How To Charge Lithium Iron Phosphate (LiFePO4) Batteries
If you’ve recently purchased or are researching lithium iron phosphate batteries (referred to lithium or LiFePO4 in this blog), you know they provide more cycles, an even distribution of power delivery, and weigh less than a comparable sealed lead acid (SLA) battery. Did you know they can also charge four times faster than SLA? But exactly how do you charge a lithium battery, anyway?
Power Sonic recommends you select a charger designed for the chemistry of your battery. This means we recommend using a lithium charger, like the LiFe Charger Series from Power Sonic, when charging lithium batteries.
CAN A LEAD ACID CHARGER CHARGE A LITHIUM BATTERY?
As you will learn in this blog, there are many similarities in the charging profiles of SLA and lithium. However, extra caution should be exercised when using SLA chargers to charge lithium batteries as they can damage, under charge, or reduce the capacity of the lithium battery over time. There are many differences when comparing lithium and SLA batteries.
Let’s go back to the basics of how to charge a sealed lead acid battery. The most common charging method is a three-stage approach: the initial charge (constant current), the saturation topping charge (constant voltage), and the float charge.
In Stage 1, as shown above, the current is limited to avoid damage to the battery. The rate of change in voltage continually changes during Stage 1 eventually beginning to plateau when the full charge voltage limit is approached. The constant current/Stage 1 portion of the charge is crucial before moving onto the next stage. Stage 1 charging is typically done at 10%-30% (0.1C to 0.3C) current of the capacity rating of the battery or less.
Stage 2, constant voltage, begins when the voltage reaches the voltage limit (14.7V for fast charging SLA batteries, 14.4V for most others). During this stage, the current draw gradually decreases as the topping charge of the battery continues. This stage terminates when the current falls below 5% of the battery’s rated capacity. The last stage, the float charge, is necessary to keep the battery from self-discharging and losing capacity.
Stage 3 is used if the battery is being used in a standby application, the float charge is necessary to ensure the battery is at full capacity when the battery is called upon to discharge. In an application where the battery is in storage, float charging keeps the SLA battery at 100% State of Charge (SOC), which is necessary to prevent sulfating of the battery that therefore prevents damage to the plates of the battery.
LIFEPO4 BATTERY CHARGING PROFILE
A LiFePO4 battery uses the same constant current and constant voltage stages as the SLA battery. Even though these two stages are similar and perform the same function, the advantage of the LiFePO4 battery is that the rate of charge can be much higher, making the charge time much faster.
Stage 1 battery charging is typically done at 30%-100% (0.3C to 1.0C) current of the capacity rating of the battery. Stage 1 of the SLA chart above takes four hours to complete. The Stage 1 of a lithium battery can take as little as one hour to complete, making a lithium battery available for use four times faster than SLA. Shown in the chart above, the Lithium battery is charged at only 0.5C and still charges almost 3 times as fast! As shown in the chart above, the Lithium battery is charged at only 0.5C and still charges almost 3 times as fast!
Stage 2 is necessary in both chemistries to bring the battery to 100% SOC. The SLA battery takes 6 hours to complete Stage 2, whereas the lithium battery can take as little as 15 minutes. Overall, the lithium battery charges in four hours, and the SLA battery typically takes 10. In cyclic applications, the charge time is very critical. A lithium battery can be charged and discharged several times a day, whereas a lead acid battery can only be fully cycled once a day.
Where they become different in charging profiles is Stage 3. A lithium battery does not need a float charge like lead acid. In long-term storage applications, a lithium battery should not be stored at 100% SOC, and therefore can be maintained with a full cycle (charged and discharged) once every 6 – 12 months and then storage charged to only 50% SoC.
In standby applications, since the self-discharge rate of lithium is so low, the lithium battery will deliver close to full capacity even if it has not been charged for 6 – 12 months. For longer periods of time, a charge system that provides a topping charge based on voltage is recommended. This is especially important with our Bluetooth batteries where the Bluetooth module draws a very small current from the battery even when not in use.
