.

If you need more precise segregation, then check the internal resistance of each cell. The cells with the same internal resistance can be used for making the battery pack. You need a testing device or charger to check the internal resistance of the individual cells.

Note: Unlike the weak link in a chain analogy, a weak cell causes stress on the other healthy cells in a battery. Cells in multi-packs must be matched, especially when exposed to high charge and discharge currents.

A weaker cell in series-connected cells would cause an imbalance. This is especially critical in a series configuration because a battery is only as strong as the weakest cell (analogous to the weak link in the chain). A weak cell may not fail immediately but maybe drained more quickly than the strong ones when discharging. On charge, the weak cell may fill up before the healthy ones and be overcharged.

To make the battery pack, you have to connect the LiFePo4 cells together by means of Nickel strips or thick wire. Generally, Nickel strips are widely used for this. In general two types of nickel, strips are available in the market: nickel-plated steel strips and pure nickel strips. I will suggest buying a pure nickel. It is a little bit costlier than nickel-plated steel, but it has much lower resistance. Low resistance means, less heat generation during the charging and discharging, which leads to longer useful battery life.

Nickel strips come with different dimensions and lengths. Choose the strips according to the current rating.

You can assemble the cells to make the pack by using hot glue or by using a plastic 32650 battery holder. I used plastic 32650 cell holders/spacers to assemble the 28 cells. The main advantages of using these cell holders are

1. You can make a custom pack of any size according to your requirement. It's like solving a puzzle.

2. It provides space between the cells, which allows fresh air to pass and the battery gets cooled easily.

3. It makes your battery pack solid and reliable.

4. It provides safety and anti-vibration to your battery pack.

First, arrange the cell holders to make an arrangement to form 4 rows and 7 columns. We have to make 2 such rectangular shape holders, one will be used at the bottom and another one will be used at the top layer.

From the previous step, it is clear that our battery pack is made up of 4 parallel groups connected in series ( 4 x 3.2V = 12.8V ), and each parallel group has 7 cells ( 6000 mAh x 7 = 42000 mAh). Now we have to arrange the 28 cells properly in the battery holder for making the electrical connection among them.

Place the first parallel group of cells (7 nos) in the plastic holder with the positive side up, then place the second parallel group negative side up, and then the third parallel group with the positive side up, and finally the last parallel-group negative side up. For better understanding, you can see the above picture.

After arranging all the cells, place the top battery holder and align the battery slightly to fit it perfectly. Now we can make the electrical connection among the 28 cells.

You have two options two connect the 18650 cells together: 1. Soldering 2. Spot Welding

The best choice is always Spot welding, but Spot Welder is much costlier than a good quality Soldering Iron.

Soldering :

You should know why Spot welding is preferred over soldering, the problem with soldering is that you apply a lot of heat to the cell and it doesn’t dissipate very quickly. This enhances the chemical reaction in the cell which damages the cell's performance. Ultimately you will lose some capacity and life of the cells.

But if you are not interested to buy a costly Spot Welder, you can solder the nickel tabs to the cell by following some precaution and tricks :

1. To minimize the contact time of your soldering iron on the cell, make sure the surface is scuffed up sufficiently and you use plenty of flux to allow for fast solder flow.

2. It is better to have a good quality high wattage ( min 80W ) iron with good thermal capacity so it can deliver the heat to the joint quickly so you don't have to hold the iron to the battery for ages and let the heat seep into it, causing damage to the battery.

Spot Welding :

The reason we spot weld because it securely joins the cells together without adding much heat to them. There are two grades of spot welders currently available in the market: hobby grade and professional grade. A decent hobby-grade Spot welder costs around $200 to $300, whereas a good professional grade may cost around ten times more.

So I will suggest buying a hobby-grade spot welder from any online store like Banggood, Aliexpress, or eBay.I am using the SUNKKO 709A 1.9kw Spot Welder from Banggood.

Now it is time to know the procedure for using the Spot Welder ( I am talking about the Spot welder that I have used in this project). The Spot welder has three welding choices: fixed welding head, fixed welding head with foot switch, movable spot welding pen with the footswitch. I prefer to use the second option. Before welding, you have to prepare the nickel strips and welder.

Successful Welding :

You can check the weld quality by pulling on the nickel strip. If it doesn’t come off with hand pressure or requires a lot of strength, then it’s a good weld. If you can easily peel it off, then you have to increase the current.

Safety: Before starting the spot welding, always wear safety goggles.

A battery management system (BMS) is an electronic system that manages a lithium battery pack and the main functionalities are

1. Monitors all of the parallel groups in the battery pack and disconnect it from the input power source when fully charged

2. Balance all the cells voltage equally

3. Doesn't allow the pack from over-discharged.

BMS takes care of protecting the battery; it disconnects the battery when it is discharged, or threatens to be over-charged. The BMS also takes care of limiting the charge and discharge currents, monitors cell temperature (and curtails charge/discharge if needed), and most will balance the cells each time a full charge is done

The two important parameters required to buy a BMS are: i) Number of cells in series - like 4S/ 6S /8S

ii). Maximum discharge Current - like 20A /30A /50A

For this project, I have used a 4S and 50A BMS board

Connect the BMS as per the wiring diagram shown above.

After soldering the balancing leads and the charging-discharging cables, the cables are scattered all around the battery pack. So, we have to arrange these messy cables properly. I have used Kapton tape to arrange the cables. You can see the above picture.

Once you have installed the BMS, the battery pack is technically ready for service. But in real life, we can't use it in the bare condition to avoid accidental short circuits. Any short circuit in the battery pack may lead to the catching of fire and explosion.

First, add a layer of insulating Barley Paper over the top and bottom side of the battery pack. Barley Paper is pure cellulose with high electrical insulation properties that have made it possible to use them for the making of portable lithium-ion battery packs.

The barley paper comes in a variety of sizes, so you have to order the appropriate width and length as per your battery pack dimension. Using barley paper is very easy because it comes in self-adhesive, so you have to use it just like a sticker. After mounting the barley paper, you must ensure that there is no bare conducting part exposed to the air.

To make a proper brick shape ( to avoid any uneven surface), I have added an extra layer of insulation over the barley paper. I have used a paper shopping bag to wrap the battery pack.

Now you can place the battery pack inside the PVC Heat Shrink sleeve and apply hot air all around the battery pack. After few minutes, you will notice the PVC wrap shrinks to form a solid cube with your battery pack inside.

Lithium Iron Phosphate batteries are charged in two stages: First, the current is kept constant, or with solar PV that generally means that we try and send as much current into the batteries as available from the sun. The Voltage will slowly rise during this time, until it reaches the ‘absorb’ Voltage, 14.6V in the graph above. Once absorb is reached the battery is about 90% full, and to fill it the rest of the way the Voltage is kept constant while the current slowly tapers off. Once the current drops to around 5% – 10% of the Ah rating of the battery it is at 100% State-Of-Charge.

You can refer to the above charging curve for a typical 12.8V LiFeP04 battery pack.

Follow the below points to set your charge controller for charging LiFePO4:

1. Bulk/ Absorb Charge:

You can set the charge controller bulk/absorb setting in between 14.2 and 14.6 Volt will work great for the LiFePO4 battery.

2. Float Charge:

For lead-acid batteries, float charging is required because of its high rate of self-discharge that it makes sense to keep trickling in more charge to keep them always charged. But at the same time, LiFePO4 doesn't require float charging, so if your charge controller cannot disable float, just set it to a low enough Voltage that no actual charging will happen. Any Voltage of 13.6 Volt or less will do the job.

3. Equalize Charge:

No equalize charge is required for the LiFePO4 battery. If equalize stage cannot be disabled from your charge controller, set it to 14.6V or less, so it becomes just a regular absorb charge cycle.

Temperature Compensation:

LiFePO4 batteries do not need temperature compensation! So, you have to switch this off from your charge controller.

Note: If you don't have adequate experience with battery charging, I will highly recommend buying a good charge controller ( EPEVER TRIROn Series ) which has features to charging LiFePO4 battery.

If you are making the battery pack for other than solar applications, then buy a good charger from Aliexpress or Amazon. The rating of the charger shall be as per the battery charging-discharging rate which is found in the datasheet. In general, a 0.5C or half of the Ah capacity charger is safe for charging the battery pack.

Example: For a 42Ah battery, the charging current is 21 Amps.

Hope you enjoyed reading about my project as much as I have enjoyed building it. If you’re thinking about making your own I would encourage you to do so, you will learn a lot. If you have any suggestions for improvements, please comment below.