So the Whole "system" has more mA and therefore lasts longer.
Stratus, you can connect your 9v batteries in parallel. That will boost you mA. If you have the space, you may want to consider using AA batteries connected in a series parallel circuit to get the target V and the extra mA that you want.
You can use the batteries to power different circuits: one for an HWB and another for motors or whatever to run from. You can put 9V batteries in parallel, but avoid mixing manufacturers.
You can use 9V batteries, but they are very poor choices for power supplies. They're expensive and have very little current capacity compared with almost anything else. You'd be much better off using AA cells in a 5 cell battery (5 x 1.5V = 7.5V). Rechargables (like NiMH cells) are also a better choice. In that case, you'd use 6 cells (6 x 1.2V = 7.2V).
We have a big box of Duracell 9Vs at work. They randomly pop open (bottom blows out with a pop) while just sitting in the box. We get about 2 pops a week. No short circuits anywhere, we put electrical tape over the contacts. They ALSO pop open under heavy drain. Not really dangerous, but plenty annoying and wasteful.
You'd be much better off using AA cells in a 5 cell battery (5 x 1.5V = 7.5V). Rechargables (like NiMH cells) are also a better choice. In that case, you'd use 6 cells (6 x 1.2V = 7.2V).
Can you explain why you recommend 7.2V (6 NiMH cells) rather than 6.0V (5 NiMH cells)?
My understanding is that the energy from the extra cell is completely wasted, being converted into heat in the linear regulator.
Additionally, the vast majority of servos available are spec'd to a maximum of 6.0V. The combination of higher available current and higher voltage will shorten the life of the servos if not run through a regulator.
I have had excellent results with my 5 cell sub-C NiMH pack on my Boe-Bot.
@W9GFO,
I suggested using 6 NiMH cells (7.2V) because Stratus didn't mention servos nor did he mention the board he's using (presumably a BOE). With the BOE, you get an LM2940 voltage regulator with a 0.5V dropout voltage at 1A. That gives you a minimum input voltage of 5.5V for regulation to occur. Although NiMH cells do have a fairly flat discharge voltage curve, they're not perfect and there can be noise on the power line. 0.5V "headroom" (between 5.5V and 6V) is not a lot. The extra 1.2 (nominal) voltage is a tradeoff between wasted energy and extra battery life.
Mike makes a good point. A lot of modern devices stop working when the battery voltage drops even slightly, digital cameras being some of the prime offenders. It pays to build in some voltage overhead to keep things moving longer, even if there is some power dissipation.
That could be an interesting project to see what the optimum number of batteries would be in a mobile robot. More batteries is more weight to drag around and more energy dissipated to regulate to 5V, but as a whole the batteries would have a longer life in a given product since they each can contribute less to keep the total voltage in the green.
It is interesting to note how Duracell and Energizer "achieve" their amazing battery life claims. A D-cell has 15,000 mAH. WOW, that's 15 AMP HOURS! But they trickle the current out at less than 50 mA, and they call the battery "dead" at 0.8 volts. Pretty optimal and highly unrealistic conditions!
Stratus
Posts: 12
Comments
Stratus, you can connect your 9v batteries in parallel. That will boost you mA. If you have the space, you may want to consider using AA batteries connected in a series parallel circuit to get the target V and the extra mA that you want.
Phil
-Phil
Can you explain why you recommend 7.2V (6 NiMH cells) rather than 6.0V (5 NiMH cells)?
My understanding is that the energy from the extra cell is completely wasted, being converted into heat in the linear regulator.
Additionally, the vast majority of servos available are spec'd to a maximum of 6.0V. The combination of higher available current and higher voltage will shorten the life of the servos if not run through a regulator.
I have had excellent results with my 5 cell sub-C NiMH pack on my Boe-Bot.
Rich H
I suggested using 6 NiMH cells (7.2V) because Stratus didn't mention servos nor did he mention the board he's using (presumably a BOE). With the BOE, you get an LM2940 voltage regulator with a 0.5V dropout voltage at 1A. That gives you a minimum input voltage of 5.5V for regulation to occur. Although NiMH cells do have a fairly flat discharge voltage curve, they're not perfect and there can be noise on the power line. 0.5V "headroom" (between 5.5V and 6V) is not a lot. The extra 1.2 (nominal) voltage is a tradeoff between wasted energy and extra battery life.
That could be an interesting project to see what the optimum number of batteries would be in a mobile robot. More batteries is more weight to drag around and more energy dissipated to regulate to 5V, but as a whole the batteries would have a longer life in a given product since they each can contribute less to keep the total voltage in the green.
It is interesting to note how Duracell and Energizer "achieve" their amazing battery life claims. A D-cell has 15,000 mAH. WOW, that's 15 AMP HOURS! But they trickle the current out at less than 50 mA, and they call the battery "dead" at 0.8 volts. Pretty optimal and highly unrealistic conditions!
http://www1.duracell.com/oem/primary/alkaline/alkcapacity.asp
One reference says a new D-cell has a short-circuit current of 28 amps:
http://hypertextbook.com/facts/2001/WilliamCruz.shtml
(MAYBE for a second or two!)