[RE-wrenches] 24 volt Battery Bank comparison

[James Surrette]

Hi Ray, 

Just a quick chime in on your series string comment. 

If you hook up 4 x 12V monoblocks for a small 48VDC system and begin charging at 58.8V, total voltage will be 58.8V very quickly but individual battery voltage will not be 14.7V.  Battery voltage will have a large range from 14 to almost 17V.  This problem is only exacerbated in higher voltage systems, ie, Australian 120VDC.  Stealing from the Telcos, the best solution is an initial activation to balance all cells.  When this is completed and assuming all leads stay tight, no bad cells, etc the bank will charge and discharge in relative harmony.  However, if the cells / batteries are bolted together and discharged right out of the gate, there's a good chance SG  & Voltages will be all over the map in short order.
 
Jamie
>>> Ray Walters <ray at solarray.com> 6/3/2011 2:53 PM >>>
I've set electric vehicle parallel strings up like this too. Once a month I charge each string separately. At higher charge/ discharge rates, the problems of unequal batteries increases dramatically. BTW, single strings are not the magic bullet either; I've had single strings with equal current through each battery, but some batteries would be at 15.5 volts, while others next to it would be at 13. The charge controller shuts off when the sum of the voltages hits the bulk charge V, but meanwhile some cells are chronically overcharged or undercharged. (parallel strings no where to be seen)
Manzanita Micro makes a device that shunts current past batteries that hit full charge in a string. This monitoring of each individual battery is the state of the art right now for Li+ batteries in EVs.
I agree, we need much better battery management and safety devices for solar. Its ridiculous some of the mundane issues we discuss here sometimes, while hundreds of huge battery packs are just waiting for one of us to drop a wrench across the terminals, with zero safety to interrupt it, and its all NEC compliant.
Batteries themselves need to have a current limiting device built into the positive terminal. Possibly the same device that controls charge current to it too?

Dreaming up future BOS equipment,

Ray Walters

On 6/2/2011 7:44 PM, dan at foxfire-energy.com wrote: 


What I like best about Mark's set up (the retired phone co. dude w/ half a hand), is that he can select individual strings at random. he can eq an individual string, or top off a few strings and park them. he can even run strings of T 105s, or even nicad (individually of course) in the same system as L-16s and the like.. he just reprograms the chargers (and logs it). I think he got the design from his days in the Navy. 



So boB, how about a controller that can be user programed to charge multiple battery configurations with a soft switch? i.e. Bank A, Bank B...? and while you're at it, maybe a multiple string DC box? Something with a shunt and a breaker for each string? A four string set up would be nice.



I could use 2% of your first million. 



db


Dan Brown 
 


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