Concept for a Novel and Improved Battery Networking System

By Ryan Hunt and Paul Hunt
Hunt Utilities Group, LLC
Pine River, MN 56474

Summary

For owners of RVs, off grid homes, and other battery backup power banks who require multiple batteries for backup power, the Battery Socializer is a battery networking system that will provide unrivalled flexibility, battery life, and reporting to make sure they get the most out of their batteries.

Unlike traditional battery banks in use today that are only as good as the weakest battery, the Battery Socializer employs microprocessor control of each battery along with a voltage abstraction system to monitor, optimally maintain, and protect each battery in the bank individually.  The result is extended battery life, the ability to replace only defective batteries, add capacity as needed, as well as the ability to mix batteries of different types and conditions. The Battery Socializer provides all of this, as well as continuous, detailed, real-time reporting on battery condition and remaining stored energy capacity.

Introduction

  Large battery banks  have been in use for over a hundred years for backup power. The oldest and still the most popular type of batteries for that function are lead acid cells.(1)  They are the workhorses of the battery world and will be around for a long time.  They are still very economical even if they do not have the same energy density as other types of batteries like Lithium Ion or Nickel Metal Hydride. (2) The thing about them, however, is they do not get along all that well in a group.  So far, people have just accepted that as the way things are.  Our engineering team envisions a more cooperative way for batteries to work together.

Current Challenges with Batteries

There are many challenges associated with battery banks:

1.Batteries in series or in parallel must be well balanced or they destroy one another. If one battery begins to fail, it is stressed the most, accelerating the failure process. This can result in a catastrophic failure, and possibly fire or property damage.(3)

2.Under current practices, batteries cannot be swapped out individually, the whole bank must be replaced. This generally is a large expense to the owner of the system.  Batteries in series are only as good as the weakest battery installed. For instance; If you add a new 100% capacity to a bank that the other batteries can only put out 80% capacity, then the new battery will end up chronically undercharged; shortening its life.  The older cells become the new weak cell and deteriorate quickly, as well.

3. Lead Acid cells require careful charging and maintenance to maximize their life cycle. Most systems require a technician to manually analyze, monitor and perform this maintenance, with significant labor expense.(3)

4.All types of batteries have their own maintenance requirements and charging parameters. Technicians and engineers have to be specially trained for each type of battery that they are required to maintain.


Traditional Battery Bus Wiring
Traditional battery bus systems require heavy conductors and connectors which are capable of handling the maximum current of the batteries. Typical applications today employ batteries that are wired in series, or some combination of series/parallel.

  • Batteries in series are only as good as the weakest battery in the bank.






Strategy for overcoming battery challenges

What is needed is a very economical package that functions as a:
  • Smart Charger
  • Battery Analyzer
  • Battery Conditioner
  • Battery Isolator
  • Current Limiter
  • Over Temperature Limiter
  • Voltage Converter
  • Remote capable metering/reporting package


To allow the batteries to contribute what it can without the risk of being damaged, a socializer must incorporate the following features:
  • Function as an auto-disconnect switch for each battery to protect it from over-discharging.
  • Function as a current limiter for each battery so it can discharge at a safer and more efficient rate.
  • Provide precision power metering to measure power into and out of the battery, and characterize the performance of each battery.
  • Provide smart charging and conditioning for each battery individually, based on its individual needs.
  • Abstract the actual battery voltage from the power bus with a DC to DC, bi-directional power converter to help protect the battery when needed, and to allow a redundant parallel installation for higher voltage busses.  
  • Include a digital data connection to report the status and measurements from each battery in the bank.









Battery Charging, Testing, and Reporting


One huge advantage of the battery socializer is that it can put energy into or take energy out of the battery independent of bus voltage. This allows it to test, characterize, exercise and recondition batteries while they are in service.  Ideally, in a given bus, no more than one battery will be in testing mode at any given time.  If the bus voltage drops below a critical point, the test can be aborted so the battery can immediately get back to work.

Smart Charging
For lead acid batteries, a good charger uses three modes,.  This would be a key piece to implement.  (4)

Max Life
Every battery type has its own specific (but similar) list of requirements.  For these examples, we will talk about lead acid batteries. In short, a lead acid battery needs the following conditions to have a long life:
•  Float at a voltage accurate to 0.01 volt and temperature compensated.
•  After a reasonable discharge, charge to a specific higher than float voltage for a set interval; then reduce to float voltage.
•  Every 6 months, discharge battery 70% if it hasn't happened during normal cycling.

State of charge
Disconnect battery for 5 hours. Measure voltage. Voltage is reasonable indication of charge level.

Battery impedance
Change current going into or out of the battery. Measure resultant change in battery voltage.

Battery capacity
During normal charge and discharge cycles, measure current and voltage drop. Express as volts per second per amp.
If it hasn’t happened during normal service, occasionally discharge battery 70%. Record impedance and time at 10 places on the discharge curve.
With experience we should also be able to predict capacity by looking at impedance curve.


