I installed two SOK 12 volt 206 AH batteries on trays under my Roadtrek 190 RV in September 2021. I live in Pennsylvania, and so far, the various charge and BMS systems seem to be dealing with the cold below freezing weather just fine. My question involves battery capacity and locations, so some background detail is necessary. One battery is located on a factory installed tray up under the middle of the RV, and the second battery is located in a protected tray that I constructed in the space where a rear mounted factory installed Onan generator used to be. Out of necessity, the positive from each battery is connected to a frame mounted (and isolated) common positive connection point, rather than connecting the positive of one battery to the positive of the second battery, and then connecting the positive of the second battery to the common positive connection point. Four zero cable is used for the long run, and one zero cable is used everywhere else. The three sources of battery charging (solar MPPT controller, shore power charger, and RV alternator via a Victron DC-DC controller) also connect the positive of each device to the common positive connection point, as does a 3000 watt pure sine wave inverter. The significant one at a time loads on the inverter are a microwave oven, a toaster oven, a coffee maker, and an induction cooktop. While the inverter can power each device separately, the battery voltage appears to drop to 11.9 volts very quickly, like after just 10 minutes of continuous cooking, before I need to run the RV motor to recharge. I thought I would have more useful power with the two 206 AH batteries. I am planning to purchase a third SOK battery to improve my power situation, and the third battery will be mounted next to the second battery in the custom tray that replaced the generator. Now for the question. Because two of the batteries will be in the same location, I was planning to connect the positive of one battery to the positive of the other, and then connect the positive to the common connection point. I would connect the negative terminals and then connect the negative terminal (of the battery not connected to the common positive terminal) to the RV frame ground. This is the way I typically see RV batteries connected when all batteries are in the same location, and the method requires less cable. Is there any difference in available energy when each battery positive is connected directly to the common positive connection compared to connecting the positive of one battery to the positive of the battery next to it? Also, would it matter that two batteries would be connected as a pair and one distant one be connected via a separate cable to the common positive connection point? Roadtrek and my RV dealer have both voiced the opinion that either configuration should give the same end result. I also have a second question regarding charging batteries while using the inverter. My solar controller charges whenever the voltage drops. In the past, if we parked in a rest area to brew coffee, we would often leave the RV engine running to keep the air conditioning on and the batteries charging as we use them. My recently acquired pure sine wave inverter vendor recommends that the charging stop when the inverter is being used. Any thoughts regarding charging while using the inverter?
LiFePO4 Battery Locations in RV
- Thread starter robert540
- Start date
MisterSandals
Participation Medalist
What is the starting voltage? And what are your charge parameters, specifically the top charge voltage?
Lets start with some rough math, will assume ~1200W microwave which uses about 1500W and 85% conversion from AC to DC:
1500W / .85 = 1764W
1764W x .167h = 294Wh
294Wh / 12.8V = 23Ah (~6% of your battery bank)
MisterSandals
Participation Medalist
Ya kind of lost me. Sounds like you have a bus bar at the midpoint between your batteries. If those are symmetrical, it sounds fine.
Your battery charge voltage and starting (measured at rest, not being charged?) voltage sound good. But depleting 412Ah batteries with what should be 6% discharge sounds wrong. Do you have a clamp meter to check discharge of each battery when microwave or other big load is running?
I appreciate your thoughts on this. One battery is about 10 feet from the common positive connection point. The second battery is less than three feet from the common positive connection point. I have ordered the third SOK battery, and it will be right next to the current rear battery. Roadtrek designed the RV to have two locations for batteries, and I am not sure that is an electrical advantage, although it may be great for balance and weight distribution. I understand the calculations you provided, and believe that the third battery will improve my power utilization. With the batteries mounted under the RV, I cannot place a clamp meter near either battery unless the RV is in the air on an alignment rack, and then I cannot be in it. The inverter I previously used was a 3000 watt modified sine wave inverter, and it never seemed to run the battery voltage down. The newly installed inverter is a new pure sine wave inverter, purchased because the old inverter could not power a recently installed induction cooktop. The new cooktop was installed because it was always difficult to use the original equipment propane stovetop without disconnecting the smoke and heat detectors. I am still testing the new inverter, as it was just installed. In this situation, I need to trust that with proper wire sizes, each battery is contributing equally to my inverter load. Otherwise, the less than 30 day old pure sine wave inverter may not be as efficient, or as capable, as the modified sine wave inverter.
MisterSandals
Participation Medalist
Can you place a clamp meter anywhere on the wire (either + or -) between battery and connection point?
Are the voltages of the 2 batteries the same? Do you have cell level access to voltage?
It seems mighty odd to be draining what sounds like 2 newer batteries with 6% discharge. I don't see how a 3rd battery would solve this unknown problem.
I can place a clamp meter between the rear battery and the common positive connection point, or between the connection point and the 3000 watt inverter. As soon as I can, when the temperature is above freezing and the rain/sleet ends, I will test again with the clamp meter. Before I purchased the first two SOK batteries, my theoretical calculations supported that three 206 AH batteries might meet my usage expectations better than two, but I wanted to be sure that my low temperature charging would be correctly managed before I increased my expense. Prior to installation, the two SOK batteries were at exactly the same voltage. I have not ruled out the new pure sine wave inverter as the reason for my observed issue, as two trips this past fall with the two SOK batteries and the modified sine wave inverter gave no indication that I did not have enough battery power.
