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Old 30-01-2010, 17:39   #1
R_C
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One of Three Batteries Is Bad

I have three 200Ah AGM batteries for my house bank and I noticed the voltage was too low after only about 100Ah discharge. I load tested each individually (500A load for 15 seconds) and two tested just fine but the third could not even handle about a 200A load and immediately dropped to less than 8V.

So what would you do in this situation? I can disconnect the bad battery from the battery bank and run a 400Ah bank for the remainder of the season knowing I am shortening their lives with a greater depth of discharge. But they are four years old and I thought I'd likely have to replace all three after five years. I know the drill about using the same age and chemistry batteries in a battery bank. Should I replace the bad battery with a new AGM and try to get another year or two out of the bank?

- Rick
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Old 30-01-2010, 17:57   #2
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Separate the good batteries from the "bad" one. Apply a mild load for 20 min or so. Charge the battery by applying 14.5 V or more and note the charge current acceptance. When the charge current acceptance drops to about 5A apply 13.9V for 24 hours and note that the float current drops to below 100mA.

If the charge current acceptance does not exceed 20A on that battery after it has had a decent load for 20 min then it is sulphated and you may need to apply 15V or more (up to 16V) in an attempt to reverse the sulphation. You have nothing to lose by making this attempt. Merely observe that the battery does not gas and vent when charging high currents.

If the battery was not defective then I suspect that you have not been charging the bank with a sufficiently high voltage during acceptance (14.4V or more) otherwise the three batteries should track. If you do not apply 14.4V or more then they will not track.
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Old 30-01-2010, 18:50   #3
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This might be a good place to insert my recent experience, as I don't want to start a whole thread about it.

I have 2 golfcart batteries on my boat.

According to my hydrometer, 2 cells of one of them recently showed in the red although the battery voltage was floating at 13.2 and easily went to 14.2 with minimal bulk charging current.

The engine has always started fine and there were no other signs of trouble.

I needed a way to apply (as Rick said) 15 volts or so at small current to the batteries for an equalization charge, so I used my laptop's power supply! It's rated at 19v and 4.7A. Worked like a charm.

I immediately got 15.3 volts @4.5A, and after about an hour, it had dropped to 15.1v.
A few hours later it was back to 15.25-15.3v.

I checked the batteries with the hydrometer and they were all in the green.
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Old 31-01-2010, 00:45   #4
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Don't bother - toss the bad one, use the existing 2 that are left til they die, then replace the whole set when needed. By adding in a new battery to the old set, the new one will deteriorate a lot faster than normal.
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Old 31-01-2010, 18:10   #5
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Rick, your advice is excellant for non-sealed lead acids, however its not good for AGMs. Equalisation is generally not reccomended for AGMs and where it is you need to follow the manufacturers procedure carefully.

AGMs are chronically susceptiable to under or overcharging. Your capacity loss could be as a result of either of these situations. All your charging sources need to be 3-stage intelligent systems , including your alternator regulator. If you overcharge AGMS, plate shedding of active material occurs and this causes shorts and equalisation will not cure this, permanent loss of capacity results. ( also venting can occur which permanently reduces capacity).

AGMS really need to be 100% recharged regulary and should not spend more then 30days at less then 100% capacity. Chronic undercharging is often a feature of sailing boats , that spend long times underway as really without a sophisicated recharge system they will be tend to be undercharged. Sulphation occurs, which can and sometimes cannot be cured by equalisation. Equally entended equalisation will cause gassing and in an AGM that cant be replaced.

charging of AGMS must take into account temperature and all charging systems that involve AGMS needs temp sensors on the batteries. ( this is true for all lead acid, but AGMS are very sensitive).

In my opinion, good quality open celld wet lead acid batteries are still the best compromise. AGMs have a place but only in a recharge system designed for them. GEL batteries have no place onboard a boat IMHO.

The problem comes when people buy AGM or other sealed lead acid systems and dont realise that ther can only be used as part of a system designed for them. They are not "drop-in" replacements.

AGMs and GEL batteries came out of the telecoms industry for backup power, where high recharging rates and low self discharge were important as was the low maintenace of sealed systems. Charging was designed to suit and these batteries spend a long time sitting at 100% charged. These conditions are not what is generally seen on boats. The advantages of AGMS are a lot less in thoese conditions.
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Old 31-01-2010, 18:50   #6
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AGM's seem like a major hassle...the more i learn about them the less i want them.
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Old 01-02-2010, 04:16   #7
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Originally Posted by goboatingnow View Post
AGMs are chronically susceptiable to under or overcharging.
...

charging of AGMS must take into account temperature and all charging systems that involve AGMS needs temp sensors on the batteries. ( this is true for all lead acid, but AGMS are very sensitive).
Not really true. They have that reputation, but it is not warranted. The reason that proper charging is so highly recommended for AGM's is because they cost 2 to 3 times as much as flooded, which can be a substantial investment in money.