Intelligent Power Bus Rules
The unique bi-directional voltage converter, combined with a voltage based mode control system creates what is essentially a voltage abstraction system that is the basis for the socializer. Based on voltage of the power bus and each batteries’ needs, the abstraction system allows for the batteries to charge when there is enough power available, discharge to the bus when needed, and disconnect from the bus to protect the battery after full discharge.


Rules for 12 volt bus

Bus Voltage

Meaning

16.0

Allow dump loads at max level. Never allow bus above 16 volts.

15.5 - 16.0

Proportionally dump excess current into dump loads such as heaters.

13.8 - 15.5

Power is coming from outside so charge up.

The target voltage of 15 is to allow an equalization mode without voltage conversion

10.5 - 13.8

Connect the batteries to the bus so they can contribute power when needed.  On a 12V bus, this minimizes the need for voltage conversion and increases efficiency of the system.

10.5 or less (battery voltage)

When the voltage of a battery goes under 10.5V, it is removed from the bus to preserve the integrity of the battery.


All specific voltages are parameters in the software and may be adjusted in practice - the important part is the structure of the rules that allow the batteries to contribute when they can and protect themselves when necessary.

Similar bus voltage control strategies can be established for higher voltage power busses such as 24, 48, 36, 48, 60, 72, etc..  By adapting the power converter to be an AC inverter, this architecture can be extended to 120VAC, as well.

The voltage based rules above allow a simple non intelligent and non boosting lead acid battery charger to be used.  Any power source that can deliver 14+volts can be used to provide power to the batteries.

More intelligent versions will communicate and do more sophisticated stuff like taking turns being the main battery on the bus, or boosting voltage for a flatter discharge curve or more complete charging.

Limits and tradeoffs
  • Current Limits through the device - ideal for long term backup (typically 8 hour discharge or longer), not ideal for short, high current applications such as electric vehicles/golf carts
  • Some inefficiency in the power conversion process. That is why the bus rules are designed to minimize the need for conversion, at least on a 12V bus.


Battery network
The functions of the socializer allow a new level of thinking when wiring a house for 12 volt operation. Batteries don’t have to all be in the same location.

A battery may be located near a high current, short term load allowing very short runs of heavy wire from battery to load. The long wire from this battery to the other batteries in the house can be small gauge wire because it charges this battery at limited current over a long period. This reduces voltage drop losses and can greatly reduce the amount of copper required.

More batteries may be added to the existing network at any time.

Batteries, or a battery/load combination can be treated like plug-in appliances. They may be removed from the network, used in the field, and returned to the network.

Economics

The economics of a package to do all the described functions can be estimated in a few different ways.

Extend the life of a battery
In the case of new batteries, the socializer keeps it at optimal conditions from day one; extending the batteries service life; perhaps doubling it.

Old or used batteries can remain in service at reduced capacity until they are no longer worth the shelf space they occupy; squeezing the last nickel's worth of service out of each battery.

Batteries can be added or removed from the network at any time; eliminating the need to replace a whole expensive battery bank at one time.

Battery banks are often oversized because the engineer doesn’t really know what the battery bank’s real capacity is as it ages. The testing and reporting function means that you always know the real capacity of the system. The ability to individually replace cells allows you to adjust the capacity as needed.

All the above add up to saving money; likely doubling a value of each dollar spent on batteries. Additional values come from:

 The reliability of many redundant units,

 Safety of smaller batteries and limited currents.

 Simpler and easier to instal; Reduced need for professionals.

Reduce the bus wiring expense
Copper is expensive compared to 20 or 30 years ago..  In some cases, having a protected, current limited power connection can allow much lighter wires to be used, reducing the cost of wire and installation labor..

Key Components
Microprocessors are now less expensive than heavy copper connectors and high speed, high efficiency power switching components used in computer power supplies are now benefiting from economies of scale in mass production.  Together, that means the functionality needed can be done at price points that are only now becoming practical.

Second Life for Second Hand Batteries
Batteries from all manner of industrial equipment from electric forklifts to floor scrubbers to golf carts are currently replaced on a schedule just to prevent potential problems.  In many cases those batteries may have half of their service life remaining, which represents a lot of value and embodied energy in their manufacture.  Today, those batteries are destined for recycling because there is no way to re-deploy them.  A battery network such as this may open a whole new market for second hand batteries that does not exist today.  Because of the voltage abstraction and individual battery care, a battery network like this could allow for a second life for used electric vehicle battery pack, as well.  

References
  1. “Can the Lead-acid Battery Compete in Modern Times?” http://batteryuniversity.com/learn/article/can_the_lead_acid_battery_compete_in_modern_times
  2. “BU-103: Global Battery Markets” http://batteryuniversity.com/learn/article/global_battery_markets
  3. “Basics about Batteries”  http://www.cadex.com/_content/Battery_Basics_Series.pptx
  4. “BU-403: Charging Lead Acid“ http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery