High Voltage NiMH battery charger

[Ioannis]

I am trying to get info on building a high voltage hybrid car NiMH battery charger.

They are based on 28 cells of 7.2 volt/6.5Ah in series resulting in a 201,6 volts (much higher when charged).

One idea is to seperate the battery cells and make 28 discreate chargers but this leads to higher cost.

The other idea is to have one high voltage charger of about 300 Volts max at 0.1-0.2C with timer and voltage circuits to monitor the process.

Any ideas welcome,

Ioannis

[Charlie]

What about charging with current pulses? Then you can use a transformer to step up the voltage, and do the current limiting on the low voltage side.

[pedja089]

This might help
http://www.linear.com/product/LTC6804-1
http://www.linear.com/product/LTC4070
http://www.linear.com/product/LTC3300-2
http://www.linear.com/product/LT8584

[Ioannis]

Thanks for the interesting ideas.

It gets more complicated than I thought.

Well, my approach initially was more direct (and maybe dangerous). Make a high voltage power supply with current limiting and some more extras from a PIC to control and check the time and voltage of the battery cells.

The client needs also to discharge the battery under control to make a couple of conditioning cycles and check at the same time the capacity of the battery pack.

The discharge I guess is going to be a very large array of transistors on big heatsink, to act like a electronic load, controled by the PIC and also keep a record of the discharge rate.

Too much power to convert into heat, about 1KWh. And then to charge back the pack.

Charlie, I did not quite understood how your idea would be implemented. Could you give more details?

This is what I want to accomplish: http://www.hybrids.co.nz/testing-complex/28-channel-battery-analyser/

Ioannis

P.S. These batteries are "small" cells of 7.2/6.5Ah and can be disconneted from each other. So one other idea would be to charge and discharge independently. Of course this means 28 seperate circuits...

[pedja089]

Much better solution is to use oven heater element, or similar. Then just use PWM to set to desired current/power.
Dissipating 1KW in silicone, is very bad idea...

[Heckler]

you may also want to consider making it possible to balance the batteries over time. As you probably know the cells will not always have the same exact capacity and voltage. Over time some batteries will become discharged before others and could suffer polarity reversal.

Most remote control car/airplane/etc chargers have the capability to re-balance the pack. Or they use an external balancer. In order to do this you would need to extend a wire from each cell to a balancing plug. This is usually a low current process so smaller gauge wire is used.

I see this more in the LiPo type batteries so I'm not sure if NiMh types would benefit from it.

Just more to consider

[Ioannis]

Oven heater element? Great idea. I too was concerned about the 1KW in silicon...

I think the balancer is needed in LiIon batteries because they are dangerous when charged in series. In NiMH will not do harm but is not absolutely necessary.

I do not think that the car itself is balancing the battery pack. Info on this are very limited. Sure is monitoring each cell though.

Ioannis

[AvionicsMaster1]

Many years ago I worked on a project where we had battery backups. Some of our batteries were NiCad and others were some weird metal combinations. The NiCads were charged and discharged as a unit but the weird ones required many little chargers that monitored each individual cell. It was the only way to ensure each cell didn't over charge or discharge. If they did over charge or discharge it ruined the cell.

My point, Ioannis, is that I agree with your last post in that you'll need to treat the cells individually. Not only for safety but for efficient charge and balanced discharge.

It wouldn't surprise me if the car monitored the battery using some proprietary bus scheme. Seems like cars get more and more complicated each year. Not necessarily better just more complicated.

[Ioannis]

OK, looking at the battery pack, seems that the battery controller is much more than a charger circuit. It checks each cell and gives driver a warning if something is not good.

Other cars just check the pack as a whole cell.

Ioannis