Charging a LiFePO4 Battery
Charging a LiFePO4 battery basically means applying an external voltage to drive current from the anode to the cathode of the battery. The lithium battery charger acts as a pump, pumping current upstream, opposite the normal direction of current flow when the battery discharges.
When the charger’s applied voltage is higher than the open-circuit battery voltage, then the charging current flows. During this process, the battery’s open-circuit voltage increases, approaching the applied voltage of the charger.
Bulk Vs. Float Charge
The lithium battery charger can behave in several different ways during the charging process. First, the charger can steadily increase its voltage in order to keep the current flow constant. This is the first stage of the charging process – typically called the “bulk” charging stage. During this stage, the charger adjusts its applied voltage to deliver the maximum current to the battery.
For example, a 10 amp charger will deliver its maximum of 10 amps during this bulk charging stage, and the applied voltage will increase up to a maximum voltage, or “bulk voltage.”
Once the bulk voltage is reached, the charger enters a second stage, called the “absorption” charging stage. During absorption, the charger applies a constant voltage, called the “absorption voltage.” As the battery’s open-circuit voltage approaches the absorption voltage, the current flow steadily decreases down to zero.
At this point, the battery is fully charged. However, a lead-acid battery will rapidly lose charge when the charger is disconnected. So, instead of turning off, the battery charger enters a third stage called the “float” stage, in which the charger drops to a lower voltage and holds at that voltage. The point of this stage is to keep the battery topped off, and account for the fact that lead-acid batteries tend to drain, even when there is no load connected.
The Charging Algorithm
These stages combined sequentially form what is commonly called the “charging algorithm.” A battery charger may generally be classified by a charging current (i.e. the max charging current) and a target battery voltage (12 V, 24 V, 36 V, 48 V, etc.). But battery chargers may also include multiple charging algorithms (typically classified as “AGM,” “SLA,” “Gel,” “Wet,” etc.). A closer look reveals that each algorithm has its own unique parameters, including:
Bulk voltage Absorption voltage Absorption time Float Voltage
There is a wide variation among values for charging algorithms for lead-acid batteries. The bulk and absorption voltages typically vary between 14.0 and 14.8 V, and the float can vary between 13.2 and 13.8 V.
The 12V Battle Born Batteries sit comfortably right in the middle of these ranges. We recommend a bulk and absorption voltage of 14.4 V. A float is unnecessary, since li-ion batteries do not leak charge, but a floating voltage under 13.6 V is fine.
Here are a few FAQ videos that talk about charging LiFePO4 batteries.
In this blog series, we will post the results of our tests for a variety of LiFePO4 chargers – including converters, inverter chargers, and solar charge controllers. In each case, we will report on the uniqueness of the charging algorithms for each brand, explore the efficacy of using the factory default settings for charging Battle Born Batteries, and determine what can be done to achieve the optimal settings.
If you have any questions or concerns about charging LiFePO4 batteries, please contact us at any time at (855) 292-2831 or email us at [email protected].
Step 2: Right Solar Cell
solar cell should be maximum 6V, because TP4056 has maximum input 6V. It is better then 5V.
current from solar cell (or power) can be variable, because TP4056 eat as much as it need. So you can choose 500 mAh solar cell or 1 Ah solar cell.
For Li. Ion battery I choose solar cell with 5V and 160 mA. For choosing solar cell, you must choose:
voltage of solar cell 1.5 x voltage of battery, so 3.7V to 4.2 V of Li-Ion is equivalent of 5.55 V to 6.3 V of solar cell.
current of solar cell should have 1/10th of capacity battery diveded by 1 hour (for Ni Mh batteries). I use same rule for Li. Ion battery. It is called C. rate rule. So If I have 500 mAh battery, I should choose 50 mA sollar cell. Good Li- Ion batteries have 2000 mAh, so current should be around 200 mAh or 1.2 W.
I use bad Li. Ion battery with measured around 600 mAh. For that, I should choose solar cell with 60 mA peak, or 0.360 W (POWER = CURRENT X VOLTAGE).
Step 5: LED Diodes on TP Board
On board, there are 2 diodes, which also consupt some power. I remove them with knife. Check picture.
