Before we begin, let me tell you why this is the main photo:
This is a battery pack built by EVE with laser-welded poles, true grade A cells, superb compression plates and insulating foils.
I take it as an example of the highest quality currently available, although expensive it has the characteristics to be used as a “power battery” frankly for storage it would be wasted......
Lifepo4
Lifepo4 batteries are everyone's wish, a wish that we often put on the brakes because of the high costs and the modifications that necessarily have to be made to our motorhome's system.
Before talking about costs, I would like to tell you about batteries, how they are composed and the various features that make them a more or less good product.
There are now many companies on the market that sell pre-assembled lifepo4 batteries of which neither the internal components nor how they have been assembled are known.
We find ourselves buying something ”sight unseen” let's see what the ideal battery should look like:
- It should be assembled with grade A batteries (by which is meant free of any peculiarities that make them deviate even slightly from the data sheet)
- It should only be assembled with the cells positioned vertically (clearly this applies to prismatic cells and not to cylindrical ones)
- They should be tablets as per the data sheet
- They should be completely insulated from the walls of the case if made of conductive material and insulated from each other as their casing normally has a positive charge
- They should be equipped with BMS (Battery management system) with balancer and smart (connectable to PCs, phones, screens) to be able to control all the functions of the bms and see the progress of the individual cell.
Is there such a battery on the market? NO!
how can we choose our Lifepo4? the most economical and valid solution from the point of view of longevity is certainly the assembly of the individual components, choosing from those most suitable and economically closest to our possibilities.
Index of Topics
China:
The biggest cell manufacturers are Chinese, here is a table of the top ten battery manufacturers.
How to choose our Cells:
The Asian market is a jungle of offers that often hide nasty surprises.
Possible shenanigans may be due to:
- Used cells passed off as new
- Defective cells with casing damage
- Capacity below the minimum declared in the manufacturer's data sheet
- Bulges
To overcome these problems, we have 2 choices:
- Buying directly from the producer
- Rely on an honest Chinese or European dealer or one who at least claims to be one.
Clearly, the first solution is less feasible as companies do not deal with private individuals but only sell in batches that often consist of thousands of cells, so we are left with the second solution.
I often read of people buying on sites such as Aliexpress receiving batteries with reduced or inflated capacities. i have identified a few Chinese cell suppliers who have always supplied cells with capacities HIGHER than stated in the data sheet.
Cell capacity must always be checked with suitable instruments such as
- a shunt (we need it to read how many amps are coming in and out of our battery)
- A bench charger to be able to charge and then discharge them up to the minimum and maximum voltages expressed in the data sheet (typically 2.5v and 3.65v)
- A tester capable of reading the internal resistance of the cells, which must always be lower than stated on the data sheet (a higher resistance is synonymous with a defective or used cell)
- An inverter (to which a load can be connected) or an electronic load to discharge the cell in order to test the actual capacity
What are cell degrees?
You often hear about Grade A, Grade A- or B are nothing more than classifications unofficial to define the cells according to their correspondence to the characteristics given in the data sheet, but do you really need grade A cells for storage? the answer is no.
Exclusively for power systems typically used in electric vehicles, these cells are able to deliver without excessive imbalance all the power stated in the data sheet in a shorter time frame.
We are never interested in “spurts” we need energy stored and ready to be used gradually.
So good cells, even if A- grade, are more than good enough for our purpose.
Compression:
Prismatic cells have a tendency to “swell” even under normal use; this tendency must be contained by compressing the cells and keeping them pressed together. If they swell by even a few millimetres, they can lose some of their capacity.
The picture below shows a battery pack with the cells compressed by 8mm aluminium plates and threaded rods, a clear example of DIY compression.
Clearly this battery was still in an early stage of assembly not yet protected by insulating foils, which we will discuss in the assembly phase.
The Bms
The battery management system is an electronic board that manages our cells.
It monitors charge and discharge currents, interrupting them if one or more cells exceed preset values.
In the case of smart bms, it gives us the ability to see how each individual cell behaves during charging and discharging, alerting us if things are not going well.
Interrupts charging and/or discharging if cell temperatures are too high or too low.
It compensates through balancing (almost always passive and of a few millivolts) the imbalance that can occur when faced with large loads and/or strong discharges.
Records the life cycles already carried out (cells are not eternal and on average last about 3000 cycles)
The active balancer:
It is an instrument consisting of an electronic board that actively passes current from one cell to another, keeping them well balanced.
Usually a balancer can move up to 5A, which is much more effective than the passive balancer of bms.
I would recommend it given the low cost and the good work it does
How to test cells:
It is often (too often, unfortunately) the case that one realises during testing that one has had the classic “catch”, so it is good to equip oneself with the right tools and test the cells.
Internal resistance test:
It can be done with suitable testers that are successful in such a measurement, the internal resistance must always be lower than stated in the data sheet if the result is different, it means the cell is used and/or faulty.
