AGM v. Gel

[danielgoldberg]

I was at the Defender's warehouse sale last week and a vendor told me something I didn't know, and it makes me wonder. We have gels now, but I always had assumed we'd move to AGM's when the time came because they are less sensitive to overcharge and can be charged faster. This vendor had an interesting take though. For starters, let me say that he sells both, they both are roughly the same price, and he appeared to have no reason to favor one over the other (which doesn't mean he knows what he's talking about, just that he at least did not appear to be biased).

He says that AGMs have far fewer (up to 50% less) cycles in them as compared to gels, particularly if you tend to discharge close to the 50% mark. So, though the initial cost is roughly the same, AGMs are much more expensive because they last roughly half as long.

He also says that the hype about faster charging is overstated. You get that faster recharging only if your charging system can get you to and sustain voltage in the upper regions of 14 volts. If your charging system tops out at around 14.4 volts, then there is no difference in absorption rates between gels and AGM.

He says AGMs are best used when you don't really have regulated charging, such as with larger outboards, or if you can set your charging system to give you 14.8 volts.

EDIT: I should have said this first. I know the topic about AGMs has gotten a lot of airtime here. My points for discussion (less cycles and no faster charging as a practical matter) don't seem to have been covered, and if so I missed them and apologize.

Thoughts?

[camaraderie]

Daniel..while his claim for # of cycles is a bit overblown it is true that you can get more cycles out of a gel. That said...they are FAR easier to destroy with even one bad charge cycle. Furthermore...there is no further development going on with Gels...it is all AGM and AGM/foam for the new technology stuff like the Odyssey and Firefly technologies which outperform ANY other type of battery in terms of cycles and can be FULLY discharged.

the rep is ABSOLUTLEY wrong about charging quickness as AGM's can be charges at 100% or more of their rated amp capacity at NORMAL bulk charging voltage and that makes them WAY less expensive than the 20% max rate engine running time you pay for by selecting gels. Th only thing I will say is that once your bank gets to be 600ah's or more..this is largely a moot point as no reasonable charging system can supply the amps that the AGM batteries can take. I expect that this may change in the future as we get more electric driven boats and use hi tech generating systems to charge new generation AGM batteries to drive the engine and provide for house needs.

[seabreeze_97]

Also, it is said by the manufacturers that the AGM's can be fully discharged for a month and then recharged with no damage. I dunno just how true that is, but I will say that when I got my boat, she had two AGM's bought in 1998. They are both still getting the job done. They had been run down after the boat sat 5 months without shore power after Katrina. I pulled them out and they recharged easily. I use one in the dink with a trolling motor now. The other is in one of my trucks. These batteries are 10 years old. I wouldn't believe it if I hadn't seen it myself, but they are amazing batteries.

[Quickstep192]

This is completely unscientific. I have four AGMs. Two in my powerboat, and two in an Antique car. Both spent the entirety of the winter with no re-charging. The boat is in freezing temperatures, the car in in near freezing temperatures. Last week after a sign of spring, I drove the car and plugged in the automatic charger on the boat while I removed the cover. Both AGMs on the boat charged up to full charge in just a few minutes, telling me that there was extremely little self discharge over the winter. Although the motor in the car was reluctant to start after sitting all winter, the battery cranked it over until it finally started. I'm sold on these things. The conventional battery on my sailboat seems to be toast after a winter outside. I'll be buying an AGM for it as soon as I can get to the battery store.

[gershel]

Not to burst anyones bubble, but my Prevailer gels are also just as strong as they were when I stored the boat for the winter last October, without being charged since. I purchased them in Nov. 97.
Marc

[seabreeze_97]

I got to thinking about it today, and I had made a mistake on the batteries. They are, in fact gels, SeaGels in fact. Sorry for the snafu.

[SimonV]

I have 400amps of West Marine gels. I will consider AGMs if their cycle number’s improve but as my charging is all regulated for gels it will be a big call.

[donradclife]

The last time I had to replace my gel batteries, I looked at AGM's, but stayed with the gels. The primary reason was that the AGM users were reporting that AGMs (like wet cells) had to be kept fully charged and/or equalized to prevent loss of capacity. Living on the hook, my 500 ah battery bank probably averages 80% of full charge, and hardly ever gets charged back to 100%. The gel cells don't seem to mind this treatment, and have typically lasted about 7 years.

[ericroline]

I installed AGM's in both sailboats. Low self discharge and quick charge is what I was looking for. Frequent overnighters and not having reliable shore power is less than desired and I wanted the batts to be able to be a little abused. After 5 yrs and no loss of performance the price was worth it. Gels are good, but don't mistreat them. They never forget.

[hellosailor]

Dan-
Gel cells have a "gelling agent" added to the electrolyte. What does it do? It is like food starch, all it does is make the electrolyte into "goo". And in order to do that--it dilutes the electrolyte. It's an inert filler that bulks things up.
Now, compare to an AGM battery, which is a "starved" electrolyte. I'd have to think that the "goo" makes the gel cell bulkier for the same capacity. But there's something more telling to me: The gel cells that used to be universally used in backup power supplies, burglar alarms, and exit lighting? Have almost all been quietly replaced by AGM BATTERIES!
Perhaps that is because they are cheaper in some way...perhaps it is because they are better, more robust in some way. Dunno. But when any vendor starts to play "charge cycles", ask them why "brand x" is good for 500 cycles, and "brand y" is good for 200 cycles, if they are all the same for the same technology.
It just ain't so. Even in wet lead batteries, there's a huge range of difference in how many charge cycles you'll get if you look at a premium commerical battery (Rolls, Surette) compared to a "chandlery $50 special". A huge difference because of the plate thickness, the composition, and the puffery (aka lies) from the vendors.
If you are all set up for gels and they keep you happy, by all means, stick with what works. But AGMs appear to be a better way to go--provided you get a good price on a good battery, just like any other type. Are there some lousy brands? I'm sure there are. Mattresses, tires, batteries...some things are just designed to keep the rubes confused, right?