Sunday, Dec 03, 2006 at 15:52
Hi David,
I can't be very specific about AGM, because my experience is confined to wet cell and gel cell types, however the chemistry is very similar (voltages are a bit different), so my experiences may throw some light on your situation.
First thing is to establish just what your load actually is. My 40 litre Waeko consumes about 3.6 amps when running, and I reckon (educated guess) that it probably runs for about 30% of the 24 hour day, hence in a day it consumes about 25 amp hrs. How big is your flouro? A little one, with 2 tubes 9" long, will consume about 1 amp, runs for say 3 hours at night = 3amp hours per day. At this point it's worth noting that 90% of the load is the fridge. My
battery also charges the laptop, cameras, phone, and runs the hf and cb radios, but in the daily tally they don't take much - the fridge remains by far the major load. Your Fridge is bigger than
mine so probably takes a bit more.
On the above basis, I require at least 30 amp hrs, possibly 35 Ah each day. You probably need 35 - 40 Ah
Batteries - you've already been to the windsun site (
http://www.windsun.com/Batteries/Battery_FAQ.htm),
which is the most informative source of which I'm aware.
I've done discharge tests with my 130 Ah wet deep cycle
battery, applying a constant current load and monitoring voltage as the
battery discharges. In my case there is a roughly straight line decline from the nominal 12.6V down to about 11.5V, accounting for about 100 Ah, then another straight line decline (much faster) from 11.5 down. For this reason I regard 11.5 as my preferred end point, and 11.0 as the point where any non-essentials (eg cold beer) are cut off.
Battery life is substantially reduced if the
battery is discharged too much further than this, especially if it is allowed to remain deep discharged for long. My key finding here is that you actually get only about 75% of the nominal
battery capacity before you reach an end point where the rate of voltage drop becomes quite high.
In your case, I think your
battery has probably provided about 60 to 70 Ah during its 40 hours operation, and so I would expect it to be approaching a conservative end point.
Regarding the fridge cutout voltages V's measured
battery voltage - there will be some voltage loss in the cable and connectors (resistance) when the fridge is drawing current (see ohms law), so the fridge isn't seeing the full
battery voltage. This is particularly true during starting, when the fridge draws much higher current than it does when running. Note too that the internal resistance in the
battery means that the terminal voltage depends on the load current too, so measured voltages will vary with load.
Suggest measure currents and get a better idea of just what your load actually is. I'd be a bit disappointed with the apparent
battery capacity, but really need more data to suggest that there is anything wrong.
HTH
John | J and V
"Not everything that can be counted counts, and not everything that counts can be counted."
- Albert Einstein
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