AGM Batteries

Although a fair indicator, I don't believe the Voltage to be an accurate measurement of the state of charge. Point one of a volt would not be any big deal to worry about & could be explained by the temp on the day, poor connection of the test leads or even inaccuracy in the multi tester. Each & every deep discharge does some damage so over time it would be expected to lose a little performance.
Just for interest, how did you arrive at the Voltage verses % of charge?
ie: (12.1V = 55%charge)
Cheers Craig............

Don't put doubt in my mind before I use the damn thing Jim...

Jimbo

Bear in mind that the charging process is an electro-chemical reaction and therefore heat-sensitive. The colder the day the longer a battery will take to charge from a constant voltage charger (which is that of a standard vehicle charging system) and the higher the voltrage across the battery at full charge.

It is likely that this is all that you are experiencing.
Trust this helps
Collyn Rivers

If you go to this website it has lots of info on agm , jel and wet cell batteries
http://www.fridge-and-solar.net/index.html
Derraux

Jimbo, have been researching the pros/cons of batteries for a looooong time now, and today went and bought 2x 80amp hr AGM , can fast charge like a start batt, can use as deep cycle ,zero maintenance ..... actually allways thought that 11.5v was 50% ,,,no no no no ,,, on AGM never go lower than 12v , check out Springers 12v specialists ......www.springers.com.au

Dd64

There are no problems re charging AGM batteries from a standard alternator as such. However AGM battery makers advise against charging them in parallel with conventional starter batteries.

As a professional engineer and writer in this field I cannot advise you to go against the makers' advice, but your might care to consider the following comments and form your own view.

The main problem with charging and AGM battery and a conventional battery in parallel is that the AGM battery absorbs a high charge. There is thus a risk of the AGM battery charging heavily, but the starter battery receiving only a light and inadequate charge. If however the two are initially separated, via (for example) a Redarc relay that will only charge the AGM when the starter battery is adequately charged - then that risk is removed.

Val Rigoli at Fridge and Solar has a heap of experience re this and I suggest you give him a ring - 07 5500 5561.

AGM and gel cell batteries are vaguely related. The concept is to build a battery that has the charge acceptance and delivery characteristics of a starter battery, but with the ruggedness etc of a deep cycle battery. This essentially requires multiple but strongly constructed plates so that a large surface area is presented to the electrolyte. This enables the battery to accept (and deliver) high current levels without damage. Techically-speaking they have low internal resistance (or to be strictly correct - low internal impedance).

Conventional starter batteries are made that way, but over time 'bits fall off the plates' and eventually build up on the base of the cells until the deposits reach the bottom of the plates. At which point the battery dies. Just like that!

This is avoided (with AGM batteries) by having the water/acid electrolyte held within a glass fibre matrix that is pressed against the plate material - hence the name Absorbed Glass Mat.

Gel cell batteries have the electrolyte mixed with a material that sets to a candle-like consistency - this again serves to hold the lot together.

(The above is a very simplistic view of the technology - there are many other differences - but containing the electrolyte in some form is the main difference).

Gel cells will absorb and provide massive amounts of current - in fact they can be charged at more than 100% of their amp/hour capacity! Their only downside (apart from cost) is that they are wrecked if charged at higher than 14.4 volts. It is safer to use 14.1 and they will in fact charge very well indeed - and to 100% - from 13.8 volts.

Both AGM and gel cells charge best from a three-step charge routine, but this is less necessary than for conventional batteries.

Re the 40 watt 15-amp modules. I'm sure that David meant 15 amp/hours - not 15 amps.

Alternatively they must be for 2.4 volt batteries!
Collyn Rivers