So when choosing an amp rating, do I go with the requirement of one battery, or the total system?
When you connect more than one battery together, it becomes one big battery. You have a 310 amp hour battery. So, the total system.
The batteries are rated at no more than a 35amp charge, ideally a 15amp charge.
The recommended charging current is between 10A and 35A. It's odd that they would recommend a range for AGM batteries, since the only number that matters is the maximum current, not any kind of minimum (you can charge an AGM battery with a 1 amp charger). With standard wet cell batteries the max charging current is 0.1c. That means 10 percent of the capacity. So, for a 100 amp hour wet cell, a 10 amp battery charger is what you want. The reason is, if you use higher current, the electrolyte will just boil and the chemical reaction required to fully recharge the battery doesn't properly occur.
AGM batteries do not have free-floating electrolyte, so there's really no boiling that can happen. They also have much less internal resistance than wet cell batteries, which means they can handle charging currents of much higher amperage. Theoretically, an AGM battery can handle 1.0c current, meaning a 100 amp hour AGM battery can be charged with a 100 amp charger. Concord Lifeline (the inventor of AGM batteries) places no upper limit in the charging current. Most AGM battery manufacturers nevertheless will state something like a 0.2c limit, or 20% of capacity. That's the safest, and most practical way to go (on some large installations you'd need a charger the size of a bulldozer and battery cables the thickness of telephone poles to put 1.0c amps back into the battery bank).
With the 155 Ah battery, it's in reality still the same 0.2c, or 20%. 155 times .2 = 31 amps. They list the upper limit as 35 amps, because you can do 2 or 3 or even 4 times that and still be fine.
But in your case, with two of the 155 Ah batteries, at 310 Ah, that's a 62 amp charger at 0.2c. You certainly could use a 100 amp charger, or a 200 amp charger. You can use a 10 amp or 15 amp charger, but it would take longer to charge. I would recommend anything between a 30 amp (for reasonable recharge times) and a 150 amp charger, it all just depends on how much you want to spend. You'll regret getting a smaller charger, like a 10 or 15 amp charger, but you won't go wrong at anything in the 30-150 amp range.
Ive found a few, but they all seem to have an equilazation charge, which from what ive read can be bad for agm batteries.
All decent battery chargers worth their salt will have an equalization charge, or have the ability to manually set the charging voltage high enough (usually in the 16volt range) to perform the equalization. Wet cell batteries need that. AGMs don't. A charger that has an equalization charge will rarely (of ever) engage the equalization function when set for AGM. Don't worry about it. But you can perform an equalization charge on AGM batteries, and they do benefit from it in certain installations, particular that of off-the-grid setups. But it's not really something we need to concern ourselves with out here if you have an AGM battery.
I guess these dc dc chargers just aren't as common as ac dc chargers.
Depends on where you look. They are ubiquitous in marine world where they are ubiquitous on boats.
I would recommend the one Moot is using (max output of 40 amps), or the
Sterling Power charger (variable, user-selectable, up to 130 amps). They perform the same way, are similarly priced, and most importantly can be set for the correct charging regimen for AGM batteries, which allows you to mix battery types between the house bank and the cranking battery and charge them using the same alternator.
I would also highly, highly, highly recommend a
battery monitor. With those batteries having only a one year warranty, and costing what they do, I'd want to make sure I monitored them carefully and correctly. If you won't monitor and charge them properly, they'll be doing good to last 18 months. Otherwise they could last 5-8 years out here, at least.
