If the battery can no longer be charged, the cause is sometimes a so-called deep discharge. This happens if the battery is sent into winter hibernation improperly, usually with too low a charge level, and/or stored incorrectly. How this can be prevented is explained in the Safety instructions to the battery, the Operating instructions or under BATTERY – 'Care: Mechanical / in operation / storage / safety'
Stromer writes the following regarding storage:
Why does deep discharge happen, can it be prevented?
Self-discharge is a physiochemical process that fundamentally unstoppable Each individual cell in a Li-ion battery experiences self-discharge due to its chemistry. These processes occur even without any external circuitry – purely internally within the cell. This self-discharge is called calendar aging. It arises from several mechanisms:
- Electrochemical side reactions at the electrodes (e.g., SEI layer growth)
- Ion diffusion through the separator
- Parasitic leakage currents within the cell itself
In a battery pack, several cells and the BMS are connected. Even a modern BMS consumes standby power, around 10–50 µA, so it's an additional consumer on top. It doesn't „see“ the internal cell currents because they don't flow through external wires.
When a lithium-ion battery stored long enough over time
- The storage temperature is high (greatly accelerates self-discharge)
- The BMS itself draws power (typically 20–100 µA)
- The cells are already aged
This leads to irreversible damage, loss of capacity, and a potential fire hazard due to internal short circuits.
A good BMS, correct handling, and storage can delay but not prevent deep discharge if no counter-measures are taken.
You can find more details on this in my compendium on Li-ion batteries under AKKU - 'Theory & terminology compendium for Li-ion batteries' read more.
The Self-detonation A Stromer BQ983 battery has a lifespan when stored properly ~1% per week or ~4% per month.
On YouTube, videos are shown on how to revive a deeply discharged battery. Such a „zombie battery“ is unstable, a fire hazard, and a safety risk. This is strongly discouraged!
What can you do with a Stromer if the battery can no longer be charged?
The following points can still be tried without security risks before a new battery becomes necessary.
- With a bit of luck, a battery may only appear to be deeply discharged, with the cell voltage still above the critical 2.5V but below 3.2V. Above 3.2V, a Li-ion cell is considered empty (0% capacity). This threshold is relevant for most chargers. They will not charge the battery for safety reasons if the voltage drops below 3.2V.
The Stromer charger CR245 (& CR65) has a hidden capability. It can revive such a battery, if necessary. This is referred to as «nurturing.».
Simply leave the battery connected to the charger for several days. and see what happens. With a bit of luck, the charger will begin the regular charging process after a certain waiting period. The newer CR246 does not have this «trickle charge function,» but it does offer a «float charge.». - One can no further Send the battery to Stromer via the dealer. Stromer doesn't repair batteries, but they can reset the self-protection and reactivate the battery. The same applies here: if the cell voltage has dropped below 2.5 V, Stromer will no longer reactivate the battery.
- As last chance Is there still the Battery replacement. However, the respective company must offer the option to reset the SoH and charge cycle counter. And a reset is only possible with HTE batteries with 3 screws.
Further information can be found at AKKU - ‚Repair / Battery Refurbishing / Used Items‘
Conclusion
The typical self-discharge rate of a Li-ion battery at room temperature is about 1–3 % pro Monat (Stromer = 4%, see above). At 20°C and with a properly stored battery, it therefore takes many months to years, until the critical cell voltage limit of 2.5 V is reached.
Tips for Long-Term Storage:
- Charge level at ~40–60 % store (not full, not empty)
- Store in a cool place (10–15°C ideal) – significantly slows down the chemistry
- Battery management system with sleep mode choose one that reduces self-consumption to <5 µA
- Reload periodically (every 3–6 months) or use the CR246 with floating charge.
The BMS protects the cell from the Damages through deep discharge (by disconnecting the consumer in time), but cannot prevent the chemical-physical self-discharge of the cells.
Anyone storing a battery for a very long time must therefore check and recharge it regularly.
An alternative is to connect the battery to the CR65 or CR246 charger. Both models allow for float charging.