Charging a deep cycle AGM battery

[Daydream believer]

I'm not sure this is quite right and it is certainly out of step with Victron and Mastervolt.

First, no I don't think one does need 14.8V to charge, rather that is around the top end for a fast charge in normal temperatures of 25C. In warmer conditions, and certainly proximate to a hot engine installation, this is too high. The normal AGM charging voltage is 14.2-14.6V.

Similarly, the recommended range for float voltage is 13.5-13.8V and 13.65V just happens to be the mean.

Finally, for long-term storage (i.e. any extended period of time), the float voltage should be somewhat reduced: 13.2-13.5V is recommended. The same applies to most lead batteries.

This is exactly what the suppliers of my batteries told me and my domestic AGMs have lasted 5 years so far.
however the suppliers of my Red Flash battery ( Tanyia i think) told me that was OK for Red Flash and the first one failed after 18 months. (12 month warranty?) I checked with them again and i have now bought another so i will see how that goes
i think one needs to look at the specs from the actual manufacturer rather than supplier as they do vary a little between batteries

 

[PeterBoater]

The label on my Rolls AGM battery says:

Float charge 13.6 - 13.8V at 25C
Cycle charge 14.5 - 14.9V at 25C
Max charging current 29A

Although the latest Rolls specification for the same battery says:

Charge Voltage Range 14.7V @ 25°C (77°F)
Float Voltage Range 13.8V @ 25°C (77°F)
Recommended Charge Current Capacity (String) 30 A
Maximum Charge Current (String) 40 A
Self-Discharge Rate Less than 2% per month at 25°C
(77°F)
Internal Resistance 3.6 mΩ

These tie in fairly well with the gel setting, which is also the default setting, specification for my Victron Phoenix charger:
Absorption voltage 14.4
Float voltage 13.8
Reduced float voltage 13.0
Maximum absorption time 4 hours

My four batteries are now eight years old, so seem to be doing OK on this regime, which is just as well considering their cost.

 

[rszemeti]

Rolls are excellent, built properly and will last a looooooooooooong time of correctly cared for.

Personally, if I was spending the £££ on AGM's I would go for an intelligent charger with a dedicated AGM charge scheme, the motorhome world has plenty of stories of AGMs failign far too quickly, they are a very tricky battery to charge right, and many fail before they should.

I use "EFB"'s myself as they are *supposed* to be a it more tolerant of abuse, and my alternator controllers have a dedicated programme for charging them, I'm hoping that is going to pay off!