An EV battery pack usually consists of a number of large battery cells connected in series in order to get high enough voltage.
When a pack of batteries age the individual cells tend to do so at different rates. Some batteries in the pack will have less capacity then others and as you repeatedly charge and discharge them the cell voltages will start to drift apart, some cells may even start reversing on deep discharge. Reversing a cell will kill it quickly.
To avoid this the pack need to be balanced from time to time. There are several methods for doing so, the simplest is to slowly overcharge the pack until all cells are full. This isn't possible with most battery chemistries as overcharging the cells will kill them. Some chemistries can handle being overcharge but it still tends to shorten their life.
Resistive Balancing
Another method is to switch in resistors in parallel with the cells that are reaching full state of charge first. The resistors shunt current past the full cell so they don't overcharge while the rest of the pack continues to charge. This method naturally wastes some energy in the shunt resistors but not very much as balancing will only happen towards the end of the charge cycle.
The most simple form of resistive balancing is the Lee Hart zener shunt regulator.
TimK just posted a circuit on his blog that uses a third method. This circuit takes charge from the cells at a high state of charge and shuttles it to the cells at a lower state; thereby eventually balancing the pack. TimK uses a switched capacitor scheme to shuttle charge from cell to cell, charging the capacitor on the higher voltage cells and discharging it on the lower voltage cells.
It is an elegant circuit and I like it for its simplicity and low cost. I did however have an idea for how you could potentially make it even cheaper. By replacing the dedicated gate drivers with a capacitor lift up scheme the most expensive component can be removed.
Charge Shuttling
I just ran a quick simulation on it and it seems to work as planned.
The upper N-channel MOSFETs (Q2,Q4,Q6 in TimK's schematic) are replaced with P-channel devices. Resistors keep the capacitors on the gate charged to the same potential as the source pin of the MOSFET.
V5 and V6 generate square wave signals with some dead-time between them to eliminate shoot through. They essentially do the same job as TimKs 555 circuit.
As V5 goes from 0V to 15V all the N-channel MOSFETs turn on due to the voltage across the gate capacitors pushing the gate voltage up an equal amount. As V5 goes low again the same effect acts to turn of the MOSFETs.
When V6 transitions from 15V to 0V the gate of the P-Channel devices gets pulled down consequently turning them on.
The charge shuttling capacitors (C1,C2,C3) thus gets switched between adjacent cells and will eventually even out any charge difference between them.
Balancing in action
The graph shows the circuit in action. The green trace is the voltage across charge shuttling capacitor C3, the blue trace is the current flowing in and out of the capacitor as it is switched back and forth. The red trace shows the gate voltage of M8, the lower MOSFET.
Disclaimer
This circuit was just a quick test to see if it would work, the component values were just wild ass guesses so nothing has been optimized. I wouldn't recommend anyone builds this circuit without doing some more careful analysis and optimization.
The simulation files can be downloaded below, I use LTSpice from linear technologies to simulate my circuits. The software can be downloaded for free from their website .
| Attachment | Size |
|---|---|
| balancer.asc | 5.24 KB |
Hi Peter,
Thanks for the feedback and suggested optimization for the battery balancing circuit. I think your capacitor optimization will work great for small numbers of batteries; however, my EE gut feel is that the edges driving the switching capacitors will degrade when driving 18 different batteries distributed around the car. The IRS2117/2118 chips also have several safety features built in such as automatic shutdown. I'm willing to spend an extra $3.00 per battery for the robustness, but some people might prefer your circuit due to it's economy.
Have a great week!
TimK
I think you may be right about that, but it is easy to get around. I would probably design it as a modular system capable of handling about 4 batteries per module, each module would have it's own gate driver for driving the lift up caps. For bigger packs I'd use several modules running off of a common clock source. The system would be more flexible this way, wiring would be simpler and you can distribute it around the car with the batteries.
Anyway, it looks like I'll be getting my hands a set of EV-95 NiMH batteries so my own battery monitoring system will be a bit different.
Thanks for the comment.
Hey there. AudiMouse from YouTube here. Very informative site you've got here!
Reading about your balancer design, I think I understand what it's doing, but I just wanted to clarify the actual concept of it.
As I understand it, what I think is happening is this:
A pulse connects a capacitor across each battery, charging that capacitor to the voltage of that battery, then that same capacitor is connected across the next battery in the series, and will either discharge to it, or charge from it, depending on if the voltage is higher or lower. Do I have that right? It's a very cool idea, and quite simple really.
Is this normally done during charging of the pack, discharge of the pack, or all the time?
Thanks!
Hey there. AudiMouse from YouTube here. Very informative site you've got here!
Reading about your balancer design, I think I understand what it's doing, but I just wanted to clarify the actual concept of it.
As I understand it, what I think is happening is this:
A pulse connects a capacitor across each battery, charging that capacitor to the voltage of that battery, then that same capacitor is connected across the next battery in the series, and will either discharge to it, or charge from it, depending on if the voltage is higher or lower. Do I have that right? It's a very cool idea, and quite simple really.
Is this normally done during charging of the pack, discharge of the pack, or all the time?
Thanks!
I think the weakness of this system is it gets exponentially slower at balancing the batteries as it gets closer to fully balanced. For my AGM batteries, this could be a problem. What if you used a transformer and attached the primary coil to the current source, and the secondary to whatever battery you're trying to bump up?
BTW I'm wondering why you don't use this on your NiMH system you are using.