Test you charging, you can connect your multimeter to solar cell, or battery.
cloudy, with a little sunny 10 mA (output current from TP4056), 24 mA (from solar cell)
cloudy, not direct to sun 0.87 mA (TP4056), 5.1 mA (solar cell)
sunny, direct sun 26 mA (TP4056), 89 mA (solar cell)
According pveducation.org website, you can calculate direct solar radiation in kW. Just fill your home lattitude and longtitude. And remember time, because radiation during day vary. I got around 1 kW/m2.
So, solar cell give me 89 mA, and 5V, so it gives 445 mW, or 0.445 W. Surface of solar cell is around 70 cm2 (basically only small lines make energy, so around 30 cm2).
Solar cell output = 0.089A x 5 V = 0.445 W
TP4056 output = 0.026 A x 4 V = 0.104 W.
To calculate how much solar radiation fall on 30 cm2 according pv education website, we must convert surface to m2, it is 0. 00 30 m2. Incident radiation is 1000 x 0.003 = 3 W.
Incident radiation = 3W
Efficiency of solar cell = 0.445 W / 3 W = 0.1483 = 14.8 %.
Efficiency of TP4056 = 0.104 W / 0.445 W = 23.37 %
Total efficiency of system = 0.104 W / 3W = 0.034666 = 3.46 %.
So total efficiency is not much, but helps. Do you remember C-rate? For this project, the bigger solar cell is necessary. I test on september, which is average between winter and summer. I use battery for my esp logger, which must survive during winter, summer is good. I will test others solar cells, in future, and show my results.
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Комментарии и мнения владельцев
Couldn’t you use a dc dc 5v 1a boost converter so that the current is 1a and put a schottkly diode on it?
appreciate your DIY on solar charging. how are we addressing the safety of the circuit, esp when 18650 is being used, which is sensitive to voltage fluctuations and might result in fire hazard, if handled improperly. my concern / query : TP4056 input voltage range from 6 volts to 8 volts. you are using 6V solar panel.during peak time of charging. how do you ensure that your input voltage doesnt cross 8v? if it does then its going to be dangerous to the circuit right? i mean safety aspect of the circuit.(since 6v solar panel can deliver up to 9volts at its peak.).
in addition to input diode. should we have something else as addition to drop the voltage to 8v incase it exceeds the same? in this case we know max is 9v for solar panel output.
I think diode drop some voltage. That drop depend on current, bigger current from solar cell, higher drop. So during very shine, current is high and drop also. Try to check graph. drop vs current in diode datasheet
I have a 3watt solar cell.it gives 10v on no loads and approximately 7v 300mah on loads on sunny day.i want to make a garden light with this.i want to use 3.7v 4000mah li-poly batery to charge from solar cell with tp4056 module.can i use 7805 or lm317 voltage regulator ic?
I read here (https://www.best-microcontroller-projects.com/tp4056.html) that If you have a load connected to the battery then this will change the current detected so the TP4056 may never terminate the charging process.
I would like to know your opinion, sice i think most users will use it for charging with load.
hello, i dont know. I think best is testing with multimeter. It is possible to damage battery because TP4056 will always pump battery with current?
That is my point. Even if the module with output pins is used, it seems it will still charge the battery when load is connected. Overcharging LiPo may damage them or even worse.
and I think, better TP4056 module is version with 4 output pins, 2 pins for load, 2 pins for battery. So battery can be fully charged and load will be powered from solar cell.
Could you give me some further explanation why you choose solar panel voltage to be 1/5 of battery voltage and solar panel current to be 1/10 of battery current divided by 1 hour.
I hope you understand, and basically choosing solar panel depend on TP4056, not battery.
voltage. input of TP4056 must be higher then 4,2 V so I choose 1.5 times bigger voltage.
Current. C-rate law. I just check what usually is charging current for lions battery. it is around 500 mA, but smaller currents are good. too. Also, during summer, the battery is charge fully as temperature increase, the voltage of battery is higher. I dont want bake battery.
hi why not use scotty diode? example 1N5819 5819 1A 40V SCHOTTKY DIODE?voltage drop not 0,6-0,8V only 0,1-0,2 V ?
