OP 2 Energy Downloadable article  Batteries

KEY CONCEPTS
	1	Batteries like to be cool.
	2	Batteries like to remain as full as possible as often as possible.
	3	Batteries don't like to be discharged more than 20% nightly.
	4	Batteries don't like to be discharged more than 50% regularly (> 2x per week). 
	5	Batteries don't like to stay discharged for a long time. 
	6	Batteries are the most dangerous aspect.  Do not short circuit (connect positive to negative) a battery = 16000 amps.
	7	Batteries are the most expensive aspect.  Consider a generator that kicks on with a low battery voltage reading to save your batteries from misuse.

DESIGN 
	1.	Good design means a battery system that will be discharged 20% each night and down to 50% occasionally (<8x / month) -- Sizing your array size/ production in your area /battery bank /consumption is a critical aspect of good design.  
	2.	Ideally batteries are the same make, model, and life history as the rest of the batteries in the series.  Never replace 1/2 of your batteries.  New batteries will quickly discover equilibrium  with the level of health of the weakest battery. 
	3.	
	4.	Create a separate space -- cool and ventilated and close to the inverter.  
	5.	You have the capacity to set your float, bulk and absorption set points.  When absorption set points are too high -- batteries will gas more leading to corrosion and losing water.
	6.	I strongly recommend a generator back up  rather than battery back up for grid tied systems for the occasional power outage.   I recommend a generator back up for a battery system for the times when you need to run power tools at dusk or more days of bad weather than you expect.  A generator is cheaper than new batteries.  
Other advanced set points include
	•	Diversion relay -- Set a relay to a water pump that pumps when your batteries are full.
	•	Load Shedding relay—Low battery Disconnect – turn off certain appliances when batteries get dangerously low.
	•	Low Battery Disconnect can also connect to grid or generator when batteries get low. 

MAINTENANCE
Good design means less maintenance.  A battery bank with proper setpoints and appropriately sized for your needs will need less maintenance through the years.
TOOLS
	•	Hydrometer -- turkey baster that tests the water in each cell to see if it's still good.
	•	Voltmeter   -- assesses individual battery voltage.
	•	Trimetric 20/20 -- better than a voltmeter at telling state of charge of entire bank
	•	Terminal Cleaner

Source or load will skew the voltage reading so the voltmeter is most useful at dawn when there is no source or no load (throw disconnects) and to see the voltage at the lowest point  after powering you through the night.

Every 4-6 Weeks
	•	Check the water in all three cells of a 6 volt -- water should never go to the lead plates and should be at the bottom of the plastic tab near the top.  Don't fill too high or you will lose battery acid during absorption/equalization.  
	•	If you are adding water monthly or every 6 weeks, lower absorption set point  in charge controller or inverter  or don't fill your batteries so full. 	
	•	Clean the terminals when there is corrosion.  Spread with dialectic goop or vaseline. If there is corrosion after 1-2months, lower your absorption set points.
	•	Tighten stainless steel bolt connections for the wire jumpers between batteries  (stainless steel bolts and nuts on batteries are less affected by corrosion from battery acid).  Tighten bolts for all connections in your system every 6 months. 
	•	Wear gloves and old clothes when moving batteries.  

SAFETY FIRST
40 amps of short circuit potential for every amphour of capacity
16000 amps through a short on a 400 amphour L 16

When unscrewing wires to clean terminals, touch the tool to the lead post before touching the bolt to discharge potential energy build up.   Not common but a lot of electricity if you touch a built up bolt and you won't be able to pull away like with the lead.

No wires comes to or from battery bank without a disconnect within sight.

KEY CONCEPTS expanded
TEMPERATURE -- Batteries like to be cool. Some people store there unused AA in the refrigerator.  The opposite is leaving your ipod or cell phone in your glove box on a hot summer day.  

FULLNESS  -- Batteries like to remain fully charged.  Batteries don't like to be more than 50% discharged regularly.  It's not just how low they go but how long they stay there.  
There is widespread confusion that being fully discharged is good for a battery.  Everyone tells me how the guy at the phone or apple store told them to fully discharge their unit.   YES, The FIRST time you use your computer or phone battery, you can fully discharge it.  This gives the battery a sense of range or capacity.  After this initial discharge, keep batteries topped off when possible and you will get more years out of your battery and it will last longer when you are away from outlets.

