This instructable is to help troubleshoot a malfunctioning LiFePO4 (Lithium Iron Phosphate) eBike battery. These batteries are commonly sold online through various sellers. Though they often differ in size, voltage, capacity and cell type, they are electrically similar and the troubleshooting techniques below should hold fast for any multi-cell LiFePO4 battery pack with internal BMS(Battery Management System).
Step 1: LiFePO4 Battery Basics
Lithium Iron Phosphate (AKA - LiFePO4) is increasingly being used for electric vehicles (and as a replacement for Lead-Acid batteries in general) due to it's long lifespan ( >1000 charge cycles), light weight, flat discharge curve and awesome chemical stability. LiFePO4 batteries are far less prone to the fire/shorting issues that Lithium Ion batteries are now famous for. LiFePO4 cells also hold their charge indefinitely. They don't leak off their charge or go dead over time.
Modern eBike batteries use Lithium Iron Phosphate chemistry with multiple cells arranged together in a grid to provide the correct voltage, current and capacity. Batteries packs are described by the number of cells they have in series (S) and in parallel (P). So a 16S4P battery will have 64 cells total (16x4). Battery pack voltage is the sum of single cell battery voltage (about 3.33V-3.35V charged) times the number of cells in series. So our 16S example battery would have a voltage of about 53.5VDC charged. Battery pack amperage is the sum of the individual cell amperage times the number of cells in parallel. So if our example cells were 2000mAh, then out 16S4P pack would have a capacity of 8000mAh.
In addition to the cells there will also be a BMS (Battery Management System) board inside the battery. This board monitors the cell voltages inside the battery and ensures that no individual cell voltage charges to above 3.65V or discharges below 2.0V where they would be damaged. It also balances the cells during charging (ensures each cell receives the same charge) and disconnects the battery from it's load if short-circuit or over-current conditions are detected.
Step 2: Battery Problems
Battery Problems are generally of 2 types-
- Charging Problems
This is when the battery either won't charge or won't hold a charge. (i.e. after charging all night it's still dead/empty)
- Discharging Problems
With discharge problems, the main symptom is the bike completely dying under load and not starting again until the battery is disconnected then reconnected to the motor.
The other common discharging issue you may see is that the battery works, but does not last very long. This is indicative of an imbalanced or damaged battery and can be caused by damaged cells or by a broken balance circuit on the BMS not charging all of the series cells equally.
Step 3: Digging In
The video above gives a look inside an ebike battery and describes the basic procedure used to troubleshoot a battery that is cutting out or failing to charge. Also included is a schematic of the BMS circuit. It's not required to troubleshoot, but is there for anyone who is curious...
You will need:
- a Digital Volt Meter
- scissors or utility knife to open battery (usually heat shrink)
And the basic troubleshooting steps are-
- disconnect battery from eBike.
- Open the battery and expose the BMS board
- Test DC voltage at the 3 negative cables on the BMS (the cables are the negative terminals for charging discharging and the battery) The difference between each should be 0 VDC (see video)
- If charging problems exist repeat 3 with charger attached to battery
- Test battery voltage at main discharge connector (should be full battery voltage)
- Test battery voltage at main charging connector (should be full battery voltage AND match 5)
- Test individual cell voltages at balance connector.(should be between 2.5-3.4 VDC per cell depending on charge. All cells should also have close to the same voltage. Anything under 2.0 or over 3.65 VDC per cell indicates permanent damage to the cell.)
- (if still faulting) Bypass Discharge protection and test under load (see video)
These steps will help you determine if you problem lies in your BMS, your cells or your eBike motor and will hopefully help someone rescue their broken battery without having to spend an arm and a leg...
Step 4: Longevity Tips
For your LiFePO4 battery to have a long and happy life-
The more wear and tear the internal electrodes of the batter see, the faster the battery will fail. Charging slowly places less stress on the battery. Using that 'Quick Charger' when the bike is going to sit overnight anyway will over time reduce cell lifespan. Charging LiFePO4 batteries quickly also places more wear on the BMS. (My own eBike BMS runs 20 deg. C hotter on a 5A 'quick' charger than it does on a 2.5A 'standard' charger.) Batteries charged slowly balance better too.
Do Not Store Fully Charged
If you are storing your bike for an extended period, run the battery down some first. Storing a dead battery can hose it. But Storing a 100% full battery causes internal battery wear also. (The internal electrodes of the battery wear more quickly when more chemically active. And they are at their most active when fully charged.) So before you put your bike away for winter, run it down to around 2/3 of full.
When storing you should check the battery every couple months. While LiFePO4 cells do not leak or lose charge, the BMS usually steals some . The Signalab V1 BMS steals juice from only the first 4 cells for it's logc/MOSFET buffering. So theoretically over long term storage those cells can imbalance... (You can see this in the included schematic. The line coming from charging circuit #5 to R36 powers the quad NAND gate IC on the back of the BMS.) Newer versions steal equally from all cells. Either way, checking every once in a while to make sure that the BMS has not run the battery down is prudent.
Do NOT Leave On the Charger
This is probably the most important longevity tip and is for the same reason we don't store it fully charged. Keeping a LiFePO4 cell at ~3.5VDC per cell 'top-off' voltage keeps the internal electrodes at their most active and causes unnecessary battery wear. Once the battery is done charging, unplug the charger.
This is a work in progress and I'm going to keep adding info so feel free to post any questions and I'll answer them if I can.
Thanks for viewing!