The Cessna 120-140 Association Forum

Last time I flew my plane didn’t crank over very fast. Battery was weak. I put a brand new J16 battery in this summer and it turned the prop amazingly fast right out if the box. We hooked up a multimeter during flight and the generator was charging it to about 14 volts. Ammeter always shows a charge after start and about 1400 rpms. There is nothing hotwired to the battery like a clock etc. Just wondering if anyone has any clues as to why it’s not staying fully charged? I’m talking like after 3 weeks of sitting.

Was thinking maybe the contactor solenoid on the battery box might be letting a slow trickle of electricity by over time?

(No I didn’t leave the master switch on)

Those batteries should hold the charge for a good long time. First, is the battery fully charged? Second, it's unlikely (pretty much impossible) the solenoid contacts could "leak". Do you have a suppression diode on the solenoid coil? Does it go to ground or to the solenoid side of the master switch? It should be the latter, but if it's the former, a leaky diode could be your problem.

Remove the ground cable from the battery. Put the (+) probe of a volt meter on the cable end and the (-) probe on the negative terminal of the battery. If the meter registers anything, you have a draw somewhere.

Update: I've been leaving the battery box cover off and the battery seems to be holding a charge now! Inside the box lid are 2 rubber pads that are supposed to provide insulation for the terminals. The box and pads have been painted and sit close to the terminals with the box lid on - might even touch. I read on the old forum years ago that a tiny bit of current could "leak" over time (especially painted) so I think this could be the cause.

We did the multi meter check and found no evidence of current draw.