Batteries

Selecting a Battery
For model locomotive use a battery must be rechargeable, have large energy storage in small size, be available in convenient shapes (rectangular, for example), have high cell voltage and be safe, especially while charging. Battery technology continues to evolve, but at present, only Lithium Polymer (LiPo) and Lithium Iron Phosphate (LiFePO4) cells are relevant. LiPo is the best choice for model loco battery power due to greater energy density, higher cell voltage, and best variety of cell shapes and sizes. 
​​​LiPo cell: The useful voltage range of a LiPo cell is 2.7 to 4.2 volts. Since voltage decreases as a LiPo cell discharges, 3.7 volts is used as nominal voltage.   
Battery Pack: In most contexts, including "deadrail" discussion, "battery pack" (or simply "battery") means two or more LiPo cells connected is series so their voltages add to produce a higher voltage battery. 11.1 or 14.4 volt LiPo battery packs (3 or 4 cells in series) are readily available for RC model aircraft and cars. They are not suitable for on-board locomotive power.
Warning: Selecting a​n 11 or 14 volt LiPo battery for a 12 volt model seems reasonable until battery charging is considered. There is no way to do this safely without a complicated battery charger that will not fit in a HO sized model. The best option is remove battery from loco and charge externally. Not only is this incredibly inconvenient, the battery leads are bulky and difficult to fit in limited space. For example, an 11.1 volt LiPo has 4 wires and a large connector for charging. RC batteries typically have 2 extra heavy leads for battery load. A small model won't survive the handling.
Conclusion: Batteries selected for S-CAB battery power use 1 or 2 LiPo cells in parallel. Nominal voltage is 3.7 volts and 2 cells in parallel has twice the capacity of a single cell.

Characteristics of LiPo Cells


​Size

Rectangular shapes are best suited for model locomotive applications and manufacturers often use dimensions (in millimeters) when defining battery part numbers. However, there is no standard format. 

Voltage

Some manufacturers may specify maximum cell voltage of 4.3 or 4.35 volts, but the norm is 4.2 and max. voltage is a critical limit. Discharge below 2.5 volts reduces cell life. A voltage of 3.7 to 3.85 volts is best for storage.

Maximum Discharge Rate

​A battery's 'C' rating indicates its maximum discharge rate. A 10C rating means the battery's maximum discharge current is 10 times its mAh rating. For an 850 mAh battery, 10C means the battery can safely deliver up to 8.5 amps.
​A "raw" cell has terminals, but no protection, no leads, or connector. This 850 mAh cell which measures 6 x 24 x 64 mm has part # HD6024064.
​"HD" means high discharge.

Cell Capacity

Cell capacity is defined in amp-hours (Ah), or milliamp-hours (mAh). For example, 850 mAh means the cell can deliver 850 mA for 1 hour. 

Energy

​Energy is amp-hours times voltage and somewhat ambiguous because voltage decreases as a battery discharges. For convenience, and now required on LiPo packaging, battery energy is based on nominal voltage, i.e. 3.7 volts. Using the above example, its energy rating is 0.85 x 3.7 = 3.15 watt-hours (WH).

S-CAB Batteries

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​S-CAB batteries are 1 or 2 LiPo cells connected in parallel with a 6 amp fuse for short-circuit protection. Leads are extra flexible 24 AWG, 2.5" long with JST PH crimp-style connector. 1" leads are used for motherboard installations. Red lead is positive.

Battery Protection

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A fuse is included for short-circuit protection. With or without the fuse, a short-circuit disables the battery. With the fuse nothing else is destroyed and the cell is still good, although unusable due to blown fuse, which can be replaced if battery is returned for repair.

Leads, plugs and polarity

Beware. If searching the Internet for batteries, there are no standards for plugs or polarity. Accidentally connecting reversed polarity is likely to destroy battery, the connected device, or both.
​​Incompatible plugs and sockets is another frustration.

LiPo battery charging

For S-CAB, commercial LiPo battery chargers are not required since charging is performed by BPS-v5.
​If used, the following charger functionality is desirable:
  • ​Detects and blocks reverse polarity.
  • Controls charging rate: typically 1 or 2C. Often ignored at the expense of reduced battery life.
  • Performs under-voltage recovery: Uses slow charge (a few mA) until cell is ready for full charge rate.
  • Limits maximum cell voltage. Enforced as hard constraint.
​Warning: A LiPo cell exposed to charging current while in an over-voltage condition heats and pressure builds within its sealed package, damaging the battery. More significantly, over-heating and swelling of the battery package are serious risks inside a model loco, including the possibility of a battery fire.

Battery Care and Maintenance

​BPS batteries are intended for use only ​with the S-CAB Battery Power Supply (BPS). They use one or more lithium-polymer (LiPo) cells, singly, or connected in parallel, and are protected by a fuse within the battery package.
​Warning: LiPo cells store a lot of energy in a small volume and come with a warning to use responsibly. By deciding to purchase these batteries, the user accepts all liability for any consequences resulting from opening, puncturing, or misuse of the battery package. A blown fuse cannot be replaced by user and the battery must be returned for repair or replaced with a new battery.

