Resistor to dump load

Passives Resistors
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1

Hey guys looking for some feedback with regards to a problem we have been facing.

We use a 48v lithium battery pack (fully charged voltage 58.4v) in our vehicles and we have designed them with regen braking. The challenge we have is that when the vehicle is fully charged and we have customers who live at the top of a hill, when they set out down the hill the regen braking kicks in and charges the battery pack, however it doesn’t take long for the battery to reach max capacity and for our system to shut down.

As a temporary measure we have limited the charger so that it doesn’t fully charge the battery allowing room for the regen braking, but the long term effect on the lithium is not ideal.

So it is our thought to use a 1000w or 2000w 10R chassis mount resistor, with a timer delay relay. The customer would depress the timer button and set off down the hill, it is our thought that the resistor would offset enough amps to not have the lithium cells spike and have the BMS turn off the battery pack.

This would only need to be used once a day on the first run down the hill because after that the vehicle uses more power then the regen can generate and it is no longer an issue.

We would be interested in some feedback, thinking of using TE1000B10RJ or TE2000B10RJ

Thanks in advance

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Hi SimoloCustoms,

Welcome to the Tech Forum!

That’s an interesting problem that had never occurred to me before, but which seems obvious, in retrospect. I presume most anyone making a vehicle with regenerative braking has run into this issue, so there’s almost certainly a commonly used method of dealing with it.

Your method seems plausible, but relying on human intervention to prevent regenerative braking when the batteries are full seems a bit dubious. I assume your system monitors the state of charge, right? If so, it would seem more appropriate to have it automatically disable regenerative braking whenever the charge exceeds some level, and re-enable when it drops below this level.

In terms of the appropriate power rating for the resistors, you’ll have to calculate that yourself, based on the maximum expected current and power produced by the regenerative braking system. Relevant factors would include your generator/motor specs, maximum loaded vehicle mass, and maximum predicted velocity. You would want to add significant fudge-factor as well, to account for unexpected scenarios and to extend the life of the system. Also keep in mind that these resistors will get really hot, so keep heat susceptible materials far enough away, and try to position them for maximum cooling.

Finally, in general, lithium-based batteries do not like to be fully charged. Fully charging them regularly will reduce the life of most, if not all lithium batteries, as I understand. Therefore, it would seem prudent, from my perspective, to have charging stop at around 90% rather than 100%, to extend battery life. If a user thought that they would need maximum range, for some reason on a particular day, that’s where it might make sense to have an override switch to tell the charging system to charge up to 95% or 98% in that one instance, to extend the range.

Having a system that only charges to 80% - 90% would also be more economical, from a usage standpoint, compared to charging fully and then having to use resistive dissipation rather than regenerative braking.

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Hi David thank you very much! And thank you for your input. You are correct that this is a common problem amongst ev’s which all have regen braking. Tesla handles it as you mentioned, by having software turn off the regen braking temporarily. The problem for us with this method is that it then allows the vehicle to completely freewheel which our customers are not quite accustom to. Further to that we rely on the regen for several other uses on the vehicle such as directional control of the vehicle when starting.

I have recently sat down with an engineer from Audi’s electric vehicle division and they are combating this issue with GPS integration, the battery monitoring system will monitor the elevation change in the vehicle and reduce the charge level depending on elevation. Unfortunately our system is not that advanced.

This problem that we have really only affects about 5% of our customer base, these customers being people that literally live on a side of a hill and are all down hill to reach their destination before reaching a point in which they will actually starting drawing amps providing room in the battery pack for the regen braking.

The remaining 95% of our customers, either live in a place where they will drive on the flats or up a hill prior to regen being used and it is never an issue.

We have designed our lithium system to not fully charge the cells to their maximum as you have pointed out for the longevity of the life of the cells. However for these customers applications we have had to drop the charge level down to 55-60% capacity what we are fighting is that it is such a steep application that that it is not just the fact that the pack becomes fully charged but also that the volt in individual cells spike and exceed our safety settings in our BMS which triggers a shut down of the system to protect the cells. The long term problem with reducing the charge so low is that the cells do not top balance correctly during the charging process.

My thinking behind using the resistor is that the 5% of our customers will understand that they have to press the button on their first ride of the day to go down the hill and then after that it is a non issue.

The resistor will simply act as a load as they are driving down the hill with the regen braking kicking in and should eliminate the spiking of the cells as they will have a load on them. Typically we only see one or two cells climbing to the “danger” zone.

A more sophisticated way of doing this with less human interaction would be to install a voltage sensor on the lithium battery that would sense the high voltage and then trigger a switch to open the solenoid to allow the resistor to start drawing power. Essentially doing what we are asking the human to do with the press of the button.

It would just be a matter of determine which components are needed for the voltage sensor, as the rest of the components are quite simple.

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An auxiliary mechanical braking system has advantages, but I’m inferring that to be out of the question here.

A solution may be as conceptually simple as a linear comparator measuring the divided battery voltage against an accurate reference, driving a transistor that connects the resistor when the pack voltage exceeds some setpoint below that at which the BMS disconnect occurs. Careful assessment of tolerances and temperature dependencies would be needed, in order to thread the needle between failure to operate and unintended operation. Honestly, it’s a function that would probably be best implemented within the controller/BMS itself.

Total braking power dissipated would be reckoned as vehicle mass (kg) multiplied by gravitational acceleration constant (9.8m/s2) and vertical descent rate (m/s) the latter of which is a fraction of vehicle speed determined by slope. A resistance value sufficiently low to achieve that dissipation figure at the expected voltage, and a dissipation rating somewhat greater than this figure would be a good starting point for development.

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Hi Rick I appreciate your insight on this, and do agree with what you are saying.

I’m thinking to proof this concept out in a simple format, we could potentially use a voltage switching regulator connected to the lithium pack, when the regulator senses the over voltage it will switch a solenoid on letting the resistor see the battery pack and add additional load leveling out the spike in the cells. Once the voltage drops back down, the switch will close and the resistor will not be used again until the predetermined voltage is reached again.

As previously mentioned this is only for about 5% of our customer base and it is literally only an issue for their very first drive after they charge.

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Please see the private message sent in response to your last.

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I just wanted to clarify for that our vehicles do have mechanical braking, so when I mentioned that they freewheel I was referring to the fact that with out the regen there is no longer any hold back provided by the motor itself.