LiFePO4 Battery Charge Settings Explained
The following are some of the most common specifications you will find in charge controllers. Check your controller instructions for more detailed information.
Boost charge mode. The controller charges at the highest power level until the boost mode value is attained. The controller will attempt to draw max power until it reaches the target voltage. The duration can be adjusted.
Boost reconnect voltage. When the system is at float, the voltage can change due to solar output. The system goes back into boost if the voltage drops below the boost reconnect voltage value
Charge limit voltage. The controller stops charging the battery if the battery voltage is higher than the charge limit voltage.
Discharging limit voltage. Sends a warning at the given voltage set.
Equalize charge voltage. Refers to the voltage used over a specific period. This is applied after the boosttarget voltage has been attained.
Equalize duration. This is the absorption phase. When the boost period is reached, voltage is now constant.
Float charge voltage. Once the boost stage is finished, the controller adjusts the power search. The panels are set to generate a constant voltage float.
Low voltage disconnect. When this voltage is reached, the battery load output is disconnected.
Low voltage reconnect. This is turned on if the load is disconnected because of low battery power.
Over voltage disconnect. The load output gets disconnected if the battery voltage goes over this value.
Over voltage reconnect. If the load is disconnected because the battery goes over voltage, the system will reconnect at the given value.
Under voltage warning. This is where warnings are set.
Under voltage warning reconnect. The warning is turned off at this value.
Boost, Bulk Charging and Other Settings
Some charge controllers use the terms boost and bulk interchangeably. Others consider them two different settings.
In some charge controllers, the bulk is the first part of the charge cycle. A controller remains in this phase until constant charge voltage is attained.
Constant charging follows and consists of boost and equalize. During the equalize cycle, the battery electrolytes are stirred and gassed. The boost cycle prevents too much gassing and overheating.
You can think of it this way. When you charge a LiFePO4 battery, the controller commences with the highest setting the solar panel can generate. The voltage will remain constant when the boost level is reached. The boost period can be any duration but usually it is two hours.
Boost duration is the same as the absorption phase, and absorption voltage is the same as boost charging voltage.
After the charge reaches the float phase, the controller will try to keep the voltage constant. The voltage will drop to boost reconnect under certain conditions. For instance, unfavorable weather might affect solar performance, or the load might be too much for the system. If it drops to boost reconnect the charging process will restart.
The equalization duration period is usually at zero for LiFePO4 batteries. The equalization voltage must be lower than boost or equal to it. in most cases it is better to have the equalization voltage lower.
Important Reminders for Charging LiFePO4 Batteries
- Avoid 100% SOC charging whenever possible.
- Avoid a 100% SOC float.
- Cycling under 10%-15% SOC is not recommended.
- The battery temperature should be kept above 0 C / 32 F when you discharge.
- Discharge and charge currents has to be below 0.5 C / 32.9 F
- The battery temperature has to be under 30 C / 86 F
Majority of charge controllers will have no problems charging a LiFePO4 battery. its voltages are similar to AGM, gel and other lead acid batteries. All high quality LiFePO4 batteries including the BTRPower 100ah also have a BMS (battery management system) that protects it from overheating and overloading. The BMS also makes sure the battery operates at the ideal temperature and the cells are properly balanced.
To recap: when aLiFePO4 battery is charged, the system tries to maintain the current. If you are using a solar array, that means the system tries to send as much current as the solar system can deliver (without overcharging the battery).
The voltage then starts to rise until the absorb phase is reached. At the absorb level the battery is around 90% filled. For the rest of the charge the battery current tapers while the voltage remains the same. The battery reaches 100% SOC (state of charge) at 10% to 5% of its ah rating.
Compared to lead acid, v batteries are simpler to charge. Just make sure the voltage is high enough and the charge will proceed. There is no equalizing or sulphating to worry about. You do no even have to charge the battery 100%.
Lastly, do not purchase a LiFePO4 battery without a BMS. This is very important as it can mean the difference between a long lasting battery and one that dies off quickly. Buying from a reputable manufacturer is always a good thing as well.