For my electric vehicle I currently use a battery management system where each module uses an Atmel Tiny13 chip to manage the charge and shunt (the chip looks like a triple5 and has a simple C program for managing the voltages).
If you have a similar system, you may experience charging issues, i.e., plugging your electric vehicle into the wall socket and switching on does not engage the charger.
This simply means the daisy chain arrangement is broken. The chips work fine, however the master system supplied by BEV (Blade Electric Vehicles) does not have a reset feature (unlike some later systems provided by other suppliers). Consequently, when a module drops from the gestalt for whatever reason (perhaps from oxide buildup on battery terminals), the unit needs to be reset. Since the master control box is sealed and difficult to get access to, this involves physically removing the offending battery board, disconnecting the fly-leads and re-attaching. Of course a connection may be so bad that physical removal is the only way to fix the problem as a reset will not have any effect.
In my case, it’s not a big deal since I soldered the charge LEDs ‘the wrong way around’ (but I love the serendipitous Christmas tree effect from my lithium battery pack). For my pack, when charging completes for each battery the pilot LED shunts off and comes back on when the battery voltage settles.
From the following video, you can see that it’s easy to pick out the tripped module…
Those I know who have the same BMS have wired LEDs to switch on only when charged and this would be the expected way to do it. Luckily for me however, it’s easy to locate the offending BMS board straight away by its lack of illumination.
Just another story for anyone experiencing a similar situation, to ensure all connections be tight and dry. I may upgrade the system when funds permit; EVWorks have some neat compact battery management designs for Thundersky batteries that are sealed and use compact SMD (surface mount) components. Their master controllers also have reset switches. Despite this, if a connection is not making it:
- Ensure battery terminals are clear of oxide buildup. If you remove a board, or battery for that matter, clean the terminals (CRC spray) and/or use a fine wire brush.
- Ensure BMS boards are tightened to within manufacturer’s tolerance, not too tight, not too loose. Your battery supplier will recommend the correct number of turns, or even Newtons if they want to get picky. While many DIY EV builders won’t have a high quality torque wrench, it is not super critical to ‘count Newtons’, though I know a couple of people who do :) *.
- Always use spring washers so nothing comes loose.
- NEVER let lithium ion battery voltages drop below 33% safe level.
Putting the minor issue of BMS tripping aside, my car still proves to be way more reliable and economical than the gas guzzling equivalent sitting next to it in my garage.
*For those not familiar with newtons, it’s a SI measurement of net ‘force’, the amount needed to accelerate a mass of one kilogram at a rate of one meter per second: F=ma, multiplying m (kg) by a (m/s2), the dimension for 1 newton unit is therefore: 1N=1(kg*m)/(s^2). Fun, eh?
{ 3 comments… read them below or add one }
BMS should be built into the batteries, and customized for each cell/ah, it would save us all a lot of headache sourcing the correct management for the type of batterys we have. its crazy that thundersky don’t sell management, just stupid monitoring
Hi Carmel! Just getting into your blog, a friend of yours posted a link while commenting on a story on carsales.com.au.
Wanted to let you know I’m finding your work really interesting. I’ve been wondering what might come in the future, if/when petrol becomes prohibitively expensive, if somebody might come up with an effective way to convert one’s much-loved older car to electricity — seems you and your mates are blazing the trail! Well done, and best of luck with it :)
yep, it’s all in the lights, you never quite get over them.