DIY Powerwall 3 – discharging and selecting cells
Diffulty level: Intro – Easy – Intermediate – Expert
Last time, we built cell chargers. This time, we will build a discharger use the tools we built to select cells.
Building a discharge station:
Dear battery fan. In previous articles I talked about charging 18650 batteries. We need 2 things before we can test the capacity. First, we need a plan, or a procedure on how to test the batteries. It’s very important to follow the procedure, if not you risk unbalanced or even faulty packs. Second, a discharger board is key to knowing the capacity of your cells.
Let’s start with the procedure:
Start charging the cells. If the cell gets warm or the cell doesn’t start charging or gets very warm, your cell is ready for recycling. Store your cell for minimum 10 days, I usually go for 2 weeks.
After 2 weeks, if the voltage is still above 4.1V your cell is ready for a capacity test. If the voltage is between 4.00V and 4.09V the cell gets a second chance (charged and rests again). If the voltage drops below 4.00V it’s a discharge cell and it should be recycled. My experience (after testing >4000 cells) is that cells that don’t make it in the first test are often lower capacity cells, so you can even discard the 4.00-4.09V cells.
The +4.1V cells are tested with the ZB206 discharge modules or the cheaper ZB2L3 modules. It’s important to use the same modules for all your cells and stick to the plan/procedure for all cells. If you don’t follow the procedure you will never be able to make properly balanced packs. Cells below 1200mah should be recycled, and between 1200mah and 1850mah can be kept as spares. Anything above 1850mah is good to go.
Mount the discharge module with a little screw on a piece of plywood.
Connect a battery holder to the module and connect a 12V power source to the board. 12V power consumption is very low, a regular 1A or 2A power supply will do the trick for +10 modules.
Insert a battery in the battery holder. With the first and second button you can select the discharge speed. I always use 1A. Press the third button to start the test. Keep in mind a 2000mah battery will take 2 hours to discharge. If you want to make a powerwall with 500+ batteries make sure your test wall has several test units. I had 13 of these testers in use.
When the battery hits 3.0V the discharging is stopped and an the buzzer will alert you. You can read the capacity of the cell on the display.
P.S. If you reverse the polarity of the battery you wont break the unit, you only get an error message.
The units are very accurate. When testing batteries from the same batch, capacity of the cells are often within a few mah from each other.
When batteries are tested I always write the capacity on the cell. If capacity is lower then 1200mah they end up in the recycle bin. Values between 1200-1850 can be used for smaller projects. If capacity is above 1850mah they can be used for a powerwall built or powerpack.
Once tested, I sort the cells in the well known 4*5 holders. I sort them by values of 100mah, so I have a view on the number of cells and the expected capacity of the powerwall.
Good luck testing yours.
Franky goes green – http://www.easypowerwall.com
Note: Franky is a valued contributor, however, his advice and opinions are his own.