If you have solar and took the choice to have a battery, you really need to know the facts or it could cost you thousands.
There are many options of batteries on the market and solar companies are selling packages with grossly under sized battery packs. When solar batteries are installed that are small we call these buffer batteries. The reason for the bugger battery is that solar power is not a constant, it can jump and down a few hundred watts every few seconds, so the battery buffers the solar power to give you a more constant supply.
Where the problem starts is in winter when there is no to little sun and the battery is at a constant low state of charge. This is NOT only limited to winter as undersized batteries will be discharged every night.
Therefore the settings of the battery and inverter really need to be carefully reviewed. high SOC will erode the positive electrode but the negative at the high and low is not effected to such a degree.
Older installations or undersized battery packs that store solar power come in a few versions but where capacity meets quality, tends to differs. The battery cells used in a lot of lower cost batteries are not able to provide the power demands. This was noted within Pylontech batteries which could only provide 1.5kw of power, while being attached to a 3.6kw inverter.
The rate of which a battery is charged and discharged effects the life of the battery. Other batteries using a full DOD would of course be shortening the cycle life of the battery packs, however the rate of charge and discharge also effects the life cycle on the battery.
The sweet spot, which was later designed into battery systems was a buffer of capacity hidden from the user. Typically this was 5% of the full AH capacity, this with a voltage range limit to -20% would deliver the effect of using the battery, and to be discharged to 80% DOD (Our batteries anyway) This mitigated user problems or inverter problems, to use the full capacity and therefore greatly reduce the life of the battery down to as little as 5 years from 15+ years. Also in design, we use the battery life system was enhanced by 28%.
The battery C rate, is the power in amps relative to the capacity in amp hours of the cell, and typically a .5 rate is fine, above this and your battery life is shortened. But below this charge rate, the life is extended. But that is relative to the rating and claims from the manufacture.
However recent tests showed that a lower charge rate benefitted the battery life and capacity, if it were kept below 0.4C however the lower charge current had no positive effects on the battery.
25 degrees is a good temperature for the battery, yet in winter temperatures below 10oC you will notice a drop in the capacity of the battery, meaning you are going to get less from the battery, and it can be 30% less.
The colder batteries are harmed by a high charge rate, therefore is it important to start the profile with a lower rate to put heat into the cells, this can be achieved by using a discharge before the charge cycle.
Depth of Discharge (DoD)
DoD refers to the percentage of a battery’s capacity that is discharged. LFP batteries generally have a higher tolerance for deep discharges compared to other lithium-ion chemistries. However, frequent and deep discharges can still accelerate aging and reduce the overall cycle life of the battery.
State of Charge (SOC)
SOC indicates the amount of energy remaining in a battery. While LFP batteries can handle a wider operating range of SOC compared to other lithium-ion batteries, it’s still recommended to avoid consistently operating at very high or very low SOC levels. Extreme SOC levels can lead to increased stress on the battery’s internal components, potentially reducing its lifespan.
The Argument
The argument regarding LFP batteries and their relationship to DoD and SOC is that while they are more tolerant than other lithium-ion chemistries, optimal performance and longevity can be achieved by:
- Avoiding frequent and deep discharges: Limiting the depth of discharge can significantly extend the battery’s cycle life.
- Maintaining a moderate SOC range: Operating within a reasonable SOC range, avoiding both extremely high and low levels, can help preserve battery health.
By considering these factors, it is possible to maximize the lifespan and performance of LFP batteries in various applications and energy storage systems.
LFP batteries, while robust, are still subject to the effects of charging rates.
Higher Charge Rates:
Argument:
- Faster Charging: Higher charge rates allow for quicker replenishment of the battery, making them more convenient for applications like electric vehicles. Many users will have a limited charge window.
Detriment:
- Reduced Cycle Life: Rapid charging can increase internal stress on the battery cells, potentially leading to a shorter lifespan.
- Increased Heat Generation: Higher charge rates can generate more heat, which can further accelerate aging and degrade performance.
- Potential Safety Risks: Excessive heat can pose safety risks, especially if not managed properly.
Lower Charge Rates:
Argument:
- Longer Cycle Life: Slower charging reduces stress on the battery cells, leading to a longer lifespan.
- Improved Safety: Lower charge rates generate less heat, reducing the risk of thermal runaway and other safety hazards that occur with certain batteries.
Detriment:
- Slower Charging Time: Lower charge rates mean longer charging times, which can be inconvenient for some applications.
Balancing the Trade-off
The optimal charging rate for LFP batteries depends on various factors, including specific battery chemistry, ambient temperature, and intended use. A balanced approach that considers both charging speed and battery longevity is crucial.
Key Considerations:
- Temperature Management: Effective temperature control during charging can mitigate the negative effects of higher charge rates.
- Charge Profile Optimisation: Carefully designed charging profiles can help minimise stress on the battery cells.
- Battery Management Systems (BMS): Advanced BMS can monitor battery health and adjust charging parameters to optimise performance and lifespan.
By understanding the trade-offs between higher and lower charge rates, and by implementing appropriate strategies, it is possible to maximise the performance and lifespan of LFP batteries.
Our Recommendations.
Keep the battery at a stable temperature – above 10degrees. but ideally room temperature (21 degrees).
Maximise the charge time. Working out the charge rate to be the lowest will extend the life of the battery.
Ensure your load is balanced. Adding more batteries will divide the current and rates, putting less stress on the battery packs. One big battery is not the answer.
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