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Bought a s/h NXT from ebay and it came with a nice, white battery pack, but no charger.

Now 10V chargers at a reasonable price are rare as hens teeth, so I wondered why a 7.4V battery needed 10V to charge it.

Has anyone tried a standard 9V DC power source with one of these?

The battery pack has a built-in charger (the power from the supply doesn't go directly to the cells). There is likely some voltage overhead required for the charger, not to mention that usually you need to supply a higher voltage than the cell voltage anyhow.

I don't think a 9v DC supply would hurt it, but I won't guarantee that it will work.

Ah, never realised that! Thanks Matt.

Seen a 10 v charger for the Power Function batteries, might try that.

Edited

Might be cheaper to get a 10V Zener diode and use that in conjunction with a 12 supply.

Edited again!

According to Eurobricks,com, you can use any DC supply up to 20v, one member regularly charges his with a 18v supply.

I tried, 9V is definitely too low to start charging - the bare minimum value is about 9.5V. PF and NXT batteries seem to share the same charging circuitery, a switcher (TI BQ24123) which is 5W power limited (it charges at about 500mA under 10V).
I wouldn't go much above the recommended 10V. Though the absolute maximum rating of BQ24123 is 20V, we now nothing of the surrounding circuitery, and especially voltage rating of input capacitor. 12V is probably safe, but to make sure I used 3 silicon diodes (1N400x) in series to drop voltage to around 10V.

Thanks Philo.

There is a 10v Power Functions charger on ebay at a reasonable price, but I can't see whether it is for the UK or USA voltages.

Still have a few 1N4001s left so, if all else fails, I'll buy a 12 v unit and use your trick.

AFAIK it can be used on all voltages. Only the mains plug should be different.

I have a variety of chargers from appliances which are no longer extant so bought one of these convertors so that I can use any of the higher voltage models (or a car battery/ power pack).

http://www.ebay.co.uk/itm/DC-4-5-24V-To ... OC:GB:3160



One model had three separate outputs with a total of 2 amps, so presumably you could charge 3 batteries at the same time.

Found an old 'Sagem' wallwart with dc output in my bits'nbobs box.

Said it was 13v, so was tempted to plug it straight into the NXT battery.

Tested the output - 19V!

Connected it into my dc convertor from ebay and set the output to 10.1v.

Battery seems to be charging ok; red & green lights, no smoke or overheating.

The LED display on this device is very handy, saves messing about with meters (yes, I did check it with a meter to start with just in case).

19 volts (no load) on a transformer rated for 13v is expected. Most wall power supplies (other than the new switching ones) are non-regulated. Usually it's just a step-down transformer, and then a bridge rectifier and a capacitor. Usually it will say something like "13VDC 700ma", and in my experience, it seems that usually the rated voltage is when you are pulling the rated current (roughly). With no load, 19VDC is probably about right.

A lot of newer power supplies are switching power supplies. The Lego 10VDC charger is, as are pretty much all modern phone chargers (i.e. USB power supplies). A switching power supply can usually accept a huge range of input voltages (e.g. 110 - 240 VAC), and then regulate the output to within 100mV of the rated voltage, regardless of the load (as long as it isn't more than the rated current).

Been searching for info on charging the newer (white) NXT battery.

I've seen statements that -
the red light goes out when the battery is charged
red light starts flashing when it is nearly charged then goes out when the charging is complete
the red light starts flashing when fully-charged.

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

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

Any advice would be appreciated.