Nickel-metal hydride (NiMH) batteries are a type of rechargeable battery commonly used in consumer electronics like digital cameras, cordless phones, toys, and more. When it comes to storing NiMH batteries long-term, there is some debate over whether they should be fully charged or partially discharged.
Quick Answer
The quick answer is that for long-term storage, NiMH batteries should be stored partially charged, at around 40-50% of capacity. Fully charging a NiMH battery before storage can lead to permanent loss of capacity over time.
Why Should NiMH Batteries Not Be Stored Fully Charged?
There are a few key reasons why fully charging a NiMH battery prior to long-term storage is not recommended:
- Overcharge – When fully charged, a NiMH battery reaches its maximum voltage. Leaving it in this state for extended periods can cause damage from overcharging.
- Self-discharge – NiMH batteries naturally self-discharge over time. When fully charged, this process is accelerated, shortening the battery’s lifespan.
- Loss of capacity – Storing a NiMH battery fully charged can lead to a permanent loss of capacity over time. This is known as voltage depression.
- Internal resistance – Extended full charging increases internal resistance in NiMH batteries, reducing performance and capacity.
Partially discharging a NiMH battery to around 40-50% state of charge before storage avoids most of these issues. The lower voltage reduces stress on the battery and slows the self-discharge process.
What Happens When Storing NiMH Batteries Fully Charged?
Here is a more detailed look at what can happen to NiMH batteries stored long-term while fully charged:
Overcharge Damage
When a NiMH battery reaches full charge, it hits its maximum voltage of around 1.4-1.5V per cell. If left in this state continuously, the electrolyte can start to break down, causing permanent damage to the internal structure of the battery. This is known as overcharge damage.
Accelerated Self-Discharge
All rechargeable batteries naturally lose some of their charge over time when not in use. This is called self-discharge. In NiMH batteries, self-discharge occurs faster when the battery is fully charged. Storing at full charge will cause it to self-discharge at an accelerated rate.
Voltage Depression
Voltage depression is a phenomenon in NiMH batteries where the battery permanently loses some of its maximum voltage capacity when stored long-term while fully charged. This voltage loss lowers the battery’s overall capacity.
Increased Internal Resistance
Extended storage at full charge allows crystalline formations to develop on the battery plates. This increases the battery’s internal resistance, meaning the battery will deliver less current and have reduced capacity.
All of these effects compound over time when NiMH batteries are stored fully charged. The longer the storage duration, the more capacity and performance the battery will lose.
What Is the Recommended Charge Level for Storing NiMH Batteries?
To avoid the negative effects of full charging, most experts recommend storing NiMH batteries at 40-50% of their rated capacity.
At this midpoint charge level, the battery voltage and other factors are optimized to minimize stress and self-discharge effects during storage.capacitance over time compared to fully charged storage.
Some key benefits of storing NiMH batteries around half charge include:
- Reduced self-discharge rate
- Avoids overcharge damage
- Helps prevent loss of capacity
- Lowers internal resistance growth
Storing NiMH batteries at the 40-50% charge level provides a good balance between minimizing voltage-related aging while still keeping the battery partially charged. Letting the battery discharge to 0% before storage is not recommended either.
How to Store NiMH Batteries at 40-50% Charge
To properly charge NiMH batteries for storage at the optimal 40-50% charge level, follow these steps:
- Fully charge the battery as normal.
- Once fully charged, immediately discharge the battery until it reaches about 40-50% state of charge. For AA NiMH batteries, discharge to around 1.2V per cell.
- Store the batteries in a cool, dry place at normal room temperature. Fridge temperatures can damage NiMH batteries.
- Check the charge every 6-12 months and recharge back to 40-50% if needed.
Using a NiMH battery charger that has discharge capability makes this process easier. There are also some smart chargers that have storage mode and will automatically discharge to an ideal level for storage.
How Long Can NiMH Batteries Be Stored?
When stored at the recommended 40-50% charge level, NiMH batteries can generally be stored for:
- 6-12 months at room temperature
- 2-3 years in a cool environment
Colder storage temperatures below freezing are not recommended, as this can permanently damage NiMH batteries. But storage in a reasonably cool place at around 10°C (50°F) can help extend the storage life closer to 2-3 years.
