tisdag 3 april 2012

Battery Installation

Battery installation

I wanted to get the batteries as evenly distributed in the car as possible. In fact it turned out that I didn’t have much of a choice when it comes to battery installation. I had one rule, NO BATTERIES IN THE BACK SEAT! So that leaves me with the rear engines compartment and the small space under the front hood.
4*90Ah
2*90Ah
Four batteries are to be placed in the back of the car, two on each side of the Agni. The other two are to be fitted just behind the front of the car, where the original 12v battery was located.

In order to support the 60kg's of batteries in the rear I had to design a support beam that also would act as a rear motor/gearbox support. I decided to make it out of aluminum since it is light and it doesn't rust. This also gave me a splendid opportunity to learn how to TIG-weld.
After hours of puddle control practice and 20 liters of Argon I started with the battery frames. These are made in 3mm L-profile aluminum and the will hold the two pairs of batteries togheter.


The two frames had to rest on something and after consulting a friend I came up with a design that consists of a support beam made of 6mm U-profile aluminum that stretches from side to side in the engine compartment.
Rear support beam

The support beam is fastened on both sides of the engine compartment and also has an "arm" that connects to the original motor mount found on the fiat.


The battery frames fits tightly into the frames welded to the support beam and pack straps will be used to keep them from moving up and down.


This is the finished support beam with batteries mounted.





The battery frames and the support beam have a total weight of about four kilos.

Batteries - the achilles heel of your ev-project

What type of batteries should I use? That's a question that everyone involved in the EV-game needs to decide on. It basically all comes down to four things:
    
   · Type of EV

   · Planned usage/range

   · Budget

   · Complexity

Type of EV

If you are converting a boat with a displacement hull you can easily go for lead-acid batteries since that type of boat will normally perform better in the water with large ballast.

I use 100kg of lead-acid in my "double ender" and that works just fine.


Linnéa - equipped with a hybrid powertrain and lead-acid batteries.

On the other hand, if you are converting a motor cycle or bicycle you would not want the extra weight from Pb's and installation space is limited.


Planned usage/range

What is the intended use of the EV, proof of concept, daily commuter or race car? What your desired range 10, 100 or 1000km? All of these things come into count here. There are a couple of calculators available that can help you get an idea of how much power you need to do the job.



Budget

A fat budget makes all the difference. If money is not the issue you can buy top of the line cells from Kokam or A123. These cells can deliver enormous amounts of current and are made of the purest materials. We will go deeper into battery technologies later in the blog.


Complexity

4 lead-acid batteries in series and a 48volt charger, that’s pretty simple compared to a 26 cell Lifepo4 pack with BMS.


Since this blog is about the conversion of a car I guess that it falls somewere in between the two extreems. I could use six lead-acid batteries in the fiat, it has been done before, but it is not optimal. The plan with my conversion is to create a daily commuter that has a minimum range of 50 km.


My Solution

In order to keep the cost and the complexity on a human level I have decided to go with six 12 volt, 90Ah LiFeYPo4 batteries. Now these "blocks" are made up of four 90Ah cells that share the same characteristics.

15Kg 12v 90Ah

The producer claims that these four cells, if not abused, are so stable that the need for individual, per cell BMS, is nonexistent. Now I know you might think that is sound too good to be true, well I'm here to find that out for you.


I will use a simple kind of BMS but its one board per block, i.e. six boards. These "AKUMON" boards will monitor each block and will issue a 12v signal if the charge voltage exceeds 17 volts or if the battery voltage goes below 10 volts. There is also a bleed-function that will limit the current in the last stage of the charge.
The AKUMON board. Provides high and low voltage protection for LifeYPO4 blocks.

The charge to be used is a 3KW (Vmax 87,6 | Imax 32A) Lithium battery charger. The charger will have a relay connected on the power chord towars the 240v grid. If one of the AKUMON's detect a high voltage, > 17v, the relay will open and the charge will stop.

Batteries, charger and Akumon's where all purchased from www.ev-power.eu great service and good prices!