THE GRID
In many respects, the grid is a great infrastructure, metaphorically similar to a public transportation system as opposed to highways.  It would be great if more local substations (like several kilowatt solar systems on roof are what powered this wonderful web).   Then one central manufacturing point power plant that supplements renewables by combusting natural gas (easier to ramp up and down than other fossil fuels and less toxicity and carbon emissions to the environment)  is my long term national energy plan. 
Lets move away from battery storage.  Indeed a 1 kilowatt grid tied system would produce up to 20% more energy than a comparable battery based system.   There are simply electricity losses involved in batteries + toxicity to the earth.  

FORMULAS -- HOW BIG DOES MY BATTERY BANK NEED TO BE?

139 amphours /day DC consumed    
x 3 days autonomy   (if you get 3 days without sun regularly)
417 useable amphour/bank (i want to regularly be able to cycle 417 amphours)
/ 50% max depth of discharge   (recommended maximum discharge)
834 amphours / bank
/ 400 amphour /string (and per battery -- volts add in series amps stay the same)
2.1 strings

48 volts/ string  divided by  6 volts/ battery  = 8 batteries per string

16 battery bank for 3 days autonomy if 6.6 kWhours per day consumed  
and you want to discharge maximum to 50 % after 3 days

I may feel good about 2 days going to 50% and the random 3 days that sometimes happen means that my batteries discharge a little more than 50% on those days. 

In laymen terms, "If you want to be able to use 6.6 kwatthours per day and cycle your batteries no greater than 50% on 3 days with no incoming energy, then you need to buy 16 6 volt 400 amphour batteries."

(Notice that if you are fine with whipping out the generator during inclement weather or using less electricity when the sun don't shine, then you can decrease your battery bank)

USEFUL BATTERY NUMBERS
BATTERY VOLTAGES 24Volts
Daily Charge   29.6
Absorption 28.2 - 29.4 (set lower if you are gassing too much or losing lots of water)
Float 26.4
Equalize 31.0 (supercharge 1x per month)

24 Volt L-16 batteries STATE OF CHARGE (varies with different batteries slightly)
SOC	100%	80%	50%	20%	
Voltage	25.5	25.0	24.2	23.32	

BATTERY TYPES 
RV marine  100 amphours @ 12V       (3-5 years)  == 1200 Watts $100
Golf Cart     200 amphours @ 6V	(5-6 years)  == 1200 watts  $200
L16		400 amphours @ 6V	8-10 years == 2400 watts  $350
Big Individual Cells  600 amphours – 2000 amphours) @ 2 V 

I would recommend starting a beginner home scale situation with the cheaper Golf Cart. Half the size for half the money.  Half the loss if poorly maintained.

Ideally
no more than 1 string of RV Marine batteries  (max 4 on a 48 volt system)
no more than 16 Golf Cart batteries
no more than 16 L16

AGM & Gel -- 
	•	sealed batteries 
	•	less maintenance -- never need to add water
	•	 2x the price as the deep cycle such as L-16 Trojan\
	•	vented/wet batteris get stiff if you don't cycle them
	•	desert maintenance 
	•	Summer/weekend home in a hot area is a perfect candidate for sealed.
	•	Critical with all sealed batteries that your charging high point is below gassing-   set your absorption point lower 28.2 vs. 29.4.  

Good reasons to wire your battery bank to 48 volts
1   Half the voltage needs 4 x as much copper wire (i.e.  a 24 volt system needs 4 x the wire as a 48 volt system and a 12 volt system needs 16 x as much.)
2 Slight friction between multiple strings of batteries
When you do have more than 1 string in a battery bank, go from the positive of 1 and the negative of the other to the inverter. This helps balance the batteries.

 HISTORY
Battery was coined by Benjamin Franklin to describe a series of capacitors (i.e. Leyden Jars that can store electrical charge.)   We are still basically living on 150 year old battery technology while solid state electronics are getting better and better.  Let's plug into the grid when possible.