AGM or Gel Batteries?

[leveller1642]

I need to replace the batteries on my outboard engine powered 25 footer. It has about 150 watts of solar panels which output around 4 to 5 amps on average.
I am going to get 2 x 100 a/h batteries but need to decide between absorbed glass mat or gel.
Can anybody tell me which would be best for my needs?
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[Symmetri]

Yes they are better, AGM will take a charge faster than flooded cell and recover from deep cycles better. They'll also cost 2-5 times more than common flooded cells. You decide if that's worth it.

I'm amazed that you found a way to mount 150 watts of solar on a 25 foot boat. How did you do it?

[leveller1642]

I bought a new 150 a/h AGM a few months ago. It was a rather cheap chinese made one. It is now virtually dead (10.5 to 15.5 volts in a few hours) i thought the slow charge may have been a factor in its demise or that I given a dud by the store.
This replaced 4 x215 a/h golf cart batteries that lasted 6 years. They would have lasted longer except that I was a bit slack on checking the electrolyte level, so i am reluctant to get flooded cells again.
I have almost decided to get 2 x Lifeline 100a/h AGMs but want to be certain that the slow solar charge rate is not an issue. If so would Gel Sealed lead Acid be better?
The charge rate for a AGM is to 15 volts and float on 13.8 volts? Is this right?
My panels are not really mounted at least not properly. I have a Uni Solar 64 watt (these panels are excellent) on the deck in front of the cabin. It has stayed there but is a bit of a pain when mooring or anchoring. I have 2 x 32 Uni solar flexible panels on the cabin roof and sides. A 20 watt BP marine panel is mounted on the rear rail. I also have a Air Marine 403 400 watt wind turbine but I am getting rid of that to cut down on weight.

[SG]

I think that the issues really revolve around

i) your use of the batteries? -- e.g., how often do you draw them down to less than 50% of capacity, etc.
ii) What charging source(s) if any, beyond the solar array, you have -- and whether they are "smart regulated"?
iii) Your sensitivity to spending another 20% or so for the batteries.

I would say that my GUESS is that the gel cells would be fine -- IF you have adequate charging control and sources.

[Westcoaster]

On my old Freedom 25 I had only a 20w solar panel but here's what I did. I bought my panel, solar controller, and 105ah AGM battery at Arizona Wind and Solar for one thing. It was cheaper to buy my AGM and have it shipped than to buy it at West Marine locally..... The solar controller I chose had the feature of auto disconnect of house demand when the battery reached 50% discharge thereby protecting the battery. I also had a charging loop on my Suzuki 6hp that charged the battery when it was running. The system worked perfectly for me for the three years I had the boat. I recommend Wind and Solar, have no ties there, for their knowledge and helpfulness. I made a panel that with master switch and little window for the solar controller and a digi flow through volt meter. I'd post a pic but don't know how........[/img]

[Symmetri]

Do you have a charge controller? If not you should get that along with your battery replacement. Probably the biggest advantage of AGM vs. wet or gel is that the AGM will charge up to full capacity faster, whereas the gel/flooded will charge at a slower rate up to 100% once you get past about 75-80% charged. Again -- you need to decide if that's worth the extra cost.

I use generic wet cell GRP24 batteries from my local Costco. $47 for 85AH and they came with a 5-year warranty. By the time I add shipping they're one-third the cost of Lifeline AGM -- so even if they don't last long I'm ahead of the game. The AGM charge-rate performance advantage isn't' worth the cost to me, because my boat recharges using solar when I'm away from it.

[Andina Marie]

If you switch from lead-acid to AGM it is important to have a regulator for the alternator and solar cell. While lead acid batteries can take voltages up 15 volts or more with no damage, charging voltages above 14.2 can permanently damage an AGM battery. Most small outboards and unregulated solar cells will exceed the 14.2 volt limit.

[Dale Jadus]

All this talk about battery charging voltages is potentially useless unless you have a charge controller that senses the battery temperature AND you have programmed the controller for the temperature coefficient of the particular battery type.

[Andina Marie]

Temperature sensing is not essential, especially in this installation where you are charging from a low current source or solar cells. Temperature sensing only becomes important when you are increasing the battery temperature due to forcing in maximum charging rates that raise the temperature. Now you can safely force the charging knowing that the battery will be protected if the temperature rises.

But in installations without high power charging where you want maximum charge in minimum time, temperature sensing is an unnecessary luxury.

[sail445]

I just replaced my Gel Batteries with new ones, (2-4D's). They lasted 11 years. I have a few friends whose Gel batteries have lasted from 8 to 12 years. It's very important to to make sure your charger is designed for Gels and not to charge above 13.8 volts..