Mindboards

h-g-t wrote: I was charging mine @ 10.1v, disconnected when it started flashing and found the voltage to be 8.05v, is this a full charge?

Seems OK. Nominal voltage is 7.4V, but just after a charge it goes a bit higher.

Also, one site said that the Lego '10v' charger actually produced 10.5v - is this correct?

Mine outputs 10.4V no load, 10.3V loaded at 400mA. This voltage is not critical, the battery will charge with voltage greater than about 9.5V. Charge is controlled by a charging IC which is probably the same as the one used in PF rechargeable battery, a TI BQ24123. Charging current is about 500mA (depends on exact voltage).

I don't have that exact battery pack, but in my experience with other Lego battery packs, the red light goes out completely when the charging is complete. Since it's a LiPo battery, it has a dedicated charging circuit. It should be safe to leave it on the charger for extended periods of time, or even permanently (as I have done on stationary robots that run for days). Next time you charge it, leave it on the charger for several hours after the red light starts flashing, and see if the red light turns off completely.

Each LiPo cell is rated at 3.7v. The NXT battery packs use two of them in series (I assume), so the rated voltage is 7.4v. When fully charged, the voltage should be absolutely no less than 8v, but probably more like 8.25v or 8.5v. I just tested one of my Lego battery packs. It has been charged recently, but it has also been used slightly since. The voltage was 8.25v.

I tested my Lego 10VDC charger, and the voltage was 10.41v. Typically even very sensitive electronics can handle up to 0.5v higher than they are rated (don't ever exceed rated limits). However, most high-power things, such as batteries and battery chargers don't care if the voltage is off by more than 1v (again, don't ever exceed rated limits).

Even though the Lego 10VDC charger is regulated, the cable resistance will cause a slight voltage drop when there is a load. Some manufacturers intentionally bump the output voltage up by .25 or .5 volts to compensate.

To sum it up, charge using a 10 - 10.5 VDC regulated power supply rated for no less than 700ma (which is what the Lego charger is rated for).

Sorry for the duplicate info, Philo beat me by a few seconds.

Thanks for the fast responses.

Never expected to have a battery like this so had no chance of doing any research before it arrived.

It has only cost me about £6 for the dc convertor (great bit of kit) and case - I'm quite happy with it so far!