regulator without regulator

[Whiskymonster]

hi.

possibly something that has been covered before, but i cannot find any information around here that matches what i want to do.

possibly my google fu is weak, or i am searching the wrong keywords.

i have an old cruiser/chopper ebike i was given before lockdown. built in 2004, old lead acid with a geared brush motor and a cheap generic controller.

it was originally 36v, but i had a bunch of good lead acid batteries knocking about so i added an extra battery. the controller is 48v anyway so no problem there.

there is a 3 speed connector on the controller, and putting it into the second 'gear' makes the bike fast enough that i am happy, and i dont want or need any more speed or power. no point in riding a cruiser too fast to say good morning to pedestrians now is there?

with a full battery anyway. as the battery depletes, so does my cruising speed. this is what i would like to fix, as it occurs to me that there is plenty of available power im not using.

the speed connector is 3 wires, two leading to the comparator inside the conroller through 10k and 20k resistors, and the third to ground. obviously grounding either of the two wires results in a slower speed, and with nothing connected i get full speed.

if i connect a 10k pot to either of these wires and to ground, i can vary the speed in a granular fashion, and i can limit the bike to a speed and power i am happy with, but obviously that drops as battery level drops.

i have looked at big ass regulators, too expensive and inefficient as the concensus of opinion round here would seem to agree. i could do it with a good old fashioned lm317 and a bunch of power transistors but that just seems stupid in the age of switch mode, and i have no desire to cook my man parts with wasted heat under my seat. maybe come winter i will change my mind...

it also seems completely pointless seeing as half the kit i would need to build a regulator is already inside of the controller to begin with.

i could just break out a 10k pot somewhere and adjust top speed as the battery drops, but theres too much potential for myself the mrs or the kids just to leave it on full and thrash the poor thing into the ground. also not acceptable.

i would guess the comparator is fed from a regulated supply inside the controller (there are 5v and 15v reg chips inside, but the board is too fine for me to trace) and thus not coupled to battery voltage, just allowing a set proportion of available power to be used.

does anyone have any smart ideas on how to link the battery level to the speed control feature? i cant be the first to want this type of thing.

the only idea i have so far would be a voltage dependant resistor made from an ldr and an led, but there must be a smarter way. is it as simple as using a potential divider somewhere to change throttle input value according to battery level?

looking forward to your responses, even if its just reasons this wont work.

thanks.

 

[goku]

with a full battery anyway. as the battery depletes, so does my cruising speed. this is what i would like to fix,

I'm not an expert. But I think that is unavoidable as long as you use the battery voltage directly. As the battery depletes, the voltage across the terminals also go down.

But like you said, I think a switched power supply that regulates the voltage AND also pump the required current is needed between your controller and the battery. And I'm not sure how easy/difficult that is.

 

[Whiskymonster]

there is the point, im not using battery voltage completely directly.

im using a 48v battery, but limited to a lower (and unknown) power using the 3 speed feature built in to the controller.

it seems the selector is reducing the power by a set amount, say to 70% power. as the battery level drops, so does the speed. i can then stop, switch the limiter off by removing the jumper installed, and use 100% of available battery voltage which gets me home just a bit faster than than the previous 70% of full battery voltage.

what i am asking is whether there is a method of varying the available power in line with the falling battery voltage.

as i say, i can control the top speed in a granular fashion using a trimpot, and twitch it up bit by bit as power falls.

the more i think about it, the more i think there may be an atmega chip involved in the solution.