I am updating a lot of my gadgets to use arduino, one of which has an internal nicad charger, with a simple constant current charger, and I use a pair of thermistors in series to detect when the battery pack is charged.
One thermistor is in contact with the side of one of the batteries, and the other is mounted slightly apart from the battery pack.
I can obvlously take the centre tap to an analog input of the micro, and switch off the constant current transistor when the batteries start warming, which is basically what I have been doing with the discrete version.
But I might be missing an opportunity to use the micro as a truly intelligent charger.
I have done a search on the arduino site, but can only come up with a PIC version, and I dont speak the language :-).
Should I rather stick to the simple temperature rise detection ?, or has anyone seen a decent arduino charge project?
Well, I did something a bit like this with a robot project a while back. The obvious thing to do is to sense the battery voltage as it charges by having a potential divider connecting to an Analog input.
I then used a PWM output to control, the charging current. Well when I say control, I mean I just switched it over to trickle when the voltage reached the right value.
Thanks guys, the Maxim route is very expensive here in South Africa, so as there doesn't seem to be an existing Arduino charger around, I have sketched one based on my existing design.
The advantage this has for me is that I can program the chip not to switch the charger back on again when the batteries have cooled down, which used to happen which keeps them topped up, but not needed most times.
I could have it recharge every couple of days if left on charge...
One of the thermistors touches the battery to sense the heat as the charging cycle is finished. There are 2 thermistors to cancel out any temperature changes in the unit. With a red LED and a 1 ohm resistor it will charge at about 750mA, so the 12v supply should be able to handle this, or increase the 1 ohm to 2.2 ohm to reduce the current to about 350mA.
This Reference Design is a battery charger that fully implements the latest technology in battery charger designs. The charger can fast-charge all popular battery types without any hardware modifications.
This application note is based on the ATAVRBC100 Battery Charger reference design and focuses on how to use it to charge Nickel-Metal Hydride (NiMH) batteries. The firmware is written entirely in C language and is easy to port to other AVR microcontrollers.