Coach Battery Question

[Cardinal Pilot]

I own a Winnebago Aspect 30c with two Group 31 batteries from NAPA (number 8231). These are each marked CCA@32 degrees of 810 amps. I looked on the NAPA website and found these specific batteries but I was not able to find the amp/hr rating. Looks like they are not true deep cycle batteries, not that it matters. I am trying to determine the amp/hours rating. Is there a way to convert the CCA 30 second rating to amp/hours?

[jackfish]

Unfortunately there is no conversion between a cold cranking amp rating (starting battery spec) and an ampere-hour rating (deep cycle battery spec). If a battery does not have a 20-hour amp hour rating it is not an appropriate deep cycle battery for serious RV duty. Also, it can be assumed that a Group 31 12-volt Marine/Deep Cycle battery that weighs 59 pounds is not a true deep cycle battery. A 115-125 amp hour Group 31 12-volt deep cycle battery will weigh more than 70 pounds.

[Cardinal Pilot]

Thanks, Jackfish. I suspected that it would be cost cutting by Winnebago. But I am not going to change them out, just trying to calculate my hours of use with certain devices running for boon docking.

[jackfish]

You could infer a possible range of capacity from comparable batteries specs. I'm guessing they could provide 85 to 100 amp hours. With an electrical meter or battery monitor you might be able to make further inferences/calculations.

[clyon51]

If it were me, I would take voltage readings every hour or so while booning until you get a handle on it. A fully charged battery at rest with zero load on it will be in the 12.6 12-7v range. At 12.1v you are at 50% discharged and time to re-charge.

By 11.0v the battery is dead. Do that 5-6 times with starting batteries and they will be shot. Deep cycles can take this 100's of times and why they are used in RVs. However, for longest life in even deep cycles, re-charging at 50% is a must.

[Cardinal Pilot]

So Googling for NAPA 8231 brought me to a website that spec'ed the battery at Reserve capacity of 210. Googling Reserve Capacity reveals:

'Reserve Capacity (RC) is a very important rating. This is the number of minutes a fully charged battery at 80 ° F will discharge 25 amps until the battery drops below 10.5 volts.'

So it would seem logical if you discharge at 12.5A, it would last twice as long or 420 minutes (7 hours). Likewise at 6.25A, you would get 840 minutes or 14 hours.

Why am I suspicious that the relationship is not linear??? BTW, another site said: 'RC is generally almost DOUBLE what the usual Ah rating is. There are a few theories on how to determine Ah capacity from RC but none that I have found spot on. Some say to divide RC by 2 then to add 16 to that number while others just say to divide by 2.'

[camaraderie]

Ed...EastPenn/Deka DUAL purpose wet cells provide 100 A/H at the 20 hour rate and weigh 57 pounds in their Marine Master packaging. The SAME battery type/brand in their true deep cycle marine configuration gives 105 amp hours at the 20 hour rate and weighs 59 lbs.. It is MY experience that national brand group31 batteries like yours in wet cell configuration weigh between 55 and 60 pounds. If anyone knows of a group 31 wet cell weighing more...I would be interested in seeing the spec sheet.
Here's my spec sheet backing up the data above. http://www.dekabatteries.com/assets/base/0194.pdf
The major thing you lose by using a dual purpose wet cell compared to a deep cycle is cycles in the average lifetime. The main thing you gain is more cranking power for starting use. No free lunch!

Note that their AGM weighs 69 lbs...but you don't need to move to AGM next time to get a TRUE deep cycle battery. Wet cells are fine unless you NEED the specific advantages of AGM.
Hope this helps...
***********
Cardinal...my 20 hour amp hour ratings are the ones typically used in the marine industry for true deep cycle batteries and represent the the total amps a battery will discharge over the course of 20 hours. In a group 31 battery with a spec of 100 amp hours that means it will deliver a steady 5 amps for 20 hours closely duplicating steady state house use. It is absolutely correct that a higher amp draw will both deplete the battery faster than simple math would suggest. (i.e. a 10 amp draw is more likely to kill the battery in 5 hours rather than the 10 you might think.) I also note that the same 100 amphour battery has a practical limit of 50 amp hours for cycling use unless one wishes to shorten the battery's life. It doesn't hurt much to go to 30 or 40% occsasionally...but when the lights dim...you've gone too far!

[RFritz]

IMO many marine industry maintenance and after-market upgrade products are superior to RV industry. I use marine safety wire for 12v, SS nuts and bolts, marine grade caulking, etc.
Keep smiling...

[clyon51]

I am far from a fan of any "marine" multi purpose battery for RV use. The GC2 battery is far superior for $ per AH than anything out here. The below link is for the batteries I use, which gives 20 AH rating (which we use in RVs), but also gives some RC rating for those interested in that.

http://www.trojanbattery.com/products/t-1056v.aspx

Someone mentioned a group 31. Trojan makes a group 31 12v gel deep cycle that weighs 70 lbs and has 102 AH rating. The spec sheet can be seen at http://www.trojanbattery.com

[Murf2u]

In my case it's more a case of what you need not the biggest, shiniest or most expensive.

Now having said that, I rarely need more than 3 or 4 days of reasonably light battery use at a time, the only time were not on full hook ups is at an airport or air show. For us, in our little C, a pair of Group 31 marine deep cycle / starting batteries is plenty of power unless its very cold (furnace running) or very hot (Fantastic Fans running) in which case we need to run the genset for a few hours to keep them topped up.

We have discovered though that by using the genset normally, I.E. running it at meal time to use the microwave and such the batteries get an automatic top off of from 1 to 3 hours (depending on the meal) and that will extend battery capacity out to an almost indefinite stay.

[camaraderie]

Originally Posted by clyon51

I am far from a fan of any "marine" multi purpose battery for RV use. The GC2 battery is far superior for $ per AH than anything out here. The below link is for the batteries I use, which gives 20 AH rating (which we use in RVs), but also gives some RC rating for those interested in that.

Trojan Battery Company

Someone mentioned a group 31. Trojan makes a group 31 12v gel deep cycle that weighs 70 lbs and has 102 AH rating. The spec sheet can be seen at Trojan Battery Company

Nothing wrong with the T105's except that you need 2 of them and they won't fit in the space designed for the OP's Group31. I'm not a fan of marine multipurpose batteries either...or any multipurpose...but the true deep cycle marines are built to take a pounding and provide better plate isolation from vibration than standard deep cycles.
As the the Trojan GEL's...in a group 31...that would prove a VERY unwise choice as they require a completely different charging regimen. Trojan does not make a wet cell group 31...but their Group 27 would fit in the same space the 31 fits in and provide nearly the same amp hours. I'd still stick with the wet marine deep cycle in a group 31 or AGM Grp31 if the benefits are important enough to justify the huge price differential for the buyer.

[jackfish]

I would get every bit of life out of your two NAPA 8231 Group 31 batteries (most likely around 180-200 AH) with a proper charging regimen and never discharging below 60% SOC. Then when they give up the ghost, I'd replace them with the two Lifeline GPL-31XT AGM batteries for 250 AH. expensive but worth it