Blog: batteries

Electric Renault Twingo: Battery considerations ¦ EV Conversion

Choosing the right battery is dependent from many factors. The most obvious one is of course the desired range, which translates into a target kWh value of energy.

But in practice it is very hard if you start off from a specific target range, as the main restricting factor is usually the available space, and then also the target Voltage of the system.

So these three factors (space, capacity and voltage) create a magic triangle with sometimes conflicting goals. Now we need to solve the dependencies in the right order.

In conversion projects, the space is usually the most important factor that dictates. The second factor is usually the target voltage, and the capacity (=range) is therefore the last and dependent variable.

So let's go through these factors step by step:

Our goal is to get the most kWh into the space available. The measurement here is kWh/Litre of space.

The existing battery box in the back of the Twingo is 800 x 520 x 280 mm and was designed to hold the ZEBRA Molten-Salt Battery. The total volume of the battery is therefore 116 Litres. We would now need to pack this space as densely as possible to get the most energy in, not forgetting about cooling etc. in the process.

The target Voltage in our system is 270V. This is the original nominal Voltage of the Molten-Salt-Battery, and we need to stay in that region because of all the other components in the drivetrain such as Inverter, Converter, Motor, Charger, Heating etc.

The target capacity would ideally be equal or higher than 20 kWh, as this is the capacity of the existing Molten-Salt Battery and also the capacity of the first series Renault Zoe, which proved to be a very capable car that brought my family of five all the way from the Swiss Alps to Barcelona on holidays back in 2016 without any problem - so that is about the behavior that I would need!

See the next articles for different battery options.

Battery, Inverter and Motor System Voltage

A few thoughts on the motor and battery choice - and related system voltage.

When I started the conversion, my original plan was to stay as close as possible to the drivetrain of the Renault Twizy, which I had the opportunity to study in-depth beforehand. Here's a picture of my "research vehicle" to show you how the drivetrain looks like:

So my original plan was to work with a LETRIKA/ISKRA motor of the type AMV7118 coupled to a SEVCON Controller GEN4 4845 (450A). The Letrika motor would have delivered about 13kW which would have been pretty close to the nominal requirement (15kW ) of my build (I had to stay within +/- 10% nominal power for homologation). The battery system of the Twizy is at 60V voltage.

Problem with this configuration was that:
- It wasn't clear if the Letrika motor would have a CE approval for automotive use when used on its own without incorporation into the rest of the Twizy system.
- The Sevcon-controllers are very proprietary and hard to program, some functions are locked.
- The battery voltage of 60V is a bit odd to achieve with some standard battery modules.

So at the same time two other things happened: I got to know a motor manufacturer (FIMEA) in Milan, Italy, who produced a fully CE-compliant automotive motor with 96V/15kW AND my original concept included the use of 4 tesla modules in series for a total of... 96V too! So that was the starting point of the 96V system concept.

I couldn't use the Tesla modules for various reasons in the end, but the 96V system voltage (and the motor) stayed, and I am quite happy about it for a small car like the 4CV. The only downside of the 96V system is that it will never be able to use DC charging as both the Chademo and CCS protocol are requiring voltages of well above 200V, for CCS preferably even over 300V. But the 4CV is not a car for long distance drives anyway, and 96V is still a rather safe voltage to work on. Higher voltage than on a Twizy also means less strain on the HV-system, even when driving steeper mountain roads, which is important in my region, too.