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We think we are going to have to buy new batteries and have been researching what to get.  The problem we have is that the batteries are under the bunks and so it is undesirable to have them gassing, but I am not sure how much of a problem it actually is.

From what I have read only AGM batteries dont gas, but they are significantly more expensive than standard batteries.

Is there any significant benefit to AGM other than no gassing?  We are charging of solar panels and a small wind generator so we do not need to support 'fast charging' as provided by an alternator on an engine.



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No need to check or add electrolyte, can be installed in any position. If they do get punctured, they do not spill electrolyte like wet cells would.

From the Wiki:

  • AGM construction allows purer lead in the plates as each plate no longer needs to support its own weight based on the sandwich construction with AGM matting. Traditional cells must support their own weight in the bath of acid.
  • un-spillable
  • high specific power or power density, holding roughly 1.5x the AH capacity as flooded batteries based on purer lead[citation needed]
  • low internal resistance allowing them to be charged and discharged quite rapidly
  • never requiring addition of water
  • acid is encapsulated in the matting
  • will operate well below 0°F or -18 °C.
  • UL, DOT, CE, Coast Guard, and Mil-Spec approved types
  • vibration resistant based on the sandwich construction.
I just discussed some el.-matters with a professional electrician.  Deep cicle AGM maintenance free batteries are more expensive coz there is much more lead in them. They are also heavier than ordinary maintenancefree automobilbatteries and are supposed to last longer. I,m not an expert but check out if your solarpanels can realy charge your batteries to 100% within a considerable time!

My first post here but here goes.

First off, I've seen the damage that can be done by exploding batteries, it's not pretty.

You need to have a sealed box with an external vent for unsealed batteries.

AGM batteries are not more expensive than good quality deep cycle lead acid batteries.

AGM's are more expensive than poor quality or automotive batteries.


Charge rate is dictated by the size, type and temperature of your battery bank.

You should size your battery bank on 30% usage between charges i.e. calculate your amp hours used between charges and multiply by 3 to give you your required battery bank size.

I know you said your using solar and wind charging but how long do you go without wind or sun at any one time.

Auto batteries have lots of thin plates to enable high currents for starting engines, however thin plates will deform and break down more easily when deep cycled.

Deep cycle batteries have thicker plates that should, if looked after, last much longer. The thickness of the plates and construction methods are the reason for their greater expense.


Personally I would and have purchased Rolls Surrette batteries, as they are the longest lasting batteries I've come across so far. 25 years is not unheard of.


Buy cheap, buy lots!!!!!

Thanks for the input.  The batteries we are replacing are Elecsol 125 Ah standard batteries which are only now dying after 10 years.  The only time they have posed a problem is when we have tried to run the autopilot, but we never pinned down whether it was the battery or someother problem.

I agree about buying quality, but we are not sure whether we are going to keep our current boat or trade up for a larger one, something we intend to work out over the next 9 months, so I was wondering whether to go for the sealed leisure batteries as a interim fix.  However, I have found that Elecsol are doing an AGM battery at a competitive price and have a 10 Ah greater capacity so I think we will go with that.

For information: we have 2 batteries, one in each hull, which are currently independant and each is charged from 2 solar panels.  We carry a battery charger to use when we have mains electricity, but did not need to use it until recently.   Now we have a pod I am rewiring so that each battery has a feed into the pod and we can select which, or both, to use.  I worked out our Ah and we should never exceed 10 amp draw unless we start using an inverter with a lot of kit hanging off it which we have not plans do do as the laptop seems to run OK off a step up converter plugged into a cigarette lighter socket.

I've had reasonable success with elecsols early batteries, but there have been quite a few complaints made about their more recent offerings . TBH I would put elecsol in the middle of the range for batteries.

I did try contacting Elecsol to get some advice about AGM versus leisure and got no response, which kind of fits with what I found on the websites just now - sounds as if a good one is excellent but if you get a faulty one you have a problem!  I think I will look at alternatives, unfortunately our dedicated battery space constrains the battery shape ...

The 12 Volt Bible for Boats, by miner Brotherton, from Adlard Coles Nautical, gives   very easy to understand, non nonsense, straight forward information about batteries and charging. I am an electrical engineer, so I can afford to state this comment. Unfortunately Adlard the old fox doesn't allow any website to appear other than his own, so ... you will have to dig deeper on your own ...

Tested out the other battery and found that it was OK so have copped out and got a sealed leisure battery to replace the other one and see us through the next year or so.

I have read extensively on the subject now and in a nutshell it would seem as follows: You probably get the best value and amps for your money with a good quality flooded battery that you can top up, but you must spend time maintaining it.  If maintianing it is going to be a problem (e.g. difficult access) and you have a reliable regulator then AGM is the best option as Gel appear to be more vulnerable to damage from overcharging.  The other option would seem to be the Rolls Surrette batteries which have some form of special venting cap which appears to give the best of both worlds.

I am definitely not an electrical engineer so please do not accept this as definitive!!

Exactly, Robert,

AGM require a smart charging system.

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