Battery Protection

​The BPS includes battery protection that would otherwise be included in a PCM (Protective Circuit Module) integrated into the battery package. Specifically, the BPS prevents over-charging and over-voltage, as well as protection from under-voltage and battery overload. When combined with short-circuit protection provided by the battery fuse, BPS design provides a very safe battery power source.

Battery Fuse

​A BPS battery includes a 6 amp fuse. Maximum battery current occurs when battery is at minimum voltage (about 3 volts) and BPS output is greatest; 1 amp or more. With these conditions, the ratio between battery current and BPS output current is approximately 4 to 1. BPS output of 1 amp consumes 4 amps battery current.
​A common cause of a blown fuse is an attempt to measure battery voltage at the battery socket. There is high risk that multimeter probes will momentarily short-circuit battery output and blow the fuse, which clears the short instantaneously (there won't even be a spark).

Unplugging battery

​​Battery plug and socket are tight by design and not intended for frequent disconnection. It is important to avoid unplugging by pulling the leads. Eventually, a wire will pull out of the connector. An IC Extraction Tool is one simple way to avoid this problem.
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IC extractor tool

Discharged Battery

​​The following behavior by a loco that was operating normally indicates its battery must be recharged.
  • Loco suddenly or intermittently stops moving. Lights and sounds may still operate, but go off when throttle is used to drive the loco.
  • Loco stops suddenly and there are no lights or sounds. With throttle at zero, the battery can be turned on again (indicated by lights and sounds) but everything turns off as soon as the throttle is moved from zero.
The correct procedure is, take the loco out of service and recharge.

Shutdown

​Conclude a battery powered operating session by either parking the loco for battery charging or be sure to turn the battery power off and prevent slow discharge while a loco is not in use.
​To confirm turn-off, have a light on, or sounds audible and watch/listen for shut down.
​If loco is being parked for charging, it is still good procedure to turn off battery power so that shutdown is automatic when charging voltage is removed accidentally or deliberately.

Battery Charging

​Battery charging occurs whenever a BPS has power input (its gray wires) from a connector or powered track. Depending on track voltage, a loco operating on powered track can draw all or part of its power from the rails.
​​Charging occurs whether a loco is moving or stationary and charge rate is approx. 450 mA until the battery approaches full voltage, at which point, charging current drops to a value no greater than is needed to hold battery voltage at 4.2 volts. (Zero if there is no battery load.)
​​If a loco is stationary, ​a decoder continues to require some current (typically between 20 and 80 mA, depending on type of decoder). which implies something less than 450 mA is available to charge the battery. This drain can be eliminated by turning off battery discharge. A similar result is automatic when BPS input voltage exceeds 12.5 volts since, in either case, rectified input provides output power with no contribution from the battery.

Over-discharge

​Over-discharge is not safety related, but can be frustrating. BPS under-voltage protection turns off a battery when its voltage drops below 2.7 volts, which provides sufficient margin to prevent over-discharge. With load removed, battery voltage typically recovers to approx. 3 volts and can be recharged without difficulty.
​Avoid repeated attempts to use a discharged battery: A battery discharged below 3 volts can be turned on (and may operate LED lights) but BPS will shut down when motor load is applied. This drains more energy from an already depleted battery and repetition eventually leads to over-discharge. In extreme cases, the battery cannot be recharged.
​How to recover: In most cases, BPS can recharge an over-discharged battery. Use the following procedure:
  1. Turn on approx. 12 volts charging input. If a decoder is connected, it should start. BPS-v5 blue LED will light.
  2. Use shutdown to turn off battery power. (Blue LED off) Battery bypass will keep the decoder powered, but do not attempt to operate the loco.
  3. Be patient. If battery voltage has dropped below 2.7 volts, charging will begin slowly at a low level ("trickle charge"). Allow several hours for full recharge.

Questions

When to charge?

Where to charge?
​For planned/scheduled operating sessions, charge batteries immediately before the session. For impromptu operation, charge batteries at the end of a long session. For long periods without use, it's best to leave a battery partly discharged.
Since LiPo batteries have no "memory effect", recharging a partly discharged battery is okay. ​

How much track should be wired?

​​This is a trade-off between battery size and layout design. When using locos that can only accommodate small batteries, more powered track extends a loco's operating time before requiring a full recharge. So far as powered track is concerned, the most important objective is avoid the task of wiring inaccessible and/or complicated sections (lots of switches) of the layout. There's no need to wire track in locations that are difficult to clean.

What is the life of a LiPo cell?

​Manufacturers' typically specify life as 500 full charge/discharge cycles. This could imply 10 years for a model loco that has heavy use once a week.
​Reliable electrical contact between wheels and track is important if using rails for battery charging. The good news, track cleaning is only necessary at track locations used for charging. For full recharge, choose a location where a loco can be parked for several hours.
For large layouts and longer operating sessions, power a few "refueling" areas that are easily wired and where locos are likely to spend extra time.

How long will a LiPo stay charged?

​​LiPo batteries have a very long shelf life (measured in years). When a BPS is turned off there is a tiny, unavoidable, leakage current. Typically, a battery will hold adequate charge for 3 to 6 months. Best practice is to charge a battery briefly before operating a loco that has not been used for a month or more.

Long-term storage

​​For storage of a year or more, it's advisable (though not essential) to remove and store each battery separately in its own plastic envelop or wrapper. The battery should remain good for a number of years.