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I hope that this is the right place to ask this.

I have a small RC car that I opened and added a raspberry pi in.

Now I want to pick a battery. First of all I heard that Lipo batteries are really good. But I can find lipo batteries only with 11.1V, 7.4V and 3.7V.

My driver motor (l298n) can only take 5v to power up my motors, so the 3.7V is too small but the 7.4V is too big.

Out of that I want to power up my raspberry pi board with the same battery. I don't want something too expensive. I want to drive the car on house with a decent speed. Not like that competition cars.

And the last problem is the charging, and protection. I saw that if you are not careful you can damage the battery if you discharge too much or overcharge.

I am not sure if this is a noob question and I need to have physics knowledge but I want some help because I can't spend all my money just to test batteries.

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    \$\begingroup\$ "My driver motor (l298n) can only take 5v to power up my motors" - the L298N can take up to 48V, and most L298 modules can take at least 10V. Which motors do you have? \$\endgroup\$ Commented Nov 12, 2019 at 16:59
  • \$\begingroup\$ The L298 is very lossy. If you have an RC servo output from the pi then consider using an ESC. You should use a battery pack with approximately the voltage of the original battery; if that's lower than the 5V needs, get a boost converter module. \$\endgroup\$
    – TimWescott
    Commented Nov 12, 2019 at 17:29
  • \$\begingroup\$ @BruceAbbott I have some stock motors from the RC car. I don't see stikers or something that can help me with the name but that is how it looks like: electrobist.com/wp-content/uploads/2018/08/toy-motor.jpg \$\endgroup\$ Commented Nov 12, 2019 at 17:30
  • \$\begingroup\$ @TimWescott so is better to get a lower voltage (3.7v) or a higher voltage (7.2V) \$\endgroup\$ Commented Nov 12, 2019 at 17:31
  • \$\begingroup\$ "I have some stock motors from the RC car" - What is the stock battery used in the car? (need to determine what voltage the motors can take and what current they might draw). \$\endgroup\$ Commented Nov 12, 2019 at 17:35

2 Answers 2

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I want to drive the car on house with a decent speed. Not like that competition cars.

For similar performance to the stock setup you want a similar voltage. AA cells put out about 1.5v to 1.1V each depending on load and discharge state, so to replace a 6 cell AA battery you want something that delivers from 6.6V to 9V. It also needs enough capacity to run the Raspberry Pi (with any addons) as well as the motors, for a reasonable time.

I saw that if you are not careful you can damage the battery if you discharge too much or overcharge.

That's true. Lipo batteries need to be charged with the right type of charger, and must not be allowed to over-discharge. Over-charging or charging a damaged Lipo may cause it to catch fire and burn up everything around it (including your house!). NiMH and Nicad have more relaxed requirements and are safer.

The standard battery for RC cars is a 6 cell 7.2V NiMH or Nicad pack. Batteries for smaller cars typically use AA or 2/3A size cells. eg. ProTek RC 6-Cell 7.2V Speed Intellect NiMH Battery (1600mA 2/3A cells). If your car used individual AA cells in a holder then another option might be to use 6 AA size NiMH cells.

To run the Raspberry Pi from a higher voltage you need a regulator to drop the battery voltage down to 5V. In the RC car world this is called a 'BEC' (Battery Eliminator Circuit). A switching regulator or SBEC 'transforms' the voltage down through an inductor, so it can be very efficient and draw less current from the battery. Here's an example:- HobbyKing™ Micro BEC 2A/5v (Switch Mode).

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  • \$\begingroup\$ So is more recommanded and easy to use some NiMh cells? \$\endgroup\$ Commented Nov 13, 2019 at 9:55
  • \$\begingroup\$ Yes, NiMH cells are safer, easier to use and easier to charge than Lipos. Downsides are lower energy/weight ratio and slower charging (if you use a low cost 'trickle' charger). \$\endgroup\$ Commented Nov 13, 2019 at 10:05
  • \$\begingroup\$ For protected Li-ion or LiPo batteries, I'd disagree that they're less safe than NiMH. \$\endgroup\$
    – forest
    Commented Dec 24, 2019 at 7:56
  • \$\begingroup\$ A protection circuit won't stop a lipo from being punctured or shorting out internally, and when it does there is a good chance of it catching fire. BTW the "Hobby" Lipo batteries used in RC racing cars don't have protection circuits because they would reduce performance. \$\endgroup\$ Commented Dec 24, 2019 at 10:13
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You are WAY underspecified to provide a meaningful answer. To choose a battery, you start with how much current it needs to provide, and for how long. Then you probably look at dimensions and weight.

Raspberry Pi's tend to use a lot of current to do the sorts of things you're probably trying to accomplish, and might not be the best of choices.

All told, you need to provide the info, and state what you consider to be a viable battery life. You might well be looking to solve something not easily solvable.

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  • \$\begingroup\$ I am not that advanced in this electrical part. I don't know how to measure and calculate all the electrical current that I need. But anyways I'm not building a tesla. \$\endgroup\$ Commented Nov 13, 2019 at 9:56
  • \$\begingroup\$ Depending on the type of Pi and what peripherals are attached, it could draw as little as 160mA. Current draw for +/2/3/0 models. That's more than a typical RC receiver, but not unmanageable. \$\endgroup\$ Commented Nov 13, 2019 at 10:13

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