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Battery charger
On Nov 18, 2:46*am, Grumpy The Mule wrote:
Howdy, The saturation voltage will probably be less than a volt, likely only a few tenths of a volt. *When the vehicle is running the output voltage could be too high. *A transistor that comfortably handles the ten amps is going to be something like a 2N2955 or 2N3055 there are curves on the data sheets that tell the story. As the current souce runs out of compliance and the current drops, the output voltage rises because the saturation voltage decreases. There is an easy fix if you use a PNP pass transistor. *Clamp the base at 13.6V plus a base emitter drop, say 0.8V, the output can't rise higher than the float voltage. Better yet, use a TL431 to sense the battery voltage to end the constant current charge by reducing the bias and enter float mode. (The TL431 is a very handy device and cheap. *Check the app notes and I'm sure you'll find many other uses for it.) *Then either NPN or PNP will work, choose your poison. I modified a junky old eight amp Kmart charger like this with a PNP device for charging the six volt battery in my tractor. You'll need some sort of short circuit protection. *During a short the ten amps times the supply voltage will be dissipated in the pass device. *Could be a fuse or breaker or active protection. *I left this out of my first attempt and the pass device didn't like being shorted for even a moment. *Luckily I tested this before putting it all back together rather than finding out later on in a snow storm. The existing thermal breaker was much too slow. LEDs would be a huge improvement. 73, Grumpy JIMMIE wrote in news:1fa6ec49-6eab-4cb6-8eeb- : I think I am going to try charging it through a 10 amp constant current source. If it works the way I am thinking as the battery nears full charge the pass transistor should saturate applying nearly full generator voltage across the battery topping off the battery. Im thinking(hoping) that when this occours the voltage drop across the transistor may be just enough to yield nearly the ideal float voltage for the batteries. The batteries normally power my heater and lights for my popup camper and a couple of radios that are mostly *used for rx. With only an initial charge from the battey charger before leaving home I can camp about 4 days. With some additioal charging I am hoping to extend it another 2 oor 3 days. Since our trips usually include a few hours a day of driving I thought I would use this oppurtunity to top off the batteries. I have leaned against the idea of just running the engine for that purpose. The incandescent bulbs I am using in the camper are the main draw on the batteries perhaps going to LEDs would help.- Hide quoted text - - Show quoted text - Saturation voltage my oh my. A simple DC to DC convertor on the gate of a FET of like 4 milli ohm so no more Vsat. I've done it with the UC3906 which is designed to charge lead acid. All you need is a source that'll will handle all that current. All the tricks I've seen done with lead acid charging constant current will fawk up ur battery unless you know when to shut it off like the UC3906 does. Maybe 3 TL431's as sensors and you can do it. But the UC3906 can sense a shorted cell and will not charge the battery if there is a shorted cell. 73 OM n8zu |
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