Category Archives: Batteries

Batteries, charging and management

Finishing a PCB

Controlling Electrons continued 1

Controlling the Electrons

After last weeks message, I was busy (kind of) to try to tame the batteries.

This week (and the following), I will assemble the PCBs I have just received.

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There are three boards, one having the flyback converters, one with all the logic, piggy-backed to the power board. Those two boards are stackable. For cost reasons, I start to test with one half string, that is, it will be a string of 36V thus a 3,6kW pack.

The rest is self explanatory, just watch the pics:

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… baking electronics 🙂

How to Contain Electrons

Hopefully, the next weeks will keep me busy building my BMS. Hey, what is a BMS. Probably everyone understands different things under that synonym. Wikipedia gives a good overview.

My BMS will be a modular one. I will have several stacks of cells distributed through the vehicle. Each stack will be controlled by a BMS handling the following tasks:

  • monitoring the cells in the stack
  • balancing the cells in the stack
  • communicating with the vehicle
  • operating the contactor

maybe, if time permits, I will also implement a health check, a charge control and a charge counter.

This week I got the most important things for the BMS, the containers for the electrons also called batteries 🙂

They were used in another project (not mine) and sitting in storage to be used again.

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To be used again, they needed to be unpacked:

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Each energy block was assembled into 4 cells in parallel, giving a total capacity of 400Ah.

My requirements are slightly different, so they were taken further apart

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until I have 12 cells. This is the number I need for a half stack. My full stack will have 72V. My half stack already has a beefy 3.6kWh of stored energy (My ZERO S electric motorbike has 4kWh).

To be honest, I am blown away by the size of the half stack, I probably will have to reconsider my range specifications, because I might not be able to integrate 6 full stacks into my small vehicle…

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How to Power Up

 

 

 

Now I am still waiting for my PCB for the Battery Management System (BMS) so I still have plenty of other things to do. Like working on the problem of cold start the EV. All battery stacks will be self contained and disconnected from the main power rail. Each stack will have its own micro controller (uC) that will measure the total voltage of the stack and control its current flow. The uC will also control the contactor. While the BMS itself will be connected to the individual cells, they can only be put to deep sleep. All the uC however shall be turned off when not operated. They will get their supply from a dedicated power net.

So if everything is off, how can we turn it on?

My solution will be a master battery, that will be hard wired to a mechanical switch. That switch, once pressed, releases current to the contactor of the master battery. This will power up the rest of the controller network and connect the other battery stacks to the power net.

The diagram below might help to visualise the logic: initial on logic

So far for theory, on to the prototype.

Below are the schematics and the first layout on the breadboard.

I picked the IRF 9610 because it can operate on the full stack voltage of 72V and I hope I will be able to find a contactor that will not draw more current that the MOSFET can supply.

latchingSwitch

 

If anyone has a tip on a suitable low power contactor that can sustain 1200A, please leave me a message.

The next step is to draw the layout for the analog part and the digital logic and then to demonstrate its behaviour.