Tuesday, September 16, 2014

Lower wiring harness for lithium BugE.

The lower wiring harness diagram is available as a downloadable PDF that can be seen by clicking HERE .  The text below explains how this design came into being. So far, the circuit has been working well. I have color coded the wiring to make the circuit easier to follow.  Upper wiring block is negative (both 12V and 48V).  Lower wiring block is positive. Red is 12V, Yellow is 48V and so on. Also, although both grounds use the same wiring block, they remain isolated from each other.  After burning out some expensive components trying to match these grounds, I finally learned that attaching the 12V ground to the traction battery ground  is just an overall bad idea!  With that being said, let's discuss the circuit as it is now.

The whole circuit appears to be quite complicated due to the special needs of the lithium pack and it's battery management system. So, I've divided up the circuit for easier understanding. 

This is the basic lithium drive circuit, extracted from the main diagram.  Although the cells are lithium, the drive circuit isn't that much different from the instructions that come with the BugE for a lead-acid pack.   When the red key is turned on, it engages the safety contactor that allows pack voltage to reach the speed controller (we need a safety contactor and heavy fuse so a short doesn't blow up the pack). However, before power can pass through the speed controller, there is a second requirement to be met.  Federal motor vehicle standards require a headlight interlock to prevent the vehicle from being powered if a headlight isn't on.  To do this, the 12V headlight circuit (not shown) engages a relay.  The relay then completes the circuit that allows voltage to at last reach the "key" input on the speed controller.  Once the "key" input is turned on, the speed controller then can pass pack voltage proportional to throttle position.  After the speed controller is the reversing switch.  Depending on it's position, the reversing switch then passes power to the DC motor (and powers it's magnetic field) to create forward motion or reverse motion.

When  it comes to charging the BugE, lithium batteries can be quite problematic.  If a lithium battery discharges too low, it can be ruined. If the cells are charged too much, there is a possibility of fire.  So, a battery management system (powered by 12v) has been installed.  Below shows the battery pack with the BMS installed.  

  To prevent cells from being ruined by undercharge, electronic modules are put on each cell to monitor voltage. They participate in a loop which is monitored by a circuit board powered by a 12V battery. If any of the cells get below a threshold voltage, the loop is broken and the circuit board sounds an alarm with an on-board buzzer.   When charging, each cell must not go over a threshold voltage. So, in charging mode, if any one module detects an over voltage, it breaks the loop.  When that happens, the control circuit cuts power to a solid state relay that is used to allow 110v to reach the battery charger.  Without power from the BMS, the solid state relay passes no power to the battery charger.  Unfortunately, the BMS control circuit isn't too smart.  It must be put in either "driving mode" or "charging mode" which is accomplished by a key switch. Unfortunately, if left in driving mode, while charging, there will be no over voltage protection.  This can  possibly cause a fire.  So, a BMS indicator lamp has been added to remind the driver that the BugE BMS must be taken out of driving mode before batteries can be charged.   Also, BMS systems take energy to run.  So, a toggle switch has been installed to turn off the BMS when it's nether charging or monitoring the pack while driving.

Although the 12V lights could be powered by a DC-DC converter they are not.  Instead, all 12V loads are powered from the same 12V motorcycle battery that powers the BMS.  This means if the main pack becomes depleted, the 12V system can still power lighting (which includes hazard lights).  The 12V circuitry in the cowl has been separated from the lower wiring circuit such that you can "unplug" the cowl, lift the upper body off the vehicle, then test it's 12v circuit elsewhere simply by connecting a positive and negative to any random 12v battery.  Due to this circuit independence, I have represented the cowl lights, speedometer and horn simply as a box called "Cowl lights".

Currently, the BugE, uses two external chargers.  One for the 12V battery and the other for the main pack. To eliminate the need for two chargers, I will soon be installing a 12V DC-DC converter (and charge controller) to charge the 12V battery from the main pack while the vehicle is in driving mode.

1 comment:

Vijaya Prabhu said...

Your post is really good. Useful content.

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