dwh
Tail-End Charlie
Thanks for the tips. I've been considering a pair of Odyssey PC1200s, if I can fit them in the factory location. Otherwise, it would probably be a combo of a DieHard Platinum P2 for the main, and a DieHard Platinum PM2 (marine) for the auxiliary.
Both the Odyssey and DieHard Platinum are AGM batteries, so I should be OK, right?
I'm not a big fan of engineering by the "should be" method. If ya gotta, then ya gotta, but better to avoid it if possible.
Looking at the Odyssey spec sheet I see a couple of things.
http://www.odysseybattery.com/autospecs.html
For one, the PC1200 is only 42 amp*hours (at the 20 hour rate). 42ah * 12v = 504 watt*hours. So you could run a 50w load for 10 hours, or a 100w load for 5 hours.
(More or less - the actual numbers vary as the load goes up or down. That battery is rated at 42ah at the "20 hour rate". So it would run a 25w load for 20 hours. A 100w load (faster drain) would lower the amp*hours number, so it would run for noticeably less than 5 hours.)
If you actually did that, the battery would be completely dead. That would be bad news. Generally, you want to discharge no more than 50% if you can for long battery life. So cut the runtimes in half. The truth is, 42ah is just not a whole lot of juice. You need to figure out your loads vs. run times. "A man's got to know his (battery's) limitations."
Down at the bottom of that page, to the right we see some important numbers. Standby charger voltage 13.5v - 13.8v. That would be the voltage range you would want from a portable charger or an RV converter/charger. Basically, that's the voltage range you want if you are going to feed a constant charge to the battery (float charge). Like an RV plugged into shore power and the converter/charger is supplying voltage 24/7.
Cyclic voltage 14.4v - 14.8v. That would be the MAX voltage range you would want from your alternator at highway RPM. It would be perfect if the charging system put out 13.5v - 13.8v at idle. Unfortunately, many alternators put out almost nothing at idle since they are designed to operate "in the zone" at a higher RPM.
Those are the normal voltage range specs for most modern "12v" FLA and AGM batteries (GELs often have different voltage specs). Older RV converter/chargers are usually lower than that. I looked at a 70's Winnebago that had a converter/charger that put out, IIRC, 12.2v. That would be too low to keep a modern battery topped up. My old Shumacher 10a charger puts out a constant 12.6v, which is almost 1v too low to fully charge a modern battery. (But that's okay. I get it close with that charger, and then let the truck's alternator finish it off. Also, I don't have to worry about overcharging my battery if I forget and leave that Shumacher turned on.)
So what voltage range does your alternator put out? You ought to know the answer to that question.
Another VERY important thing to note at the bottom of that page is the Typical deep-cycle life. Take it down to 80% depth of discharge (20% remaining) and it's good for 400 cycles before it's toast. If you don't go below 50% it'll probably last a couple of thousand cycles. Take it down to fully dead and who knows...a dozen cycles? Two? Maybe more, maybe less.
(Actually, it won't usually be completely ruined. Normally, what happens over time is that the battery can only absorb less and less power. So after too much deep drain abuse, your 42ah battery might only hold 5ah - which I would call "toast".)
Also note that on the charging voltages it says, "no current limit required". That's very good, as it means that you can recharge those batteries with high amperage. You could recharge that 42ah battery with a large alternator. For example the 100a alternator in my truck could provide an amperage 250% of the battery capacity and those batteries should handle it no problem.
The Platinum PM-2 has a 68ah capacity. Quite a bit more than the PC1200. So if it will fit, and the price is close, I'd go with more amp*hours. I don't find any specs on charging voltages. Sears probably has an owner's manual for that battery that will almost certainly have those numbers. Very likely they'll be pretty much identical to the Odysseys.
Both of these batteries have cranking amps ratings, which almost always means that they have thin (starting) or medium (rv/marine) plate thickness. True deep cycle batteries have thick plates and almost never have a cranking amps rating. If you keep looking, you might find a "true deep cycle" that will fit and give even more amp*hours capacity.