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Battery Pack Configurations

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There are many ways to implement a battery pack or traction pack. In this article we will try to clarify the distinction between simple, parallel, and hybrid implementations. We realize that the use of the word hybrid may be confusing, especially when you add battery pack, which might seem to describe the standard traction battery in a hybrid vehicle rather than a particular type of battery pack configuration.


A simple pack consists of a single series string of batteries or cells, this is by far the most common type of battery pack. If no other modifier is used to describe a battery pack you can usually assume it is a single, simple string. Ideally the individual batteries are chosen such that a single string of them will satisfy all the requirements of a projects design goals. However there are instances in which a satisfactory battery is not available to meet all of the demands, or there may be other reasons to use a more complex battery pack configuration.


In the EV world with regards to battery packs Parallel usually indicated parallel strings of batteries. They might be disconnectable or hard-wired but probably don't have any advanced electronics between them. There are special considerations to observe with regards to the interconnects such that one string's total internal resistance is as equal as possible to the other's. Such parallel strings are usually made up of the same type of batteries. They are normally designed to share the load equally and are used to either increase the available power or energy content of the pack.

See also:


The use of the term "hybrid" in relation to a battery pack simply refers to a pack that is a combination of two different types of energy storage devices. It does not refer specifically to a hybrid vehicle, although this section discusses its use in a hybrid vehicle.

A Hybrid battery pack is too advanced a setup to be simply called parallel. It would likely be made up of two strings of roughly equivalent voltage, but may be entirely different chemistries or even use caps as one of the strings. There would usually be a DC-DC device between the pack or some other way to manage the flow of power from one string to the other. Such a Hybrid Battery Pack is used to take advantage of the strong points and mask the weak aspects of the various strings that make up the whole. This is similar to reducing engine size and making up for it with electric motors, as is done with hybrid vehicles. One might have a high capacity but low power string and a low capacity but high power string and combine them in a manner to utilize both of these strengths and eliminate the weaknesses.

Background Information
In short, a new larger capacity (source) battery pack and charger device would float-charge the stock (target) battery pack at a voltage near the top of its normal operating range, taking care to consider regeneration headroom. A contactor based setup might simply parallel a properly sized source pack with the target on a SOC based duty cycle, additionally a large resistor might be used to limit the current transfer when the source pack is fully charged and the voltage differential is at its greatest. Such dump charging is simple and effective but can also be risky if not properly configured. A DC-DC converter or DC input charger with load following capabilities might also be used to continuously replace the energy removed by the motors while taking care to not overcharge the target battery pack. It's important to keep the stock battery pack at a voltage which corresponds to a high, yet not full SOC. The charger would also need to stop discharging the source battery pack when it reaches a particular desired SOC. Once the source pack is depleted the vehicle should revert to normal operation using only the original battery pack. A properly configured Hybrid Battery Configuration will exploit the State Of Charge Drift portion of the vehicles battery management routines in order to accomplish State Of Charge Manipulation. The use of a Battery Tap Emulator should not be necessary with these implementations.

See also: These Searches may take some time to complete:

PHEV Applications

Conversion topologies

At the very least, to convert a non-plug-in HEV into a PHEV, you need a power cord and a battery charger. Beyond that, there are many topologies depending on the battery pack(s) used, and on the Battery Electronic Control Unit (ECU) used.

This is a tree of possible PHEV conversion topologies. It also gives examples of conversions that use a given topology.

  • Stock pack and Battery ECU only
  • One or more additional stock packs, in parallel with stock pack
    • Using the stock Battery ECU
    • Using a custom Battery ECU
  • New pack only
  • New pack in addition to stock pack (note 1) (e.g.: Hymotion, but which topology?)
    • Two packs connected in parallel (note 2)
      • Using the stock Battery ECU (e.g.: original PiPrius/PriusBlue)
      • Using a custom Battery ECU
    • Two packs connected through a DC-DC converter (note 3)


  1. Called the "Hybrid pack" method elsewhere in this page
  2. Called the "Contactor" method by the people who use a contactor to selectively connect the two packs in parallel
  3. Called "PFC method" by the people making the PiPrius conversion, because it uses a product called the "PCF-30"

Here are some of the advantages and disadvantages of the various topologies:

  • The "Stock pack only" topologies are pretty pointless if the range achievable by the stock pack is only a couple of miles.
  • The "New Pack Only" topologies do not need to deal with batteries of different chemistries. However, if the same Battery ECU is used, the ECU has to be fooled into operating with the new pack. If the new pack has better characteristics than the stock pack, removing the original pack results in a lighter vehicle for a given range.
  • Using the stock ECU is cheaper and somewhat easier, but there may be hassles trying to fool it into working with a different pack. Using a custom Battery ECU avoids all the hassles required to fool the stock ECU, but you have the design effort and cost of a new ECU
  • Topologies that use a new pack in addition to the stock pack need to deal with how the two pack are connected (in parallel or through a DC-DC converter). Also, if the new pack has better characteristics than the stock pack, keeping the original pack may be an inefficient use of space and weight.
  • See also some useful documents prepared by Ron Gremban on Feb 17, 2006: New spreadsheet of projected battery performance in PHEV conversions, Excel version, pdf version


See also Prius PHEV#Hybrid-Pack Method for specific conversion examples using this method.

See the stock Toyota Prius Battery Specs and Prius PHEV Battery Options#Hybrid-Pack Method pages.

With regards to the Prius, the term parallel might be used to describe a pack that uses additional stock batteries which are always connected to the original stock battery. Though such projects exist (using both stock NiMH and PbA chemistries), most do not charge their parallel packs from an AC outlet.

Ron's original PriusPlus History and the EDrive systems do not use a parallel nor hybrid type pack, rather these implementations replace the stock battery with a simple string. Though they may be capable of switching back to use the stock string, the two packs are not used at the same time. An advantage of this implementation is that there is no chance of damaging or degrading the stock battery pack which could be removed for weight and space savings or even sold to offset the conversion costs. If the stock battery is removed, then its replacement must be as robust, in order to prevent failures. The risk can be mitigated by leaving the stock battery in place, so that it can be used should the need arise.

The Hymotion system may (I'm speculating) be using a Hybrid Pack which is made-up of the stock NiMH string and an additional Li-ion string with some level of management between them. Such a hybrid setup has also been suggested by a number of EV folks as they have discussed and even implemented such setups for use in pure EV's over the years. Some benefits of such an implementation may include simplicity of installation because there is no need for a Battery Tap Emulator: the existing battery and its taps are used. State Of Charge Manipulation may still be required using State Of Charge Drift. Drawbacks might include the need to keep the stock battery which might not be ideal for the tasks of a PHEV and there may be lighter batteries which could better utilize the space which the stock battery occupies. Though there is a risk of damaging the stock battery, such an implementation should actually reduce the load and number of cycles to which the stock battery is subjected, thus further extending its service life.