Raspberry Pi 5 battery

Hi @jeffreyjene,
   I do not have an R-Pi 5, nor most of the other load components you mention, so all I can offer is a bit of thought speculation, mainly based on a bit of web digging I did when R-Pi 5 first came out. Hence, please regard my comments as 'things to check up on and consider', rather than hard facts. Also, my memory isn't always 100% reliable. So, I am hoping someone else, with 'real' experience will jump in, but this is the best I can offer ... apologies in advance for any errors.

R-Pi 5 specifies 5A supply, albeit that includes an allowance of around 1.5A for peripherals. The actual consumption of the R-Pi 5 itself depends on what software it is running, as well as what is connected to it, so the actual current demand predictions are rather vague, but it is normally sensible to provide more current than you expect the system to demand, as failing to provide enough typically results in a system that crashes at every opportunity, usually prioritising times when it is most embarrassing. Hence, I would recommend allowing at least 5A supply, as Raspberry Pi suggest, even though it may work with a bit less.

Using a buck regulator, to reduce the voltage from about 7.4V to 5V is sensible, but the above recommendation means it should have a current capability of at least 5A. Looking at the TI LM2596 data sheet

https://www.ti.com/lit/ds/symlink/lm2596.pdf

shows the 'headline' capability of the device is only 3A. 

Furthermore, I am guessing you have a 'low cost' LM2596 module, that probably originated from China. Whilst such modules often offer great value, in my limited experience, I think it is wise to regard their 'real' current rating as somewhat lower than the 'advertised'

Hence for your 5A requirement, I would probably look for a buck regulator with at least 7.5A output 'advertised' capability, in the hope of getting a reliable service. In general, the electronics components (from every source) that exhibit a high failure rate, are those that are run near their limits.

Your choice of battery cell, is also worthy of a comment, with reference to current demand they will meet. From your description, the cells must supply the buck regulator for the R-Pi 5, plus the motors. The motor current will depend upon both the motor design, and mechanical load on the motor, so I recommend you attempt to measure the actual current demand, in as close to the final system configuration as possible. As for the buck regulator, you will be aware:
      Power Output = Power Input * Efficiency      or more usefully:   Power Input = Power Output / Efficiency 

The TI data sheet for the LM2596 shows efficiency graphs for different voltages, no doubt using the highest quality inductors, etc. to give the most favourable impression. Although, I do not recommend the LM2596 for your application, I'll 'borrow' the efficiency value of 80%, as an example for this discussion.

Then for 5V 5A output, Power Output = Current * Voltage = 5 * 5 = 25 Watts

Hence, Power Input maybe estimated to be Power Output (25W) / Efficiency (80%) = 31.25 W

Battery voltage will fall as it discharges, so taking a value of 7V

Battery current = Power Input to regulator/ Battery Voltage = 31.25 / 7 = 4.46 A

The total motor current should be added to this figure. e.g. if each motor takes 1.2A, then 

Total current = 4.46 + (2 * 1.2) = 6.86 A

So may expect battery consumption to be 7A 

The current capability of a cell is usually quoted as xC, where x is a number like 0.5, or  1, or 2, or 30, which can vary widely, depending on the internal design.

Thus "1C" discharge capability is linked to the battery capacity ... say a 3000 mAhr (which is 3AHr) battery, has a discharge capability of 3A. A "2C" 3AHr cell could supply 6A (for about 30 minutes) and so on. Some cells are clearly designed for higher discharge rates, but I have the impression many 18650s are around 1C rating.

I don't know what the "C" rating of your cells is, and unfortunately they are often sold without the value being obvious, but clearly, if your system is going to demand 7A, then you would need cells with rating of at least 7/3 = 2.3C.

(Please remember, I have no idea how much current your motors will take - the value I used was imaginary. Make your own measurement/estimate and redo the above calculation with your values.)

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My final comment relates to the R-Pi 5, itself. My understanding is that its software has been set up to expect a 5A USB supply, with a Power Delivery handshake on boot, to indicate the presence of a 5A power supply. I am not completely clear of the consequence of that handshake being omitted, but I vaguely recall that it may show warnings and/or limit the current available for peripherals. I think there is a software change to deal with this situation. Sorry, I don't know the accurate details, and only mention it as a subject that you may wish to research further.

Good luck  and best wishes, Dave

@davee Thanks a lot, that does help! The 18650s do well for my size robot. I ordered some high end 3500mAh batteries and I'll see if those work out. The 3000mAh batteries I have now test out ok (with the occasional glitch). Hopefully this will work out. the real challenge later will be powering the Jetson Orin for a different design I'm doing.

Hi @jeffreyjene,

  Thanks for the kind reply. I confess to having little real experience about how Li-ion cells are specifically designed and manufactured for a discharge (and similarly charge) rate of (say) 1C, compared with one specified to cope with 20C, which I briefly discussed. However, I note that some cells on sale do provide such information, occasionally also adding hints like 'not suitable for power tools'. Note that '18650' only specifies the approximate physical size of the can ... 18mm diameter, 65mm high, and even those figures vary quite a bit. The same size can is used for cells with widely different maximum charge and discharge rates, depending on the internal structure and chemistry details.

Note that the 'more reputable' sources quote both a charge and a discharge rate. The two values are commonly different.

Also note that the 'xC' charge and discharge rates only apply when the cell is within a certain 'middle' temperature range, and within a certain 'middle' charge level range.  At low or high charge levels, and when cold or hot, the allowable current flow levels can be much lower. I recommend you do your own research to put specific values to the conditions that require reduced current flows.

Li-ion cells can turn very nasty when provoked, including producing lethally toxic gases, as well as fire and explosion risks. Hence, I recommend a cautious approach.

So please take care and good luck with your interesting project. Best wishes, Dave