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I don't know where else to ask this question and I think this is the best place so I'll ask here.

I'm making a remote controlled boat that uses two DC motor controlled by a L293D motor driver chip. It takes power for driving the motors from 4.5 to 36 V. To supply this, I'm planning on using two 9 V batteries in parallel. Would this be fine, or is there a better option like 4 AA batteries in a series?

I don't know much about batteries and the like, so there might be something that would make this not a good idea that I don't know about, so that's why I'm asking here. Thanks.

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  • \$\begingroup\$ What load is on these motors? How much operation time do you need before it flattens the battery? \$\endgroup\$ – Phil G Oct 24 '18 at 16:40
  • \$\begingroup\$ Your question is asking what is "better", but you haven't told us what you want/need. So how can we tell you which is better? How do we know if space/volume is a factor? How do we know if cost is a factor? You also haven't described the motors. Are they big/small 1A/100A motors? Be sure to provide as many details as possible without making us have to ask for each of them individually. \$\endgroup\$ – Bort Oct 24 '18 at 16:40
  • \$\begingroup\$ Doesn't need to be a long operation time, maybe 20-60 minutes. I just want the supply to be able to power the motors enough (they are rated at 1.5 to 4.5 volts) and for a decent amount of time. Space is a factor because it's going to be on a relatively small boat, but not a big deal. Sorry for not providing enough details first time. \$\endgroup\$ – vastqud Oct 24 '18 at 16:51
  • \$\begingroup\$ Do you have multimeter? That may help with some of the details that you don't have yet. \$\endgroup\$ – Bort Oct 24 '18 at 17:54
  • \$\begingroup\$ a 9V battery consists of six AAAA cells .... do not use a 9V battery ..... use AA or C cells instead \$\endgroup\$ – jsotola Oct 24 '18 at 18:08
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In terms of longevity, the 4xAA solution would be better you have more amp-hours.

I don't know what your motor current load is, but for arguments' sake 100mA. At that discharge rate a typical 9V alkaline will yield about 400mAH, so you have about 800mAH of capacity. An AA battery on the other hand has about 2500mA hours (since it's in series, you don't x4).
The bigger problem with using 9V batteries at higher loads is that internally they are just a stack of 1.5V cells, so the internal resistance is a bit worse because there are 6 of them to get the 9V. The only upside is you have a little more margin on the voltage, but at a really high penalty especially on price.

My personal preference would be to use a AA rechargeable system (NiMH), the capacity is almost as good as the alkalines, and you will save money. Energizer charger/battery packs sell for literally the same price as the rechargeable batteries, so you can buy a couple to swap out if you're running a long time. You can also add one or two more AA batteries in series if you have the room to give you some margin on voltage.

Just an addendum:

In terms of battery chemistry/technology it depends on what you want your operating cost to be. Generally I find alkalines to be ridiculously overpriced especially "ultimate lithium" types which are -not- the same technology as Lithium-ion (which is what generally most people are familiar with).

Lithium ion -is- a good alternative, if you can find a good RC hobby shop they should be able to get something set up for you, but one word of caution- Lithium batteries have very high energy density so make sure you don't get an electrical short.

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  • \$\begingroup\$ Alkaline capacity drops very quickly with higher discharge rates (over 25 mA). A "9V NiMH" has very low capacity and is 8.4V rather than 9. The capacity is only 175 mAh rated down to 1V. Where alkaline is discharge is rated to 5V. Datasheet 8.4V NiMH: data.energizer.com/pdfs/nh22-175.pdf \$\endgroup\$ – Misunderstood Oct 24 '18 at 20:41

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