Which lithium battery to buy?

Which lithium battery to buy?

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

What are the differences between all these batteries and how do you choose the best battery for your application?

This article gives you an overview so that you can make your choice with a better understanding of its possible consequences.

 

Let’s start with the bad news…

 

The vast majority of resellers try to make you believe that their lithium batteries are a “plug & play ” replacement for lead acid batteries. Their websites advertise their product as “ready to replace your heavy lead acid battery”, “this makes it a great alternative to lead acid batteries”, or even “…is a direct replacement for lead acid batteries”. Plus, they feature charging 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 volts and keep it charged most of the time, as is good practice for lead, your new battery will die very quickly.

 

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

 

You do not have to replace all the equipment in your electrical installation. With a little planning, you can have an installation that will last a very long time. Simply review the features and settings of your charging equipment and decide how you manage it so that conditions are optimal for your new lithium battery. A suitable BMS (Battery Management System), a few relays and the right advice can get you quickly in the right direction.

 

Now that you’re ready to buy a lithium battery, how do you know what to buy ?

 

From drop-in batteries to cell packs, there are so many choices with a huge range of prices. Below is an overview of the main differences and their implications.

The basic component of every lithium battery is the cell with a nominal voltage of 3.2 volts. The cells are assembled in parallel then in series to form a battery of the desired voltage and capacity. A BMS is added to protect and hopefully manage the entire installation.

 

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

  1. The type of cells used in the battery
  2. How the battery is assembled
  3. How the EMS “Manages” the battery and the entire installation

 

1. The different types of lithium batteries

 

There are three major technologies used to manufacture LiFePo4 cells:

 

Benefits Disadvantages
Cylindrical Low manufacturing cost
good mechanical stability
Low capacity per cell
Prismatic/hard shell Large capacity per cell
can work under high load
More expensive to make
a little swelling
Prismatic / pocket Lower cost than hard shell Flexible bag with high swelling performs best 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” : the cells are assembled between compression plates with a BMS

One is not better than the other. You just need to know what you are paying for, and above all the real characteristics in relation to your needs and your expectations. The problem is that many online resellers give false, incomplete or inconsistent specifications showing that they do not have technical knowledge of their products.

 

“Drop-in”
“ Cell pack »
Benefits
  • Everything included in one box
  • Ease of connection

 

  • Flexible size and capacity
  • Easier to build a large capacity pack
  • Choose the BMS that matches your installation and your needs

 

Disadvantages

 

  • We don’t know what’s inside
  • Most BMS only protect against major disasters
  • Rare to be able to control external equipment

 

  • Must add external BMS

 

 

 

What does this mean – Our point of view:

 

Connections are the weakest link in an electrical installation

We are therefore cautious with a battery made up of hundreds of small capacity cells connected together. Some are very good and the manufacturer does not hesitate to show you how it is done inside (this is how Tesla batteries are made). But for low priced ones, it is impossible to find information on how they are assembled.

 

Golden Rule #1:

 

Don’t buy a battery made up of hundreds of small capacity cells (cylindrical or pocket) unless the manufacturer is serious and open about how they are constructed

 

“Prismatic/pocket cells” are not well suited to high capacity/high load installations

They will swell quickly if subjected to stress. A simple way to tell if a battery is made up of hundreds of pouches: look at the weight of the battery and the recommended charging current. For example :

  • A 200 Ah – 12 volt battery manufactured with a cell known as “prismatic/hard shell” weighs approximately 32 kg with a recommended charging current of 100 A to achieve the advertised performance (but can go much higher)
  • The “same” 200 Ah – 12 volt battery made with pocket cells weighs approximately 16 kg with a maximum charging current from 30 to 50 A.

They cannot be the same product and certainly will not provide the same service!

 

Golden Rule #2:

 

Don’t buy pouch cells for high capacity/charging applications (WARNING: they are never advertised as pouches, so look at the weight and the recommended charge current…)

 

The manufacturing process of lithium batteries is not 100% well controlled

Although all cells look the same, they do not come off the production line with the same characteristics (like capacity and internal resistance). The differences can be significant and it is essential that all cells in a battery 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 effort testing and sorting the cells to keep only the best ones and ensure they are all compatible in one battery. Others don’t go through this expensive process and might even use second-grade cells… while still maintaining the same characteristics as the good ones.

 

Golden Rule #3:

 

If the price is too good to be true… It’s because you are not buying the same product.

You may be very disappointed in a few years when one or more cells enter a death spiral that first reduces the effective capacity of the battery and then kills the good cells that should carry all the load.

