How long can locos sit before replacing/charging batteries

That depends on the type of battery.

I was once told that MTH 8.4 volt NiCad batteries (the white ones) lose approx. 20% off their charge per  month of not being charged. NiMH batteries typically lose charge at a slightly faster rate. The benefit from NiMH batteries over NiCads is the lack of a memory effect, not a longer charge hold period.

Your mileage may vary.

You could do the track battery test with proto-2 .   Power up the track   in conventional mode and let the engine sit for 15 minutes or so. Turn off the power and the engines sounds should continue on for at least 6 or 7 seconds.  If  they don't  you may need to charge quote a bit  longer or replace the battery. Yep no battery with proto-3.

It's the proto1 engine that I might have concern. We don't want to scramble the chip.  I suppose  I might do the same test but let the track voltage charge the engine longer.  Someone on here will know about the proto-1s.

I have one PS2 running on its original battery from 2000. It was in storage for more than 12 years. According to the DCS battery test, it's OK.

Charge retention can be considerably shortened if the batteries are overcharged.

Mike,

    IMO you should always remove batteries from your P1 & P2 engines and replace them with BCR's.  The P3 engines have heir own super Caps, just remove the additional battery if the engine has one, when you store it.

PCRR/Dave

I bought an engine that had a battery that was so corroded, it fused to the wire! needless to say the boards were shot. 

Chris

Was the battery Ni-Cad or NiMH? These batteries shouldn't leak. I stored 8 locomotives for more than 12 years and not one leaky battery.

Had a few leakers over the years but most that have sat for as long as two years charged up okay without corrosion problem.  Slowly converting to BCRs as the batteries finally die.