# Current regulator - Impossible?

Thread Starter

#### SimonFili

Joined Jun 20, 2022
5
Hi there, new here!

I'm trying to find a current regulator for the following need.

My source is 2 Lithium batteries of around 20V, they are 5Ah 20Vmax batteries for Dewalt power tools. So I have a total of around 41.5V full charge for 5Ah.

The thing I'm trying to power is an electric bike, which, according to my test with my bench PSU, cannot take more than 2A.
- I'm trying to supplement the internal battery of 42V to extend the range of the bike.

When I connect the external battery directly to the bike, the current goes to 13 A and the bike controller shut down the input and stops the charge.,

Therefore, I'm trying to regulate the current to a maximum of 2A.

My first look was at something like the LM317, but the voltage is limited to 36V and Amp to 1.

I looked at this page https://www.ti.com/power-management/non-isolated-dc-dc-switching-regulators/overview.html
and did not find any chip with those ranges, 40V IN at 2A.

I also saw some circuits with MOSFET but did not find enough info to help.

Am I asking the impossible? Any ideas?

Thanks!

Simon

#### MrChips

Joined Oct 2, 2009
26,493
Welcome to AAC!

That is not how circuits work. Electrical circuits are governed by Ohm's Law:

I = V / R

If V and R are fixed then I is predetermined.
If V = 40V and I = 2A then R must be fixed at 20Ω.

That is not how an electric bike works. The ebike will demand whatever current it requires.
The ebike controller is shutting down for another reason.

#### crutschow

Joined Mar 14, 2008
30,114
Not completely clear.
So where is the 13A coming from that shuts down your bike controller?
You shouldn't get that much charging current if the battery you add is the same voltage as you bike battery.

#### Audioguru again

Joined Oct 21, 2019
4,798
40V at 2A is a little electric bike for a child.
Maybe that battery has a protection circuit that limits its current to only 2A and the new battery has no protection circuit.
Maybe the bike current is not what is talked about but it is the battery charging current instead?

#### MrChips

Joined Oct 2, 2009
26,493
My electric bike has a 350W motor.
This is approx. 10A @ 36V.
It will run for about 1 hour which works out to 10Ah.

Thread Starter

#### SimonFili

Joined Jun 20, 2022
5
Not completely clear.
So where is the 13A coming from that shuts down your bike controller?
You shouldn't get that much charging current if the battery you add is the same voltage as you bike battery.
Sorry, you are right, not clear. When I did my test, the bike battery was around 36V. Thus I was trying to push 41.5V to 36V battery, thus a 5.5V difference.

My goal is to connect the external battery in the charging port, which seems to be limited to 2A, which is the max amp of the charging transformer at 42V.

Thus, I want to limit the external battery to 2A to not overload the battery controller of the bike in the hope to extend the range of the bike.

The internal battery is 250Wh with a 250W motor.

My initial goal was to simply put a diode between the external battery pack to connect to the charging port.

Make sense?

Thread Starter

#### SimonFili

Joined Jun 20, 2022
5
In theory, when we go for a ride, the internal battery is fully charged at 42V. The external is at 41.5V. So with a diode drop, the external would be around 41V. After a while (riding), the internal battery would be at the level of the external one at 41V, from that point on, the external battery would supplement the internal one at around 2A max. Thus the hope would be to extend the range. Many people did this with a 36V pack with a diode. I choose to use two 20VMax to get closer to 42V of the internal battery.

Maybe I don't need the current regulator, since in real use I would be supplementing at the same voltage level?

The current limiting would be useful if I want to use the external pack to charge the internal pack, depleted to a low level.

In theory, the internal motor (250W) would consume a max of 6A at 42V. This is at max power. Thus, if my external pack is supplementing, I would assume the amp would be divided in 2, so around 3A, consumed from the internal battery and 3A from the external pack.

Last edited:

#### ag-123

Joined Apr 28, 2017
238
LM2596 has current limiting protection, and it is a buck converter (driver)

there are lots of ready made modules in the online "flea" markets
https://www.aliexpress.com/wholesale?catId=0&SearchText=buck+converter
the thing is, you would need to get a real original LM2596 from TI to get that feature I'd think.

I'm messing with the idea of making a buck converter with a microcontroller.
In practical terms, the output voltage can be managed by varying the PWM duty cycle and frequency.
To regulate currents additionally, some additional circuits to sense current is needed.
Then it is a matter of programming the control algorithm to achieve the goals.
I'd think this is a most 'flexible' way but not necessary one with the best regulation response. You can vary the duty cycle and frequency in nearly any way you decide with your algorithms.
Op Amps can possibly react faster than a microcontroller.
https://forum.allaboutcircuits.com/...ing-simulation-differential-equations.187476/

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#### crutschow

Joined Mar 14, 2008
30,114
If both batteries are lithium-ion, then they are likely the same voltage.
The difference is, some batteries are rated at the nominal voltage and some at the full-charge.
So if you use the nominal cell voltage of 3.7V, 10 cells will give a 37V battery. while the full-charge cell voltage of 4.2V gives a 42V battery.

So it would seem that you should be able to charge either battery with your charger.
Don't understand why the charger wouldn't work with your external battery and where the large current came from. (?)
A charger should normally limit the current to a safe value.

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Thread Starter

#### SimonFili

Joined Jun 20, 2022
5
Thanks for all the help and brainstorming. I connected my external pack to the bike charging port when the difference between the internal and external was a few volts. The good news, the charge controller on the bike worked at 5A.

Therefore, I will not need a current regulator in normal conditions, as explained before.

@crutschow Yes, I agree that the internal pack of the bike is probably 11 3.7V cells and my external pack 2x Dewalt 20Vmax are probably 10x 5 cells each. The charger that came with the bike is 42V 2A max. The external pack is 40.5V. The high current I got was when I connected the external pack at 40.5V to the bike at below 36V, which peaked at 13A and tripped the bike controller.

@MrChips @ag-123 those buck/boost regulators are not good for an output of 42V, from what I found. Anyway, for this project, I would need it. But I looked into a MOSFET circuit (linked above) that seems to have been ok at that voltage.

Thanks and the case is closed!

#### crutschow

Joined Mar 14, 2008
30,114
the internal pack of the bike is probably 11 3.7V cells and my external pack 2x Dewalt 20Vmax are probably 10x 5 cells each
No, I meant that the batteries are both probably 10 cells (5 cells each for the Dewalt) with each cell varying from 3.7V nominal to 4.2V fully charged.

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