The regulator canít figure it out and Iím not sure which amp reading youíre referring to. A charge controller builds to a defined voltage in Bulk, then holds that voltage until the bank can no longer accept a defined amount of amps. You need a shunt at the house bank that can tell how many amps are actually being accepted as charge into the bank, regardless of what the charge source can produce. The charge source output is somewhat irrelevant, as it has to put charge into the bank, plus provide current to whatever appliances are running at the same time. Iím sure there is waste in top of that.
Iím no pro, so there is probably a better way to describe this.
Resting voltage can be a reasonable way to know you donít discharge too far, but itís hard to know youíve fully charged. Of course, no boat that has anything turned on is ever resting.
I think this whole charging thing has been probably way over thought as it applies to most people with auxilliary diesels on sailboats.
These boats have an increasing appetite for electricity over the years and require more and more power for these devices.
I've observed the development/process over 35 years I have owned a sailboat. Yes I added more and more things which needed power... then added a larger batteries to feed them.. and a higher output alternator to top off the batteries faster... and then some solar to replenish the batts when the engine was not on and the sun was shining and I was away. The along came these very accurate measuring devices and monitors and smart regulators which had profiles which take into account the performance characteristics of various batteries. These measuring devices required a shunt.
"An ammeter shunt allows the measurement of current values too large to be directly measured by a particular ammeter. In this case, a separate shunt, a resistor of very low but accurately known resistance, is placed in parallel with a voltmeter, so that all of the current to be measured will flow through the shunt. The resistance is chosen so that the resultant voltage drop is measurable, but low enough not to disrupt the circuit. The voltage across the shunt is proportional to the current flowing through it, and so the measured voltage can be scaled to directly display the current value"
Amps in the circuit are derived from precision voltage measurements. So now you can tell how many amps you're adding from charging devices... or using by load devices.
People who are big time electricity users need to do something. Bigger and bigger batts often can work. So more charging is chosen... generators, high output alts... fields of solar panels... co generation from spinning props and so on.... and of course we now have high efficient LED lighting and low consumption electronics.
If you have a smallish boat you need to design a system and your USE of electricity which FITS your boat... this includes the load side and the charging side.
My hunch is that your boat's architecture will be a voice in what works. But everyone seems to be addicted to electrons and what they can do.... and that work comes at a cost.
I have 2 8D - AGMs and a Optima blue top for engine starting. I have a high output alt with a smart regulator a couple of yr old 55 watt solar panels. I don't use an entertainment system... nor a 12v frig. My AP is very low draw. All lighting is LED... and I pretty much have no problem with having enough power... or recharging the house bank. My alt is running a few hrs per day when I am on board and I am never using shore power as I am on a mooring or anchor.
Best advice is to keep it simple and sail more.