Which lithium battery to buy?

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.

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.

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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

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1. The different types of lithium cells

There are three major technologies used to manufacture LiFePo4 cells:

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.

“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 – 12 volt battery made with reputable “prismatic / hard shell” cell weighs about 32 kg with a recommended charge current of 100 A to achieve announced performance (but can go much higher)

• the “same” 200 Ah – 12 volt battery made with pouch cells weighs about 16 kg with a maximum charge current of 30 to 50 A.

They cannot be the same product and will definitely not deliver the same service!

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 (like 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.

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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 and optimise energy availability / battery life.

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 cell	High voltage per cell	Range description
2.00 V	4.20 V	extreme (outside of this range the cell may not survive)
2.50 V	3.65 V	recommended by the manufacturer
2.85 V	3.45 V	optimal 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!

Important feature to look for in a BMS: provide an advanced warning before it cuts off the power to your installation and leaves you in the dark (could be challenging on a boat when there is no autopilot, no navigation lights, no vhf, no charts…. and if that happens at night in bad weather and close to the shore)

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 three 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 3.4 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).
   
3. Balancing is triggered based on the measured voltage difference between cells. But there are variances in cell internal resistance and with high current this measure is useless to evaluate the state of charge. Therefore the BMS activates balancing on wrong information and may in fact unbalance the cells. Check that your BMS triggers balancing based on open cell voltage (measured voltage compensated for internal resistance) – if not, it might be better to disable the balancing function.

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 cell voltage, temperature 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 (or reduce) 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 VERY IMPORTANT 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 with a simulation function…

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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 designed for lithium):

I would choose a single 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 (very few drop-in batteries do have those features).

For a 12 volts and 200 Ah capacity battery, it will 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 build a good quality “cell pack” made of “prismatic / hard shell” cells assembled between compression plates, with an external BMS and two heavy duty 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.