 

Not all BMS are born equal

All BMS are described as “high performance,” “advanced,” “intelligent,” or even “comprehensive.” But very few provide detailed specifications. The main functions of a BMS are to protect and optimize energy availability/battery life.

 

Golden Rule #4:

 

If you cannot find detailed specifications of the BMS… do not buy it or the “drop-in” batteries that use this BMS

 

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

 

Protect 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 these ranges as a guide:

 

Low voltage per cell High voltage per cell Description of the range
2.00V 4.20V Extreme (outside this range the cell may not survive)
2.50V 3.65V Recommended by the manufacturer
2.85V 3.45V Optimal for long battery life (outside this range, cells age more quickly)

 

A quick internet search shows that many BMS have a fixed cut-off voltage:

Between 2.00 V and 2.50 V on the low side

3.75 V or more for high side

This voltage range will keep your battery “safe” so it doesn’t die instantly. But that won’t stop him from aging quickly. These recommended ranges are quite similar to the acceptable range for lead acid batteries (It is quite convenient to present lithium batteries as a direct replacement for lead acid batteries…). The same goes for temperature ranges…

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

 

Golden Rule #5:

 

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

 

Important feature to look for in a BMS: warn in advance before it cuts the power to your installation and leaves you in the dark (this can be difficult on a boat without autopilot, without navigation lights, without vhf , without maps… and if this happens at night in bad weather and near the shore)

For short circuit protection, regardless of what the battery manufacturer claims, you should mount a quick fuse within 30cm of the battery. It will cut power faster than a temperature-controlled (PTC) fuse or electronic system. Check your local electrical regulations and confirm with your insurance.

 

Balancing the cells:

 

Maintaining cell balance when full is essential for long battery life and maximum usable capacity. If a cell has a higher voltage at the end of charge, it will age faster than other cells, reducing its capacity over time and therefore increasing the imbalance…slowly entering a death spiral. In a previous article, we discussed the different approaches to balancing the cells of a lithium battery. Active balancing is what every lithium battery needs.

 

The three pitfalls of most balancing systems are

  • Balancing only occurs when a cell’s voltage is above 3.50 – 3.55 V. In a well-managed system, this should never happen unless the imbalance is very large (your battery is probably already breathless). In any case, the charger should stop charging before or shortly after this, without giving the system time to balance.
    Look for a BMS that is capable of balancing as soon as the cell voltage is above 3.4 V and a certain voltage difference between cells is detected.

 

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

 

  • Balancing is triggered based on the voltage difference measured between cells. But there are variations in the internal resistance of cells and with high current this measurement is useless for assessing the state of charge. Therefore, the BMS activates balancing on erroneous information and can actually unbalance cells. Check that your BMS triggers balancing based on open cell voltage (measured voltage compensated for internal resistance) – otherwise it may be better to disable the balancing function.

 

Optimize battery life:

 

This is the key area where a BMS will make a difference to the life of your battery by keeping it within the optimal voltage range. In most cases, it activates relays to allow charging and/or discharging based on the cell’s voltage, temperature or state of charge. It also alerts you to any drift in key battery parameters, giving you the opportunity to resolve the cause of the problem before the battery suffers.

 

Some examples :

  • Stop charging once battery is full (voltage at upper limit of optimal range)
  • Stop (or reduce) charging if temperature is too low
  • Disconnect non-critical loads when voltage is too low
  • Warn you if there is a cell imbalance

 

The “Optimize” feature of a BMS is what gives you peace of mind and allows you to enjoy energy knowing your battery is well maintained.

 

One last VERY IMPORTANT thought about BMS… How do you know if the BMS will protect and manage your battery when needed? With almost every BMS on the market, you wouldn’t know. You just hope it will do its job when the time comes!

 

The only way you can be sure is to put the BMS in real-world conditions and see how it behaves. TAO EMS is the only one on the market that can do this with a simulation function…

 

Lithium battery for off-grid energy storage …

 

Low capacity (200 Ah or less) and simple installation (one charging source designed for lithium):

We recommend a single battery “ drop-in » good quality with an external EMS. The EMS must have adjustable warning and protection levels , and be able to control external equipment (very few battery backups have these features ) .

 

For a 12 volt, 200 Ah capacity battery, it will cost between USD 2.000 and USD 3.000 depending on the 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 charging sources):

Choose a good quality “cell pack” consisting of “prismatic/hard shell” cells assembled between compression plates, with an external EMS and two heavy-duty relays.

 

For a 12 volt, 400 Ah capacity battery, this should cost you between $3,000 and $3,500 ($2,000 for the cells plus about $1,500 for the two relays and the TAO EMS with current shunt and monitor). The usable capacity is greater than an 800 Ah AGM battery and will last at least twice as long.

 

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

 

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