Monday, September 28, 2020

Building a new lithium battery

For years I have been using lithiun batteries to power my radio equipment when portable, both LiPo and LiFePO4.
If you want to know more about these different types read this post I wrote a few years ago.

I started out buying batteries from HobbyKing in China. Then I decided to build a few myself with pouch cells I bought from AliExpress. 

Recently I lost two LiPo pouch cells when I abused a battery - forgetting to attach the under voltage alarm. Another LiPo cell of a different pack turned out dead without a clear cause. I ordered a new cell but they are quite expensive. Losing three of those cells in a short period of time I suspect I might have to replace more in the future. So I started looking for other solutions with cheaper building blocks: smaller cells.

LiPo cell after abuse

Looking at smaller cells I first stumbled upon the 18650. A popular Li-Ion battery and quite small but you need a lot of them to get a useful capacity as they are around 2Ah a piece. However I found more practical LiFePO4 cells with a 7Ah capacity: 32700. I bought a set of those with strips of nickel attached to the ends for soldering as I do not have a spot welder (yet).

The voltage of four of these LiFePO4 cells is too low for my amp - that is why I opted for LiPo up till now - but as the amp is designed to handle up to 18V a battery of 5 cells would do the job. So I decided to go for a 5s3p design (5 cells in series and 3 in parallel).

Building a 5s3p battery

I bought a BMS as well with the idea that it will protect the battery from under voltage. And it allows me to charge the battery even with a simple (non balancing) charger. I was hoping that a BMS is more effective in balancing than the chargers I have - that take quite some time to balance an unbalanced pack.

Completed battery with BMS

Testing the battery my first lesson was that you cannot charge a battery that has a BMS with a smart charger. The BMS starts kicking in when the first cell approaches its maximum. The charger notices the increasing resistance and shuts down with an error ("connection break"). 

Charging through the BMS with a smart charger 

Time for Plan B I used a 12V source and a step up converter to 18V to act as a "dumb" charger. Meanwhile I checked the charging current and the voltage of the individual cells to see what was happening. 

This time the charger patiently waited for the BMS to allow for a current to run. The BMS would drain the cells for a bit and then allow a charge current until one of the cells hit around 3.5V and then the cycle repeated. However it did not seem to be able to bring the cells closer together. After an hour there was still a spread of 0.2V between the cells. 

After letting the pack sit for a while I found out that the BMS aims to keep the cells at around 3.4V (on my V-meter). So during charging - aiming for 3.6V - a cell might go higher but the BMS will continue to discharge it until it is back at 3.4V again.

I will have to see if this noticeably impacts the capacity of this battery pack. When I have some more time on my hands I will run some tests and see how it behaves under the stress of powering a 400w amp.

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