Any way to figure out amp hours?

[drivebyandy]

I currently work at NAPA (pretty handy place to work while building a van for living) and get a considerable discount so I'm trying to check out the batteries we offer but I can't find any amp hour ratings.. Only a 'Reserve Capacity (in minutes)' with no info on what that actually means.<br /><br />Can anyone help me out with this? <br /><br />Might just have to buy them somewhere else but it's surprising what NAPA carries so I check everything through work first to see if I can get it cheap. Haha.

 

[Guest]

Andy, isn't the reserve Capacity just on the starter batteries, not what you want for a house battery...? I think if you look at the marine or rv batteries, they will give amp hours....<br />Bri

 

[drivebyandy]

That's the thing.. The batteries I'm looking at are Group 27DCM (I'm guessing not the same as Group 27?)&nbsp;and Group 8D. They're listed as Marine and RV right on the side but for some reason they don't give an amp hour rating.

 

[SoulRaven]

Look for the CCA # (Cold cranking amps)... Remember that what ever the amps show on the battery, you will only be able to use half of them, or easier dont let any battery go below 12.2V which is half of the batteries amps (Aprox), below this level the battery will start to be damaged, thus a shortened life...<br /><br />I run a group 8D battery, which are commonly used in heavy equipment... They are big and heavy but mine is powered by solar and fits perfectly between my van seats... No problems yet and going on 2 years now...

 

[Seraphim]

Group 8D, from what I've seen recently, appear to be about 280 ah.&nbsp; 4Ds are about 220. the 12V versions weigh about 129 to 170 lbs. One reason we just decided to get 2 100ah batteries and run them in parallel - only 60 lbs apiece lol.<br /><br />Do a google search for '8d AGM' and you'll find some for comparison purposes. You might find the ones you carry at NAPA

 

[IGBT]

Reserve capacity rating is the number of minutes the battery can maintain a 25 amp discharge with a voltage above 12V (roughly).<br /><br />Thus if you see a RC of 180 minutes, it means the battery can deliver 25 amps for 3 hours.<br /><br />Note that amp hour ratings are for a discharge over 20 hours.&nbsp; So a 100 amp hour rated battery can deliver 5 amps for 20 hours.&nbsp; Because all batteries have a internal resistance that wastes power as heat, and because power is a function of current squared times resistance ( P = I^2 * R), a battery can deliver more watts if it is discharged slowly than if it is required to provide a large current.<br /><br />What all that means, is that the reserve capacity of 180 minutes which indicates a amp-hour rating of 75 (25amps x 3 hours) is actually a very conservative measurement compared with an amp-hour rating of 75.&nbsp; The 75 amp-hour rating battery would probably only do 25 amps for 2 or 2.5 hours.<br /><br />Was all that confusing enough?&nbsp; I can try and make it more difficult <img src="/images/boards/smilies/biggrin.gif" alt="" align="absmiddle" border="0" />

 

[wandering mike]

This <a href="http://www.phrannie.org/battery.html">page from Phred's Poop Sheets </a>has lots of good info on batteries and charging and things like that. &nbsp;His other sheets are good too on all manner of things related to living on the road. &nbsp;It is focused on RV's, but the underlying info as regards batteries and electrical matters is the same.