DIY Electric Car Forums banner

1 - 6 of 6 Posts

·
Registered
Joined
·
11 Posts
Discussion Starter #1
Not an EE as this question will make clear :)

Something I've noticed as a limiting factor to the performance of EV systems is voltage drop-off under high load situations. A battery system that starts off at 400v, might drop to like 340 under heavy load. As the battery gets drained, it reaches its minimum voltage even sooner. You can't add more battery modules in series because you can't go over a certain starting/resting voltage.

What if the battery could dynamically reconfigure itself in response to load? So you'd start with 400v so your 400v gear is happy, but you've got a couple extra charged modules on standby. Then when you're pedal-to-the-metal, and voltage has dropped 50v or more, the battery dynamically introduces one or more of those standby modules to recover 10, 20, 30, or roughly however many volts were lost under this high-load condition. Then when the pedal is lifted and voltage recovers, the standby modules are taken back out of the series so things don't rise over 400v. As the pack gets drained overall, they could be re-introduced on a full time basis.

When charging, the battery could/would dynamically take certain modules out of rotation in the pack so the unit as a whole was never resting over 400v, but still ensuring everybody gets a full charge.

I am certain this must be a terrible idea, just not educated enough to know why yet :) If this could work, it would extend the rpm range at which an electric motor could make peak power, thus increasing its peak horsepower, and area under the curve.
 

·
Registered
Joined
·
196 Posts
The cells' rated discharge gives you a maximum power output per cell, whichever way you configure them. The datasheet for the cells will tell you whether the output current needs to be reduced at lower states of charge.

If you want a more consistent output, you can cap the current at what your pack can sustain below 30%.

Taking cells out of the pack under lower loads means you're putting more stress on the cells that are connected, but also when you reconnect them you'll have inconsistent state of charge across the cells.

Your idea makes the system more complicated, adds potential for short circuits if your transistors or relays for dynamic reconfiguration fail closed, but doesn't actually increase the total output of the battery.
 

·
Registered
Joined
·
4,580 Posts
I wouldn't call the idea "terrible"; I would have used a less severe term. Emyr has explained why it's not good.

There was a variation which avoided the problem of an inconsistent state of charge: configure the pack in two strings so it can be used with the strings in parallel or in series, so all cells are always used equally in either configuration. That idea is long obsolete.

Here's the solution which every production EV already uses: the pack is configured to produce the highest voltage needed, even at the lowest state of charge and sagging under discharge. This means that when fully charged and not sagging the voltage is substantially higher than needed... and that's okay because the inverter is designed for that voltage, and reduces the voltage to the motor to the level suitable for the conditions of the moment. The limitation on inverter output in almost all conditions in a modern EV is current (at low speed and high load), or power (at most speeds and full accelerator), or temperature (of any component), not available voltage.
 

·
Registered
Joined
·
1,442 Posts
How long is the duration of the "heavy load"? And how much current is needed for this?

There already exist a "dynamic battery" solution, also known as capacitors. These can be used to stiffen up the power supply to handle short burst of high current load conditions.
 

·
Registered
Joined
·
180 Posts
Are you running at the Bonneville Salt Flats for 3-10 seconds? That voltage sag affects top speed, in theory, not acceleration. Which is why manufacturers don't care.

If you really must, get your wallet out, buy a trailer to tow, and load it up with capacitors as KennyBobby suggested.

Realize that the caps and trailer add mass, which means you'd need more caps and get a bigger trailer, which means...
 
1 - 6 of 6 Posts
Top