Lithium batteries | Battery Management System (BMS)

Which lithium battery to buy?

A quick search on Internet for a 12 Volt 200 Ah lithium (LiFePo4) battery returns prices ranging from USD 406.00 to USD 4,185.00… all with seemingly the same great benefits and similar characteristics!

What are the differences between all these batteries and how to choose the battery that is best suited to your application?

This article provides some insight so that you can make your choice with a better understanding of its possible consequences.

Lets start with the bad news…

The vast majority of resellers try to make you believe their lithium batteries are a “plug & play” replacement for lead-acid batteries. Their web sites present their product as “ready to replace your heavy lead-acid battery”, “this makes them a great alternative for lead-acid batteries”, or even “… is a “drop-in” replacement for lead acid batteries”. In addition they show charge characteristics that are compatible with the equipment you already have for your lead-acid batteries.

The truth is that if you regularly charge your lithium battery at 14.4 volt and keep it charged most of the time, as it is a good practice for lead-acid, your new battery will die very fast.

But the good news is that you can do something about it without spending too much money:

You do not have to replace every piece of equipment of your electrical installation. With a bit of planning you can have an installation that will last a very long time. You just need to review the characteristics and the settings of your charging equipment and decide how you manage it so that conditions are optimal for your new lithium battery. The right BMS (Battery Management System), a couple of relays and the right advice can get you going rapidly in the right direction. Feel free to post a message on the forum and ask for advice on converting your current installation to be lithium friendly.

Now that you are prepared to buy a lithium battery, how do you know what to buy?

From “drop-in” batteries to packs of cells, there are so many choices with a vast array of prices. Below you will find an outline of the major differences and their implications.

The base component of every lithium battery is the cell with a nominal voltage of 3.2 volt. Cells are assembled in parallel then in series to build a battery of the desired voltage and capacity. A BMS is added to protect and, hopefully, manage the whole installation.

 The three key aspects to consider when buying a lithium battery are:

  1. the type of cells used in the battery
  2. how the battery is assembled
  3. how the BMS “manages” the battery and the whole installation

1. The different types of lithium cells

There are three major technologies used to manufacture LiFePo4 cells:

low cost of manufacture
good mechanical stability
low capacity per cell
Prismatic / hard shell
large capacity per cell
can operate under heavy load
more expensive to manufacture
some swelling
Prismatic / pouchlower cost than hard shell
flexible bag with significant swelling
performs better under light load
life is shorter at higher temperatures
low capacity per cell

2. How batteries are assembled

Two different approaches:

  • “drop-in”: cells and BMS in a single container
  • “cell pack”: cells are assembled between compression plates with an external BMS

One is not better than the other one. You just need to know what you pay for, and more importantly the real characteristics in relation to your need and expectations. The problem is that many online resellers give false, incomplete, or incoherent characteristics showing that they know nothing about their products (feel free to post characteristics on the forum and I will help you read through the lines).

“Cell pack”
  • all included in one box
  • ease of connection
  • flexible size and capacity
  • easier to build large capacity pack
  • choose the BMS that fits your installation and needs
  • do not know what is inside
  • most BMS protect only against major disasters
  • very few can control external equipment
  • must add an external BMS
What does that mean – the way I see it:
Connections are the weakest link in electrical installations

So I am cautious about a battery that is made of hundreds of small capacity cells that are connected together. Some are very good and the manufacturer do not hesitate to show you how it is done inside (that is how Tesla batteries are made). But for the low cost ones it is impossible to find information on how they are put together.

Rule of thumbs #1:

Do not buy a battery that is made of hundreds of small capacity cells (cylindrical or pouch) unless the manufacturer is serious and open on how they are built

“Prismatic / pouch cells” are not well suited for high capacity / high load installations

They will swell rapidly if placed under stress. An easy way to know if a battery is made of hundreds of pouches: look at the battery weight and the recommended charge current. For example, a 200 Ah reputable “prismatic / hard shell” cell weighs about 8 kg with a recommended charge current of 100 A max, pouch cells assembled to get the same capacity weigh 4 kg with a recommended charge current of 30 to 50 A max. They cannot be the same product and will definitely not deliver the same service!

Rule of thumb #2:

Do not buy pouch cells for high capacity/load applications (WARNING: they are never advertised as pouches, so look at the weight and the recommended charge current… if they do not lie!)

The manufacturing process of lithium cells is not 100% under control

Although all the cells look the same, they do not come out of the production line with the same characteristics (mainly capacity and internal resistance). Variances can be significant and it is critical for all the cells in one battery to have exactly the same characteristics. You can see the challenge when there are hundreds of cells in a battery… The best manufacturers spend a lot of efforts testing and sorting the cells to keep only the best ones and be sure that in one battery they are all matched. Others do not go through this expensive process and even could use second grade cells… while still promoting the same characteristics as the good ones.

Rule of thumb #3:

If the price is too good to be true…
it is because you are not buying the same product and you will be very disappointed in a few years

3. Not all BMS are born equal

All the BMS are described as ‘high-performance”, “advanced”, “smart” or even “full featured”. But very few provide detailed specifications. The key functions of a BMS are to protect, balance and optimise.

