Blog

Monday 20th November 2023

Aligning motor and gearbox

Attaching an electric motor to the clutch housing of a vehicle requires checking to make sure the gearbox shaft is closely aligned with the motor shaft ... otherwise the two shafts will fight each other and cause early bearing failure.

Below is a video showing my process for checking the alignment of the motor and gearbox.

Sunday 21st May 2023

Battery Box  design issues

Although it may seem simple to simply create a box to hold a set of batteries, when you read the detailed specifications from the battery manufacturers, things get a little more complex.

To get the full life from prismatic LiFePO4 batteries, it is important to provide what the manufacturer terms "300kgf fixture" which appears to mean that batteries need to be constrained from swelling as they charge by applying a compression of up to 300kgf as the battery charges.

Some people attempt to deal with this by using springs to constantly squeeze the batteries together. However, the problem is that if you have a collection of batteries connected with busbars, the busbars will have to allow for approximately 1mm of movement during charge/discharge cycles and there is a risk that pressure on the terminals will damage the battery internally.

Some people try to arrange the batteries so that the swelling does not affect busbar distances (i.e. batteries are connected end-to-end in series rather than parallel).

I my design I am going for a fixed volume design in which batteries are fixed (using flexible adhesive) beside each other with space between them when discharged and are under compression when they are fully charged they experience a compression of 300kgf imposed by the physical volume they are contained in.

Monday, 3rd April 2023

Designing a battery box for my conversion

I have been grappling with how to construct the battery boxes for my conversion of a Daihatsu Rocky to electric.

I will need more than one battery boxes for several reasons:

• space - there is not enough room in the engine bay

• weight distribution - batteries are heavy and I need to keep the weight distribution fairly close to the current weight distribution so that it handles properly

The 72 3.2V batteries (EVE LF304) will weigh just under 400kg and store 70kWh of energy, and should give me a range of about 300km if driven gently (80km/hr).

The design rules for battery enclosures specify that each box must be able to withstand 20 X the weight of the batteries in the event of a front collision, 15 X the weight of the batteries in a side collision and 10 X the weight of the batteries if impacted from above, below, or from behind. 

Clearly, securing the batteries in a sheet-metal box with rubber straps is not going to be sufficient. The box needs to be seriously engineered and secured in the vehicle.

For example, as our 4X4 can expect to be driven over corrugated roads, the batteries have to be protected from the wear and tear of vibration and bouncing around on rough roads.

Batteries with Lithium Cadmium chemistry also need to be cooled to protect against risk of fire from overheating. Batteries with LiFePo chemistry may not need cooling if charging is kept down to Level 1 or 2 charging.

Most commercially produced vehicles meet the design rules by packing the batteries between strong beams beneath the floor of the vehicle with strong pressed metal panels above and beneath the batteries.

I have not found any company that designs and builds battery boxes for custom vehicles.

One option was to get strong aluminium boxes welded up to fit into the spaces provided. The problem with this approach is that it is difficult to stop the batteries vibrating around in the box because it is difficult to insert them into the box tightly.

The option I am taking is to use a bolted construction where the battery box compresses the batteries slightly when they are assembled. It is made from 10mm aluminium plates bolted together and sealed with silicon to make it water proof. Each battery will be separated by a resiliant membrane to reduce wear and abrasion and absorb the compression.

I have drawn up the dimensions of the aluminium plates using a CAD package, and am waiting for them to be delivered. I will have to drill and tap the 250 holes to assemble the boxes.

Tuesday 7th March 2023

Effect of temperature on range

After 12 months of keeping records while driving an electric Mini Cooper  a pattern is beginning to emerge relating daytime temperatures and power consumption.

The blue lines are power consumption (kWh/100km) and the orange line is average daytime temperatures in Perth.

Most of this driving is along the same route, similar traffic conditions and speeds (usually around 33km/hr in suburban driving). As the blue line shows, there is quite a bit of variability. Some of this is due to battery charge lost when the car is not driven for a week or so. I am somewhat suspicious about the accuracy of the Mini's average power consumption meter as it has strange behaviour when I reset it. It begins with an average of 100 kWh/100km and then gradually drops down to the real average (between 10 and 13 kWh/100km) over the next 20km, so is not reliable for distances less than 20km.

Despite this variability,  there is a 20% increase in power consumption on average between summer and winter - corresponding to a drop in temperature from around 30 degrees C to around 15 degrees C.

Saturday 11th February 2023

Monitoring State of Health of an EV Battery

Batteries degrade over time, so it is important for vehicle owners to know how healthy their battery is for a number of reasons:

(a) warranty - most new vehicles warranty their battery to retain at least 70% capacity in the first 8 years or 160,00 km (whichever comes first) - except Mercedes who provide a 5 year warranty.

(b) used car purchasers - want to know whether the battery is still healthy

(c) used battery purchasers - for people purchasing a used battery when converting their EV to electric - need to know how healthy their battery is.

(d) converted vehicle purchasers - for people purchasing a vehicle that has been converted by a conversion company, or who have their vehicle converted - there are variable warranty periods - for example, we originally planned to have our vehicle converted by Perth-based company Unique EVS - which specified a 2 year warranty - but no information to indicate what degree of battery degradation was covered by the warranty - so hardly a satisfactory warranty.

How do you measure state of health of a battery? You need to know this if you believe that your battery has lost capacity and wish to claim it under warranty. 

Very few vehicle manufacturers provide you data about the state of health of your battery, although there is legislation proposed in the USA to make this mandatory in the future. So, there is no easy answer.

Most vehicle manufacturers install systems in the vehicle that transmit information to the manufacturer about your vehicle as you are driving. For example, in my Mini I can download my charging history that is stored by BMW. However, there is a lot of data that BMW have collected about my Mini that is currently not accessible to me which would help answer the question about the state of health of my battery.

What can you do?

One method is to keep records. For example each time I charge my Mini I record state of charge, range estimate, amount charged and distance travelled between charges. This allows me to keep track of how much range my Mini has on a full charge. Over time this range will decrease as the battery degrades, and I will be able to determine how much capacity has been lost.

Recommendations:

(a) when you purchase a  new vehicle - keep records of your charging and range data as soon as possible after purchase

(b) request all of your vehicle data from the manufacturer (under FOI legislation)

(c) lobby your member of parliament to legislate for requirement for State of Health information to be automatically provided to you.

Monday 23rd January 2023

Calculating Acceleration after Conversion

Acceleration and time to reach 100km/hr are often quoted as key information about the performance of an electric vehicle. 

I have developed a simple Excel spreadsheet tool to enable you to get a good estimate of acceleration performance of your vehicle after conversion.

To use this tool you need four items of information:

BEFORE CONVERSION

• top speed of vehicle (km/hr)

• power of the original motor (kW)

AFTER CONVERSION

• weight (GVM) of vehicle

• peak power of electric motor (kW)

The tool will provide you with a graph of acceleration and times to reach speeds up to (and beyond) 100km/hr.

Download link is here