WhyManageBatteries

From EvTechWiki

Why battery management is important

One of the basic questions one has to ask when designing a battery management system is why manage batteries. The answer is that EVs tend to have large battery packs that are arranged as a series string or buddy pairs. Either arrangement means that the pack is only as strong as the weakest battery. (Some EV drivers actually keep track of which battery is their weakest and use it as their indicator battery).

Many people have come up with different techniques to make the weakest battery stronger. One of the most famous is Lee Hart's, which actually has a 20A DC-DC driven off the entire pack that will 'prop up' the weakest battery. Other people prefer to make the strongest battery weaker - this is the techniue used by Rich Rudman's Rudman Regs. (Actually, it's not so much making the strongest battery weaker as it is permitting the pack to be charged beyond the point that would normally be possible without damaging the battery that has the highest state of charge by shunting some of the energy away from the batteries that are stronger or more fully charged.

All of this, of course, is to extend range and to make the car require less maintenance and attention. Ideally, a EV - any form of powered transportation - would not require any maintenance. You would just turn on the power and go.

Another reason for having a battery management system is that most batteries are badly damaged by being deeply discharged, and even more so by undergoing cell reversal. A good battery management system can alert the vehicle's drive to shut down before any actual damage is done. A *great* battery management system can watch for trends (a battery often running low before the others do) and start adapting before any problems occur, as well as warning the driver that they have a problem before it becomes too severe.

One can easily envision a future in which EVs have 'smart' batteries that have battery management modules built into them that watch voltage and tempurature on a cell-by-cell basis, and display little messages on a display when a battery needs changed. Changing a battery would probably just involve removing two large cables and one small one, and be something that any end user could do.

One can also envision a future in which EVs have really smart batteries, and if one battery fails the car continues to drive - or even each battery contributes as much as it can without damage and then stops contributing.

Basically, the goals of a good battery management system are:

• Keep all of the batteries at approximately the same state of charge.
• Time permitting, make sure all of the batteries are full at the end of the charging process
• Scale back or cut power to the drive if a battery gets too low, or too hot
• Keep the user informed about the capacity remaining in the battery pack
• Keep the user informed about any other important information concerning the battery pack

The goals of a great battery management system are:

• Do anything possible to make the batteries last longer
• Learn about the batteries and keep detailed logs of their history
• Assist technicians in performing tests (i.e. capacity test) on the battery pack
• Waste as little energy as possible