Build your own battery (DIY battery kit)

There is a lot of myths in respect of batteries and it when you are in the industry and have had hands on experience, you soon see that mines bigger than yours or better, is what they are all saying, but when it comes down to it, there all the same.
We always have issues selling batteries because we are not for profit, so we have low price batteries and the “too good to be true” comes into minds of the weak and so we sell less of the lower priced batteries. But when it comes to DIY batteries we come to moral impasses.

Hardware and the components to build batteries is one of the problems as it does cost as much or more to buy the individual items as it does to buy a retail battery. So you have to ask yourself if it is worth building your own battery?
Like for like, RenewSolar is around £1000 less than many other “select batteries” for the same hardware. So as we are not over charging for the hardware and our batteries, the prices are pretty much the same.

Building your battery

Content:
Battery tower case
BMS
LFP Cells
Cell Bus bars
PCB and cables.

Case

it maybe just me, who happens to know that the battery case is over priced in particular rack cases after all a server rack case regardless if is it a battery or not so im not convinced in the prices that are charged for the cases. The highlighted image at the top is a vertical battery, and as such you pay for wheels and they are more of a custom design that the typical server case come battery case. With some of the batteries that we sell, there are clear price differences in the ABS cases, the metal boxes, and the “proper battery” cases, and so you have the looks and the practicality of the case.

Cases and BMS can go hand in hand, with the different brands of BMS they have different screw locations, so you case will have to match the BMS, This applies more to proper battery cases, as ABS cases or metal boxes tend to have the BMS attached to the inside somewhere.
If you have a battery already and want to add more, then the selection of a BMS is more important, as the BMS communicate with each other. This is typically backwards compatibility, so the old master battery will now be the slave.

What is interesting is why a BMS, you see a BMS should manage the battery, as a stand alone, therefore all the communications is rather pointless.

AI – Said this is why a BMS needs to talk to an inverter. But this is what a BMS should do regardless.

The communication between a Battery Management System (BMS) and an inverter is crucial for the safe, efficient, and optimal operation of battery-based energy systems. Here’s a breakdown of the key reasons why this communication is essential:  

1. Enhanced Battery Protection:

• Preventing Overcharge and Over-Discharge: The BMS constantly monitors the voltage of individual cells and the entire battery pack. By communicating this data to the inverter, the inverter can adjust or halt charging and discharging processes to prevent overcharging (which can lead to safety risks and reduced lifespan) or over-discharging (which can also damage the battery).  
• Temperature Management: The BMS monitors the temperature of the battery. If the temperature goes outside the safe operating range, the BMS can inform the inverter to reduce or stop charging/discharging, preventing thermal runaway and extending battery life.  
• Current Control: The BMS measures the charge and discharge current. It can signal the inverter to limit the current within safe limits, preventing damage from excessive current draw or during charging.  

2. Optimized Battery Performance and Lifespan:

• Accurate State of Charge (SOC) Management: The BMS precisely tracks the battery’s SOC. By sharing this information with the inverter, the inverter can optimize charging and discharging algorithms to maximize efficiency and prevent unnecessary stress on the battery, thus prolonging its lifespan. Without this communication, the inverter would have to rely on less accurate voltage-based estimations of SOC.
• Cell Balancing: The BMS ensures that all cells within a battery pack are equally charged. It can communicate the need for balancing to the inverter (or handle it internally and inform the inverter of the status), which is vital for maintaining the overall capacity and health of the battery pack.  
• State of Health (SOH) Monitoring: Advanced BMS systems can estimate the battery’s SOH (remaining capacity compared to its original capacity). Sharing this data with the inverter can allow the system to adapt its operation as the battery ages, optimizing its use over time.  

3. System Efficiency and Safety:

• Dynamic Adjustments: Real-time communication allows the inverter to dynamically adjust its operation based on the battery’s current state. For example, it can optimize charging rates based on temperature or reduce power output if the battery is nearing its discharge limit.  
• Fault Detection and Response: The BMS can detect various faults within the battery (e.g., cell imbalance, high temperature, overcurrent) and immediately communicate these to the inverter. The inverter can then take appropriate actions, such as shutting down the system to prevent further damage or safety hazards.  
• Single Point of Monitoring: With BMS communication, users can often monitor the entire system (battery status, inverter output, etc.) from a single interface provided by the inverter or a connected monitoring system.

4. Advanced System Features:

• Specific Battery Charge Profiles: Different battery chemistries have specific charging requirements. When the BMS communicates the battery type and its real-time status, the inverter can utilize the most appropriate charging profile, ensuring optimal charging and extending battery life.  
• Multi-Battery Systems: In systems with multiple batteries, BMS communication is essential for coordinating their operation, ensuring balanced charging and discharging, and providing a unified view of the entire battery bank.

In essence, the communication between the BMS and the inverter creates a “smart” energy storage system where the components work together intelligently to maximize performance, safety, and longevity of the battery while providing reliable power. Without this communication, the inverter would be “blind” to the detailed status and health of the battery, leading to suboptimal operation and potentially compromising the battery’s lifespan and safety.

In reality and from first hand experience. I can say that there are only two things,
Current balance: Where the BMS and inverter balances or shifts the load and charge in and out of a battery pack within a battery bank (more than one battery).

