# Connecting EV Batteries

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March 18, 2013

You may have seen some of my previous blogs regarding Lithium batteries. In my blog Lithium Battery Fires Why I highlighted some of the various fires that have occurred in the last few years. In blogs {doclink 259971} and The Care and Feeding of a Lithium Battery I discuss some of the important properties of these batteries, and in effect, how to avoid the catastrophes detailed in that first link.

Connectorization of cells is absolutely critical. Why? Aside from heat and other losses (more below), arcing due to poor contacts can cause some massive problems with control electronics and circuit protection, not to mention safety.  As we all know, poor, arcing contacts can cause voltage spikes, just imagine what those spikes are like at hundreds of amps with a nominal DC voltage in the hundreds of volts.

In the EV world, connectors are large. For the most part, you will find cells inside of a pack (most traction battery packs are made up of dozens, possible hundreds of cells in series or a series-parallel combo) bolted together with bus bars. Why? Quite frankly, it is simple and effective. Due to the risk of arcing, the mechanical reliability of a bolted joint, and the high current requirements (again, typically hundreds of amps), large copper bus bars are typical. Cells also are usually installed once at the factory and not touched again for many years, possibly ever.

Figure 1 Prius Battery Cells

How do you size bus bars? This question can be a complicated one, based on your allowable cost, voltage drop, and power loss. I have yet to find a good tool for this, so have had to create my own in the past to answer this question based on the variables I cared about. I would suggest building a spreadsheet to calculate your size based on what matters to you. Using system voltage, peak (and sustained) current, use the following voltage drop formula to determine the Circular Mils of cable you need:

CM = (2*K * I * D) / VD

Where:

CM = Circular Mills needed

K = 11.1 (conductivity of copper, multiplied by 2 for both legs of circuit)

I = Max load current in amps

D = 1-way cable distance in feet

VD = Voltage Drop desired

Using this, and a chart to convert wire gauge (AWG or mm) and Circular Mils to cross sectional area, you can then determine the cross-sectional area of a bus bar that would be equivalent. For the sort of power levels that are experienced in EVs, I have typically used 0.5% voltage drop (as a % of source voltage) as a maximum value. Any higher than this and you start to dissipate a lot of watts (you can start to dissipate hundreds of watts quickly) in the bus bars. Always err on the high side, and I highly recommend testing. As you may know, loads can be created cheaply enough using light bulbs, heater elements, or even spools of wire.

Interestingly, once the battery pack is assembled, most EV makers switch to a more traditional connector. These may be rather large in size (contact wiping surfaces measuring in the square inches), but nonetheless are connectors that can be fairly quickly disconnected. This is due primarily to the need to disconnect a pack from a vehicle (electrically) for service or inspection procedures. Many vehicles will also feature a purpose-built emergency disconnect (independent of cable connectorization) and may also incorporate a fuseholder in such an emergency disconnect. Such a product is available from TE, the AMP+ Disconnect.

Figure 2 AMP+ Connector

Figure 3 AMP+ Disconnect

This product line offers a variety of connector types for the EV world, in various circuits and ampacity, all with a low contact resistance and ease of operation, in the SAE designated “safety orange” color used to designate high voltage connections in EVs.

Figure 4 LEAF Battery Cells, Bus Bars, and Output Connector

As with any connector, identifying your critical parameters is vital to proper selection. EV connectors are no different. You have to understand your system and quantify your needs before you can begin the selection process.