AGM Batteries

[rawsondsr]

How do these differ from lead acid, and how do i look after them?

[Alan de Enfield]

The leading advantages of AGM are a charge that is up to five times faster than the flooded version, and the ability to deep cycle. AGM offers a depth-of-discharge of 80 percent; the flooded, on the other hand, is specified at 50 percent DoD to attain the same cycle life. The negatives are slightly lower specific energy and higher manufacturing costs than the flooded.

 

As with all gelled and sealed units, AGM batteries are sensitive to overcharging. A charge to 2.40V/cell (and higher) is fine; however, the float charge should be reduced to between 2.25 and 2.30V/cell (summer temperatures may require lower voltages). Automotive charging systems for flooded lead acid often have a fixed float voltage setting of 14.40V (2.40V/cell); a direct replacement with a sealed unit could overcharge the battery on a long drive*

 

*You will need to alter the settings on your battery charger and MAYBE replace your alternator with one with a more suitable voltage..

 

AGM and other sealed batteries do not like heat and should be installed away from the engine compartment. Manufacturers recommend halting charge if the battery core reaches 49°C (120°F). Table 1 spells out the advantages and limitations of AGM.
 

 

Advantages

Spill-proof through acid encapsulation in matting technology High specific power, low internal resistance, responsive to load Up to 5 times faster charge than with flooded technology Better cycle life than with flooded systems Water retention (oxygen and hydrogen combine to produce water) Vibration resistance due to sandwich construction Stands up well to cold temperature Less prone to sulfation if not regularly topping charged Has less electrolyte and lead than the flooded version

Limitations

Higher manufacturing cost than flooded  Sensitive to overcharging (AGM has tighter tolerances than gel) Capacity has gradual decline (gel has a performance dome) Low specific energy Must be stored in charged condition (less critical than flooded)

Table 1: Advantages and limitations of AGM. 

 

Source : Battery University.

Edited May 12, 2018 by Alan de Enfield

[Detling]

They are sealed so no topping up, slightly lower resistance, so they accept a higher current when charging above 80% but still need a long tail charge to reach 100%.  There are two types it seems for charging voltages, one type (which I have) have a max voltage of 14.4 which must not be exceeded and a 100% rested voltage of 12.9 rather than 12.8 for wet open cell if you monitor SOC by voltage you need a chart for AGM not normal lead acid. The other type can be charged at 14.8 but I don't know any more about them. Exceeding the max charge voltage will cause the gel to break down so you permanently loose capacity, you cannot do an equalisation charge on them. They have a significantly higher cycle count, some wet cheapies are only 70 cycle life, AGM's range from 200 to over 1000 cycle life specification. Provided you can be sure of not overvoltage charging, some MPPT controllers do an automatic equalisation charge every month, as do most combi chargers, they are a drop in replacement for wet cells with a longer cycle life generally, which may charge in 5 hours instead of 6, but will still be damaged by prolonged undercharging.

[WotEver]

 

25 minutes ago, Alan de Enfield said:

Table 1 spells out the advantages and limitations of AGM.

The advantages listed includes “Less prone to sulphation”.  However to balance that the limitations should also include “Unsuitable for desulphation in most cases”.  

 

So don’t allow them to become sulphated. 

Edited May 12, 2018 by WotEver
Remove huge table

[Alan de Enfield]

43 minutes ago, WotEver said:

 

The advantages listed includes “Less prone to sulphation”.  However to balance that the limitations should also include “Unsuitable for desulphation in most cases”.  

 

So don’t allow them to become sulphated. 

It does say 'less prone to sulphation' AS LONG AS THEY ARE KEPT FULLY CHARGED ('topping charge')

which is not dissimilar to FLA's

[WotEver]

1 hour ago, Alan de Enfield said:

It does say 'less prone to sulphation' AS LONG AS THEY ARE KEPT FULLY CHARGED ('topping charge')

which is not dissimilar to FLA's

Indeed, but FLAs can be slightly overcharged to help desulphate them, hence my point. 

[cuthound]

My Lifeline AGM's can be equalised and have beeno at least for the 4 years I have owned the boat. They are recombination cells, where oxygen and hydrogen evolved from the plates during charging is recombined back into water for the electrolyte, which is held captive in a glass mat between each plate.

 

http://lifelinebatteries.com/2015/10/can-i-equalize-agm-batteries/

 

Mine are 11 years old and still have around 60% of their original capacity left, despite being housed on the swim in the same compartment as the engine.

 

They are however eye wateringly expensive, so may not be replaced like for like when they expire. 

Edited May 12, 2018 by cuthound

[Alan de Enfield]

15 minutes ago, cuthound said:

 

They are however eye wateringly expensive, so may not be replacedar ikebana or like when they expire.  

replacedar Ikebanana ?

 

I have heard that Stokies have a weird language.

[cuthound]

1 minute ago, Alan de Enfield said:

replacedar Ikebanana ?

 

I have heard that Stokies have a weird language.

 

Bluddy spill chucker,  or possibly the auto wrong feature of my tablet. ?

 

I will correct the original, if I can work out what I meant to say.