The OVP OCP OTP are all individual to the battery itself as a stand alone even with balance function ( charge)
This balance I mention above does away with the need to pack balance as often the case is that you can have cell drift when the batteries are not maintained ( AKA held voltage for absorption and balance).

Putting the battery together is fairly simple, as you can see its + to – all the way though the pack and then at the end( top) you will have a + and – terminal which is connected to the terminal and BMS.
You will install the Balance cables, as you can see in the above image the BMS is a Daly, with the 2a Active battery balancer and coms module, which give you the com ports ( left side) which is used to talk to the inverter and other batteries.
– Note our cases battery bars do have an optional BMS for the balance leads.

Work Safe.
DC batteries and complete battery packs can discharge a massive amount of current – heat. This means that a short, by accident or careless tool placement can lead to critical injury. While building the first battery you should take care when handling wires and making connections that you are not creating a short circuit. Equally tools can make contact, fall on the battery and create a short circuit and this is the big danger.
Once the battery is together they do become less dangerous. but you should always take care in building a battery.

BALANCE and PREP.
I see so many posts about ” how do I balance my new cells?”
You can align all the cells and connect all the + together, and connect all the – together, to create a big parallel connected battery.
This will Level the voltage between the batteries – IT DOES NOT balance the capacity.

Let’s break down cell voltage and cell capacity, and then address why you can have different capacities even with the same voltage in parallel cells.

Cell Voltage:

• Definition: Cell voltage is the electrical potential difference between the positive and negative terminals of a single battery cell. It’s the “push” or “force” that drives electric current.  
• Characteristic: The nominal voltage of a cell is primarily determined by its chemistry. For example, a typical lithium-ion cell has a nominal voltage of around 3.7V, while a lead-acid cell is around 2V, and an alkaline cell is around 1.5V.  
• In Parallel: When cells are connected in parallel, their voltages remain the same. The overall voltage of the parallel combination is equal to the voltage of a single cell. Think of it like multiple pipes connected to the same water source; the water pressure (voltage) in all pipes is the same.  

Cell Capacity:

• Definition: Cell capacity is a measure of the total amount of electrical charge a cell can store and deliver over time. It’s typically measured in Ampere-hours (Ah) or milliampere-hours (mAh). A higher capacity means the cell can supply a certain current for a longer duration.  
• Characteristic: The capacity of a cell is determined by the amount of active materials within the cell and its design. A physically larger cell with more electrode material will generally have a higher capacity than a smaller cell of the same chemistry.  
• In Parallel: When cells are connected in parallel, their capacities add up. The total capacity of the parallel combination is the sum of the individual cell capacities. Using the water pipe analogy, parallel connections increase the total volume of water that can be delivered.  

Why Different Capacities with the Same Voltage in Parallel?

If you have four cells connected in parallel, and they all exhibit the same voltage, it means they are likely of the same chemistry and are at a similar state of charge at that moment. However, their capacities can be different due to variations in their:

1. Physical Size and Design: Even if the cells are of the same chemistry, they might be physically different sizes or have internal design variations that allow one to store more active material than another. A larger cell will typically have a higher capacity.
2. Manufacturing Tolerances: During the manufacturing process, there will always be slight variations in the amount and quality of the active materials used in each cell. These small differences accumulate and can result in minor variations in capacity between cells from the same production line.
3. Age and Usage History: Even if the cells started with the same capacity, their capacity degrades over time and with usage (charge/discharge cycles). If the four parallel cells have experienced different usage patterns or are of different ages, they will likely have different remaining capacities, even if their current voltages are the same. A cell that has undergone more cycles or has been subjected to harsher conditions might have a lower capacity than a newer or less stressed cell.
4. Internal Resistance Variations: While the voltage might be the same, subtle differences in the internal resistance of each cell can also indirectly lead to different effective capacities under load. A cell with lower internal resistance might be able to deliver more current for a longer period before its voltage drops significantly.

In Summary:

• Voltage is primarily determined by the cell’s chemistry and its instantaneous state of charge. In a parallel connection, voltage is equal across all cells.  
• Capacity is determined by the amount of active material and the cell’s design. In a parallel connection, the total capacity is the sum of the individual capacities.  

Therefore, you can have cells in parallel with the same voltage because they are of the same chemistry and have a similar state of charge at that point in time. However, their capacities can differ due to variations in their physical size, manufacturing tolerances, age, usage history, and internal resistance. LFP capacity by voltage, is difficult as the voltage curve is flat, therefore a 0.1v difference could be 40% capacity difference.

To charge the battery, simply apply the voltage and add current until the current level drops, this means that the current ( chemistry absorbing) has leveled out between the actual cells.

You cannot find a charger?
We make them, The most common charger is our 8 amp charger as it is very cost effective. check out the shop.
You can also charge cells with a higher voltage by connection them in series, and using an active balancer. This is often the faster way to charge the cells and “level them up”. In this respect we use a 700w charger.

Our balance charger is for 4s or 8s, with a 47amp charge and 5 amp balance.
If you have 280ah cells your total balance time will be 5 hours. Simply plug in the charger to your AC supply, connect the + and – and then the balance leads and away you go. the display will show you the voltage and current passing to the battery as it charges, just wait for the current to drop to 0.5 (or less) and then the cells will be balanced.
As you are group charging, ensure that the current drop is equal between the groups.