# How to find the right battery for my project

I am working on a project that requires 12v 12A of power. But I can't find the right battery. I have been looking at a few online tutorials about choosing batteries but they have not been very helpful. The battery I am requiring is 12v 12A DC. and it needs to be rechargeable. It also needs to be less than 2.5 inches tall, since there is not much clearance space in my robot. Any help I can get would be greatly appreciated.

• First off, a "rechargeable" battery is called a "secondary" battery. See this discussion from the University of Washington. So that's one keyword. You should then also study pages written by robot enthusiasts about their battery choices. I'm sure a lot of good material is out there. For example, this tutorial focuses on the topic and may provide a segue into better questions/thoughts. – jonk Aug 7 at 22:32
• There seem to be some missing requirements here. You want 12V 12A, but haven't said how long for. You have specified a maximum height of the battery pack, but not the maximum length or width. – Simon B Aug 7 at 22:57
• please do not post at multiple sites robotics.stackexchange.com/questions/21042/… – jsotola Aug 7 at 23:32

There are only a limited number of battery chemistries and each one has its own voltage so there are only a limited number of voltages. Also, battery voltage drops as they get discharged so you won't find exactly 12V. Search radio-controlled hobby websites.

The most likely chemistries available to you, in order of increasing cost, increasing performance, decreasing weight, and decreased tolerance to electrical and mechanical for abuse, are:

• NiMh
• Lithium-Polymer or Lithium Iron Phosphate (or the other lithium variants, just not lithium-ion...too low power to move a robot)

You will need a specialized charger for the better types of batteries such as Lithium Polymer and its variants such as Lithium Iron Phosphate since they real fire hazards (due to their high energy density) if mishandled, both electrical and physically.

A battery's energy capacity is rated in Amp-hours (Ah).

A battery's power capability is a C-rating and tells you how much current the battery can supply relative to its Amp-hour rating.

7Ah means (theoretically) it can provide 7A for one hour before it is dead. Drawing 7A out of a 7Ah battery is a discharge of 1C, and it should run for 1 hour.

If that same 7Ah battery is rated for 3C, then that means it can supply a maximum of 21 Amps. Drawing 3C worth of current from that battery will runs it down three times faster, so it will only run 1/3 of an hour.

So the fact you say you need 12A only tells us the power you need the battery to supply, not how long it needs to run for.

$$Max.Amps = C.rating \times Amp.hours$$ $$Run.Hours = \frac{1}{discharge.C.value}$$ $$discharge.C.value = \frac{Discharge.Current}{Amp.hours}$$

• @X builder note the very important distinction between "Power" (how much energy your battery needs to supply every second) and "Capacity" (the total amount of energy your charged battery can supply until it is fully discharged). – BeB00 Aug 7 at 22:42

12V 12A or 144VA is about the storage capacity of 3 lead acid car batteries for 1 hour!

Lithium secondary batteries must consider the cost to achieve this performance of expect range Volumetric energy density: 250 to 680 Wh/L

Consider that batteries are rated for 20h capacity and Peukert's Law dictates that will decrease with reduced discharge times.

As with any moving device, the increasing weight of batteries diminishes the available energy left above moving the weight of the batteries. Thus choosing your energy demand and Volumetric and Specific power density: 300 to 1500 W/kg requires careful Energy or power/weight planning to meet your requirements.

• Yeah, I am assuming this is a ground robot and that 12A is peak current draw which won't be pulled the entire time. You can get a pretty big robot rolling at walking speed on one or two dozen watts and have it running for quite a long time even though the peak power/current might need to be higher. – DKNguyen Aug 7 at 22:54

A lithium booster pack for automotive use could give you enough power. Many only have a protected output, but some have a switch for full 12 volt unrestricted output. In my case, with a smaller unit, I added a 20 amp inline fuse and an output connector. I use it to provide test power to security cameras while troubleshooting. My unit is rated at 7000 joules. It is 2 inches by 4 by 8.