There are a gazillion options when it comes to purchasing LifePO4 (or any other Lithium-based) batteries. Some (maybe even MANY) of the options are so well priced that they are too good to be true. There are certainly good deals to be had, but if a deal is too good to be true it probably is.
Why is this?
Put simply there are great differences in the quality and build of batteries. There are different grades of cells that make up the batteries and of equal importance, there are so many options of battery management systems (BMS) that are of varying quality and functionality. So in a nutshell: Cells and BMS!
In this article, we're going to unpick some of the aspects of buying LifePO4 batteries.
Cells
A 12v battery has 4 or more cells. Each cell has a nominal voltage of 3.2v which when combined in series make up a 12v battery.
Let's look at the cells. Essentially there are the following grades:
A. These are cells that fall within a factory’s own standards when checked for quality and consistency. These are good cells that hold a uniform voltage, internal resistance and capacity (amongst other things). They should cost a reasonable amount (i.e. not cheap as chips). Some factories have two categories of this grade:
A+
A-
B. These have failed the particular standards of the factory that made them, probably due to a higher self-discharge rate or something similar. They are usable but not as good as grade A.
C. Not all factories have an official grade C category, probably because they can't be sold as working cells anyway. They are destined to be destroyed.
Important: NOT all new cells are grade A. Some people look at their serial numbers, determine that they are consecutive, and assume they must be grade A. This is not true. You can purchase consecutive serial numbers of grade B cells for a nice cheap price.
The most important thing is probably this: a chain is as strong as its the weakest link and if one of the four cells in a battery is not good it determines the overall performance of the whole battery. This is due to the BMS, to be discussed below. To get the most out of your battery all four cells must be matched! If you have three grade A cells and one grade B the whole battery will act as if it's grade B. Basically you will have diminished usable capacity whilst not using all of the capacity.
OK at this point we need to discuss the BMS and then take a holistic approach.
BMS
The BMS is the Battery Management System. Basically, it's a bit of hardware and some handy software. The BMS provides the following essential functionality:
Maximum charge cut-off. It cuts off the charging process when any of the four cells reaches it's maximum voltage. Most BMS are pretty standard in this regard, mostly varying in the mechanism used to stop the charge (different methods of isolators)
Minimum discharge cut-off. It cost off the discharge when any of the four cells reaches it's minimum voltage. Again, most BMS are fairly standard in this regard.
Cell Balancing. It attempts to balance the voltages of the four cells. It is able to read the voltage of the individual cells and to slightly increase or decrease the voltage. This is known as top balancing (and bottom balancing). The thing is that there is considerable variation between different BMS when it comes to balancing.
Temperature cut-off. It cuts off the charging process when the battery becomes too cold to safely charge. The same is true of discharge.
BMS Connectivity and Info capability - Apps and displays
We need to discuss the information that is provided to you by the BMS. This ranges from:
Nothing!
Elementary or sophisticated display units
Super-duper Bluetooth apps which tell you everything and allow you to set your own parameters
Why do we bother mentioning this? If something goes wrong and you are in the "Nothing!" category, how on earth do you solve the problem? Let's say you're a third of the way through your three-week road trip. Habitation power goes down. Pretty much all you can do is disconnect and reconnect your battery. You're probably going to need to buy a lead-acid or AGM from the nearest store just to keep lights and heating going while you try to figure things out.
We strongly recommend that you should be able to see what is going on in the battery, especially if it was a bargain-basement cheap deal of the week. No offence meant!
A Holistic Approach
When purchasing a Lithium (LifePO4) battery please consider one of two options:
Spend good money on a quality solution (based on reviews etc)
Spend reasonable money on a solution that has the ability to tell you, via app or display, what is going on in the battery.
Additional Info
This is a really good Youtube to watch, he explains how grade B and C cells occur:
