When is it time to replace house/engine battery?

We knew when I ours failed to run the furnace overnight.

I've had two battery's simply short internally when they got old. At that point there is no way to even jump them and load testing would have showed they were fine beforehand.
I won't push mine beyond 8 years because I don't want to be stuck anyplace.

When is the time to replace batteries: TWO answers: If you have lots and lots of money and the down time is more costly than a bit of reduced battery life you replace them based on the calendar. However your name also ends in Company or Incorporated.

If you are an RVer and can afford to do it this way you replace them when they fail, mine lasted like 8.5-9 years range, About normal(7-10)

Your idea to load test is good.. My chassis battery, when I replaced it was load tested by the battery store, (me too) The report was: Battery fully charged (no shorted cells or anything like that) but only 50% rating before it flunked. (Exactly what I found with my harbor Freight special).

The chassis.. house,, Well they were getting anemic on the test, but a shorted cell ended the discussion.

Man are those new DEKA's heavy!!!! (they are a touch bigger than the Interstates they replaced (Taller).

How to load test depends on your load tester.. ONE way is to run every electrical thing you have and see how fast they run down, but frankly.. That is gonna take a while on house batteries.. I use a Chicago Tools "Dual meter" adjustable carbon pile load tester from harbor Freight, One meter is voltage and the other current, claims it can suck 500 amps, actually peaks at 450.

Disconnect from RV (This is important)
Hook up tester,
Crank up the amps (For GC2's the proper current to test is like 700 amps, well beyond the ability of this tester) and 15 seconds later when it BEEPS, read the voltmeter. Green is good, Red is bad

At 450 amps on a pair of golf car batteries Green is ok or good, Yellow is bad.

Crank the current back down and WITH INSULATING GLOVES ON unhook tester,, After 15 seconds at 4,500 watts plus.. It is going to be HOT!

It is time. Now......

If you lived in Aridzona, you would have already replaced the battery.

The rationale is that acid does things for a living. Like eat lead battery plates for one.

An ideally maintained flooded lead acid battery living in a 0C 32F environment will last around 30% longer in a storage environment, than a battery in an average of 25C environment. Ideally maintained means maintenance charging, top charging, and stirred once in awhile.

Ice cold battery electrolyte will not affect much until it warms up (body heat). 50C (122F) 1.275 electrolyte is DANGEROUS. Like several hundreds of times more reactive.

5-years is at the end of the down-slope ramp of reliability. Definition: You will get more service time out of the battery. Just don't put yourself in a position where a failure could cause hardship or hazard, if you decide to press for 100% of service life.