It's been awhile since the last update but I've been busy on the next step in the project which is the cell balancers for the lithium cells. I wasn't sure what I was going to do for balancing as anything on the market is not cheap. Dimitri, a buddy of mine from Florida who is working on his second conversion, ask what my plans were for balancing, but I had none. He had found a few schematics of shunting balancers that people had designed. I messed around with the schematics in prototypes and although a good stepping stone they didn't work the way I wanted so I began designing my own circuit.
The cells are charged in series and viewed to the charger as one large cell. The charger has no way of equalizing the SOC (state of charge) for each individual cell. I've designed a simple cell balancer that takes advantage of the nature of lithium during charging. LiFePo4 cells are 3.2v nominal and like other lithium technology stay right around this nominal voltage until the end of the charging cycle when they climb in voltage very quickly. The cell balancer is designed to start shunting energy at 3.6 volts that would otherwise be going into the cell. The balancer shunts 1.5 amps at 3.6v and goes up from there as the voltage increases. Let's say you are charging your pack at 10 amps, then each cell is receiving 10 amps. If a cell reaches 3.6v 1.5 amps of this energy is shunted and the cell is now only receiving 8.5 amps. The remaining cells which are below 3.6v still receive the full 10 amps which allows them to catch up over time and perhaps multiple charges. The lower the overall charging rate then the higher percentage of the energy can be shunted allowing for a complete cell balance in one charge. For example if you can limit your charge to 1.5 - 2.0 amps you can simply leave the charger on and wait for all balancers to indicate they are shunting and therefore the pack is now balanced.
Here is a picture of a completed cell balancer. They simply need to be connected to each cell only. There is no master unit or mess of wires to tie them all together. I named them Volt Blocher. My my last name Blocher (pronounced Blocker) and the fact they will keep the voltages down by shunting away the energy.
Here is a picture of them installed in the car. I needed to make 45 units for my setup. My wife helped out on the assembly line and really saved me some time.
Here is a picture of the units in action. In the closer four units you can see that three of the four units are shunting energy indicated by the red LED. At this point the picture was taken only about 6 or so of the 45 units were in this state meaning the remaining cells needed to catch up.
This looks much cooler in person but thought I'd add a shot with most of the cells now shunting. Lots of red and green lights. After about three hours of charging I had just four cells remaining that weren't balanced. I'm going to continue to charge at a slow rate and see how long it takes to bring them level with the others.
There will soon be two others using the Volt Blocher units. Dimitri and Jim have ordered their DIY kits and will hopefully have them up and running soon.
For more information and ordering please visit VoltBlocher.com