CAN AMPTRON LiFePO4 BATTERIES CHARGE FROM A STANDARD COMMERCIAL SOLAR MODULE OR DOES IT REQUIRE A SPECIAL MODULE? AND IS THERE A NEED FOR A CONTROLLER?
The battery can be charged from a standard commercial solar module, but it definitely requires a solar charge controller / regulator. When it comes to solar charge controllers, our batteries can be charged from most solar charge controllers, however there would be compromises if the charge controller does not have a Lithium/LiFePO4 charge profile (see more about this below).
When it comes to modern solar charge controllers, there are 2 main characteristics that needs to be considered:
Solar input conversion type
- PWM (Pulse Width Modulation) controllers – Cheaper and lighter / smaller, but less efficient controllers. The PWM controller is in essence a fast switch that connects a solar array to a battery. The “switch” is “switched” ON and OFF as needed (pulse width modulated) to increase the battery voltage to the absorption voltage. The result is that the voltage of the solar array will be pulled down to near that of the battery. The controllers slowly lower the amount of power applied to the batteries as the batteries get closer and closer to fully charged, but the excess energy is “wasted”.
- MPPT (Maximum Power Point Tracking) controllers – More expensive but more efficient controllers. The MPPT controller could be considered to be a “smart DC-DC converter”, i.e. it drops the panel voltage down to the voltage required to charge the battery at the most optimum charge current, using Ohm’s law. The charge current is increased in the same ratio as the voltage is dropped, just like a conventional step-down DC-DC converter. The “smart” element in the DC-DC converter is the monitoring of the “maximum power point” of the panel which will vary during the day with the sun strength and angle, panel temperature, shading and panel(s) health. The “smarts” then adjusts the input voltage of the DC-DC converter. This controller makes optimum use of the available energy, and can typically produce 20% or more current into the battery than a comparable size PWM controller under the right conditions.
- Battery charge regulation
Just like regular battery chargers, the solar charge controller regulates the voltage and current flowing from the solar panel into the battery bank to fully charge the batteries while also avoiding overcharging the batteries. As with a regular battery charger, often various battery types can be accommodated. The differences come with the different charge profiles that are accommodated by the particular charge controller. Many charge controllers only have charge profiles for lead acid (Gel, AGM, Flooded) batteries, and only some charge controllers have specific charge profiles for Lithium, and in particular LiFePO4 batteries.
Amptron’s LiFePO4 batteries can potentially be charged with conventional lead acid charge profiles (due to the built-in Battery Protection system), however an LiFePO4 charge profile have advantages as explained below:
- A Lithium charge profile vs. a Lead Acid profile usually has a slightly higher charge voltage and a “deeper” constant voltage phase at the end of the charge cycle, which is able to get the Lithium batteries to a full 100% State of Charge. A Lead Acid profile will not normally be able to achieve this, however it will still get close, say maybe 95% +. Research has shown that you can extend the life of Lithium batteries if you prevent charging to 100%, however the effect only has a pronounced impact if charged to less than 80%. Between 80% to 100% State of Charge (SOC), there is not much observable difference to the battery life. For this reason in real life cyclic applications, it is not practical to reduce the charged SOC to less than 80%, and therefore you may as well aim for close to 100% SOC to obtain the maximum usable capacity out of the battery. However, using a Lead Acid charge profile will not cause damage to the LiFePO4 battery if not charging to a full 100% SOC..
- The more significant issue is that the Lead Acid vs Lithium charge profiles have different “return to boost” behavior. After the charge controller completes its full charge cycle (i.e. it reaches the end of the absorption phase and changes over to the float stage), the charger will then start to monitor the battery voltage, to decide when to switch back to the boost/bulk charge mode and restart a full charge cycle again. The issue is that most Lead Acid charge profiles will wait for the voltage to drop too low before it restarts the charge cycle – that is because a Lead Acid battery has a lower resting voltage than a Lithium battery. The implications are that the Lithium batteries’ State of Charge may already be quite low before the Lead Acid profile decides to switch back into a full bulk charge again. On the contrary, a Lithium profile will start to recharge the Lithium battery much sooner, therefore keeping it topped up. The Lead Acid charge profile will eventually switch back into a full charge cycle again, but often not until the Lithium battery has been drained to a low state of charge first, whereas you may have expect the battery to have been fully charged. This could also mean that when charging from solar panels, energy is wasted that could have been used to top up the batteries because the Lead Acid charge profile recognised too late that the Lithium batteries need charging.There is a potential work-around for this issue which is to disconnect the solar charge controller from the batteries temporarily, (either via switching the charger off or via a kill-switch installed on the positive line between the charge controller and the battery), allowing you to temporarily disconnect the connection to the batteries. Disconnecting the charger and then reconnecting, should reset most solar charge controllers and force a full charge cycle again, but this will have to be done manually and can be inconvenient.
- Further, a Lead Acid charge profile will enter a float stage at the end of the charge cycle. This is designed to trickle charge lead acid batteries to compensate for self-discharge, and any excess energy is dissipated as heat. However, LiFePO4 batteries have a very low self-discharge rate which do not need float charging. The issue is that when float charging LiFePO4 batteries for long periods of time, especially if the voltage is above the resting voltage of the LiFePO4 cells, the electrode is likely to produce pure Lithium out of the Lithium ions which causes metallic plating and reduce the capacity over time. This can have a negative effect on the lifespan of the batteries. Similar to point 2 above, a work-around is to switch off / disconnect the charger from the battery when it is not being used, to avoid long term float charging. Alternatively, if this is too much of a hassle or you don’t want the compromises, you can change the charge controller to a model with a specific Lithium profile.
Thus, there are advantages for the lifespan of the batteries to be charged through an Lithium charge profile, and if you want to get the full capacity and performance out of the Lithium battery, you will need a charge controller with a Lithium profile.
To sum all of this up, yes you can charge our Amptron LiFePO4 batteries with standard commercial solar panels, but a solar charge controller will be needed. As for charge controllers, an MPPT charge controller would be the preferred option unless lowering cost and/or size is important to you. Further, it is highly preferable to use a solar charge controller that can charge the battery with an LiFePO4 profile.