We have a (possible) problem with the standby battery of our G1000 C172S. The standby battery should be load tested for 20 seconds before engine start. During this time an indication light should be on to verify the battery voltage. The POH doesn’t say what you should do if the light goes off early, but supposedly it is no-go.
I’ve noticed that that this light frequently goes off slightly early (1-2) seconds. This hasn’t bothered me much, but yesterday it went off after 15 seconds. The aircraft hadn’t been flown for 11 days and was parked in an unheated hangar with temperatures between -5 and 0 °C. The aircraft has a electric engine (oil) heater which has been on the whole time, but I guess it doesn’t affect battery temperature much.
Since I was planning to do only circuit work in VMC, I went ahead with the flight anyway as I could be on the ground within a few minutes in case of a main bus electrical malfunction and the standby battery should certainly last that long.
About three hours after the end of the flight, I went back to the aircraft to do another check. This time the light stayed on for more than 30 seconds.
Does this suggest an actual problem with the standby battery or is it to be expected due to self-discharge over 11 days and/or reduced capacity because of the cold? In that case, how can you do a proper check of the standby battery in such conditions?
What is the “standby battery”? Is it the one that powers the standby instruments?
I am curious because on the DA40-180 w/G1000 there is no check such as the one you describe. There is a non-rechargeable battery powering the standby attitude indicator for 1 hr once the sealed Emergency switch is turned on. That battery is to be replaced if the seal is broken, or every two years. There is also a 100 hrs inspection.
What you describe sounds like a rechargeable battery. Such batteries loose the ability to hold the charge over time.
Yes, it is a rechargeable battery. It is used both to provide stabilised power to the G1000 system during engine start and as a short-time backup in case of failure of the main electrical bus. It can supply the PFD, one NAV/COM/GPS and illumination of the standby instruments.
The standby instruments are either unpowered (ASI, altimeter) or powered by the engine vacuum pump (AI).
Yes, rechargeable batteries do lose capacity over time. My question is really if I should take the poor result of the load test as a sign that the battery has aged enough to need replacement or if it can be explained by self-discharge and/or reduced capacity due to low temperatures.
My second check shows that the battery is capable of providing more than enough capacity when it is warm and has been recently charged.
Isn’t load testing it before every start draining it?
Isn’t load testing it before every start draining it?
Yes, but it’s recharged in flight.
At what battery voltage did the light go off? I can’t remember…
I understand but I mean that aren’t you at some risk of reducing its life by adding more discharge cycles?
At what battery voltage did the light go off? I can’t remember…
The POH doesn’t say.
My experience with batteries that start acting up like that is that they’re toast and you’d should replace them the sooner the better. They may be perfectly good for a while after recharge, but they will lose it within an increasingly short period of time.
When I lived in Norway, the first signs of weakness of a car battery occurred often when the temperatures dropped. The oil is thicker, and the battery capacity is reduced by low temperatures. The combination prevents the starter from cranking.
I understand but I mean that aren’t you at some risk of reducing its life by adding more discharge cycles?
That shouldn’t actually be a deep discharge cycle.
The POH doesn’t say.
Yes, but the G1000 is supposed to display bus voltages, I think
My experience with batteries that start acting up like that is that they’re toast and you’d should replace them the sooner the better.
Yes if it’s really the battery and not the voltage warning light.
I dimly remember that last time I flew the 172SP G1000, that particular warning light looked quite flaky.
Yes, but the G1000 is supposed to display bus voltages, I think
This test is done before the electrical system (and thus the G1000) is turned on. The POH advises that the test should not be done with the G1000 in operation.
Yes if it’s really the battery and not the voltage warning light.
It is not a voltage warning light. It is a special indicator light used only for the load testing of the standby battery.
The standby battery has an TEST/OFF/ARM switch. In the TEST position it is connected to the test load and to the indicator light. In the ARM position it is connected directly to the essential bus. The essential bus is also powered via diodes from both main electrical buses, so as long as one main bus provides power, the standby battery is charged. If none of them do, then the standby battery powers the essential bus.
