Performing final quality and performance checks
At this point your BMS is connected and your battery is finished! Well, electrically speaking anyways. We’ve still got to seal it up and make it pretty, but that’s for the next chapter. Now it’s time to double check that everything has been connected correctly.
Use a digital multimeter or a voltmeter to check the voltage of your battery through both the charge and discharge wires. You should get something reasonable in the range of your finished battery. A 36 V battery might not read exactly 36 V if the cells weren’t fully charged to begin with. Many li-ion cells ship from the factory at 3.3 V per cell, so a 10s 36 V battery built from these cells should read 33 V at both the charge and discharge connectors. If for some reason you get an odd number, especially a number that is too low, it’s probably due to an incorrect connection somewhere with
the BMS. Double check all of your main connections and balance connections.
If you get a reading of 0 V or an open circuit reading, it may be a BMS issue, but it may also be a cell connection issue. To determine which, measure the voltage from the negative terminal of your first cell group to the positive terminal of your last cell group. If it’s still 0 V or an open circuit, that means you likely forgot to make a series connection somewhere. If you get the appropriate voltage then you have a problem with your BMS, likely an incorrect BMS connection somewhere.
If you’ve checked everything and still get an incorrect voltage reading that isn’t within the range of your fully charged to discharged voltage, you might just have a bad BMS unit. This happens occasionally, especially with cheaper “China specials”. If you’re going to buy a cheap BMS, you might want to buy two just to have a replacement. But if you’re going to buy two, you might as well spend twice as much on a good one to start with.
Checking the voltage is a good way to make sure everything is correctly connected, but it won’t tell you how well everything is connected. For that, it’s best to test your battery with a real load. If possible, testing with the actual load it was designed for would be optimal. For some applications though, it’s not possible to put the bare battery in the device, or the device isn’t ready yet. you don’t want it to get too much hotter than that.
An infrared or laser thermometer is great for checking the temperature of different parts of your battery. They can be bought for less than $5 on sites like AliExpress. Pay careful attention to your series connections to make sure they aren’t getting too hot either. A bright red glowing piece of nickel strip would be a dead giveaway that you’ve messed up, but even just a single series connection that is much hotter than the others can be an
indication that that connection isn’t sufficiently strong (i.e., it needs more current carrying capacity).
If you can’t use your intended device, there are other ways to test a battery.
12 V batteries can be plugged into a wide array of 12 V DC devices, such as lights or heaters. Other voltages don’t lend themselves as easily to powering the devices you have laying around, but they can be plugged into a voltage converter to output 12 V DC for a similar effect.
You can also create a “dummy load” by using a heating coil, power resistors or a chain of light bulbs to create a resistive load for the battery. Halogen light bulbs and power resistors are often used to build homemade battery discharging devices. A quick google search for either will find many different sets of instructions to build a battery discharger. I even posted a video on the EbikeSchool.com YouTube channel showing how to make an adjustable voltage halogen lightbulb discharger.
Testing your battery under a load isn’t crucial, though it’s a nice extra step to ensure quality. But as long as your battery is providing the proper voltage from its charge and discharge connectors, then you’re ready to move on to the next step.
Chapter 12: Sealing the battery
Once you’re finished with your connections and confirmed that your battery is in good working order, you can go ahead and seal it up. In my opinion, all lithium batteries should be sealed in some way. An unsealed lithium battery with exposed terminals is simply an accidental short circuit waiting to happen.
The only case where I commonly see lithium batteries left unsealed is in the DIY powerwall community. They generally use snap together plastic cell holders which create a nice battery case and keep the cell terminals lifted off of the surface they rest on, but they don’t actually cover the exposed terminals.
I’m not sure why these folks don’t seal their batteries. Maybe they want ease of access to replace bad cells, since they usually build their batteries with salvaged cells. Maybe they want the benefits of passive air cooling and don’t want to design in an active airflow system. Maybe they just figure that the batteries are up on a shelf or hidden in a closet and therefore they’re probably safe.
Either way, such batteries with exposed terminals represent a fire hazard if anything were to ever short them. A mouse in the closet would turn pretty crispy if it stepped in the wrong place and shorted some high capacity home energy storage batteries.
The method of sealing your battery will depend on a number of factors including the type of cells you used, the size and shape of your battery, and the environment in which your battery will be used.