SLUAAR1 july 2023 BQ24630 , BQ25170 , BQ25180 , BQ25300 , BQ25620 , BQ25730 , BQ25798
2 Charge Profile and SOC vs OCV
LiFePO4 and Li-ion batteries share the same charge profile shown in Figure 2-1. This charge profile is a standard Pre-charge, CC, and CV charge profile, however, since LiFePO4 and Li-ion batteries have different voltage profiles, these stages in the charge profile happen at different voltages. For Li-ion batteries, VOREG≈ 3.9-4.2 V, VPrecharge ≈ 3.0 V, and VShort ≈ 2.0 V. For LiFePO4 batteries, VOREG ≈ 3.5-3.65 V, VPrecharge ≈ 2.0 V, and VShort ≈ 1.2 V.
Furthermore, LiFePO4 and Li-ion batteries have similar charge rates, but Li-ion typically has a discharge rate of 1C whereas LiFePO4 can have discharge rates of 3C. This makes LiFePO4 good for higher current applications.
Figure 2-1 Standard CC/CV Charge Profile
Another key difference between LiFePO4 and Li-ion batteries are their SOC (State of Charge) vs OCV (Open Circuit Voltage) profiles. As can be seen in Figure 2-2, Li-ion batteries have a fairly linear SOC vs OCV profile whereas LiFePO4 batteries are fairly linear for the approximately 85% to 100% SOC range, but has an abrupt change in slope for the approximately10% to approximately 85% SOC range. This becomes significant when choosing what charge voltage accuracy is needed in a design and the SOC the battery will be charged to [1].
[1] A designer can choose to charge their battery at a lower SOC to reduce the capacity degradation due to decomposition of the graphite on the anode.
Figure 2-2 Li-ion SOC vs OCV Profiles
Figure 2-3 LiFePO4 SOC vs OCV Profiles
Small charge voltage inaccuracies can cause a large percentage of the battery’s capacity to go unused especially if the designer decides to try to preserve the cycle life of the battery by charging the battery to a SOC lower than 100%. Table 2-1 and Table 2-2 illustrates this quite well by showing how much battery capacity you can lose for Li-ion and LiFePO4 batteries due to charge voltage inaccuracy. Table 2-1 this when charging to a SOC 100% and Table 2-2 illustrates this when charging to a SOC of 80%.
To explain how we got the results from Table 2-1 and Table 2-2, we give the following example from Table 2-1. If the designer uses a charging design with a charge voltage regulation accuracy of +/-2% to charge a Li-ion battery, then the charge voltage needs to be set at 98% taking in consideration of the +2% tolerance if the design target is not to let the battery voltage surpass 100% of charge voltage. As a result, the minimum Vbat can be 96% of the maximum charge voltage because of the negative end of the charge voltage accuracy. So, with a charge voltage accuracy of ±2%, The battery is possible to be charged 4% below the maximum charge voltage and this can result in up to 13.2% of the battery’s capacity to go unused.
As can be seen in Table 2-1, even a charge voltage accuracy of ±0.5% can result in a 3% loss of battery capacity. This only gets multiplied more with worse charge voltage accuracies. When comparing LiFePO4 to Li-ion batteries, LiFePO4 perform better when they are charged to a SOC of 100% because LiFePO4’s SOC vs OCV profile has a smaller slope for higher SOC compared to Li-ion batteries.
However, as can be seen in Table 2-2, charging LiFePO4 to a SOC of 80% to preserve its cycle life becomes essentially impractical. Even with a charge voltage accuracy of ±0.5%,you can be losing up to 41.4% of the battery’s available capacity. This is an additional 21.4% of battery life on top of the 20% you are losing by deciding to charge to a SOC of 80%.
| Charger design | Charge Voltage Accuracy | Battery | Minimum Vbat (mV)(1) | Capacity at Vbat | Maximum Capacity Loss | Capacity Loss Compared to TI design |
|---|---|---|---|---|---|---|
| Texas Instruments | ±0.5% | Li-ion | 4147 | 97.0 % | 3.0 % | - |
| LiFePO4 | 3612 | 98.3 % | 1.7 % | - | ||
| Competitor | ±1% | Li-ion | 4342 | 93.6 % | 6.4 % | 3.4 % |
| LiFePO4 | 3576 | 96.6 % | 3.4 % | 1.7 % | ||
| Discrete | ±2% | Li-ion | 4298 | 86.8 % | 13.2 % | 10.2 % |
| LiFePO4 | 3503 | 93.2 % | 6.8 % | 5.1 % | ||
| Discrete | ±3.5% | Li-ion | 4232 | 75.5 % | 24.5 % | 21.5 % |
| LiFePO4 | 3393 | 87.9 % | 12.1 % | 10.4 % |
| Charger design | Charge Voltage Accuracy | Battery | Minimum Vbat (mV)(1) | Capacity at Vbat | Maximum Capacity Loss | Capacity Loss Compared to TI design |
|---|---|---|---|---|---|---|
| Texas Instruments | ±0.5% | Li-ion | 4111 | 75.8 % | 24.2 % | - |
| LiFePO4 | 3297 | 58.6 % | 41.4 % | - | ||
| Competitor | ±1% | Li-ion | 4091 | 71.8 % | 28.2 % | 4.0 % |
| LiFePO4 | 3263 | 30.6 % | 69.4 % | 28 % | ||
| Discrete | ±2% | Li-ion | 4049 | 63.6 % | 26.4 % | 12.2 % |
| LiFePO4 | 3197 | 12.5 % | 87.5 % | 46.1 % | ||
| Discrete | ±3.5% | Li-ion | 3987 | 47.2 % | 52.8 % | 28.6 % |
| LiFePO4 | 3096 | 6.7 % | 93.3 % | 51.9 % |
When considering that LiFePO4 batteries already have a long cycle life compared to other battery chemistries, most designers charge their LiFePO4 batteries to a SOC of 100% because they are limited by the lower energy density. On the other hand, Li-ion batteries do not have as significant capacity loss when charging at a SOC of 80%, so it can be practical for the designer to try to preserve the cycle life of their Li-ion batteries by charging the batteries to a SOC of 80%.
Nevertheless, any unused capacity of the battery means that the designer has to buy an even bigger battery to support their needs. This means that if your design truly needs a 10Whr battery for your application, and your design doesn’t utilize 15-30% of the battery, either by charge voltage inaccuracy and/or in order to preserve the cycle life of the battery, you will need to buy a 15-30% bigger battery. And, since the battery is usually one of the most expensive parts of the system, the tradeoffs to save a few cents on the battery charger and/or the potential increase in cycle life typically do not outweigh added cost of having to buy a bigger battery. Furthermore, when you consider the additional features that a battery charger design can provide, the benefits of a good charger design outweigh the costs.