RC-Monster Forums - View Single Post - Maxwell Ultra-Cap's/Lithium cells hybrid pack

BrianG · 12.03.2006, 12:20 AM

It would be interesting to see how this works. I agree that it would all but eliminate the instantaneous voltage sag providing that the heavy current draw isn't continuous. I have a couple of questions though;

- What are the physical dimensions? If it is too big, it may have a hard time fitting in some already cramped areas.

- How do you charge them? A capacitor that size (even 500,00uF) should NOT be simply slapped on the batteries as the instaneous current would be all but infinite, only limited by the battery capacity. As a matter of fact, it would seem like a dead short until the cap charge level increased. Most of the time, large caps are charged via a series connected resistor. The time it would take to charge is 5*R*C. Whatever charging procedure is used must be used whenever a fresh set of batteries are installed.

In car audio, the general rule of thumb is to use 1F for every 500-1000watts (rms). Of course more is better, but there is a bang-for-the-buck aspect. I would think that 2-3F would be more than sufficient and be much smaller, lighter, and easier to install.

Also, there is an added benefit to the use of a cap; runtime may actually go up a little. Since there won't be large pulses of discharge current, the battery will actually deliver the full Ah rating of the battery.

	(#2)
BrianG RC-Monster Admin Offline Posts: 14,609 Join Date: Nov 2005 Location: Des Moines, IA	12.03.2006, 12:20 AM It would be interesting to see how this works. I agree that it would all but eliminate the instantaneous voltage sag providing that the heavy current draw isn't continuous. I have a couple of questions though; - What are the physical dimensions? If it is too big, it may have a hard time fitting in some already cramped areas. - How do you charge them? A capacitor that size (even 500,00uF) should NOT be simply slapped on the batteries as the instaneous current would be all but infinite, only limited by the battery capacity. As a matter of fact, it would seem like a dead short until the cap charge level increased. Most of the time, large caps are charged via a series connected resistor. The time it would take to charge is 5RC. Whatever charging procedure is used must be used whenever a fresh set of batteries are installed. In car audio, the general rule of thumb is to use 1F for every 500-1000watts (rms). Of course more is better, but there is a bang-for-the-buck aspect. I would think that 2-3F would be more than sufficient and be much smaller, lighter, and easier to install. Also, there is an added benefit to the use of a cap; runtime may actually go up a little. Since there won't be large pulses of discharge current, the battery will actually deliver the full Ah rating of the battery.