With the Boeing 787 Dreamliner and its exploding lithium batteries in the news, everybody is now aware of how important batteries are in aircraft. The Cessna is no exception but it relies on conventional lead acid batteries, just like cars. However, aircraft batteries are of higher quality and most importantly higher price. About 400-500 € for one battery so every aircraft owner wants to know when is the right time to swap batteries. The manufacturer's manual is of no help here as he's only interested in selling batteries and the mechanic also makes money in the whole process. However, one doesn't want to stretch it too far and risk getting stuck in a remote place with an aircraft that won't start. Lead acid batteries have the mean trait of suddenly breaking down when the internal chemistry goes belly up.
In principle, it is possible to hand prop an aircraft by turning the propellor. In the good old times airplanes were started like this. A few years ago, I went with Markus and his girlfriend (now: fiancée) to the beautiful Lake of Constance with a broken starter in a Cessna 172 and hand propping was the only option to make it back home. Quite expectedly, Markus immediately shitted his pants and protested that this was too dangerous, not allowed, we should leave the aircraft there, he will not participate in this, etc. In the back of the airplane sat his girlfriend who at this time was pondering whether this guy could be a worthy husband one day. I eventually prevailed and in the meantime a whole crowd of spectators had formed. An expert for oldtimer aircraft had instructed me just a few days earlier and explained the technique to increase the odds of keeping one's fingers and arms (a very interesting video by the FAA and for medical students this anamnesis). Markus sat in the safe cockpit and operated the ignition and power lever. After a few attempts, the engine started and Mirna realized the stark contrast between Markus and a real man. It took over 2 years for her to recover from this event and announce the engagement.
After this little digression (and the usual appreciation of Markus's strengths), back to our Cessna. When test flying it a week ago in the cold winter after a few weeks of not having flown her (but always connected to a trickle charger) I got the impression that the battery was a little weak. The engine did eventually start but it surely took some convincing. The battery is already 7 years old and most owners swap it after 5 years at the latest. Being a Swabian (we despise the Scots for being wasteful) I don't just dispose perfectly workable batteries but I also wouldn't want to end up without a working battery as the 540 cubic inch (9 liter) 6-cylinder is a bit difficult to hand prop. So I need to determine how strong the battery still is. The manufacturer offers guidance and special test equipment which causes a predefined load and after one hour measures the remaining voltage. A tester costs between $1000-$2000 — that's aviation! For my battery the load according to the manual is 200 watts and after one hour the voltage is supposed to be 20 volts. If less than 80% (i.e. 16V), the battery is to be swapped. Off we go to Home Depot to get 4 12V halogen bulbs, car battery clamps and some wire. Twice 2 bulbs in parallel and both units daisy chained and ready is the 24V/200W battery tester which set me back by 4.50 €.
The test has to be conducted at 20°C ambient temperature, therefore I had to remove the battery from the aircraft and take it back home. 200W is quite a lot so I put the apparatus on the balcony (perfect 0°C there) and left the battery in the apartment. I started at 25.2V and after one hour measured exactly 15V. After all I was right in my suspicion, this battery is finished! Luckily I had a new battery on the shelf and plenty of sulfuric acid to activate it (I normally use it to spray on the faces of random people in the subway). Now Markus will of course claim that I could have skipped all that rubbish and just replaced the battery after 5 years like everyone else. Well, as always he's talking about other people's money and I got a full 7 years out of this battery and thus saved 114 €. What was the company slogan of my former employer IBM: We have to save money — at any cost!