No matter what the storage period, it’s a good idea to check the battery voltage every 6-12 months. If the voltage has dropped significantly below the 40% level, recharge back to 40-50% again.
Do Other Rechargeable Batteries Also Self-Discharge?
Self-discharge during storage affects all rechargeable battery types, but the rate of self-discharge varies:
| Battery Chemistry | Self-Discharge Rate |
| Nickel-Cadmium | 10-20% per month |
| Nickel-Metal Hydride | 15-20% per month |
| Lithium-Ion | 2-3% per month |
| Lead-Acid | 5% per month |
NiMH and NiCd batteries have relatively high self-discharge rates. Lithium-ion batteries hold their charge much longer. But all battery types will gradually self-discharge over time in storage.
Should Other Rechargeable Batteries Be Stored Fully Charged?
For most rechargeable battery types, partial charges are also optimal for storage:
- NiCd – 40% state of charge recommended
- Li-ion – 40% charge optimal for long-term storage
- Lead-acid – 50% charge level ideal for storage
Fully charging NiCd, lithium-ion, and lead-acid batteries prior to long-term storage can lead to some of the same problems as with NiMH, like accelerated self-discharge and loss of performance. Storing at around 40-50% charge avoids this.
Can Storing NiMH Batteries Fully Charged Permanently Damage Them?
Storing NiMH batteries fully charged can lead to permanent damage and capacity loss over time. However, this depends on the length of storage:
- Less than 3 months – Unlikely to cause permanent damage
- 6-12 months – Some permanent capacity loss possible
- 1+ years – High chance of permanent damage when fully charged
For storage periods under 3 months, permanent damage is very unlikely even at full charge. But past 6 months, permanent loss of capacity becomes increasingly likely the longer the batteries stay fully charged.
Can NiMH Batteries Be Restored After Storage?
If NiMH batteries have lost capacity after improper long-term storage, some of that capacity can potentially be restored. Restoration methods include:
- Full discharge/charge cycles – Completely discharge and recharge the battery several times. This can reactivate some of the battery’s capacity.
- Deep discharge – Discharge battery fully to 0% then recharge. Helps rejuvenate the battery.
- Refresh charger – Use a smart charger with refresh mode to attempt to restore capacity.
While these methods may help restore some battery performance, they cannot fully recover capacity lost from long periods of improper storage at full charge.
Frequently Asked Questions
Should NiMH batteries be stored charged or uncharged?
NiMH batteries should be stored partially charged at 40-50% rather than fully charged or fully discharged. This midpoint charge level optimizes storage life and capacity retention.
What happens if you store NiMH batteries fully charged?
Storing NiMH batteries fully charged can lead to damage from overcharging, increased self-discharge, permanent loss of capacity, and higher internal resistance over time.
How long can you store fully charged NiMH batteries?
NiMH batteries can typically be stored fully charged for up to 3 months before more significant long-term damage occurs. But partially charging to 40-50% is still recommended even for short-term storage.
Should NiMH batteries be refrigerated for storage?
No, refrigerating NiMH batteries is not recommended. While cooler temperatures help slow self-discharge, fridge temperatures below freezing can actually damage NiMH batteries permanently.
What is the shelf life of NiMH batteries?
When stored properly around 40-50% charge, NiMH batteries have a typical shelf life of 6-12 months at room temperature. In a cool environment, this can extend up to 2-3 years before capacities start to significantly degrade.
Key Takeaways on NiMH Storage
- Fully charging NiMH batteries before storage can damage them and reduce capacity over time.
- Storing NiMH batteries at 40-50% charge minimizes battery aging effects.
- NiMH batteries have a relatively high self-discharge rate when fully charged.
- Permanent capacity loss is possible if NiMH batteries are stored long-term fully charged.
- NiMH batteries can typically be stored 6-12 months at room temperature, or 2-3 years in cool environments when at 40-50% charge.
Conclusion
In summary, fully charging NiMH batteries prior to storage is not recommended. The ideal charge level for long-term storage is around 40-50% of rated capacity. This midpoint charge helps minimize damage from overcharging effects, voltage depression, increased self-discharge, and internal resistance growth during the storage period. While inconvenient, taking the extra step to partially discharge a NiMH battery before storage can dramatically improve the battery’s lifespan and prevent permanent loss of capacity over time.
