People often say "you must stop discharging your battery at xx volts" - but that's meaningless unless you know what current is being drawn from the battery.
e.g. if you want to leave 50% capacity in an N70 size battery then you should stop discharging at 9.4 volts - IF you are discharging at 340 amps. Or stop at 12.4 volts if you are discharging at 5 amps.
Of course these voltages will change with battery type, battery age and temperature.
I've been collecting some info on Voltage+Discharge current versus capacity and not surpisingly it shows inconsistencies between different sources of information. At least these figures will let get closer than the "no current, battery rested for 24hrs" figures that every quotes, but are very inaccurate when the battery os actually powering fridge etc
Discharge Amps
---------0x-----0y----0z-----0w----3---4.5---22---90--270--340
100% 12.70 12.66 12.80 12.80 12.5 12.8 12.5 12.2 11.6 10.4
80%- 12.42 12.49 12.64 12.55 10.2
60%- 12.20 12.32 12.48 12.30- 9.9
50%- 12.06 12.24 12.40 12.18 12.4 12.2 12.0 11.8 11.3 9.4
40%- 11.90 12.16 12.32 12.06- 9.5
20%- 11.58 12.06 11.96 11.82- 8.9
--0%- 10.50 11.89 11.80 11.60 10.6 10.8 10.4- 9.5- 9.4- 6.0
Sources
0x - unspecified battery www.windsun.com/Batteries/Battery_FAQ.htm#AGM,%20or%20Absorbed%20Glass%20Mat%20Batteries
0y - Low Maintenance (Calcium Lead) Battery - www.uuhome.de/william.darden/carfaq4.htm
unspecified battery
http://www.batteryuniversity.com/partone-13.htm and www.ibsa.com/www_2001/content/faqs/tech_talk/maintenance/faq_tech_maint.htm
0z - Maintenance Free (Calcium-Calcium) or VRLA (AGM) - www.uuhome.de/william.darden/carfaq4.htm
0w - Odyssey Staring Battery Handbook.
4.5A-270Amp Absorbed Power N70-size 90AHr AGM deep cycle battery -
http://www.absorbedpower.com/battery/pdf/AP-GT12-90C-01Sep05.pdf
3A, 340A 60 AHr unspecified battery - www.cyb.com.au/automotive/batt_facts/undbatt_2.htm
Mike