AGM batteries persisting at low voltage

[Exile]

Originally Posted by Exile

No, no, you misunderstood my post and left out the pertinent part:

"[My Xantrex XC5012] applies the appropriate charge to each battery separately. So it's my understanding (could be wrong) that what I effectively have is a 50A charger applying all of its charging capacity to each of my 255ah batteries at any one time."

So it's not acting as a 150A charger by applying a 50A charge to all 3 batteries at the same time, but selecting which of the 3 batteries is most in need of charging and applying the appropriate charge to each one separately.

I'll double-check my charger's manual on this & post what it says. We're all just trying to learn here while hopefully helping out the OP. No need to get annoyed.

From RTFM :

"The XC Series has three full current rated, independently controlled outputs which enable it to charge three different batteries or battery banks. Each output can accommodate any one of the four allowable battery types, regardless of what battery types are connected to the other outputs. Each output can be in a different stage of charge, ralative to the other banks; for exampe, Bank 1 in float, Bank 2 in absorption and Bank 3 in bulk.

* * *

In multiplex charging mode the charger will check all qualified battery banks every 15 seconds to determine which bank is most in need of charging. That bank will then be charged for the next 15 seconds. Every 15 minutes the charger will query all outputs and detect which battery banks are present and healthy."

As already pointed out, my particular charger only charges each battery separately, for a maximum of 50A to only one battery at a time.

[Exile]

Originally Posted by a64pilot

Your charger will only charge each battery independently, if they are in fact independent and are not connected electrically. If they are say connected in Parallel, then what you have is one big battery and the charger cannot differentiate between them.
Doesn’t hurt to have them each connected to the charger though, so I’d not change that.

Yes, each of my 3 batts. are connected to the charger, and 2 out of the 3 are paralleled (making up the house bank) -- but through the 1-2-ALL switch and not at the batteries themselves. This allows the charger's monitor to display which of the two that are paralled are getting charged, but you're correct that the two that are paralleled are acting as one large battery, in my case 510ah (rated).

[a64pilot]

I have six batteries, but broken into two banks, one of my chargers has three outputs, I ganged two together and went to the big bank, the single to the smaller.
Then I pretty much defeat the whole thing by always leaving the switch to both making one big bank
Doesn’t hurt though.

[The Sun King]

Anyone using AGM batteries should carefully study the specs and check how they are being charged. It is very easy to damage them by charging at a too high rate. I see guys talking about their 9 year old AGMs and I know guys who have boiled them dry in less than two years. Be gentle with them!

[a64pilot]

Only way you can charge at too high a rate, is to charge at too high a voltage.
Batteries self regulate, so long as the charge source voltage is correct, the battery will only accept the amount it should.

In fact Lifeline seems to indicate that having a very large charger is good for their batteries, slow charging allows sulphation to occur

[sailinglegend]

Originally Posted by The Sun King

...Anyone using AGM batteries should carefully study the specs and check how they are being charged. It is very easy to damage them by charging at a too high rate....

You can't charge AGMs at a too high rate - by that you mean current - rate of flow of electrons. In fact it's difficult to charge any kind of battery at too high a rate because they limit how much current they will accept and this current decreases as the battery SoC increases.

I hope what you mean "it is very easy to damage them by charging at a too high a voltage." Above 14.4v batteries start to gas and any 'sealed' battery like AGMs cannot afford to lose electrolyte this way. Having said that modern sealed batteries, and especially AGMs, can recombine the H2 and O back into H2O so usually gassing does not happen.

Too high a temperature, or we should say 'overcharging' can easily happen if the battery gets too hot and the chargers are not temperature compensated to lower the charge voltage as the temperature rises. Sailing in very hot climates, or putting the batteries in an engine compartment, will shorten the life of any battery quickly.

AGMs are not just expensive plug-in replacements for FLAs because the man in the shop said they were the best. They have to be installed as part of an overall charging package that will make use of their faster charge acceptance, or they will not give value for money.

There are hundreds of AGMs brands out there that are not suitable for use on a boat and not designed to regularly fall off three metre waves. Many are just standby Telecom type batteries that may get discharged to 50% 2-3 times a year - not every day!

So beware 'not all AGMs are created equal' - and they may not be the best choice for your kind of sailing!

[MJH]

Originally Posted by Marathon1150

We have recently crossed from Mexico to the Marquesas, taking 29 days to do so. We have been anchored in Taiohae Bay, Nuku Hiva, our first landfall for a week. We have 400 watts of solar and a wind generator. Two nights ago, the battery voltage (as displayed on a BEP monitor) dropped to 10.8v. Since then, unless there is power going into the batteries (engine, solar, wind) they are hovering at 11.8 or 11.9 overnight. Normally this should indicate depleted batteries but the fridge continues to start and run.

We are planning to disconnect the batteries and measure individual voltage (2 x Lifeline AGM 4D) that are 2 years old but I am taking advantage of being on shore to send this message now.

Strangely, the BEP says we are at 100% in terms of amp hours.

Also, our fridge control unit failed about 2 days out and was replaced with a used unit by a local shop earlier in the week.

Any thoughts regarding this problem would be much appreciated. For example, is there a chance that the new fridge controller has a short or is this a coincidence?

Or are we experiencing the difficulty of getting batteries up to full charge when away from the dock for a long period of time?

Or, are the batteries bad?

Many thanks

Nello

Congratulations on your crossing.

I would first talk with the manufacturer, Lifeline. Secondly, I would decrease the load by removing the refrigeration and any other nonessential loads from the system...refrigeration is a tremendous load...get the batteries back up to 100%.

~ ~ _/) ~ ~ MJH

[hellosailor]

With AGM charging voltages, it is worth noting that all AGMs are not "the same". From maker to maker the optimum and maximum continuous charging voltages DO vary by one or two tenths of a volt, and charging sources (like MPPT controllers) will sometimes not match what the battery maker wants.

So it pays in the long term to check the numbers, program the MPPT controller (some Genasuns had to be factory customized, not user changeable) to match correctly, and to add temperature compensation because that can throw the system into "wrong" by another tenth or two of voltage. All not an issue if you are only looking for three or four years of life, but if you want eight years...this needs to be done.

[sailinglegend]

Originally Posted by hellosailor

With AGM charging voltages, it is worth noting that all AGMs are not "the same"......

This is very important and is why any charger on a boat should be user programable to set the correct charging voltages. For AGMs that could be anywhere from 14.1v to 14.9v, often with different values for the Bulk and Absorption stages.