What is an Active Balancer Module?

An Active Balancer Module (Active Balancer) is a unit which will “Actively” balance the voltage between 2 or more cells connected in series.  When charging/discharging cells connected in series, each cell will behave differently depending on their internal resistance which will result in the being unbalanced.  The more you charge/discharge the battery bank, the State of Charge (SOC) and voltage variance between the cells will increase, resulting in 1 cell having a greater SOC, while another cell within the same bank having a lower SOC.  The problem with this is the potential for the cell with the higher SOC being overcharged, while the cell with the lower SOC being over discharged and therefore those cells experiencing a greater derogation in comparison to the other cells.

An Active Balancer will monitor and transfer current between cells within a bank.  Cells with a higher SOC, to cells with a lower SOC ensuring they are better balanced and drastically reducing the chance of one particular cell in the bank being over charged/discharged.

Unlike a passive balancer, an active balancer will operate while a battery bank is being charged, discharged or even left in storage.

What is a Passive Balancer?

Most BMS’s will have a built-in balancing function, however for the majority of BMS’s currently available this will be a Passive Balancer.  A Passive Balancer will attempt to balance cells within a bank by increasing the resistance to cells with a higher voltage/State of Charge (SOC), allowing for more current to flow to cells with a lower voltage/SOC.  A Passive Balancer only works while the bank is being charged, which means cells within a bank can still fall out of balance while being discharged.

What is a BMS Parallel Module?

The main purpose of a BMS Parallel Module is to regulate the current flow between 2 or more banks of batteries connected in parallel.

When 2 or more banks of batteries are connected in parallel, the State of Charge (SOC) of the different banks will equalise.  That means, they will transfer current from the highest charged bank to the lowest charged bank until they have the same SOC, and therefore have the same voltage.  With LiFePO4 Cells, this transfer of energy can be significant, and the high current flow will most likely be too great for BMS, while also causing damage the LiFePO4 cells.

The charge profile for LiFePO4 cells is relatively flat, that means there can be a large variance in the SOC between 2 or more cells, with a small variance in their comparative voltages.  For example, you may have only 0.01V difference in voltage between 2 cells, however there will be a big variance in the SOC between these cells.

By using a parallel module, if 2 or more banks are connected in parallel, it will restrict the amount of current transferred between the banks, therefore protecting the BMS and the cells from any potential damage.

Some might say that if you have 2 more bank always connected in parallel then they will always be equalised… Although this might be true in principle, practically there are a couple of scenarios which may see 2 banks attempt to equalise, creating a situation of high current transfer, such as:

1. Before initially connecting banks in parallel, you should need to top balance ALL cells which will ensure they have the same SOC otherwise they will equalise when connected in parallel.
2. If you have 2 banks connected in parallel, 1 bank maybe disconnected by the BMS due to one of the protection parameters (like cell voltage variance) being triggered. If this happens, then one bank will continue to provide/receive current and therefore increasing the variance of the SOC between the banks.  When you attempt to reconnect the bank into the parallel circuit, the banks will attempt to rebalance transferring unrestricted current.

By using a Parallel Module with the BMS for each bank, it will govern the amount of current able to be transferred between cells and avoid any potential damage to the BMS/s or cells.

 

How to connect Batteries in Series?

 

Connecting batteries in Series is when you connect the positive terminal of one battery to the negative terminal of another battery.

Connecting batteries in series will increase the combined Voltage of the packs but the capacity will remain unchanged (or lowest capacity of the combined cells).  For example, if you have 4 x 3.2V (Volt) 200ah (amp hour) batteries connected in series, the total capacity will remain to be 200ah, however the voltage will increase to 12.8 Volts (4 x 3.2V).

When connecting cells/batteries in Series, the capacity of each battery should ideally be the same to ensure that one battery/cell in the circuit is not over charged/discharged.

Unfortunately, the answer to this question is not so simple. There are several variables that determine how many LiFePO4 Batteries/cells you would need to be self-sufficient like:

1. What is your energy consumption (watts)?
2. How long would you like to run off battery power before you needed to recharge?
3. What is the voltage of your inverter?
4. Do you have a charging source like Solar Panels or a Wind Generator?

Calculating your energy consumption

Your energy consumption is the total watts (W) of energy your appliances like a fridge, lights, air-conditioner and even your inverter will consume. The consumption from these appliances will vary as 1) not all of them will be running constantly and 2) appliances like Fridges and air-conditioners will use more, or less energy based on temperature conditions.

There are 2 ways in which I recommend you can calculate your energy consumption:

1. List all your appliances that consume energy, noting what their average rated energy consumption is (you should be able to find this information on its Specification sheet/sticker). Then, list the typical time you would run each appliance. With each appliance, multiply the wattage by the estimated time each appliance would be used for and total them up. Here is an example below:

 

From the example above you can see that we would use 5,400 Watts or 5.4 Kilowatts (kW) in the average day. This also means that our average hourly energy consumption is 225Watts per hour (Wh).

For all of you that are now thinking it’s all too hard and want to know an easier way, then see Option 2 or 3 depending on your situation.

2. Off-Grid for a Caravan, camper, motorhome, or RV, you can buy a Wattmeter from Bunnings, Jaycar, or online for around $20. See image below. A Wattmeter will not only tell you to have much energy you’ve consumed but it will also tell you the peak amps drawn which you will also need to take into consideration for selecting an inverter and wiring sizing which will not be covered by this article.

Plug this unit into a Mains PowerPoint (GPO) at home or in a Caravan Park. (Note: you may need to be a little creative if you have a 15A male plug as most of these Wattmeters will have a 10A female input and the bottom earth pin is larger). Once plugged in, run your appliances as you would in a typical day. After a 24-hour period look at the Wattmeter to see the amount of energy in Watts you’ve used.

