How to test the battery pack in your EV.
There are 1001 ways out there to test the batteries in your pack, and find potential problems. Most are ineffective, a lot of work and will not give you enough tangible data to make informed decisions or hold a battery vendor’s feet to the fire if needed.
Because I believe it to be unreasonable and expensive to run an EV without a BMS, we will assume you have one installed and we will use it to test the pack health. Furthermore, we are going to assume you are using our BMS; otherwise you would be referring to the ‘other’ vendors documentation for this information.
OVERVIEW
A single bad battery in a pack can decimate the performance of an EV. Fortunately, even the layman can quickly spot problem areas in a pack with the proper equipment. In this tutorial I will walk you through a pack test on a typical EV.
PROPER EQUIPMENT
This document assumes you are using our BMS consisting of a PakTrakr and PowerCheqs/BattEQs.
SAFETY FIRST
As with any vehicle, it is critical that you observe all standard safety precautions. Failure to adhere to proper safety protocol can be deadly. In this case, we are working on the battery pack. It is not to purpose of this article to teach you the basics of battery pack safety, but you are working with high voltage and it can kill you. We also perform test drives and document data during the drive. I recommend having an assistant drive while you document data. Become familiar with basic EV mechanic safety prior to performing service on your EV.
THE PATIENT
In this document we will be testing a new ZAP Xebra PK (72v). This PK is brand new, but it was purchased from a dealer that did not equip it with a BMS. Not surprisingly, it was had very poor range and performance on delivery. Lets find out why.
TESTING
1. We installed a GPEV BMS consisting of PowerCheq modules and a PakTrakr.
2. With BMS installed, the vehicle was allowed 24hrs for the pack to balance.
3. Because I need to document my test results, I use a Woodward Display/SD writer to capture data from the PakTrakr. With this unit plugged in, the vehicle was taken on a test drive.
4. 
The data from the test drive is captured on the SD card and charted in Microsoft Excel. The graph is showing volts/distance. As you can see from the data captured, battery #2 tanks hard at about the 6 mile mark. I normally halt the test when I see the first battery drop out, but as you can see, battery #5 takes the death-dive before I get back to the shop. Keep in mind, this vehicle was sold to the customer as “new”, and one would have expected a clean pack from a “new” vehicle off the lot.
5. Because the customer had planned on upgrades he had purchased 2 additional “new” batteries from the same dealer. Seeing the two bad batteries in the pack, we swap out the now-known bad units for the two new batteries. Pack was allowed to balance, then was charged. NOTE: Because battery #5 is under the tie-down bracket and more difficult to access, original battery #4 is slid into the #5 position and the new batteries go into the #2 and #4 slots for easier access during testing.
6. Test #2 is attempted, but both “new” batteries collapse before the vehicle reaches the end of the street:
The short drive around the block is all it takes to grab this data to use against the dealer that sold him these batteries.
STOCK PAKTRAKR DISPLAY
For those not using the uber-cool Woodward Display, it is still very easy to see the pack behavior graphed above. Set your upper display to “Batt V@” and do your test drive after proper charging/balancing. The distinctive dives in the graph above will be seen as radical “spikes” on the PakTrakr display under load as the instrument all of a sudden assumes it is dealing with a 6 or 8 volt battery because a 12 volt battery is not supposed to dive so low. So, for the graph above, the stock display was showing batteries 1,3,5, and 6 sinking low under load while batteries 2 and 4 spiked to the top of the display.
BATTERY REPLACEMENT
I recommend letting the BMS balance the pack for at least 6 hours after installing new batteries in a vehicle. After this time take a short drive to observe the pack balance before putting the vehicle on the charger. During this first charge check on the pack often to make sure in did indeed balance, and you are not over-charging your new batteries. After this initial balancing your BMS will do an excellent job of keeping the pack balanced and healthy.
WHEN TO REPLACE YOUR RUNT
Every pack has a runt. Replace the runt and a new one will appear. Somebody has to be the weakest. Particularly for those using the Woodward Display, the runt is easy to spot. Just because a battery is weaker then the rest, however, does not mean with will not live for a long time with the balancers helping him out. Once the runt starts bottoming out on the Woodward Display or Spiking on the stock display during your normal runs it is time to replace that battery.
WHY WE TEST THIS WAY
Notice in the first test results documented in this article that resting voltage bounces back to normal range when the series load is removed from the failing batteries. Meer voltage tests before, during and after the run would not show those batteries as bad. Those batteries passed “dummy load” tests at the original dealership just fine. This is an example of how ineffective some of the traditional tests are.
Questions? Comments?
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