Depleted batteries are best charged when they are of the same size and type and at the same State of Charge (SOC). So the best choice for the house battery would be the same size AGM as the starting battery.
So for recharging purposes having two AGMs in an AIO bank would be the best set-up, because they would 'always' be at the same. SOC.
Well, um...not really. Or perhaps I should say, "not always".
That definition of what is "best" applies to batteries that are rigged
as a battery bank.
In a bank configuration, it
is best to have them all the same age, type and SoC. It is also
VERY important that they be wired in a "balanced resistance" fashion, as per this:
http://www.smartgauge.co.uk/batt_con.html
The
reason for this is simply that any difference in the rigging will cause one "battery+wire" to have lower resistance than the others, and thus do more work - that battery will discharge a bit more, and recharge a bit more and wear out sooner. If that happens in a battery bank, then you've got additional problems because if you replace that one battery, it will have a lower resistance than the older batteries, and since the rigging is unbalanced to begin with, it will do even more work than the original did - and wear out even quicker.
Now, here's the punchline. If you do an AIO lash-up where one battery is under the hood, and the second is down on the frame, or where two are under the hood and 4 are down on the frame, or one up front and one in the back, or 3 in the back - WHATEVER configuration - if you have not rigged the wiring in a fashion to provide a balanced resistance for all batteries...then one of the batteries will have a lower resistance and do more work.
In which case - the very reason for making the batteries "all the same" has already been defeated.
I.e., if you have 8' of wire running down to the batteries on the frame, then you better have 8' of wire running to that battery under the hood as well. If you don't, then "all the same" is irrelevant since electrically, they are not all the same - the resistances are different and one of the batteries is going to end up doing the most work.
An AIO
is rigged in a bank configuration, so the wiring had better be rigged properly for balanced resistance or it is going to have the same problems as any improperly rigged battery bank.
But a split-charge isolation setup is NOT rigged as a bank.
Now, say you are charging one thin plate starting battery, and one thick plate deep cycle (AGM or FLA or even a mix of both - doesn't matter) with a split charge solenoid, an alternator and a normal automotive voltage regulator. The deep cycle is 50% depleted, the starting battery only 5% depleted.
You supply a charging current at some voltage - say 14.4v. Eventually, both batteries will reach the same voltage, and the charging system will bring them both up to 14.4v and then back off.
It doesn't matter if they are the same. One can be larger, one can be older, one can have a lower SoC, one FLA and one AGM, one can be a FLA starting battery and the other a bloody great bank of AGM deep cycles...doesn't matter - the charging works the same way regardless; Get the voltage to 14.4v and back off.
What is important
when charging by voltage is that both batteries require the same
voltage set points. FLA and AGM both generally require the same, but GEL no. So if you hook up a GEL that is rated fully charged at 14.1v, then keep pumping it up to 14.4v, it's not going to last.
(OFF-TOPIC: A lot of people buy GEL thinking "ooh, it's better", but don't check the voltage specs of that battery and their charging system, and then wonder ******? when it doesn't last after they keep overcharging it...and then they run around saying, "GELs suck. I had one and it didn't last at all." GELs are actually awesome - IF you charge them according to the instructions. They do generally have different voltage set points. This is why most smart chargers have one setting for "FLA and AGM" and another setting for "GEL".)
Taking into account Peukert's Law; "As the rate increases, the battery's available capacity decreases"....
...For a total AIO of 'available' amp hours of 83 for house use....
...Factor 90% SOC and we'd have 34 amp hours for house use...
Yes, of course all completely true.
I will note that your calcs assume LVD at 50% SoC. Is that where the LVD actually does trip? Or does it actually trip at some voltage higher than 50% SoC? If it trips at 60% SoC that'll make a pretty big difference in your math.
Another thing to note is, again, that 50% assumption. I regularly discharge my aux battery below 50%. I suspect that many do. Thus when comparing available amp*hours, one needs to plug in REAL WORLD figures into the formulae.
Yes, 50% is conventional wisdom for longer battery life. Yes, it would be great if we could do that. If I had an automated system to keep the batteries above 50%, such as solar or generator auto-start then I'd buy really good batteries and do what I could to extend their life as long as possible.
But I don't. I beat my aux battery up. I take it down far below 50% - often. I thrash it and then I replace it every couple of years. I don't buy the best that money can buy, because I know going in that I'm going to ride it hard and put it away wet. (I do buy high quality engine start batteries though - that's mission critical and there is no way I'm going to intentionally abuse it.)
So the ACTUAL amp*hours that I get out of my aux battery is higher than what your calculations show because I discharge more deeply than your assumptions. So yes, the AIO would get me a few more amp*hours, but not as great a difference as indicated by your math.
And as I've already said, I'll GLADLY give up those extra amp*hours in favor of the insurance and redundancy of an isolated starting battery and the option to drag my aux battery down as low as I want.
Another thing your calcs don't take into account is the Peukert effect of how I draw down my battery. If all I use is an 8w light and occasionally run the water pump (my normal usage profile), then my aux battery is going to provide more usable amp*hours than the simple formula indicates.
All in all a BB-LVD/AIO/AGM set-up is just one of many set-ups.
Aye. And it's a great setup for running a small a/c in the sleeper of an OTR rig so the driver can get some sack time.
For a weekender or on a trip with a lot of driving it should be fine, but in my opinion, it's not a great setup for long-term camping or being far from civilization. I often spend a week or more at a site if I like it, so I end up using my generator to recharge - but I don't fire it up daily, only when I need it. I don't religiously monitor the aux battery...I don't have to. I run it till the lights get a bit dim then the next day I recharge it with the generator. As I said, with the AIO that option does not exist - once the LVD trips you either recharge or you sit in the dark until you do.
I don't see how AIO would have been a better setup for Frederik and Josephine in the Congo. That's where the real world trumps the theory.
Another drawback that hasn't been mentioned about the AIO setup, is that because it's rigged as a single battery bank, if one battery goes bad (and without properly balanced wiring...that *will* eventually happen, and could happen even with balanced wiring), you have to replace them all or you run into the "one new battery in the bank" problem I described above. With an isolation setup, one can beat the crap out of the aux battery and replace it as needed without having to also replace the engine battery at the same time.
