Gel or AGM for hot climate ?

abdoudiawabdoudiaw Registered Users Posts: 12
I have a hard time making a choice between GEL and AGM deep cycle batteries.
I live in Senegal (West Africa), hot climate.
Total load about 200W including a 12/24 Volt refrigerator.
I have a Morninstar MPPT SunSaver controller.
Which would be a better choice when other criteria are taken into consideration?
Thanks.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,517 admin
    Re: Gel or AGM for hot climate ?

    You need to find the details about the GEL battery... Many will be damanged if charged faster than C/20 (5% rate of charge based on 20 Hour battery capacity--i.e., 100 AH battery charged at 5 amps maximum). There are some GEL (usually out of Europe) that appear to not have this low charging current limit.

    From our host's battery FAQ:

    http://www.windsun.com/Batteries/Battery_FAQ.htm
    Gelled batteries, or "Gel Cells" contain acid that has been "gelled" by the addition of Silica Gel, turning the acid into a solid mass that looks like gooey Jell-O. The advantage of these batteries is that it is impossible to spill acid even if they are broken. However, there are several disadvantages. One is that they must be charged at a slower rate (C/20) to prevent excess gas from damaging the cells. They cannot be fast charged on a conventional automotive charger or they may be permanently damaged. This is not usually a problem with solar electric systems, but if an auxiliary generator or inverter bulk charger is used, current must be limited to the manufacturers specifications. Most better inverters commonly used in solar electric systems can be set to limit charging current to the batteries. Some other disadvantages of gel cells is that they must be charged at a lower voltage (2/10th's less) than flooded or AGM batteries. If overcharged, voids can develop in the gel which will never heal, causing a loss in battery capacity. In hot climates, water loss can be enough over 2-4 years to cause premature battery death. It is for this and other reasons that we no longer sell any of the gelled cells except for replacement use. The newer AGM (absorbed glass mat) batteries have all the advantages (and then some) of gelled, with none of the disadvantages.

    GEL seem to be pretty easy to damage. And if you have this low of charging current limitation, then it becomes difficult to quickly recharge your battery bank and use DC loads at the same time (i.e, the "easy way" would be to only charge during the day and discharge at night--If you have DC loads during the day, there is no solar charger, yet--that I am aware of, that can limit charging current to the battery if there are other DC loads).

    GEL seem to be a very nice choice for computer UPS systems--Lots of floating (standby waiting for power failure). And the UPS turns on when the AC power fails--can supply lots of power. And when the power is restored, the UPS turns off and recharges the battery bank (no load issues and easy to manage 5% rate of charge maximum).

    At this point, if the pricing is similar, I would suggest AGM batteries instead (from what little I know--I am no battery expert).

    Is there a reason you don't want flooded cell? Typically flooded cell are cheaper and less sensitive to over charging than either GEL or AGM. There is the issue of getting "distilled water" (or filtered rain water) to refile the cells every month or so. In general, it appears that good quality deep cycle flooded cell batteries (if well taken care of) will last longer than AGM/GEL type batteries.

    The battery FAQs:

    http://www.windsun.com/Batteries/Battery_FAQ.htm
    http://www.batteryfaq.org/
    http://batteryuniversity.com/

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Gel or AGM for hot climate ?

    Cost per Watt hour would be my reasoning. In that vein, flooded cells are still probably your best bargain and most likely easily available - unless you have to get them shipped in. Then the added cost of transporting unsealed batteries (considered hazardous) will likely outweigh the extra cost of AGM's. I would not consider gels, as they are usually unsuited to RE applications (although there are some whose specifications indicate they are).

    Temperature affect on performance will be pretty much the same no matter what type you use. It would be more important to use a charge controller with a remote battery temperature sensor than to pick one type of battery over another.

    Your total load may be 200 Watts but there's two things missing from that: the time (Watt hours per day) and the peak demand of that refrigerator (assuming it is a compressor type; if it is solid state it is going to use a lot of power all the time).
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