One of the most vexing challenges for me when doing dead-rail conversions on O scale steam locomotives is what battery to select. As nearly as I can determine, lithium polymer (LiPo, see LiPo Wikipedia) batteries are the almost-universal choice among dead-railers. So, the choice of battery technology was not much in question for me, but the battery voltage and size is still a choice to be made. I’ll deal with each of these in turn.
I’m not sure how I fell into the 14.8V “camp” for LiPo batteries on O scale dead-rail conversions. Maybe it’s because CVP pushes them for their Airwire products (CVP Airwire). I think CVP has valid admonitions about using higher voltages in regards to radio control range performance and cooler operation (see for instance CVP G3 Decoder User Guide, page 12). And, and almost all of the O scale operating modes (2 rail DC, three rail AC, TMCC, DCS) seem to have 24V or so maximum operating voltage, which is getting close to damage threshold for some radio receivers such as the CVP Airwire receivers. So, would 11.1V (3 LiPi cells in series, “3S” as discussed later) work? Probably just fine in most cases. The plus for 14.8V LiPo is that vendors offer a variety of battery physical and power configurations at 14.8V, which I’ll address next.
There is probably a good deal of lore and religion surrounding the specifics of which brand and configuration of LiPo battery to choose for dead-rail. I’m going to try to stick with what I have tried, not what might theoretically be “better” or “worse.”
Let me start with the less controversial part of my decision process: configuration. LiPo batteries come in a large variety of sizes and configurations (see for instance this site), but in my personal experience with dead-rail, the LiPo’s seem to have a single cell size roughly that of an AA cell with 3.7V output with a charge capacity of around 2000 to 2600 mAh. The individual cells are connected to achieve a total output voltage of about 14.8V (four connected in series, thus the term “4S”), but what varies is the total charge capacity and its ugly handmaiden – physical size.
This is the bear: We want large charge capacity for longer running times, but we in O scale must fit the batteries in often-tight spaces (at least compared to G scale) such as tenders where we also put radio control receiver boards, sound cards, speakers, etc. Of course, HO scale has even more severe volume constraints, but with approximately one-sixth the mass the locomotives must pull.
Personal experience here: I tried to fit an eight-cell (two rows of four vertically-stacked cells, 2.6″ x 2.8″ x 1.4″, 6000mAh, CVP BATT2) configuration into several O scale tenders, and that battery pack just flat-out would not fit. Even though this configuration was only one cell high, O scale tenders are just not tall enough to fit even one cell oriented vertically – the cells must lie sideways to fit. This cell-length limitation is important to remember for O scale. You might find space in diesel locomotives, but not steam locomotive tenders. These limitations were a big disappointment for me since I wanted to cram a large storage capacity battery pack in the tender and run “forever” (forever being at least four hours).
Sigh… Backing off in size, I have found that 2x2x1 LiPo battery packs will fit in O scale tenders with the individual battery cells running along the length of the tender. See the Figure below (pardon the body parts). You can see that this configuration also comfortably fits the width limitations of O scale tenders.
A note of caution: you cannot stack too much on top of this configuration before you lose vertical clearance inside the tender. For instance, I thought this was going to work:
Stacking the receiver board on top of the battery would be a useful space-saving strategy, but this configuration would not clear vertically in all tenders I tried (Big Boys, Cab Forwards, Challengers, and Alleghenies). The receiver board manufacturers would probably dislike my mounting electronics on top of batteries, even with sufficient clearance.
My experience is confined to three battery suppliers of 2x2x1 LiPo battery packs: CVP, Tenergy, and “HJE.” All come with a Protection Circuit Module (PCM) that provides:
- Overcharge, over-discharge protection
- Overcurrent protection
- Short-circuit protection
- Voltage- and current-balance
- Temperature protection
- CVP battery suppliers: CVP, Reindeer Pass
- Tenergy battery suppliers: Garden Railroad Supply Company
- HJE (this may be an “in-house” brand): RLD Hobbies
More creative 14.8V battery configurations are possible that include one battery-diameter thickness solutions such 1x2x2 (Tenergy.com) with dimensions of 131 x 36 x 23mm (LxWxT), so it’s approximately one battery-diameter thickness, two battery-diameters wide, and two battery lengths long (~5.2″). See the picture above for an example of using this thin configuration in a very tight mounting configuration on an MTH Virginian Triplex with a PS-3.0 board operating in DCC mode. See this blog for more details.