Using A Starting Battery With Deep Cycle To Start A Refrigerator

solarpowerwindandstu

Hi i'm looking for some advice on a battery system, can a deep cycle/starting battery be used in a system of 12, 6 volt golf cart batteries? Would the starting battery make it so it's easier on the main batteries for starting a compressor if i hook the inverter running the refrigerator to the starting battery?

vtmaps

Welcome to the forum,

If you have 12 golfcart batteries in a 12 volt system, that means you have 6 parallel strings.  That is an unstable and unworkable design for a system that cycles frequently.   If you need that much storage you should be using a single string of 2 volt batteries, or raise your system voltage so you can have fewer parallel strings.

A starting battery is not the solution and may actually make your problems worse. 

btw, how have you interconnected the batteries?  When you have that many strings you should use a bus bar to combine the batteries... except that you shouldn't have parallel batteries to begin with.

--vtMaps

oil pan 4

One type of battery will try to discharge through the other.
Don't do it.

solarpowerwindandstu

vtmaps said:

Welcome to the forum,

If you have 12 golfcart batteries in a 12 volt system, that means you have 6 parallel strings.  That is an unstable and unworkable design for a system that cycles frequently.   If you need that much storage you should be using a single string of 2 volt batteries, or raise your system voltage so you can have fewer parallel strings.

A starting battery is not the solution and may actually make your problems worse. 

btw, how have you interconnected the batteries?  When you have that many strings you should use a bus bar to combine the batteries... except that you shouldn't have parallel batteries to begin with.

--vtMaps

Yes I was gonna use a big bus bar for combining the parallel strings, I'll look into the 2 volt batteries. Thanks!

solarpowerwindandstu

oil pan 4 said:

One type of battery will try to discharge through the other.
Don't do it.

Thanks! I never thought of that.

vtmaps

solarpowerwindandstu said:

oil pan 4 said:

One type of battery will try to discharge through the other.
Don't do it.

Thanks! I never thought of that.

That may already be happening... the problem with parallel batteries is that the charging current does not divide equally among the parallel paths.  That means the batteries age differently.  That means that the batteries become different from each other.  The imbalance grows until the entire bank fails.  One bad cell can bring down the entire bank. 

If one cell in one string develops shorted plates, you can go into thermal runaway and have a fire.  To prevent that, you must have a fuse or circuit breaker in each string of batteries.  This adds more potential points of failure and makes it even more difficult to keep the batteries balanced. 

Even if you had impossibly identical batteries, and impossibly perfect wiring, parallel batteries are still unstable.  If one battery is slightly warmer than another (inevitable), it will draw more of the charging current.  That will make it warmer which will allow it to take more current which will make it warmer which will make it take more current which will make it warmer... that's called positive feedback... sort of like balancing a pencil on its point.  Mathematically the pencil is balanced in equilibrium, but it is an unstable equilibrium.  In the real world it will lean infinitesimally to one direction, and then the forces on it pull it further in that direction.

--vtMaps

solarpowerwindandstu

vtmaps said:

solarpowerwindandstu said:

oil pan 4 said:

One type of battery will try to discharge through the other.
Don't do it.

Thanks! I never thought of that.

That may already be happening... the problem with parallel batteries is that the charging current does not divide equally among the parallel paths.  That means the batteries age differently.  That means that the batteries become different from each other.  The imbalance grows until the entire bank fails.  One bad cell can bring down the entire bank. 

If one cell in one string develops shorted plates, you can go into thermal runaway and have a fire.  To prevent that, you must have a fuse or circuit breaker in each string of batteries.  This adds more potential points of failure and makes it even more difficult to keep the batteries balanced. 

Even if you had impossibly identical batteries, and impossibly perfect wiring, parallel batteries are still unstable.  If one battery is slightly warmer than another (inevitable), it will draw more of the charging current.  That will make it warmer which will allow it to take more current which will make it warmer which will make it take more current which will make it warmer... that's called positive feedback... sort of like balancing a pencil on its point.  Mathematically the pencil is balanced in equilibrium, but it is an unstable equilibrium.  In the real world it will lean infinitesimally to one direction, and then the forces on it pull it further in that direction.

--vtMaps

I got a really good deal on 6v batteries is there any safe and viable way to use them effectively for a 12v system? maybe have 6 different systems that have separate panels so the batteries are apart, would that work? then I can put in a voltage controlled relay so when 1 battery gets to 50% it will switch the inverter to another.

vtmaps

solarpowerwindandstu said:

maybe have 6 different systems that have separate panels so the batteries are apart, would that work? then I can put in a voltage controlled relay so when 1 battery gets to 50% it will switch the inverter to another.

Yes, you can have 6 different batteries, each with its own array AND controller.  Then you can switch the inverter from one system to another. 

If you do that, give some consideration to whether the switch is make-before-break or break-before-make.  If the switch is break-before-make, the inverter will be unpowered briefly during the switch.  If the switch is make-before-break, the inverter will be continuously powered, but the charged battery will be briefly connected to the discharged battery.  This could lead to some unpleasantly high currents through the switch and potential arcing if the switch is not rated for that application.

solarpowerwindandstu said:

I got a really good deal on 6v batteries is there any safe and viable way to use them effectively for a 12v system?

It may not be such a good deal if it's not suited to the application.  A good deal on a half ton pickup truck is not a good deal if you really need a one ton dump truck.

--vtMaps