EMRFD Message Archive 14276
Message Date From Subject 14276 2017-09-07 15:43:33 RodWall1234@Gmail... Re: [emrfd] Re: Would one of these car starters work as a QRP supply Hi,A car jump starter with a capacitor.Regards,Roderick Wall, vk3yc.----- Reply message -----
14277 2017-09-07 15:44:26 RodWall1234@Gmail... Re: [emrfd] Re: Would one of these car starters work as a QRP supply Hi,Our local Aldi had a special purchase. A Jump starter for cars. That indicated it has a capacitor and not a battery.I don't know if this is true and or if it is a battery. Maybe someone thinks a capacitor is a battery?I wonder if it could be used to power a transceiver?I couldn't find it on the internet.Regards,Roderick Wall, vk3yc.----- Reply message -----
14278 2017-09-07 16:19:18 Bill Carver Re: Would one of these car starters work as a QRP supply? Hi Roderick. Long time no chat.
I'm TERRIBLE at "maintenance", specifically keeping batteries in a good state of charge. So often when time comes to use them one or more cells is kaput. The capacitor sounds great. I've never used a super capacitor in a circuit so have no idea what the self-discharge rate, or capacitor relaxation rate, is. At $100 AUD that box is a bit expensive to replace a Lithium battery, but perhaps conceptually worth it because the Lithium is dead when I go to use it.
300A = dQ/dt, which defines the
rate current can be extracted. All important if you're starting a car but not enough to decide whether it could replace a batter in a specific application. On has to wonder what the capacitor size is, in Farads.
It's voltage rating has to be more than a 12V battery, I'd think 15V is probable. If it was 10f and charged to 12V, then you could pull 50mA for 3.3 hours and it would drop to 11V. That's plenty of time for an afternoon of antenna work, and still not discharged to the point the VNA would care. Interesting.
14279 2017-09-07 17:40:50 Bill Carver Re: Would one of these car starters work as a QRP supply? Roderick, I looked in Mouser catalog. The biggest capacitor was
ESR 11 milliohms
nominal leakage current 0.225 mA
Q = CV, so it will store 300 coulombs.
Assuming a constant leakage current of 0.225 mA it would take 370 hours to discharge to zero. Assuming it was constant resistance of 3V/0.225mA =13.33K, then you'd lose 37% of the voltage in 370 hours.
Assuming you used it to run a VNA the self-discharge would not consume much of the stored charge. On the other hand if it say in a drawer for a month you probably would find it to be seriously discharged.
If you pulled 1A from it, delivery 3W, the internal dissipation would be 11 milliwatts......so 99.6% of the stored energy would make it to the load. Pretty efficient at 1A.
On the other hand, if you pulled 10A from it, you will be losing 1.1 watts internally (it would get warm!) and you'd be getting only 96.3% of the stored energy back.
I might have dropped a decimal point somewhere..........
It would be interesting to use five in series to get a 15V 20F capacitor. Tricky to charge all of them equally without putting them in parallel for charging. But five of them would cost $136.80 to see. Hmmm. A little rich for my blood.
73 - Bill - W7AAZ
14280 2017-09-07 18:10:08 Will Kimber Re: Would one of these car starters work as a QRP supply? Hi,
Regarding the Jaycar unit.
If it charges in one minute at 10 amps then to get 100 amp to start
vehicle you have 10 seconds available. Ignoring losses of course.
300amps starting current gives you 3.3seconds! If the first cylinder
doesn't fire would it make it to the next with inertia?
14282 2017-09-08 06:20:34 RodWall1234@Gmail... Re: [emrfd] Re: Would one of these car starters work as a QRP supply Hi Bill,Yes long time no chat. Your calculations are interesting.When there was talk about using a car jump starter to power a transceiver. I couldn't help highlighting the Capacitor type. It would be interesting to know what the ratings are for the capacitor. Or capacitors.The Aldi unit was cheaper than for the Jaycar unit.Roderick Wall, vk3yc.----- Reply message -----