Capacity test:
Cell capacity is one of the most time-consuming tests but one that allows us to better evaluate our purchase.
Charging the battery to the maximum allowed 14.6v and discharging it to the minimum allowed 10v, we can do this either with an “electronic load” that will record the amperes coming out of the battery and give us the real capacity, or by equipping ourselves with a schunt to be placed between the battery and a power inverter to which we will connect our load (which can be any household appliance)
I do not recommend electronic loads because they have a low amperage draw and this may slightly distort the test, as a serious test should be done at about 0.5C draw.
Components to assemble our battery:
To assemble our battery (after balancing the top) we will need these components:
- Case (case, made of wood, iron, plastic like automotive batteries)
- Compression plates
- The Bms
- The + and - poles where to connect to the battery
- Cables
- crimp terminals
- heat shrink tubing
- active balancer
Assembly:
After waiting about two months after shipping our coveted Lifepo4 cells and purchasing all the other items on the list, the fateful moment arrived to assemble everything.
The first thing to do is check cell voltages and internal resistance which must be lower than described in the data sheet.
Top balancing:
You have to put the cells in parallel and charge them to maximum, in the case of lifepo4 3.65v, to do this you will need an adjustable bench power supply.
Clearly, the time required for this operation varies depending on the amperes delivered by the power supply and the capacity of the cells.
Example: with a 5A power supply we want to charge 4 cells of 280Ah? it will take: 280/5= 56 multiplied by the number of cells 4 (in a 4s) = 224 hours fortunately they arrive charged to about 40%
Battery pack composition:
Let's take the example of a 4s; we should put them side by side paying attention to the positive and negative poles as we should create continuity, meaning that the positive of cell 1 should be connected with the negative of cell 2, the positive of cell 2 should be connected with the negative of cell 3, the positive of cell 3 with negative 4.
Connections will be made with the supplied busbars and pins.
Utmost care when connecting busbars:
This should always be done with shielded tools, by shielded I also mean covered with insulating tape (if a tool is accidentally dropped, the short-circuit can be very dangerous for the person and harmful to the cells)
At this point we will place the compression plates with tie rods by screwing them “firmly and firmly” without exaggerating
Installing the bms and active balancer (optional)
The bms is classically installed starting from the first cell where we connect the negative with the black cable and the positive with the first cable next to the black, then in the second cell we connect the third cable to the positive, in the third the fourth, and in the fourth the fifth cable, all always on the positive except for the first black.
For the active balancer the same applies, so much so that I usually solder the eyelets with both cables (negative bms + negative balancer, positive bms + positive balancer, etc.).
Where to buy cells:
Cells can be found for sale on every site nowadays, but it is also true that some of these sites sell used, defective cells with untrue nameplate data.
I buy in China and in particular on Alibaba from sellers that I now consider to be “familiar”, I consider them to be such because many of us have bought from these same sellers through the FB group “Lifepo4esperienze” and punctually the cells matched what was promised.
What to expect from all sellers:
- A video where you can clearly see the internal resistance and cell voltage as well as the aesthetics
- DDP (Delivered Duty Paid) transport costs are borne by the seller including import taxes in Europe
- The product data sheet must be as complete as the one I have included as an example in the box below.
How much do the cells cost?
It is difficult to answer the question because it is a fluctuating market, but more or less the prices of cells and accessories are as shown in the table below, a table drawn up (for accessories) in consideration of fast and air DDP transport, which is not convenient for cells that will go by ship, train or road.
Testing the assembled battery:
By now we are happy and jubilant to have finally assembled the battery we so desired, but now it's time to test it to see if everything has been done properly.
- Apply a load (typically a quality inverter with suitable power for testing)
- Start “unloading”, always checking the unbalance, which must not be what we usually see on the many videos of the various youtubers.... (I have seen unbalances of 0.150-0.200v) justified by the fact that as soon as the load is removed, the situation returns to normal (absurd and incompetent) An adequate unbalance must be in the range of 0.08 0.020 with a load of 0.5C (this means half the power of the battery) So if I test a 280Ah I will apply a 140A load and will not have to read decimals but only hundredths and thousandths of volts
- During the test, the busbars must be checked, the temperature of the busbars must be checked, if they get too hot, the busbars must be disassembled, thoroughly cleaned and reassembled, the busbars must be cold and only a few degrees warmer than the cell.
Good morning
I am very interested in this issue, having also installed a 100ah ULTIMATRON lifepo4 battery in my motorhome.
We have connected the battery recharge with a complete Victron system, but after only 2 years of life the battery from a 75% charge immediately switches to 100% when the engine is first started, in less than 2 seconds as if the Bms went into protection. .
Checking the state of charge of the four cells, they are very unbalanced, I do not understand whether it is the fault of his Bms or the cells that are pitiful. .