# Battery pack solution for a 5V 500mA project

My project contains:

• Raspberry Pi Zero W
• Arduino ProMicro
• 10 5V RGB LEDs
• I2S amp that drives a 4Ω 3W speaker

Currently I am powering it from a 10000mAh USB power bank. But in order to avoid brutal powerdown, I need a "smart" battery solution where the battery level and charging/discharging status could be monitored and user informed about it.

My current measurements show that at rest it consumes 220mA, max current is about 800mA, and normal use current is about 500mA.

I'd like it to be able to live from full charge something around 24h. During that time, it would be actively used for about 2h, while the rest of the time it would be in "resting" mode, consuming no more than 220mA. So, I figure I need 2*500+22*220=5840mAh (at 5V) battery solution.

I have checked for the necessary circuitry needed to monitor the battery charging (Sparkfun Battery Babysitter - LiPo Battery Manager) and a voltage regulator. But I am puzzled about the Li-Po element - for a 3.7V Li-Po output voltage, stepped-up to 5V that would mean I need 5840/3.7*5=7892mAh (if my calculation is correct).

So, do 8000mAh Li-Po elements exist? If they do, it seems they are extremely rare.

• Although not answering the question, it is possible that you could greatly reduce that 220 mA idle consumption - perhaps something on Raspberry Pi Zero - Conserve power and reduce draw to 80mA would help. Aug 13, 2019 at 20:34
• @AndrewMorton Those 220 mA are from Pi Zero, Arduino ProMicro and idling I2S amp together. Aug 13, 2019 at 20:39
• I just thought that any easy reductions in the idle power draw would be a good thing for battery life and/or size. Aug 14, 2019 at 7:41

So, do 8000mAh Li-Po elements exist? If they do, it seems they are extremely rare.

They're not rare. They're just expensive. Batteries that size are used fairly frequently by RC model airplanes and if a cell of a certain size does not exist then they put them in parallel. As mentioned, larger mAh come with increased safety issues. The chargers to charge such large batteries in a reasonable amount of time are also quite expensive, and such chargers also tend to need a separate high current AC-DC power supply which further increases cost.

Batteries with that capacity are used to fly not-exactly-small airplanes and helicopters which would suggest your went wrong somewhere in requiring such battery capacity for a much less intensive application. You don't even need this comparison to know something is wrong. Your system is idling 92% of the time but that is consuming 80% of all your energy. Your "resting mode" current is a whopping 220mA. That is far too high.

What is consuming all that current when your system is supposed to just be sitting there doing nothing? Deal with that and your battery will be 1/5th the size. Don't let things idle with power for no reason. Put things into sleep mode after a period of inactivity or power things down with MOS switches until there is a reason to power them up.

Stepping down is also easier than stepping up, so I would suggest you go for a two cells in series to get 7.4V and step that down to 5V.

• How would two cells in series be charged? The Sparkfun battery monitor docs explicitly warn it is designed to charge one cell. Perhaps some kind of switching circuit to loop through several batteries? Aug 13, 2019 at 21:32
• @Passiday You would have to use another charger if you take the route I guess. With two series cells does come the complication of balancing cells which may outweigh the stepping down. You can't loop through the batteries as you suggest easily unless things are isolated which they aren't. Aug 13, 2019 at 21:33
• With all this trouble, I keep wondering what kind of magic the (not that expensive) 10000 mAh USB power bank does in order to supply those 5V (well, more likely 4.8V) for quite a decent time (must measure yet how long). Aug 13, 2019 at 21:36
• @Passiday The magic of mass production to offset NRE costs. Also the willingness to wait 12 hours for the thing to charge. Aug 13, 2019 at 21:39

Larger Lithium Polymer cells can present larger safety problems (bigger faster fires if they fail). That's one possible reason for their scarcity on the retail market.

An alternative (popular for amateur radio use), is to use Lithium Phosphate batteries, such as:

https://www.bioennopower.com/collections/9-6v-series-lfp-batteries/products/copy-of-6v-12ah-lfp-battery-abs-sealed