If you want to set up an Off-Grid House, then go to your Electricity Box and make a note of the reading on your Electricity Meter (See the image below). In 24 hours take another reading from the meter and take the most recent reading minus the initial meter reading to find the total Watts you’ve used in 1 day.

How many LiFePO4 Batteries do I need?

Now that you know how many Watts of energy you’ve consumed in a day, you will now be able to calculate the Battery Capacity you’ll require to be fully off-grid. In the example above we calculated that we would consume approximately 5.4kW of energy in 1 day. Now the capacity of most batteries is advertised in Amp-hours (ah), so we need to convert Kilowatts (kW) into Amp-hours (ah). To do this you need to know what type of inverter you’ll be running. For those that don’t know, an inverter is a device that converts DC power into AC power. In an effort to keep this simple, we will focus on the input Voltage so we can work out how many batteries you’ll need. For people with a caravan, campervan, motorhome, or RV, you’ll usually go for either a 12 Volt or 24 Volt input, but if you’re looking to run an off-grid house you’ll probably look at a 48 Volt or higher input voltage inverter. So, for this example let’s pretend we’re got a 24 Volt 5 kW Inverter.

Now in the above example we know our daily energy consumption is 5.4kW and we know that our inverter requires 24Volts Input Voltage, so we can then calculate the amount of Amp-hours (ah) of capacity we’ll require to be totally off-Grid for 24 hours. To do this, you simply divide the number of Watts consumed by the input voltage of the inverter. So, in the above example 5400W/24Volts = 225 Amp-hours (ah). So, this means you’d need a minimum of a 225ah LiFePO4 Battery pack @ 24Volts to run your appliances if they consumed 5400 watts each day. Note that this does not take into consideration any solar, wind, or charging input to the battery yet. It is also assuming that you’re using 100% of your storage capacity which is not recommended.

To achieve 225ah of battery capacity at 24 Volts, you will need to connect cells up in series and parallel.

Connecting batteries in Series is when you connect the positive terminal to the negative terminal and so on as shown below. Understand that when you connect cells in series it increases the voltage however it keeps the rated capacity the same (eg 8 x 3.2Volts 206ah cells in series = 25.6Volts with 206ah of capacity).

 

Connecting batteries in Parallel is when you connect each of the cells in a way that all the positive terminals are connected, and all the negative terminals are connected as shown below. Understand that when you connect cells in parallel it increases the rated capacity however it keeps the voltage the same (eg 8 x 3.2Volts 206ah cells in Parallel = 3.2Volts with 1648ah of capacity).

 

 

To achieve 225ah of storage you may connect 16 x 130ah 3.2Volts LiFePO4 cells, you would connect 8 cells in series twice, and then connect them in parallel. This is known as a 2S8P configuration and is illustrated below.

 

If each one of the cells above were 130ah and you connected 16 in a 2S8P configuration, you would have 260ah @ 24Volts or 6.24 Kilowatt-hours (kWh) in storage capacity.

What are the average recharging amps available?

Most people that are looking to be off-grid will have Solar as a recharging source, however, you can also consider Wind or even a fuel-powered generator (however I personally would only use a generator as a last resort as it’s not free to power).

Solar can be good however depending on where you live, it can be limited to the number of hours in the day you have to be able to recharge your battery bank, and depending on your energy consumption and storage capacity, you may need a large surface area to be able to generate enough energy to recharge your battery bank.

Wind turbines can also be a great means of recharging your batteries, it’s not really limited to particular hours of the day, however, you obviously need to have wind for it to be useful.

Regardless of if you choose solar, wind, or a fuel generator, you should now consider what recharging amps will be available when considering how many LiFePO4 Batteries you’ll need to be off-grid.

In the example above we know we need 225ah @ 24Volts to be able to run our appliances with our 24Volts 5kw inverter for a 24-hour period. However, let’s now consider that we have some solar panels which can recharge our batteries during the day. Again, to try and keep this simple, let’s pretend that we have solar panels that can generate on average 30amps @ 24Volts of charging current 8 hours per day. This means that in the 8 hours period we would be able to recharge our batteries with 240amps.

Now because we are now generating power to recharge our batteries for an average of 8 hours per day, it means that during the day we are not using the stored battery power. As such, the 225ah of storage capacity required to be Off-Grid for a 24-hour period can now be reduced. If we are generating solar power for 8 hours per day, then it means that we need to run off batteries for the other 16 hours. So this means that we would now only need 3600W (16 hours x 225Wh) of storage capacity rather than the 5400W initially calculated. It also means that we can reduce our battery bank down to 150ah of storage capacity.

As a rule of thumb, it’s recommended that you don’t use more than 80% of your LifePO4 Cells capacity, so 195ah would be enough capacity to run off Grid for 24 hours while recharging the LiFePO4 cells during the day.As such, our 24Volts 200ah Battery Kit would be a perfect fit for this setup which can be seen here:
https://lifepo4oz.com/products/25-6v-206ah-lifepo4-battery-pack-kit-with-200a-deligreen-smart-bms

 

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There are so many options for off-grid battery systems, solar batteries, power, and storage. Choosing the right off-grid solar system for you will depend on your energy consumption, location, and period that you're looking to operate before needing to charge.

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One thing that we have not talked about here, that is cost. Although, 10kW off grid solar system price will be between $10,000 and $15,000. The lithium off grid solar power batteries will be a bit more expensive. Additionally, there are many things that homeowners thinking about going off-grid should take into consideration.

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