USB/DC/Solar Lipo Charger w/ 4x NiMH Eneloop AAs?

[photocyte]

I'm interested in making an outdoor solar powered data logger. The power requirements shouldn't be too high, so I was thinking of using Sanyo NiMH rechargeable batteries instead of Lithium Ion (Safer, more robust in different temperatures etc.). Here is where I betray my naivety when it comes to battery charging... Would it be possible to use the Adafruit "USB / DC / Solar Lithium Ion/Polymer charger" with 4 Sanyo eneloop rechargable AAs in series (1.2V nominal voltage so ~4.8V if 4 are in series). I figured that the setpoint of the Adafruit charger (~4.5 volts according to the IC datasheet and design docs?) would be close enough to work alright. My circuit is 3.3V running off a linear regulator so 4.5V should be more than enough to supply power. Is my thinking correct? Or is there something complex about different battery technologies and charging that I'm not getting. Thanks for any thoughts people can give!

[adafruit_support_bill]

No. The charging requirements for NiMH and LiPo are completely different. You need to use a charger matched to the battery type.

[photocyte]

Thanks for the quick response. But indeed, now that I'm looking at the Eneloop datasheet, a 4.2v charging termination point is quite a bit too low for 4x AA's (Only 1.05V per cell out of a ~1.5V maximum). Using the Adafruit charger with 4x Eneloops in series wouldn't exactly utilize the capacity of the Eneloops. Although, if I were to use 3x Eneloops in series, that sets the charge termination point at 4.2/3 = 1.4V per cell which is pretty close to optimal... Reading further, unfortunately the Eneloop datasheet indicates that at low temperatures (0ºC) 1.5V is actually the cell voltage of the discharged state, meaning that the CC/CV scheme set at 4.5 would really break down for NiMH Eneloops at various temperature. Too bad! But a good excuse to learn more about batteries! Has anyone come across a VPCC charger that implements a NiMH charge termination scheme (-dV/dt?) instead?

You say the charging requirements for NiMH and Li-Ion are totally different. I'm assuming you're referring to the charge termination scheme (-dV/dt for NiMH etc. versus CC/CV for Li-ion) and not the charging rate. If the charging rate is too low (Set by what the panel can give at a reasonable voltage), then maybe NiMH batteries wouldn't be suitable for a solar powered project? Here's a worrying excerpt from batteryuniversity.com: "It is difficult, if not impossible, to slow-charge a NiMH battery. At a C‑rate of 0.1 to 0.3C, the voltage and temperature profiles fail to exhibit defined characteristics to measure the full-charge state accurately and the charger must depend on a timer". At the same the Adafruit battery tutorial says: "[NiMH batteries] like to be charged at about 0.1C but can be discharged at 0.2C". The Eneloop datasheet has them charging at about 1.0C. Little bit convoluted, appreciative of any insight into this...

NiMH resources for people coming across this:
http://batteryuniversity.com/learn/article/charging_nickel_metal_hydride
http://www.ladyada.net/library/batteries.html
http://www.rcgroups.com/forums/showthread.php?t=330459
http://www.rcgroups.com/forums/showthread.php?t=527385

[adafruit_support_bill]

You say the charging requirements for NiMH and Li-Ion are totally different. I'm assuming you're referring to the charge termination scheme

Preconditioning phase, temperature monitoring, charge termination all need to be matched to the cell chemistry. Lithium cells in general tend to have more critical requirements - due to their tendency to burst into flame when mistreated.

[photocyte]

Sounds like I should just stick to a Li-ion battery + the Adafruit charger Looking at the datasheet it seems like the Li-ions are relatively robust temperature wise. Thanks for the feedback