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Looking for some help please. We manufacture small off road Land Rover Defenders for holiday parks and the likes. Up until recently these cars have run off 3x 8v lead acid batteries with a potentiometer accelerator pedal operating a DC 24v motor through a Curtis control unit. The cars only does 4/5 mph and are braked by the motor via the control unit (this can be adjusted. The problem we have is we’ve upgrade to a single 24v lithium battery. All has been good until this scenario, When the battery is fully charged and the car is driven down a hill the regen energy isn’t able to be returned to the batteries and the BMS cuts the power. This is causing major customer issues as the car comes to a very abrupt halt. We have bought a number of these batteries (not returnable) and the control we have used for years with no issues so don’t want to have to look for another type, is there any form of capacitor/diode we can use to take the regen power/spike? We don’t need to use the power, as it’s minimal, we just need it to affectingly go away!! Any help is much appreciated!!

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    \$\begingroup\$ What's the reason you cannot hire someone qualified to help evaluate your situation and consider solutions for you? This really isn't the place to correctly and fully evaluate unique safety issues for your customers. As to your question about just using a capacitor or a diode or anything that simple... especially without lots of detail provided here... I rather doubt it. Do you, at this time, understand the root cause for the abrupt halt that didn't exist before when using the same control unit? What's new other than just the battery? What's the system before and after, in detail? \$\endgroup\$
    – periblepsis
    Commented Sep 19, 2023 at 23:39
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    \$\begingroup\$ "The BMS cuts the power"- Sounds like the motor controller shorts the motor to prevent severe overvoltage damage due to the regen energy. But that's just a guess, you'd have to verify. The fix would be a resistor bank and a comparator to dissipate the energy when the voltage rises above a set point that's somewhat under the overvoltage behavior of the controller. \$\endgroup\$
    – John D
    Commented Sep 20, 2023 at 0:17
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    \$\begingroup\$ It may be that the regen energy is being abruptly dumped or even shorted. The group that did your battery upgrade should have notified you of the potential issue. Also, as Periblepsis points out this is clearly a safety/liability issue. If you do not have a fully qualified Engineer (such as a PE), it would be best to directly consult with the manufacturer/vendor of the control unit (Curtis) and/or the BMS. They should be the ones to offer the most reliable and safe solution. \$\endgroup\$
    – Nedd
    Commented Sep 20, 2023 at 0:17
  • \$\begingroup\$ @Chris I'm an admin here. This question is essentially identical to two recent ones. Both closed (so far) due to lack of a clear Q& A path. Do the answers given here and the related sicussion answer your question. If not there may be the need for a definitive Q&A to be written. || The problem he and you are having is that LA (Lead acid) batteries when fully charged will absorb brief energy peaks with minimal voltage rise, whereas LiIon MUST not absorb energy when fully charged so their BMS's turn off, the voltage peaks and the ... \$\endgroup\$
    – Russell McMahon ♦
    Commented Oct 3, 2023 at 1:03
  • \$\begingroup\$ ... controller then shuts down and applies full mechanical parking braking. || This is relatively easily dealt with. There may be enough for you in the existing answers. PLEASE advise either way. \$\endgroup\$
    – Russell McMahon ♦
    Commented Oct 3, 2023 at 1:04

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There are many options but some depend on access to variables inside your battery, charger or controller which you may not have. Others only solve part of the problem. Here are some:

  1. Limit charge to 90% (needs BMS or charger control)
  2. Add a diode so regen never goes back to the battery and add a zener type clamp above the battery voltage to handle all regen. This needs to be properly sized and may not be small or cheap and will generate heat.
  3. Adjust the controller to avoid regen over a certain voltage (not clear how the car would stop in this case). This, along with mechanical brakes are how EV cars handle this
  4. Just add a zener above the battery voltage however this probably only acts after the battery has tripped open. It may save the controller from faulting however which could be a small win (same caveats as #2 apply).
  5. A capacitor may absorb spikes but caps can’t hold any meaningful amount of energy in this case.
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  • \$\begingroup\$ I think just sending the batteries out 90% charged is the best option. It is probably possible to buy a charger that can do that. If not it can certainly be built. \$\endgroup\$
    – Drew
    Commented Sep 20, 2023 at 1:18
  • \$\begingroup\$ Thanks for all the replies. As suggested by a few we are looking at lowering the batteries charge voltage to keep it around 90% when charge completes. We charge with a Victron smart charger which you can enable user defined charge specs, once the battery manufacturer comes back to me with a voltage charge chart/graph for the battery will will adjust the charger to suit and test. I’ll update with some results hopefully later. Thanks again! \$\endgroup\$
    – Chris Tunstall
    Commented Sep 20, 2023 at 8:46

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