Lithium-Polymer Battery Information
Characteristics
The basic component of a battery is a cell. The characteristics of a battery are determined by battery chemistry; in this case; lithium-polymer. Rectangular shapes are best suited for model locomotive applications and manufacturers often use dimensions (in millimeters) when defining battery model numbers. However, there is no standard format. The battery illustrated here is 6 mm thick, 24 mm wide and 64 mm long.
The basic component of a battery is a cell. The characteristics of a battery are determined by battery chemistry; in this case; lithium-polymer. Rectangular shapes are best suited for model locomotive applications and manufacturers often use dimensions (in millimeters) when defining battery model numbers. However, there is no standard format. The battery illustrated here is 6 mm thick, 24 mm wide and 64 mm long.
Energy StorageDepending on charge, a LiPo cell's voltage varies between 2.7 and 4.2 volts. For convenience, 3.7 volts is used as nominal voltage of a LiPo cell. Since voltage varies as a battery discharges, storage is defined in amp-hours (Ah), or milliamp-hours (mAh). For example, 850 mAh means the battery can deliver 850 mA for 1 hour. Energy is amp-hours times voltage and somewhat ambiguous because voltage decreases as a battery discharges.
Battery ProtectionFor safety, LiPo batteries often include what is generally called a protective circuit module (PCM). It's purpose is to disconnect the battery in order to protect it against the following conditions: over-voltage, under-voltage, overload and over-discharge. Most PCMs will limit damage from a short-circuit, but the PCM may not survive and the battery should be replaced.
LiPo Battery ChargingSince LiPo cells can be damaged during charging, a good battery charger will impose the following constraints:
Cell balancing of series-connected cells: When LiPo cells are connected in series, they don't necessarily assume equal voltages, which complicates battery charging. Since maximum voltage per cell is 4.2 volts, suppose we ignore cell balancing and use 12.6 volts (3 x 4.2) as the voltage limit when charging a 3S LiPo battery. If cell voltages are unequal and 3-cell total is 12.6 volts, at least one cell will be above it's 4.2 volt maximum. A LiPo cell exposed to charging current while in an over-voltage condition heats and pressure builds within its sealed foil package, damaging the battery. More significantly, over-heating and swelling of the battery package are serious risks inside a model loco.
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Maximum Discharge RateA 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 our 850 mAh battery, 10C means the battery can safely deliver up to 8.5 amps.
Battery PacksBattery packs are assembled with cells connected in series (S) and/or cells connected in parallel (P). When cells are connected in series, voltages add; a 2S battery (2 cells in series) has a nominal voltage of 7.4 volts; 3S is 11.1 volts, and so on.
With parallel connection, voltage remains equal to cell voltage while storage and discharge capability add. Two 850 mAh, 10C cells in parallel can store 1,700 mAh (1.7 AH) and deliver up to 17 amps.
Batteries for use with BPSFor BPS use, cells are connected in parallel to increase energy storage. Since 17 amps is not required to operate a model locomotive, battery current is limited to 6 or 8 amps.
 Fuse connects negative terminal to battery output
Since BPS includes the same protection as a PCM, a battery package for use with BPS includes only one extra component; a fuse for short-circuit protection. With or without the fuse, a short-circuit disables the battery. The benefit of the fuse is 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.
Cell balancing is not necessary for parallel cells: The physical laws of electricity guarantee that cells connected in parallel have the same voltage. Consequently, cell balancing is unnecessary.
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Implementing on-board battery power
There are two options for on-board battery power:
- install an 11.1 volt or 14.4 volt series connected battery
- use parallel-connected battery (or batteries) with a BPS battery power supply.
When installing battery power in small-scale (HO, On30, On3) locos, it can be difficult to find space for batteries and definitely not practical to include a complicated battery charger. Frequent handling of small models while removing batteries for charging is undesirable and it can be difficult to find a convenient location to mount a charging socket on a small loco. Some handling of a model is still necessary when connecting a charger. Keep in mind that an external battery charger with cell balancing requires a 4-pin connector for a 3S battery pack (5-pin for 4S).
Parallel-connected batteries simplify battery charging by eliminating the need for cell balancing. A tiny integrated circuit chip (IC), which costs less than $2 and implements all logic for safely charging a LiPo cell (or cells in parallel), makes on-board battery charging feasible, even in HO-scale models. However, additional electronics is necessary to convert LiPo cell voltage (ranging from 2.7 to 4.2 volts) to a value required by loco motors (typically, 12 volts). The BPS battery power supply performs these tasks and adds extra functionality discussed on the Battery Power page.
Comparing Options
The following comparisons are worth considering for battery installations in HO, On3, On30, and small locos in S-Scale. For heavy O-scale locos and larger-scale modeling, series-connected battery packs with external battery charger is the best choice.
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Function/Performance
Battery switch
Battery charging
Use of track power
Battery protection
Voltage regulation
Stay-alive application
Energy storage
mAh multiplied by voltage |
BPS Battery Power Supply
Includes switch with both automatic and manually initiated operation.
Integrated on-board battery charger. Use 2-pin connector or rail pickup power source
When available, rail power is used for battery charging as well as loco power
Included in BPS with a fuse in battery package for short-circuit protection
BPS output voltage is remains constant as battery voltage drops during discharge
A small battery automatically powers loco and decoder during track power interruptions
3 cells in parallel provide
3 x 850 x 3.7 = 9.4 Wh |
Series connected Battery Pack
Requires a separate switch
External battery charger. Use a 4 or 5-pin connector for charging 3S or 4S battery
Polarity conflicts prevent use of rail power for battery charging or loco power
For safety, battery pack must include PCM
The voltage of a 3S battery pack varies from 12 .6 and 8.1 volts during discharge
Since it is the only power source, a larger battery eliminates track contact.
3 cells in series provide
850 x 11.1 = 9.4 Wh |