AGM Batteries misbehaving...any ideas?

[Sinbad1]

My recent purchase of 6 120ah of AGM (absorbed glass mat) to replace some old delco voyager house batteries has left me with a problem.

On arriving at the boat and switching on the batteries read 11.8/9 volts. I then recharge and this appears to be at a max of 13.7 volts. The batteries absorb approx 30 amps for about 4 hours and then reduce to a float charge. Battery bank is then reading 12.6/7.

Has anyone any idea what is happening. I have a link 10 battery monitor which gives me amps/volts in/out and I have been checking the battery voltage with a multimeter at the batteries.

Where has all my capacity gone?? Why are the batteries reading flat then they are full after 120 amp being put into them??

Many thanks.

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[pvb]

How are you recharging?

Recharging to a max of 13.7v isn't going to fully recharge the batteries. Is this using a simple mains charger, rather than a smart multi-step charger? If you have an Adverc or similar regulator on your alternator, try recharging using the engine. This weather, the batteries should get up to a charge voltage around 14.8v before the current really drops.

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[Robin]

I think we need more information but:-

Firstly if they really read only 11.8v initially they are FLAT. If nothing was left switched on unknown to you perhaps, (bilge pump, gas alarm???) then it suggests that one at least of the batteries is dead, pulling down the others. You could try disconnecting one of the main battery leads and reconnecting via an ammeter (some multimeters will allow up to 10A) to see if there is something drawing power even with the battery switches off.

Secondly if you have 6 x 120AH that are totally flat you would need considerably more than 4hrs at 30A to recharge them (more like 20hrs), again suggesting one or more of them is defunct.

Thirdly the float charge reading should be around 13.2 - 13.4v.

A fully 100% charged 12v battery which has been rested several hours (ie no charge/discharge) should show about 12.7/12.8v and stay at that when left though it would drop a bit over a week or two from self-discharge. No way would it drop to 11.8v unless there is a dead cell or more somewhere.

Time to look at the battery warranties!

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[steve_cronin]

Completely agree.

This subject is close to my heart at the moment as I have been toing and froing between different opinions from different charger/regulator suppliers on the subject. the selective use of information from some of these people (out to sell of course) make the 45 minute controversy look small meat by comparison.

Personally I cannot justify the large extra cost of these or "Gel" batteries. For one thing my boat doesn't experience the large "g" forces of the modern jet fighter from which the design parameters of the gel battery were promulgated.

Secondly the more tedious charging sequence required of these batteries does not lend them to my power requirements.

Thirdly, since I can buy two sets of wet batteries for the price of gel batteries and since the most cost effective improvement to a boat's (otherwise healthy) electrical system is the installation of brand new batteries, I can enjoy this experience twice in the same expenditure cycle as those with these posh batteries do. Incidentally, well looked after wet batteries can have very long service lives nowadays with all the sophisticated charging and management systems available. Beware however, some of these are more appropriate to ambulance and fire appliance use than the particular requirements of a cruising yacht. With some replacements for your original equipment there is even severe risk of fire and/or severe damage to your electrics. More on this when time is at a lesser premium.

Steve Cronin

<hr width=100% size=1>The above is, like any other post here, only a personal opinion

 

[andyball]

Re: Completely agree.

Both sterling and adverc suggest that cheapo wet lead acid batteries are probably the best choice for most people.

Tend to agree about suitability for emergency vehicles.

Price-wise for 105AH, can buy truck batteries (intended for running tail-lifts w/o engine) for £40-45. "leisure" batteries at same price , sealed wet lead acid (delphi voyager, supposedly ok for 50% dod) £75-80, Extra deep cycling delphi sealed £96 ,Vetus sealed wet for £120 , a 75AH optima blue for £258 (yipes) & the same for an exide gel 110AH.

Then they get really expensive !

How much are agm 105-110 AH ?



Back to original question, echo the other comments re. charging time too short if batteries are working ok & charging voltage too low.

Some "authorities" are suggesting sealed batt's better left w/o float charge & topped up occasionally. Supposedly very fine line between not enough & too much (over weeks & months). Certainly a lower float voltage than is commonly used seems indicated.Given low self-discharge, just enough to cover loads left connected cd be ok.

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[Robin]

Re: Completely agree.

We too use wet batteries, fortunately not through a relay! One advantage of wet batteries that I like, being old fashioned, is that you can check them with a hydrometer which easily identifies a bad cell. We did look at AGMs and Gel batteries when we replaced ours but couldn't get anything like the same AH totals within the same box volumes using them, there were less choices of case size I think, never mind the price premium.

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[Talbot]

Re: Completely agree.

Another factor that is often overlooked is that these gel batteries need a different re-charge cycle and voltages to wet cell. I use standard deep cycle batteries for domestic and have a carbon fibre impregnated for my reserve/engine start. c/f is more expensive than normal deep cycle but accepts cranking currents without damage and accepts the same charge cycle as deep cycle (they are also smaller than their deep cycle equivalents)and I can always start the engine by hand if needed (must try that one day)/forums/images/icons/smile.gif




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[pvb]

Partly agree...

Like you, Steve, I don't think most boat owners can really justify the cost of gel batteries. However, I don't think that ordinary wet lead/acid batteries are necessarily the best long-term value. If the engine alternator is fitted with a smart regulator, sealed maintenance-free batteries are probably worth buying. Their lead-calcium technology is very effective at resisting long periods of high voltage charging without gassing. My bank of sealed Delco batteries is 7 years old and still working fine.

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