A Review and Testing Using 3 Efest Lithium Manganese Oxide (IMR) 14500 size, think AA size, 650mAh Button Top Batteries in Series 4 Soshine LiFePO4 (IFR) in 2 AAA battery boxes. Note that even with the 2 battery boxes and 4 batteries a 1/4 oz. weight had to be added to bring the boxcar to the NMRA recommended weight. (Please see the updated page on sourcing batteries.)      I still believe that the best, and safest, set up for using a battery car for HO Dead Rail/Battery on Board and LocoFi™ is to use four Lithium Iron Phosphate (IFR) 10440 size batteries in two AAA battery boxes in series to create a 4S resultant battery. Using those batteries yields 60 minutes of prototypical run time on my flat demo layout.      At this time I am safely achieving an hour of prototypical run time with either 3 IMR 10440 size or 4 IFR 10440 size batteries. 3 Efest IMR in 2 AAA battery boxes with one box modified to use only one battery.      The photo shows 3 IMR, Lithium Manganese Oxide 10440 size batteries, in two AAA size battery boxes. One battery box had been modified to use only one cell.      I later learned that an AAA size connector battery is available. When that type of connector battery is used, no modification of the two AAA battery boxes, connected in series, is needed.      Using three Lithium Manganese Oxide (IMR) 10440 size batteries, in series, for a 3S resultant battery had also worked well, and the IMR batteries are relatively safe to use. 3 Efest 14500 size 650mAh IMR batteries installed in a 3 battery AA battery box.      I had previously partially tested these Efest IMR 14500 size 650mAh batteries when I had received them in June 2022 and had reported on that test in the now archived section titled, "PULLED OFF TRACK AND DERAILED BY AN EXPENSIVE MISTAKE!".      A lot of "things" were in place near the beginning of October 2022 that were not available to me in July 2022. I was now ready to put three Efest IMR 650mAh 14500 size batteries to the Prototypical Run Time Test.      The SkyRC MC3000 charger allowed me to charge and discharge three of these batteries at a time to get some preliminary data, before actually putting them to the Prototypical Run Time Test in a battery car. (Note: The EBL chargers, previously noted in this article could also be used, and would cost less than a SkyRC MC3000.) The EBL charger charging the 650mAh batteries. I do not recommend using these chargers. Photo left only for reference.      The MicroMark connectors allowed me to easily swap out the two AAA battery boxes in series for a single three in series AA battery box.      Information from a discharge test (see "The Nitty Gritty" archived here.) demonstrated that, at a current draw of 0.3A to a cut off voltage of 3.4V, an average of 640mAh was removed.      A Prototypical Run Time Test was preformed. I terminated the testing at 2 hours. I stopped at two hours, as that is what the "experts", those that create commercial Dead Rail/Battery on Board systems, had noted that they were targeting for battery run time. (see "How Long Is a Typical Operating Session's Run Time?".)      The actual Prototypical Run Time Test demonstrated that 120 minutes was easily doable on my flat demo, layout pulling a low number of cars, using three Efest IMR 650mAh 14500 size in the resultant battery.      See "The Procedure to Determine the Actual Run Time. The same test procedure is used for these larger 14500, AA size, IMR batteries.      For ME, using 4 Lithium Iron Phosphate (IFR) 10440 size batteries, because they are a safer type of battery, for a total Prototypical Run Time of 60 minutes is fine, and having a second set of 4 IFR batteries allows up to 120 minutes of Prototypical Run Time with MY chosen locomotives. The 10440 size batteries are very easy to exchange in the AAA battery boxes, so little time is lost with a battery exchange. 04/05/23: With a lot more prototypical run time on "My First HO Train Layout in a Long, Long Time", my locomotives were safely achieving an hour of prototypical run time with either 3 IMR 10440 size or 4 IFR 10440 size batteries. My Thoughts and OPINIONS Pros A single set of three Efest IMR 650mAh 14500 size batteries, creating a three in series (3S) resultant battery, in a trailing HO battery car, can, given the right conditions, run some diesel style locomotives for up to two hours. Cons Three button top 14500 size batteries are a very, very tight fit and require quite a bit of finagling to insert and even more finagling to remove from a three battery in series battery box. (Flat top, 14500 size would be a better choice.) Because of the tight fit of the button top batteries into the chosen 3 battery battery box, the battery box must be removed from the battery car to insert or remove the 3 AA size batteries. The battery box does NOT need to be disconnected. It just needs to be removed from the battery car while inserting or removing the batteries. Flat top 14500 size batteries might work better. Running the small motors found in HO scale locomotives for a continuous two hours is not good for them, nor the small decoder unit.      The section titled, "The Nitty Gritty" contains complete battery testing information and the testing for the Prototypical Run Time. It is archived here.