I've been researching here and on google regarding potential battery technologies for a toy that I want to make for my kids. I was hoping to get others' perspectives on this, keeping in mind that this is for a child.
I'm trying to look at this from all angles, but safety is the most important. Here are the things I have come up with:
- volatility: can the cell explode if mistreated, i.e. charged too long, toy thrown around, etc.
- lifespan: does my son have to keep an eye on the toy to make sure it's always charged?
- size: can I even fit the cell in the toy?
- cost: cheaper is obviously better
Have I missed anything obvious?
As far as those four points go, here is what I have found from my research so far:
- volatility: LiPo sure sounds like you have to be more careful. There are battery packs that have overvoltage and undervoltage protection circuitry built-in, but I would like to see if I can find an off-board circuit that can be built for less money since that's NRE and cells might need to be changed. Battery management ICs like the MCP73831 should help, as well as a fuel gauge like the MAX17043. Not sure if there's anything else I can do. NiMH has similar ICs available, like the DS2715 for charging and the BQ2014NS-D120 fuel gauge. Either technology would probably benefit from a temperature sensor / cutoff of some kind. LiPo looks like it does not like shock, so having the toy thrown onto the pavement might not be a good thing.
- lifespan: LiPo shouldn't be allowed to discharge below a threshold voltage. Neither should NiMH. Need to check if fuel gauge can cut off toy's circuit if below threshold.
- size: LiPo has the huge advantage here. At 3.7V per cell, I only need a 1S LiPo, and they come in all kinds of (small) sizes. NiMH will likely require 3 1/3-AAA cells, which I should still be able to fit.
- cost: LiPo batteries without protection circuitry are super cheap, like $2 in single quantities. The ones I have found with protection circuitry are larger and 4x the price. The NiMH 1/3-AAA cells I found were about the same price. No mention of protection circuitry so I don't know if that's important if I have the battery management IC (same goes for LiPo)
I'd love to hear what others have to say about these points. Did I miss anything really critical, and just as importantly, did I post any bad information about these two battery types?
EDIT -- I have added LiFePO4 as suggested by Russell and AndreKr. I don't necessarily trust myself to design a proper circuit that is bulletproof, so I am looking at the MCP73123 since its current limitations are within the range of the single cell that I want to charge. I saw the Tenergy cells previously, but wasn't sure about them and ended up ordering a few of these from a shop in the US: http://www.batteryspace.com/LiFePO4-Rechargeable-14430-Cell-3.2V-400-mAh-0.4A-Rate-1.28Wh.aspx. I really like how they can be ordered with tabs attached, which is what I did.
So right now I have a LiPo protected cell and MCP73831-based charger coming from Sparkfun so I can play with it, as well as the Powerizer LiFePO4 cell and a sample of the MCP73123 which I'll somehow try to breadboard to test its charging capability.
I'm going to look around, but if anyone knows of good app notes to make a PIC-based LiFePO4 charger that explains constant-current source circuits, I'm all ears! Thank you for your input.