Here we are with the first of several posts concerning solar design. before I go any further, let me say that most of the information in my posts will relate to off grid stationary solar installation design best practices and will therefore differ with what an RV'er might consider good practice.

I will address RV specifics as and when needed.

So first to Batteries.

when I ask most people about lead acid batteries, they almost always think of the car battery.. it's 12 volts dude, what more do you need to know ?

well that response is just like ice cream.. it's cold right ?
like ice cream, batteries also come in assorted flavors..

The SLI or car battery..

this type is designed to deliver high amperage for a short period, cranking over that big engine for several seconds at 400 amps or more..they are designed with thin lead calcium alloy plates for low resistance and low voltage drop. they are NOT designed for deep cycle use and will fall apart internally very quickly if you treat them as such.

Pros: high current typically ( high c rating )
inexpensive due to high volume manufacture
Cons: low lifespan if cycled.

Hybrid batteries ( FLA or flooded lead acid )

The hybrid has negative plates of lead calcium alloy and positive plates of lead antimony alloy, the plates are thicker than a SLI battery to give more deep cycle capability and is a trade off between high current and deep cycle useage. these have names such as Golfcart, Trolling, RV, marine etc... note that these batteries are marked with CCA and Ah. CCA or cold cranking amps is what you would see on a SLI battery..

Pros: Good all round battery for medium cycle useage
cons: much less current rating than SLI

VLRA or AGM ( advanced glass matt )

valve regulated lead acid batteries come in several flavors.
Hybrid just as the flooded battery, these are the typical AGM sold
and have very low internal resistance and therefore high discharge and charge capability. they have the advantage of no electrolyte maintenance ( it is absorbed into a glass fiber cloth ) but cost considerably more than a FLA, also they don't have near the cycle capability of FLA.

Pros: very high c rating like SLI
No electrolyte maintenance
higher cycle capability than SLI
cons: High cost compared to SLI
lower lifespan than deep cycle

Telecom or UPS AGM

this version of AGM is designed to spend most of it's useful life on float charge much like the SLI. like the SLI it's a high current battery and has a low resistance. not recommended for solar RE applications or RV.

pros: Very high c rate
cons: very low cycle capability like SLI

Deep cycle FLA.

these are the mainstay of solar RE. very thick plates of lead antimony built for deep cycle use and long life. the trade off here is the high internal resistance. these are NOT designed for high current use, they are designed to be used down to typically 50% DOD ( depth of discharge ) at relatively low currents and will withstand this use pattern for a very long time. deep cycle batteries are not rated in CCA. capacity is rated at the 20 hour discharge rate ( c/20 )

Lithium iron phosphate ( LiFePO4 )
This is the preferred lithium chemistry for us normal folks.
it's quite stable and not prone to self ignition or combustion like other lithium types. they are made in numerous shapes and sizes.

two main types to consider for us..

Prismatic cells.

The popular form ( in large capacity sizes ) is the prismatic cell. the basic cell has a voltage of 3.2 volts is available in capacity from 10 Ah to 1000 Ah and needs to be assembled into strings to achieve the desired voltage.

Drop in's

these are made as a form, fit and function replacement for SLI batteries. and are targeted at people who like the idea of lithium but neither know or want to know the technology and have cash to burn.

pros: extremely high charge and discharge rates
no electrolyte issues
very light weight compared to FLA
unlike FLA and AGM, can be placed in parallel service
without adverse effects.

cons: very expensive ( 400 to 500% higher than FLA )
for most use require a monitoring system to prevent cell
imbalance. AKA a BMS or battery monitoring system.

I'll finish up with a mention of C rates.

the C rate is just the batteries capacity divided by the charge or
discharge current.

example: a T105 of 225 Ah capacity
if we charge or discharge at say c/8 then we have 225/8 = 28 Amps

for charge or discharge there are ideal rates that battery makers suggest these are not arbitrary figures but recommended values for maximum lifespan for general use. refer to manufacture data for those rates.

personally, I would not recommend a rate of anything higher than
a c/4 rate for FLA and c/2 for AGM, anything higher than this will shorten life. just like a car, the harder you drive it the quicker you wear it out. same applies to batteries.

so that's enough of batteries, next installment will a short one on solar panels and controllers. followed by a real system design and suggestions for RV use.

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