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 Post subject: Choose the Right Battery for a Solar or Renewable system
PostPosted: Sun Aug 17, 2008 11:33 am GMT EthGMT 
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Joined: Wed Apr 19, 2006 1:02 am GMT EthGMT
Posts: 110
Location: SF Bay Area
Just a quick FAQ for new users that are confused about picking batteries.

Generally, the deeper you discharge your battery, the fewer cycles it will survive... Also, the higher the current you draw, the "less efficient" the battery is. Typically, a good rule of thumb is to never discharge your battery below 50% and to limit your "average" maximum current draw to around 5% of the Amp*Hour Rating of your battery bank (typically use the 20 Hour battery rating when sizing your battery bank). A good starting point is 6x your 1 day Amp*Hour load (3 days without sun, and 50% maximum discharge). Charge current is recommended to be around 5-13% of the batteries Amp*Hour Rating (some batteries can take up to ~20% maximum charge rate).

For a typical Car Type battery (starting battery), they are designed for about 15% discharge (85% state of charge or SOC).

A true Deep Cycle designed battery (heavy/thick plates) can probably by discharged well below 50% (perhaps even down to 20% SOC)... However, the plate sulphation begins to harden within hours--so you need to get the Deep Cycle battery back above 75% SOC quickly (within hours or a day) or else the sulfate hardens and the battery capacity is reduced.

For AGM's, there is one manufacturer (Concord?) that says their batteries will not experience sulfate hardening, so they can be cycled down to 20% SOC and not be immediately recharged. However, deep cycling will still lessen battery overall life.

For example, from the Concord website:

Solar Battery Comparison Overview
AGM Battery Technology vs Gel Cell Batteries
AGM Battery Technology vs Flooded (Wet) Solar Batteries
SunXtender Solar Battery Dissected (PDF)
Sun Xtender Technical Briefing - SOC, OCV, & DOD
Literature Manuals and Technical Information

Concorde Test Standards
20% DOD - 2,800 Cycles
50% DOD - 1,050 Cycles
80% DOD - 550 Cycles

So, you can cycle 4x deeper (to 20% SOC) for 550 cycles, or to 80% SOC for 2,800 cycles...

And, there are trade-offs... You might be able to use 1/2 the AH rating of a Concord AGM battery (if you want to go with deeper cycling), but they cost twice as much (approximately) as a plain flooded cell deep cycle battery.

The Marine and RV batteries are sort of hybrids between a car battery (starting battery) and a true deep cycle battery... In the case of solar--generally you don't have the high current peaks of starting, but need to heavy plates for long life of a deep cycle battery... Marine/RV batteries don't really do that part as well since they are also designed to supply higher peak currents for starting motors too.

It usually comes down to picking between a Deep Cycle Flooded Cell battery or a good quality AGM...

Flooded cell; is cheap(er), rugged, easy to refill if you goof and over charge. But requires monthly adding of distilled water and cleaning (plus good venting for hydrogen gas). Also, is about ~80% energy efficient (charge/discharge/charge).

AGM; expensive, pretty rugged, much cleaner (no acid mist to rot cables/terminals), no water needed, and about 90% charge/discharge efficient. Can sit discharged for longer periods without damage (sulfphation). However, can be easily damaged if overcharged (no way to add water if boiled).

Generally, if this is your first system experience--"cheap" Flooded Cell batteries are recommended for "training". Once you get your system under control and have experience, you can decide later (~3-7+ years or so) if you want to replace the next set with very expensive AGM, or more expensive (better quality) flooded cell batteries, or just stay with the least expensive flooded cell.

Here are some other resources for more information

Battery Faq Website
Aonther Battery FAQ
Commercial Solar Sales FAQ


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