Battery Pack Configurations
There are many ways to implament a battery pack or traction pack. In this article we will try to clarify the distinction between simple, parallel, and hybrid implamentations. 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 usuaily assume it is a single simple string. Ideally the individule 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 disconnect able 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 strings total resistance is as equal as possible to the other. 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.
A Hybrid battery pack is to 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. 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.
With regards to the Prius, a parallel pack might describe using additional stock batteries which are always connected. Though such projects exist using both stock NiMH and PbA chemistries most do not grid charge their parallel packs.
Rons PriusPlus and the EDrive systems do not use a parallel nor hybrid type pack, rather these implementations replace the stock battery with a simple single string, though they may be capable of switching back to use the stock string the two 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. If the stock battery is removed than it's replacement must be as robust in order to prevent failures, this can be mitigated by leaving the stock battery in place such 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. The benefits of such an implementation may be the simplicity of integration and the ability to utilize the existing battery. Drawbacks might include the need to keep stock battery which might not be an ideal battery for the tasks of a PHEV, there may be lighter batteries which could utilize the space which the stock battery occupies.