The $200 gauge will tell you how much power you have taken out,
I'm having a tough time believing you've looked at any of the links I've posted for you...?
It will tell, you how many Ah's you've removed or % removed when calculated factoring in Peukert and temp. Looking at Ah's out screen
tells you little about the batteries actual SOC, unless you discharged it at the 20 hour rate, from a known confirmed capacity at 77F. This is why they TRY and compensate for such things as charge efficiency, Peukert and temp.
Also please understand that by purposely under-programming your BM by 20% this throws another monkey wrench into your Peukert calculation... You are actually inducing more miscalculations than you are solving....
If we assume an average
10A load with an 860Ah bank and a Peukert of 1.2
860Ah @ 10A & 1.2 Peukert = Peukert corrected load of 7.5A and usable capacity of approx 1150 Ah's
688 Ah @ 10A & 1.2 Peukert = Peukert corrected load of 7.8A and usable capacity of approx 881 Ah's
The Ah counter is using the Peukert you give it to make its calculations. If the bank is not really 688Ah's then your Peukert is even further from being accurate. These devices are already disadvantaged by counting errors, due to ever changing capacity, Peukert and charge efficiency as well as temp, why confound the issue more by programming it inaccurately?
and how much you have put back, if setup correctly.
Actually they don't show you what you put back because they eventually hit zero then stop counting. In a flooded GC battery it is likely you need to replace 120-130% of what you removed in order to get to full, but the counter stops at 100% of removed. Return 100% of what you removed and you are NOT at 100% SOC as many incorrectly assume by simply looking at the Ah screen. The last 20 - 30% needed for full, beyond what you removed, takes hours & hours to replace. The BM can show you what went in
but does a pretty poor job of showing you what portion of that went into waste energy
and what went into actual usable capacity
They can't show you accurately
what you returned, in terms of usable energy, especially if you did not complete a full 100% recharge. Charge efficiency is not a static figure like an Ah counter treats it. In bulk you may be close to 98% charge efficient but the higher you go in SOC the less efficient the conversion becomes.
Ah counters have no way to calculate properly
for charge efficiency unless you get back to 100% with every cycle. These counting errors begin to add up by cycle #2 away from "full".
It will give you a solid idea how far into your AH reserve you are.
It will only be solid
if you've done the proper programming, which you don't want to do. Even then they are not very accurate after just a few cycles away from a full recharge and thus require manual known-full re-sets
I honestly think you'd be better served by a Smart Gauge or this:
DON'T LET YOUR BANK FALL BELOW 12.2V - LOADED VOLTAGE AT AVERAGE HOUSE LOADS... Done......
When your battery bank is accepting less than 1.5% in charge current, of it's Ah capacity, about 13A, while at absorption voltage, you can "consider" it full enough. This is still not full
, but full enough
Ah counters can be good tools if they are used properly. Unfortunately most owners don't want to use them properly and thus a simpler tool like a volt meter (most owners don't know how to use these properly either) or Smart Gauge is often the better choice.
Doing what you plan to do I think you can save $200.00 and just not let the bank dip below 12.2V, unless starting the engine or using the windlass.