Hello, I've been working on a hexapod project for some time now and have run into a bit of a road block. Below is a power schematic of the robot, just showing the general flow of power throughout the robot. My problem is I'm relatively new to understanding batteries and exactly how they work (how many Amps they can actually discharge, how long they can discharge for, ect.). For this robot specifically, I'm pretty sure I want a LI-PO battery as I want the robot to be rechargeable (If you have other suggestions that would be great too). I know it needs to be 12V to power my camera and its transmitter, and I also know I'll need to have a pretty beefy DC-DC buck converter as the robot could require around 45A at 5V if all servos are running at once. Because of my high ampage needs I imagine I'll need two 12V batteries in parallel to get the total amps needed.
All of that being said, looking at the diagram I've provided, what would be the best batteries to use for this project? I've found some 12V 2400mA batteries on Amazon but cant find a C rating on them so I'm unsure how many amps they can actually give me. Any advice on the DC-DC buck converter would also be greatly appreciated.
The C rating is right there, 2400mAh or 2.4A. Run time at best with those batteries is 24Ah / 45A. If they are LiPO then you need a balance charger (cheap = fires). Here is what the RC model guys use, anything less is a crap shoot. Web site is www.skyrc.com. Pick the charger that matches the battery type. There are chargers for cylindrical as well as LiPO packs that require balanceing. Have a look at the B6Neo for starters, and either go up or down a model depending on your needs.
Unless you are using lead acid, don't even think about putting batteries in parallel, it' a recipe for disaster.
Many of those 12V batteries you see on Amazon are just 3sNp battery packs. The voltage will vary from 9v to 12.6 with a nominal of 11.1V
You might consider a distributed power system with 2 or more battery packs. Maybe one 11.1V pack for the lower current 12V devices and two 7.4V packs with two bucks for the servos.
Could this be a good battery pack to use? I see your point about the two battery packs, but I'd like to try and keep it at one if I can to make recharging the robot easier. This battery, if my understanding is correct, is pretty overkill for my project as it can pull almost 500 A? As far as I know, the total ampage the battery can pull is (mAh given * C). If that's the case, then would this DC-DC buck converter work to drop the voltage to 5V for the rest of the system?
For maybe a few seconds, followed by a rest period. You need to examine the datasheet.
Battery capacity = 5A x 11.1 = 55.5Wh (Watthours)
If your system draws only half of your estimate 45A, you will be consuming 22.5A x 5V = 113W
The battery will last: 55.5Wh/113W = .49hours or 29.4 minutes
Ok THANK YOU. This is what I missing in terms of what to look for in a battery.
So, I did the same math you did but assumed a system draw of 30A average and decided on 2 hours for how long I wanted my battery to last (I know this isn't actually how long the battery will last, but its a good baseline). I got that my 11.1V battery would need to have a rating of 27027mAh which is obviously more than one battery will be able to handle at the size I need them. Earlier you said that putting LiPo batteries in parallel was a bad idea, what's the reason behind that?
Visit an e-bike shop and ask if they are discarding a weak battery that put out 36/48/52 volts, but might only have 7Ah capacity out of 10Ah to 20Ah. Some places will just dispose of the battery... I use one for a LED display that lasts two weeks, lighting all day and night.
Batteries have internal resistances that are usually very small but vary from battery to battery. That will cause them to charge and discharge at different rates. Also two batteries will never have exactly the same charge.
If the resistances and charges were the same, putting them in parallel would be OK.
However, since they are different current will flow from one battery into the other and that is not good, especially with high discharge rate batteries. They could overheat, explode, even catch on fire.
Alright, thank you for all the help. I'll keep looking around for a battery that will work best with this project now that you've taught me what to look for!