If a $60 battery croaks because of poor maintenance, not a big deal - but if that is a $200 battery you probably want to take better care of it. AGM's are no more sensitive than flooded batteries with the exception that a heavy overcharge can force some liquid out, which cannot be replaced like it can in a flooded.
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Old 01-02-2010, 11:06   #8
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The reason that proper charging is so highly recommended for AGM's is because they cost 2 to 3 times as much as flooded, which can be a substantial investment in money.
nonsense, AGMS are more temperature sensitive because overcharging using too high a voltage will cause gassing and possible capacity as well as shedding of the active material. They nned to be charged carefully and not allowed to sulphate as equalisation is not really possible.

The fact is they have no place on board a typically boat with standard anti-diluvian charging systems. Its nothing to do with cost.
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Old 03-02-2010, 14:12   #9
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Lifeline is the only major name that recommend equalization charging of their AGMs. The other makers all say DO NOT DO THIS, their batteries are apparently not made to withstand the extra voltage/pressure. They also claim that the AGM design makes conventional "sulphating" totally impossible, there is no way to have sulphates precipiate out of the solution, or fall down to the bottom of the battery.

Can't hurt to try a long patient conventional charge of the weak battery--but the odds are it is toast, to be thrown out and not replaced until the whole set is.
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Old 03-02-2010, 15:46   #10
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Lifeline is the only major name that recommend equalization charging of their AGMs. The other makers all say DO NOT DO THIS, their batteries are apparently not made to withstand the extra voltage/pressure. They also claim that the AGM design makes conventional "sulphating" totally impossible, there is no way to have sulphates precipiate out of the solution, or fall down to the bottom of the battery.
The problem with that is that sulphation does not precipitate out anyway. Sulphation acts like a high resistance barrier on the plates, so if some AGM mfg is claiming that, I would not trust them.
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Old 03-02-2010, 16:31   #11
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AGM/gel-cell myths

Windsun is correct, goboatingnow, read some of my past threads regarding AGM, gel-cell, flooded-cell attributes. For example, the ONLY lead-acid battery (keep in mind all of these types are lead-acid following the same electrochemistry regarding energy depletion and replenishment) that has proved to hold a charge for a LONG period of time has been high quality pure lead production AGM batteries (6 years without a charge and today going strong with charge/discharge cycles.

I have a lot of experience testing various batteries for marine and RV applications. I still have a gel-cell that is about 12 years old going strong and it has been HAMMERED over the years. I still recommend both gel-cel and AGM batteries.

Keep in mind that it is NOT gassing that destroys an AGM it is excessive gassing the overshelms the ability of the battery's ability to recombine oxygen and hydrogen gas and ultimately resulting in venting outside of the case that contributes to the destruction.

If one follows the Amp-hour law charging limitiations no AGM will get destroyed and, yes, 16V CAN be applied if necessary as long as one monitors the Amp-hour state of the battery and controls the charge rate accordingly. Know that most lead-acid batteries of all types are killed by UNDERCHARGING not overcharging.

ALL types of lead-acid batteries that are designed for deep-discharge capable of delivering high currents need to have an acceptance voltage equal to or GREATER THAN 14.4V during an acceptance cycle or they will die a slow death with subsequent charge/discharge cycles.

Especially in the case of AGM and gel-cell batteries they can be placed in parallel as long as they each have the same internal electrolyte specific gravity. I have mixed AGM with gel, old with new and monitored each one to determing tracking during charge/discharge. They will always track if one uses an sufficiently high acceptance voltage to guarantee complete recharge during a charge cycle (barring a catastrophic cell failure or open cell interconnect).
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Old 03-02-2010, 16:41   #12
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In my experience, And I have a lot, AGM's placed into a standard boats systems, ie dumb alternators etc, are very prone to giving problems, with a lot of users complaining of unexpected short lifetimes, ( just like the OP). Given the cost difference this is a double wammy. This is based on feedback just like the OPs

What I said was that deploying AGMs is only doable in a system matched to charging AGMS and charging them properly. They absolutley have their place and I have used many, but not in standard setup boats.

Gels are even worse as they are inevitably overcharged by the standard car type regulator on a typical alternator, and even worse with smart regulators where people dont understand the system requirements.

I stand by my comments.
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