Two or one battery packs?

[Jacky Lee]

Hi!

I'm currently doing a project for my engineering class, and I need some help deciding on how many battery packs to use. The battery packs are rated at 7.2v, 2000mAh, and a max discharge of 3.5 A.

So, I will be using: an Arduino (~80 mA), an electromagnet (450 mA), and two motors (6v, stall current @ 2200 mA). My initial plans were to use one battery pack for the Arduino and electromagnet, and the other pack for the two motors.

Are two battery packs unnecessary? I figured that if the motors start pulling in current, the terminal voltage of their battery pack will decrease, and if that happens, a 7v supply won't be supplied to the Arduino, which must have a 7v supply otherwise its digital pins may malfunction. Thus, the need for two battery packs.

Any feedback is greatly appreciated!

 

[DerStrom8]

Personally I'd recommend powering your Arduino with a dedicated power supply, and get a SLA battery for the heavy-duty work (the electromagnet and the motors). However, if you're stuck using only the packs you mentioned, then yes, the configuration you mention should be ok. However, two stalled motors are going to draw 4400mA (4.4 amps), so I'm not sure how well your other battery pack will be able to handle that. A SLA battery for the motors and electromagnet would really be your best bet.

 

[Jacky Lee]

Unfortunately, I'm limited to using NiMH batteries by my project restrictions. I'm pretty sure my motors won't stall - they will only pull about half their stall torque, and therefore nowhere near the stall current.

But thank you for the help! It certainly cleared my doubts.

 

[DerStrom8]

DC motors always start up stalled. Inertia prevents them from starting as soon as power is applied, so you should always make plans using the stall current.

 

[Jacky Lee]

Ah, I hadn't thought about that. The total stall current will be 4.4A, but the battery has a max discharge of 3.5A. Even if for just a moment, will the battery be able to handle it?

 

[audioguru]

A pretty big capacitor parallel with the battery might be able to power the stalled motor long enough to get it started.