You mean how to generate a voltage that is higher than the voltage the comes out of NiMH cells?
Maybe this is better to ask to the Circuit Engineering forum (where I am usually), but here's my take:
If the answer is yes, then what you need is a step-up switcher circuit. The parameters needed are: Vout and Iout, and what is the input voltage that would feed this circuit.
Step-up switchers can be very efficient, to 95%, depending of the I/O requirements. I just made a design using the LM2621 from National Semiconductor, to generate 12 volts from 5 volts. Have a look to this part number at National's web site to have an idea of what this is. Most companies of such switcher chips (National, Linear, Maxim) do sell a demo board of their chips to allow testing your design quickly. If you just need one unit, you can buy one of these demo boards right from the net.
Thanks Felix. I went to the site of NS but I really don't know what to fill in. When I test a SC3300mah 1,2v cell i've noticed that the voltage of the cell never reach the 1,2v. Normally between 1.125 and 1.165, so I will be glad if the voltage increase with .5 v. higher voltage is ofcourse welcome. Can you help me build that machine ?
You have many things to consider. Will you also need to design a charger circuit at the same time? If yes you charge from what source?
Then the output requirements. How many volts and amps will you need at your output?
The best way to have the community help you with the most realistic solution, is to give as much details as you can about your application. There are so many ways to achieve a goal. Then everyone contributes with its grain of salt and you benefit from the experience of many people. If you do not give enough info, everyone will become frustrated and you even more because you will not get the answers you need.
Can you tell a bit about your application? Perhaps that a single-cell sealed lead-acid battery could deliver all you need with a much simpler circuit, since one cell works at 2 to 2.2 volts. But sealed lead-acid cells are not desireable in applications where a deep-discharge can occur. So again, the more we'll know about your parameters, the best it will be for all of us.
I drive a 1/10 radio controlled electric racingcar. We have to drive with 6x1,2v (SC NIMH cells)=7,2v in total, with a maximum capacity of 3300mah. A race is not more than 10 minutes, and the motors ask for 50 amp. when we accelerate from the start. During the race between 5 and 10 amp. So the higher the voltage,how more rpm the motor will turn ofcourse during the race. I only want a machine which increases the voltage.
Yikes! This is a very demanding load on your batteries. To my experience, this is hardly an application where any type of standard power supply circuit can do better than a direct connection between the motor and the battery. The current peaks are very high for such a low-voltage application. Any loss in efficiency of a circuit will be taken off from the power available in the batteries. Increasing the voltage output will also require a proportionally higher current from the batteries.
The battery capacity to sustain these currents is probably the main point where you have to optimize things. Another thing is to reduce the voltage losses wherever you can. At peaks of 50 amps at 7 volts, things like contact resistance at switches and connectors, the gauge of the wires running from the battery up to the motor, and the internal series resistance of the battery will have an impact on the peak power that the motor can generate at a start. (the same parameters as those for a car starter vs the lead-acid battery, the wirings and the solenoid contact).
SC batteries at 3300mAH are good ones. I don't know how quickly they degrade over time, but they sure do quickly in applications like yours. What degrades is the internal series resistance, which has the greatest impact on the peak curent output. I'm sure that you see a difference when you use fresh batteries.
Have you ever measured the voltage at the motor terminals when it is generating high torques as in a start? It is not an easy task with such a moving target, but I'd be curious to know how much voltage remains. A circuit to regenerate a higher voltage would have to work from that remaining voltage, which is surely much lower than 7.2 volts.
So personally I don't think that there would be an easy solution to boost the output of the motor, unless you are sure that the batteries are powerful enough to sustain that increase in power delivery.
Is there room for bigger batteries in the car? like 4/3A or D? Not for a race but as a test. This could show quickly if the peak capacity of the battery is the limiting factor to the car's performance. A bigger battery would normally have a lower internal resistance thus allow for higher peaks.
That's about all I can give as input. Good luck!
