No idea.
What I do is I put all the loads on and check, as the RPM is reduced from the 2000 during the power checks, as it drops through 1100rpm, the current goes negative, and at idle it shows about 25A (in my case) with a few amps of fluctuation due to the strobes. That checks the alternator subsystem.
Lots of theory above but just how is a pilot to know if a battery is about to give up ?
One practical check is to turn the battery master and pitot heat on, if the pitot heat is very slow to rise the lkelyhood is the battery is not far off failure.
I have deleted the obvious advert and the totally wrong stuff about nicads being lithium 
Lots of theory above but just how is a pilot to know if a battery is about to give up ?
One practical check is to turn the battery master and pitot heat on, if the pitot heat is very slow to rise the lkelyhood is the battery is not far off failure.
That’s only if you don’t have a voltmeter
Also, not a single one of the planes I did my PPL in had any pilot heat 
All burnt out, or maybe disconnected on the order of the school.
A_and_C wrote:
Lots of theory above but just how is a pilot to know if a battery is about to give up ?
When it no longer cranks the engine, that’s a pretty good telltale
I remember that I thought the propeller on the DA40 was not turning as fast as it used to and I told the owner the battery was going south. He did not want to believe me until I twice could not turn, or barely turned the engine for start. Still I had to go get one of those instruments to measure acid weight and show him it was way low. He finally caved in, and replaced the Gill battery with a Concord.
A_and_C wrote:
One practical check is to turn the battery master and pitot heat on, if the pitot heat is very slow to rise the lkelyhood is the battery is not far off failure.
So let me get this straight: You turn on the master switch, then turn on the pitot heat then measure the time it takes to get to full operating temp ?
And just what is the “normal” time for this ? What is the “normal” temperature ? What is the variance from one pitot heat system to another ?
Practical ? Me thinks not …
Michael
If the battery is on the way out the pitot will not get any more tha warm, pilots can’t perform a battery capacity check but the pitot heat will give you a good idea of the battery state very quickly and tell you it is time to get the battery checked properly.
A_and_C wrote:
If the battery is on the way out the pitot will not get any more tha warm,
If the pitot heat wont produce anything more than warm, then the battery is severely dis-charged. That does not mean that the “battery is on the way out” since it could be discharged for any number of reasons and re-charging it and correcting the root cause would be in order.
pilots can’t perform a battery capacity check
Well in fact there is a very simple way to gauge the battery’s charge state: look at the voltmeter (if installed) if not you can get a digital voltmeter that plugs into the cigar lighter for about £2 £3.18 that does the same job.
but the pitot heat will give you a good idea of the battery state very quickly and tell you it is time to get the battery checked properly.
If no voltmeter is available, then much simpler to just crank the engine – if it’s slower than normal you can bet the battery is discharged.
No need to get out and burn one’s hand on the pitot …
Michael wrote:
If no voltmeter is available, then much simpler to just crank the engine – if it’s slower than normal you can bet the battery is discharged.
It could of course be discharged for several reasons, for example if the engine has not been run for a long time, some electrical consumer has stayed on overnight… But if you can rule out the latter, and the airplane has been flown recently, then chances are that the battery no longer holds or accepts the charge.
Whatever you do, if your battery is too low to start your engine, do not jumpstart. That is a good way to definitively kill your battery when the engine starts and the generator pumps 70 A through it.
Aviathor wrote:
That is a good way to definitively kill your battery when the engine starts and the generator pumps 70 A through it.
This will not happen, on a poor battery the internal resistance will go up, limiting the maximum charging current. Even a good battery would not charge at 70 Amp. When you have an amp meter, you would see somewhere between roughly 20-40 Amp with the alternator running at 14 / 28 Volt.
One risk of jump starting could be spikes (when using a poor GPU based on a generator combined with a poor battery). The poor battery will not be able to buffer / smooth out as good. The safest option is to use an external power unit based on a battery.
It isn’t a good idea to jump start (due to empty battery) and perform a flight which is depend of electiricity, suchs as with Diesel engine, but also for IFR flights.
Well I have been there and done that , seen the 70 A on the ampmeter (although it does come down quite quickly – within 30 s), and how the battery wasn’t itself after a few jumpstarts because someone left an appliance on between two flights.
In the DA40 there are two gotchas; the map reading light on the pilot side, and the cigarette lighter outlet which are both live even with the master switch on. So there is potential for a dead battery
