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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.

Dave
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    As for shock safety, try researching experiences from RC aircraft community. Those little ariplanes and helicopters keep crashing all the time and it's very uncommon to actually get any damage to the battery from the crash itself. – AndrejaKo Sep 30 '12 at 09:56
  • Great idea, I hadn't thought of that, but that's a pretty obvious starting point I guess! – Dave Sep 30 '12 at 14:01
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    Dave - did you build it? – Russell McMahon May 28 '19 at 15:27
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    @RussellMcMahon sure did, thanks again for your suggestion! The Microchip part worked fine for my LFP cell. – Dave Sep 17 '19 at 20:32

2 Answers2

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LiPo is MUCH easier to manage well than NimH.
Energy densities for top capacity NimH are about the same as LiPo nowadays.
(That was written in 2012. In 2021 LiPo energy densities are now typically somewhat higher).

NimH is a relatively hard battery chemistry to manage well. Charging at low rates is not usually advised and negative voltage deflection under charge or temperature rise are the usual end-of-charge detection methods. In contrast, LiPo is charged at constant-current until a set voltage is reached and then at constant-voltage until current falls to a preset level. LiPo will accept any lower-than-maximum rate of charge if desired, and can be recharged from any state-of-charge with no special conditions. (Handling very low voltage cells is slightly more complex, but all sensible charger ICs handle this - and very low voltage should never be allowed to happen.)

The ONLY reason I would think of using NimH in your context is safety - and if it was my son, I'd consider that I could make LiPo safe enough for him to use. LiPo can "melt down" very enthusiastically with flame, BUT it is extremely rare in practice and taking quite usual precautions should allow a safe result. I would have no personal concerns over LiPo safety in a competently engineered system.

HOWEVER, NEVER use unprotected LiPo cells if you care about safety. The in-battery protection IC DOES NOT serve the same roles as the charger ICs do. The in-battery ones are just to stop people from doing stupidly dangerous things to the battery. That said, IF your charger is properly implemented, and if there is no chance of short or fire potential then most of the protection circuitry is not needed. I say "most" because, if there is e.g. a catastrophic equipment failure and e.g. a short circuit occurs, the in-cell circuitry will usually open-circuit the cell and prevent a fire.

Using the proper charger ICs should allow a very safe and reliable charger to be implemented.

You do not need gas gauging per se - just low voltage cut-out. If you can stop operation at say 3V / cell, that should be enough.

Protected cells should not cost vastly more. If they do, it MAY indicate that the cheap ones are bad ones. You can get utter junk LiIon batteries (and you'd hope to get a price advantage when buying junk :-) - if you were silly enough to buy them. There are enough reputable brand cells around that buying them probably does not cost vastly more. Ensuring that the cells are genuine is another matter. As a working position I suggest you start by assuming that anything bought from a low cost Chinese supplier is fake or out of spec and THEN try and prove otherwise. (NB: Racism? - definitely not!. It's based on experience - many visits to China and time in factories, etc. China is very, very large and has a vast range of sellers in a very competitive market place. In a casual sale, expect a certain portion of the sellers to be 'dodgy' at best.)

Added:

I was going to come back and mention LiFePO4 - AndreKr beat me to it.

Compared to LiPo, LiFePO4 (Lithium Ferro Phosphate) are safer, longer life and have lower energy density. You can buy RCR123A LiFePO4 batteries with 450 mAh x 3.2V capacity. (Some claim up to about 700 mAh but are suspect.) Tenergy LiFePO4 RC123A are widely advertised on ebay and should be good. Tenergy are AFAIK a "rebadger" BUT seem to sell good product. LiFePO4 MUST be charged properly, but are as easy as LiPo to manage. A very simple charger can be built using a constant-current regulator followed by a 3.6V constant-voltage regulator. This setup charges at constant current until Vlimit is reached, and then at constant V. Setting to 3.5V is better.

Here is a randomly found seller of Tenergy LiFePO4 RCR123A batteries. They also sell chargers. NOTE:
Do NOT use Lithium Ion RC123 (3.6V nominal).
Do not use 3.0V Lithium Primary RC123.

The terms RC123, RC123A, RCR123, RCR123A etc are used somewhat interchangeably by sellers. Just be sure of what you are getting.

Russell McMahon
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  • Russell, thank you for the detailed answer, especially since it focuses on safety, which is a huge concern for me. I'll try to evaluate both battery types. I'm not nearly as concerned about cell longevity as I am unexpected fires, so there's still an advantage for NiMH. Can you recommend any particular LiPo battery management IC, or is the one I listed adequate? And as far as LiPo brands go, I totally agree. I am very suspicious of Chinese brands, but Turnigy has great reviews on the RC sites. Is $1.50 for a single cell alarmingly inexpensive to you? – Dave Sep 30 '12 at 14:06
  • Here's the cell I am looking at -- it's a Turnigy nano-tech 650mAh cell, for about $2: http://www.hobbyking.com/hobbyking/store/__23317__Turnigy_nano_tech_650mAh_15c_Round_Lipo.html – Dave Sep 30 '12 at 14:07
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    LiFePo batteries are similarly managed as LiPo and are said to be safer. – AndreKR Sep 30 '12 at 14:10
  • @AndreKR thanks, I had forgotten about those and had read about them briefly before. Thank you for the suggestion, I'm going to add LiFePO4 to my list and will research it some more. – Dave Sep 30 '12 at 15:47
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I've used AA sized 14500 LiFePO4 ("LFP") to great effect in recent months while on the road, & found them near bullet proof for such devices as instruments,shavers,flashlights,Canon digital cameras & emergency cell phone chargers. Their only concern is keeping a clear head when using place holding "dummy" cells, else several LFP could be innocently installed & thus oversupply the device! You might be alerted to this in a hyperactive battery shaver, but -YIKES-imagine 2 x 3.2V LFP AAs getting up to mischief in a camera that expected 2 x 1.5V alkaline cells...

Although such AA devices as digital cameras stop working anyway at lower voltages, a switched paralleled white LED's brightness matches LFP voltage level beautifully -cease using the device & recharge the LFP when the LED dims (~2.7V). A ~US$7 imported USB smart charger has been ideal -it's hardly worth making your own at such bargain prices. Check my Instructable => http://www.instructables.com/id/Single-AA-LiFePo4-cell-powered-project-in-a-parti/

Manuka
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