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When you say the 'battery light', you mean the red battery symbol on the dashboard?

And you also have on the orange 'IMA' light, and the orange 'check engine' light right, because your back battery cells are bad? (These bulbs may have been removed - they should come on briefly while starting)



If so, then your problem is this:

* The 'alternator' on this car is a 150 volt to 14 volt power converter behind the back right seat. If it's not functioning, the battery light comes on.

* It will stop functioning if it's input voltage goes below ~100 volts. This *should* never happen.

* Normally during car operation, the input is connected to the battery pack via a big relay which is connected to the battery pack.

* The battery pack can be charged by the motor inverter whenever the engine is running. However, that only happens if the rear ECU commands that charging be done (and the green 'charge' bars appear on the dash).

* The battery pack is also charged whenever the pack voltage is below about engine rpm*0.032. This happens whether software commands it or not - it's a function of the way the electronics work that it cannot *not* happen.



So... I would guess your issue is:

* One of your battery cells/sticks is bad, causing your battery to have a high internal resistance, which means that any load on it causes the terminal voltage to drop rapidly below 100v and the 'alternator' stops working. This won't happen at high RPM because the power will be supplied from the motor instead.

Other possible issues:

* Bad cable connection on any of the high voltage cables.

* Bad connection within the battery pack.

* Main breaker behind back seat is off

* Bad relay that opens when it shouldn't.

* ECU software which is refusing to charge for some reason.


To diagnose, I would attach a multimeter across the battery terminals while the car is running (beware, high voltage DC is very dangerous - more dangerous than mains AC - you should attach the wires with the main breaker off, then switch it on and not go near it while testing).

If you see the battery voltage wildly swinging from 100v to 170v while driving, then you have the issue described. If the battery voltage is fairly stable around 150v then it isn't the problem.

If you have the problem, disassemble the battery and test each stick or set of sticks for internal resistance. Each stick/cell should have a short circuit current of *150 amps*!! If it's down at 30 amps the car will still function. If it's down at 10 amps, you'll see the issue you are describing. It only takes one bad stick to cause that. Using a battery tester that has an internal resistance test mode is probably easier than measuring short circuit current directly - in which case you're looking for an internal resistance of 10mOhms ideally, 50 mOhms worst case, and 150mOhms and it won't work.