Not clear...need battery advice

I've read the FAQ and look around the site for other discussions about batteries. however I'm still not clear on % discharge of batteries. A true deep cell can be discharged almost 100% and recharged many times w/o damage. However what is the limit of the AGM type? Are they only rated to be discharged upto 50%? Is the only battery type made to discharge totally and to be recharged on a regular basis a deep cell?

If so, any suggestions on where to buy? Many refer to marine and rv, but these are often hybrids, not true deepcells..



  • BB.BB. Super Moderators, Administrators Posts: 30,511 admin
    Re: Not clear...need battery advice

    Generally, the deeper you discharge your battery, the fewer cycles it will survive...

    For a typical Car Type battery (starting), 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:

    [SIZE=-1] 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--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 AGMs.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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