[actofgod]

For those wondering about the charging voltage and charge time, see Namuna's post on the first page. I'll recap here:

1) Try to charge at the same voltage as the battery pack, but since they don't sell AC-to-DC adapters (that I've seen) at those voltages, go to the next step up. Ex: 2.4v dual cell = 3.0v charger; 3.6v tri cell = 4.5v charger. If you want to charge both types, get an adjustable charger.

2) Each adapter has a different current that it delivers, in mA. To figure out how long (in hours) you need to charge with a given current, divide the capacity of the batteries (40mAh) by the charge rate in mA. (ChilledOut says that rechargeable AA's put out about 1000mA if you're using them for a custom AA charger instead of using an AC-to-DC adapter.) Multiply by 1.2 to account for inefficiencies in the charging process, then multiply by 60 to convert to minutes. Finally, multiply whatever follows the decimal point by 60 to get the seconds.

Ex: using 300mAh charger, 40mAh / 300 = 0.13333
0.13333 * 1.2 = 0.16
0.16 * 60 = 9.6 minutes
0.6 (the part after the decimal in 9.6) * 60 = 36
final answer: 9 minutes, 36 seconds

Ex: using a 600mAh charger, 40mAh / 600 = 0.06667
0.06667 * 1.2 = 0.08
0.08 * 60 = 4.8 minutes
0.8 (the part after the decimal in 4.8) * 60 = 48
final answer: 4 minutes, 48 seconds

3) Once you have figured out how long you need to charge to reach *peak* capacity, keep in mind that that is the theoretically absolute MAX time you could charge. I recommend playing it safe and cutting it off a little before. It will decrease the lifespan of your batteries if you overcharge them. Each time they are overcharged, they lose some of their max capacity. Don't overcharge! Go back and do the calculations above again, and this time don't multiply by 1.2 in the second step. That's a guideline "safe" charge time limit that I use, but you'll have to decide for yourself how close to the limit you want to get.

Why does charging take so long? I did a little research on NIMH battery charging; normal, "overnight" charging is charged at "C/10", or the capacity of the battery in mAh ("C") divided by 10. Rapid, 1-hr charging is done at a charge rate of "C". For a 1500mAh AA battery, you'd charge at 150mAh for overnight charge and 1500mAh for rapid charge. They recommend charging at the high rate only until the battery is about 80% full and then "topping off" at a lower charge rate.

These tiny button batteries are 40mAh, so they'd be slow-charged at 4mA (!) and rapid charged (1-hr) at 40mA. We're super-rapid charging here, well beyond the recommendations. A 500mA charger charges at 12.5 times the recommended maximum charge rate.

Maybe they will handle this high rate, but the higher the rate you use, the harder it is on them. If you're willing to wait longer at a lower charge rate, you might be better off in terms of battery life. On the other hand, these batteries are pretty cheap if you buy several packs at a time when you order. It's up to you to decide what charge rate you will use. I'm going to use 300mA for now.

Your runtimes will vary depending on which motor you use. Tinyrc used a 1.0 motor to get the 45 minute results with a stock battery, and I'm not sure if that was driving around with steering or just running the motor while sitting in its back.

Troubleshooting:

1) Make sure the batteries are oriented correctly. The positive end is clearly marked with a + sign. That narrow end should face where the button tip of the old battery went, towards the side with the red wire. All batteries should face the same direction.

2) Make sure the batteries aren't contacting something on the PCB. If you have snipped the batteries apart and have a little space where the metal is exposed through the shrink wrap, turn it so that the metal faces sideways, away from the PCB. You can also try using clear tape on the bottom side of the PCB as an insulator. This may be your problem if it works with the PCB lifted up but does not work, or works sporadically, with everything put back together.

3) Charge at a voltage higher than or equal to the combined battery pack. If using AA's (not recommended), you'll probably need one more battery than the number of cells in the car, since they lose some voltage over time. I've had the most luck charging at 3.0v for dual cell and 4.5v for tri cell.

4) Test the car while on the charger to make sure it is getting a good connection. If it is not working while on the charger, it may or may not be charging the batteries, or your car may have a loose wire. Check the battery contact points to make sure they are all touching. Revert to an uncharged stock battery and test.

5) That's about all I can think of at the moment. If you're still having trouble, post details about what you're doing and what you've tried and maybe someone can help.