MightyMax 18Ah AGM battery

[sternwake]

Update?  The Northstar is now past your 6-8 months. 
Why not get a reman starter and see if the old one is really the problem?
Thanks for the info you've posted so far.

 The Northstar continues to degrade.  Unless fully charged, my stereo turns off while engine cranking and my voltmeter will drop into the 9's and high 8's. 
The Northstar's end amps no longer taper anywhere close to 0.4 amps at absorption voltage, but taper to about 4.8 then start climbing again. I've seen them climb back up as high as 11 amps, and the battery must have been hot.

Strangely though, the voltage maintained during overnight discharges is still close to normal.  If this was the only benchmark for performance, I would not know the battery is at end of days and be blissfully ignorant of its impending failure.

When well discharged it can still accept huge currents for much longer than expected, given my experience with sulfated flooded marine batteries that require relatively few amps to achieve absorption voltage even from a well depleted state.

The Mightymax just yesterday failed to jump start a Sprinter diesel whose newish AGM group 94 starting  battery had been drained to sub 8 volts, but that's not surprising.  It took 5+ hours at 14.7 for it to reach full charge after feeding that AGM battery for a minute or 2.
I've kept the MM battery mostly fully charged much of the time, but I've not tried it again to see if it alone can still start my overnight cold engine, but I suspect not.

I have used the MM battery to power a 24v hedge clipper and here and there powering other things, but rarely a true deep cycle and never left undercharged for very long.  

The stereo shutting off when starting the engine annoys me, so when I am out running errands with a less than full Northstar, I have been putting the MM battery in parallel for starting, and then driving, and often forget to unparallel it until well after sundown, so it has been getting shallow cycled. It provides 50 to 60 amps of the ~180 for engine starting when in parallel with the Northstar.

Recently I've acquired an 'ideal' diode rated at 50 amps, and put 45 amp Anderson powerpoles on each end. I do orient it to charge the MM battery when driving or when the solar has brought system voltage above the full charge resting voltage of the MM battery, which still seems to be in the 13.16v range, and it cannot feed any loads, such as the laptop or starter.

I think this sub 12$ Ideal Diode( which drops only 0.04v at 40 amps)  would be great for keeping a jumper pack battery at top charge and condition for when it is needed for jumpstarting.  Since I do not need or want to carry an additional house battery bank, and have proven that I do not require it, I think this is a viable strategy for the many, who do not want to go through the hassle or expense  of setting up a secondary house battery system. The only issue would be if the jumper battery does not have the health to assist the overdepleted engine starting battery, which hopefully has been upgraded to at least  the largest marine battery which can physically fit the stock location.  The Ideal diode would almost completely solve that issue for many years.

HOnestly, my workshop battery is also done for, needing water often, and 12 amps cant get it above 14.2v as it bubbles and gurgles and spews electrolyte down its sides.  The Northstar needs to replace it, and a new battery needs to replace the Northstar. its just a matter of going to get a new one, well, and paying for it too.  Its been 6 years, but the battery distributor is likely to remember my fury when told an underweight relabelled marine battery, is a true deep cycle battery.  I wound up returning that battery and getting the workshop battery, a USbattery group31 which lasted about 500 deep cycles in the van before being relegated to workshop duty.

Part of me wants to wire up a solenoid to just parallel the Mightymax battery for engine starting and cycle the northstar into the ground just to see how much longer it can go, but it will still get cycled in the workshop.  a single 100 watt panel is hardly ideal for charging it though, especially in winter.