Deciding on what battery to choose is not difficult, it comes down to what you can afford and the size of system you want.
You can build your own battery using raw Lifepo4 battery Cells, use lead acid or a ready built Lifepo4 battery.
Lifepo4 is very popular as they are much lighter than traditional lead acid, they don't need venting and can be stored in any position.
Lead acid has many draw backs in comparison to Lifepo4 but its all down to budgets and situations.
LifePO4 batteries have their own inbuilt battery management systems (BMS) to help keep everything balanced and have a fantastic life span (10 + years).
Their ability to provide constant current sets them apart from all other batteries available today.
Indeed LifePO4 batteries are far superior for solar use than any other battery type.
However, its your budget that matters!
LifePO4 do cost more initially but will actually save you money in the longer term.
Some savings may be made by building your own battery!
AGM (Absorbent Glass Matt) batteries do perform better than traditional flooded lead acid batteries as do GEL batteries which use silica to make a gel formed electrolyte instead of liquid.
For larger systems it is more viable to go with Server Rack Batteries.
These are usually in the 48V power region and are designed to work & communicate with certain off-grid & hybrid Inverters.
Batteries are sized in Volts & Amp Hours.
A 12 volt 100 amp hour battery is designed to deliver 1 amp of power over 100 hours, but there's a catch with lead acid batteries.
Because you cannot drain a lead acid battery below 50% of its capacity (Damage Occurs) you need to divide any lead acid battery capacity in two. (100/2=50).
So you can only expect 50 Amp hours, but there's another catch!
Thanks to something called Peukerts law the batteries available capacity decreases as it is being drained.
They are also very heavy!
All batteries have something called a "C" rating, this is an important piece of information that you need to be aware of.
Always look for batteries with a high C rate.
This rating is the amount of time a battery can be charged & discharged at safely.
Higher the C rating, better the battery is overall.
One battery i tested had a 130Ah rating but only if used at 0.001c which is not only misleading, its unrealistic.
Lithium ion / iron batteries work differently in that you can actually get 100% of the stated capacity.
They will deliver a constant current at a constant rate of discharge. They can be placed in any configuration you desire & they weigh half that of lead acid.
Always buy the best battery you can afford and preferably go with lithium iron phosphate (Lifepo4) batteries.
They out perform and vastly outlive lead acid batteries.
It is preferable to buy a LifePO4 battery with low temperature disconnect option, you will damage any lithium battery if you try and charge at zero degrees or below.
Low temperature charge disconnect will stop the battery from being charged in such circumstances.
Higher quality battery and solar charge controller brands offer this as standard and some batteries even boast a self heat function.
Watt Hours of any battery is calculated by multiplying the voltage by the Amp Hour rating.
So a 12v 100Ah battery will have a watt hour rating of (12 X 100 = 1200 ) Watt Hours, whereas a 24V 100Ah battery will give 2400 Watt Hours.
Similar lead acid battery would only produce (12 X 100 = 1200Wh / 2 = 600Wh)
Many people get confused about connecting batteries together in terms called series and parallel.
Have you ever put two or more batteries into a torch?
If so you have already made a series connection!
Series connections increase voltage, but amp hours remain the same.
So if you connect two 12v 50 Amp hour batteries in series then the voltage increases to 24 volts but the amp hours stay the same at 50 Ah.
Parallel connections are opposite, they keep the voltage the same but the amp hour rating increases.
Using the above example 2 x 12v 50Ah batteries become 12v 100Ah batteries.
These connections also apply to solar panel configurations!
If you have calculated how many watts you need each day, its time to size your battery bank, solar panel needs & find a suitable solar charge controller.
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