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Sunday, 29 July 2018

Paddock battery charging.

Paddock battery charging. That is charging up our racing car battery whilst it's stood in the paddock awaiting it's next run. Both Julian (with the Mini) and Matt (with the Chevette) have expressed an interest.... right...

So we've started the car, ticked over in the cue lane for a few minutes, done our timed run, and driven back to the paddock. We can now charge up the battery waiting for our next run.

The car is fitted with an acid mat battery, and we have a spare, normal lead-acid battery to charge from in the paddock... great, just connect the two together right? Well, no. The fully charged voltage of a lead-acid battery is 12.6 V (at 20 degrees C) ... the best we can hope for is the two batteries, once connected, is to equalise, and it won't happen quickly. So we need to raise the voltage of the paddock battery to 14.4 V, which will charge the car battery without causing it to gas. We'll also limit the current to 5 amps or so.

Now, we could build ourselves a small boost circuit to do this for us, but thankfully, we can purchase a module from eBay for very little money.

It has adjustable voltage output and adjustable current limiting, and rated at 90W.

I'd also like to be able to measure the output current and voltage, and the input voltage...

Voltages are easy to monitor. I don't want the paddock battery to become discharged below 11.6V to avoid it becoming damaged.

Output could be measured by looking at the voltage drop across a small resistor in the negative lead to the battery under charge (in fact, that's exactly how  the constant current mode on the boost module is done), however, I'll use another module....

....enter the ACS712 current motoring IC.
It measures the current by looking at the magnetic field created around a heavy piece of wire which is moulded into the IC. There are 3 versions of the IC, a 5A version, a 20A version and a 30A version. I have chosen the 20A version, so I've got a bit of headroom.

There are two relay modules, which are used to switch the charger on, and the other to enter "bypass mode", whereby if the battery to be charged requires more than 5A at the minimum voltage, the booster will be bypassed, until the charge current falls below 5A, at which point the booster can safely take over.

This is the monitoring circuit. Power is supplied to the ATMEGA328P and accessories via a 7805 regulator.  The uC reads the voltages of input and output using ADC 2 & 3 respectively, via dividers formed by R2 & R3, and R4 & R5. Current is read (as a voltage of  100mV / A) by the ACS712 connected to ADC0. A relay output is provided, which will connect the output from the boost converter once the micro is up and running, and the battery to be charged is connected. If the current exceeds 5A, the boost converter is bypassed, so as to protect it until current is stable. Data is displayed on a 20x4 LCD, connected via an I2C interface. In the event of the input voltage falling below 11.6V, the beeper will sound to alert the user the input battery is becoming seriously discharged. I could stop charging at this point, using the relay, but it's more important that the car battery is charged than the input battery being flat (although at 11.6V, the input battery is entering deep discharge, and permanent damage/loss of capacity may be being caused)

A small PCB is etched ... 

Here's a great website for creating a 3D view. It only works with small boards (unless you want to pay for it) ... but's its a fun thing... just drag and drop your .brd file from eagle into it! You can zoom around and look at any angle, even from below ... Website can be found at 3dbrdviewer.com. (Sadly, since writing this, the site has closed.)

It's a pity it doesn't look quite so good in real life ;)

And the unit is assembled and tested...

Shown here charging a small SLA.

Code (and eagle files) can be found on my github page https://github.com/andydoswell/paddock-charger

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