Rule of thumb #4:

If you cannot not find detailed BMS characteristics, do not buy them or the “drop-in” batteries that uses that BMS

Lets take a look at the key functions of a BMS and what to look for in their specifications:

Protect the cells against too low and too high voltage

There are three voltage ranges to consider when using LiFePo4 cells. Although there are some minor variations between manufacturers here are those ranges as a guideline:

Low voltage per cellHigh voltage per cellRange description
2.00 V4.20 Vextreme (outside of this range the cell may not survive)
2.50 V3.65 Vrecommended by the manufacturer
2.85 V3.45 Voptimal for long battery life (outside of this range cells age faster)

A quick search on Internet show that many BMS have fixed cut-off voltage:

  • between 2.00 V and 2.50 V on the low side
  • 3.75 V or more for the high side

This voltage range will keep your battery “safe” so that it will not die instantaneously. But it will not prevent it from ageing fast. These recommended ranges are quite conveniently similar to the acceptable range for lead acid batteries… and therefore present lithium batteries as a “drop-in” replacement of lead acid batteries. The same goes for temperature ranges…

Having this large voltage range for the BMS is also a good way to avoid too frequent cut-off if the cells are of poor quality with large discrepancies in internal resistance and/or capacity!

Rule of thumb #5:

Do not buy a BMS with a large range of cut-off voltage / temperature and if it cannot be adjusted…
as it may hide low quality battery / cells

Important feature to look for in a BMS: provide an advanced warnnng before it cuts off the power to your installation and leaves you in the dark?

For short circuit protection, independently of what the battery manufacturer claims, you must mount a fast fuse within 30 cm of the battery. It will cut the power must faster than a thermal re-settable fuse (PTC) or an electronic system. Check your local electrical regulation and validate with your insurance.

Balance the cells:

Keeping the cells balanced when they are full is critical to a long battery life and maximum usable capacity. If a cell has a higher voltage at the end of charge, it will age faster than the other cells, reducing its capacity over time and therefore increase the unbalance… slowly going into a death spiral. In an earlier article I have discussed the different approaches to balancing lithium cells. Active balancing is what every lithium battery needs.

The two pitfalls of most balancing systems are

  1. balancing only takes place when a cell voltage is above 3.50 – 3.55 V. In a well managed system this should never happen unless the unbalance is very large (your battery is probably already on its last leg). In any case the charger should stop charging before or shortly after that happens not giving the system any time to balance.
    Look for a BMS that is able to balance as soon as cell voltage is above 3.3 V and a certain voltage difference between cells is detected.

  2. the balancing current is too low to have any effect. I have seen many BMS with balancing currents of 35mA. For a 600 Ah battery with a 1% unbalance, it would take more than 170 hours of continuous balancing to get back into balance… but that will never happen (see point 1).
    Look for a BMS that is able to balance with a current of at least 1A (2A is better).
Optimise the life of the battery:

This is the key area where a BMS will make a difference in your battery lifespan by keeping it within the optimal voltage range. In most cases it activates relays to enable charge and / or discharge based on the measured voltage or state of charge. It also warns you of any drift in key battery parameters, giving you the chance to fix the cause of the problem before the battery suffers.

Some examples:

  • stop the charge once the battery is full (voltage at the upper limit of the optimal range)
  • stop the charge if the temperature is too low
  • disconnect non critical loads when the voltage is too low
  • warn you if there is a cell unbalance

The “Optimise” function of a BMS is what gives you peace of mind and let you enjoy energy knowing your battery is well looked after.

A final thought about BMS… How do you know the BMS will protect and manage your battery when needed? With nearly all the BMS on the market you don’t. You just hope it will do its job when the time comes!

The only way you can be sure is to place the BMS in real conditions and see how it behaves. There is only one BMS on the market that can do that…

If I had to buy a new lithium battery for off-grid energy storage

Low capacity (200 Ah or less) and simple installation (one charge source):

I would choose a good quality “drop-in” battery with an integrated BMS. The BMS must have adjustable warning and protection levels, and be able to command external equipment.

For a 12 volts and 200 Ah capacity battery, it should cost between USD 2.000 and USD 3.000 depending on features and quality. It has more usable capacity than a 400 Ah AGM battery and should last twice as long at a minimum… which make the “drop-in” lithium battery option less expensive over a ten year period.

High capacity (above 300 Ah) and more complex installation (multiple charge sources):

I would choose a good quality “cell pack” made of “prismatic / hard shell” cells assembled between compression plates, with an external BMS and two high current relays.

For a 12 volts and 400 Ah capacity battery, it should cost you between USD 3.000 and 3.500 ($ 2.000 for the cells plus about $ 1.200 for the two relays and the TAO BMS with current shunt and monitor). This is more usable capacity than a 800 Ah AGM battery and will last a minimum of twice as long.

Of course that is expensive compared to the low cost options that can be found on Internet, but I firmly believe that in the long run the options presented here will be less expensive and will give you peace of mind knowing you can depend on it for a long time.

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