When traveling by RV, what RV users care most about is the power system on the RV. All kinds of AC equipment such as AC air conditioners, induction cookers, refrigerators, audio multimedia systems and even electric water heaters, require the power supply system to work stably. In addition to AC appliances, there are also DC devices that draw power directly from batteries, such as DC air conditioners, DC refrigerators, DC LED lights, and DC monitors. If you are an RV user and hope that these devices can provide stable power for your every trip, it is very important to have a basic understanding of your RV power system.
When you fall in love with RVs and use RVs frequently, understanding the power system, internal structure and logical structure will be of great reference significance for you when choosing various products. Most importantly, if there is a problem, you will know where the problem is and troubleshoot it quickly. We know that not all riders are electrical engineers, we will use the simplest language to describe.
Now that we have talked about 12.8V, let’s deepen the understanding of lithium batteries. The nominal voltage of a single cell of lithium iron phosphate is 3.2V. When we use a 12V system, we connect four cells in series, so 12.8V is obtained. Many RV users are also very confused about the full voltage and the discharge voltage. This is very simple. The nominal value of a single battery cell is 3.2V. Generally, the maximum voltage of the built-in protection board is 3.65V, and the empty voltage can be up to 2.5V, but the basic setting is 2.8V. The conclusion is drawn: 4*3.65V = 14.6V when fully charged, and the voltage is 4*2.8V=11.2V when the battery is less than 10%.
Here a question that most concerned about are: If we feel that the battery in my RV is not enough, can I buy another set to increase the capacity? We see that many users are chaotic. For example, if they have a 400AH battery and a 600AH battery, the ideal state is to reach 1000AH, but this is not the case in actual use. When lithium batteries are connected in parallel, they will be based on the maximum capacity to calculate the cumulative value. Similar to the above parallel method, the number obtained should be 800AH, and this parallel method is not scientific and is prone to problems. So why is it so? Let’s explain. Assuming when you charge a 400AH battery, the single-cell voltage is 3.2V/400AH (multiple blocks in parallel) and a 600AH battery, the single-cell voltage is 3.2V/600AH (multiple blocks in parallel), you are charging two sets of batteries at the same time, and the capacity of the battery is related to the voltage. The built-in protection board also judges whether the battery is overvoltage. Under the same conditions, when the 400AH battery is charged to 3.65V, it starts the protection, feedbacks the battery voltage to the charger, and lets it stop charging. At this time, the 600AH battery is only charged to 400AH, it cannot reach 3.65V.
Charging power (W) = voltage (V) * charging current (A). The selected AC charger or MPPT, PWM solar control charger is nominal A. For example, the battery is a 12.8V lithium iron phosphate battery; because there are two stages in the charging process (lead-acid has three stages), the first stage is called CC mode (constant current). When the battery voltage is lower than 14.6V, the charger will charge with the maximum current until when the battery voltage reaches 14.6V, it will switch to CV mode (constant voltage). At this time, the charger voltage remains unchanged at 14.6V, while the charging current naturally decreases, until the charging current drops to 10% of the rated charging current, the charging is completed. Note that unlike lead-acid batteries, lithium batteries do not require a floating charge function (FC). The floating charge will affect the number of cycles of lithium batteries.
After understanding some basic common sense, we will know that there are actually two power systems in our RV, one is a 12V DC (battery) system, which supplies some DC loads, such as DC air conditioners, DC refrigerators, and USB chargers, 12V LED lights in the RV, etc. There is also a 220VAC or 120VAC power system (through an inverter) for some large electrical appliances, such as induction cookers, microwave ovens, ovens, multimedia audio-visual systems, etc.
MPPT/PWM solar controller (MPPT is maximum power point tracking, usually 20-30% higher than PWM power generation).
There is a certain space on the top of the RV to install solar panels. According to our survey, the solar panels of the general RV are 400W-800W. Here we will share the most reasonable RV solar panel power generation design. Usually, we choose even numbers when installing solar panels, because we need to involve series and parallel. If your RV is a 12V system, the best working voltage of the MPPT solar controller is 24V-36V. Of course, many MPPT controllers also say that it can work at 3V higher than the battery, but the RV is not like an off-grid solar system. The lighting and installation angle are both extremely demanding for RV, so we choose a slightly higher one. We have also checked a lot of solar panels used in RVs, which are basically around 18V, so we suggest 2 pieces in series and then parallel. If you use an 800W solar panel, it is most ideal to choose the 12V/80A MPPT solar charge controller. We have seen a lot of solar panels installed, but the actual controller did not reach, and it still can’t generate much electricity. Because of the different lighting in various places and the different parking positions of the riders, it is impossible to give an accurate answer to how much electricity the 800W solar panel can generate. However, we investigated most of the RV users who installed 800W, and they can generate about 2 kWh of electricity in 3 hours from 11:00-14:00 noon.
Here, let me share with you the experiments we have done. In fact, we have seen many RV users install a lot of solar panels on roofs of different shapes and sizes. Most users will find that 800W solar panel installed, but only has a charging capacity of more than 200W or even more than 100W. Here is my experience, because most of the single solar panels on the RV have a voltage of around 17-21V, so many users and even RV factories connect two pieces in series and then in parallel. An important theory here is that if solar panels with different power are connected in series, the current is output at the smallest piece. In layman's terms: 150W and 100W are connected in series, the actual effect is only 200W but not 250W, so when doing the test, the actual power generation of one group of 150W+100W and the other group of 100W+100W are the same.
We saw a lot of users installed some very small pieces on RVs. If this pieces is 20W and another piece of 100W, it can only be 40W, which will be a lot of waste. Will the parallel connection of multiple groups of different powers have an impact? For example, we have one group of 150W+150W, and two groups of 100W+100W. Seeing that the power is 700W, but the actual power generation efficiency has not been reached.
The reason is that the parallel of solar panels with different powers into the MPPT will affect the maximum power tracking. Therefore, the suggestion is to install an even number of boards of the same size on the top of the RV, so that the power generation effect can be highest. Composed of a double number in the smallest unit, for example, it is recommended not to install 100W in some places and 50W in some places. just install at 50W. In our actual test, if the installation is reasonable, use the MPPT with good performance, the power generation efficiency can reach more than 50% when the sun is not particularly good, and it can reach more than 85% when the sun is big occasionally.
Therefore, when choosing to install solar panels for RVs, you must pay attention to this detail. Don't reduce the overall power generation efficiency just because you want to increase a small piece of usable space.
Summary: When installing solar panels on the roof of your RV, choose an even number of solar panels of the same model to achieve the best condition.
Author: Rico Chen by CNBOU