When should I stop dischargung my battery ?

Submitted: Thursday, Feb 23, 2006 at 22:19
ThreadID: 31096 Views:1981 Replies:3 FollowUps:2
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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
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Reply By: Derek from Affordable Batteries & Radiators - Thursday, Feb 23, 2006 at 22:50

Thursday, Feb 23, 2006 at 22:50
If you squint your eyes it looks like a crocodile.

The table that is but not the can off worms I wish to open tonight. I'm off to Rainbow Beach for a long weekend and product testing.

I have new BP Solar 100w folding panels to test.

Have a great weekend.

Regards Derek.
AnswerID: 156774

Follow Up By: Alloy c/t - Friday, Feb 24, 2006 at 05:36

Friday, Feb 24, 2006 at 05:36
Thought the forcast was for overcast/wind/rain ? anyhow enjoy.
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FollowupID: 410933

Follow Up By: Mike DiD - Friday, Feb 24, 2006 at 06:23

Friday, Feb 24, 2006 at 06:23
Have you ever thought of starting your own "I'm going away for the weekend " thread ??? It's really quite easy - let me know if you'd like some help.

Mike
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FollowupID: 410934

Reply By: Member - Collyn R (WA) - Friday, Feb 24, 2006 at 11:43

Friday, Feb 24, 2006 at 11:43
There are no 'shoulds' with this one!

With deep-cycle batteries it is a monetary rather than a technical issue.

There is a correlation between battery longevity and depth of discharge. The deeper you discharge them, and the more often you do that, the shorter their life.

Battery makers say that 50% discharge (to a rested 12.1-12.2 volts) is the optimum and if you do that you will get 500-800 such cycles of use. If you go down to only 20% remaining (about 11.7-11.8 volts) you are lucky to get 80-100 cycles.

Starter will typically withstand less than ten very deep discharges.

In practice most people flog the battery until the lights go dim and beer or chardonnay heats up in the fridge - then winge because the battery is stuffed quicker than expected. But it's up to the individual to decide.

AGMs cop this treatment better than most.

It's really that simple!

Unless you want to consider this:

If you get a smaller and thus cheaper battery and flog it to an early death, you may end up getting more amp-hours for your dollars than paying a lot more for a big one the capacity of which you only partly use. The latter may last much longer but has still cost more per Ah over its lifetime.

I'd get the big one every time: I hate warm beer!
Collyn Rivers

AnswerID: 156843

Reply By: Alloy c/t - Friday, Feb 24, 2006 at 20:06

Friday, Feb 24, 2006 at 20:06
MikeDid ,where the --ckin--ll do you come from ,was your previous screen persona Ozi by any chance ?? ,,cause just like our old "mate " Ozi you cant stand ANY critique of your WRONG views /opinions.
AnswerID: 156919

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