dual battery problems

[tomvg]

Hi,

I recently added a new battery to my ebike. My old battery has 17 Ah and the new one has 15,5 Ah (both 36V). I connected them in parallel with the following diode in between: https://www.aliexpress.com/item/33043341733.html?spm=a2g0s.9042311.0.0.357a4c4d18na71

Everything works fine, but the new, smaller battery seems to be used much more by the system. It was almost completely empty, while the old, larger battery was still 3/4 full.

Any Idea what could cause this? Could it be due to different internal resistance of the cells? They are of a different quality.

Thanks in advance,
Best regards,
Tom

 

[tomvg]

Forgot to mention: when I connect either battery separately, everything works fine also.

 

[fechter]

I would guess the older battery has a much higher internal resistance (sag) so makes the newer one do most of the work. You could test by running one battery at a time and measure the voltage sag under full load.

If you got both batteries to the same voltage and parallel them without the diode, essentially the same thing will happen only the weaker one can pump some charge into the stronger one when there is a light load.

 

[john61ct]

Which wastes some energy.

How about a 1/2 switch at least then you know

 

[999zip999]

Just use one and check mileage then # 2.

 

[tomvg]

Hi,

Thanks for the quick replies. The mystery is solved. I accidently had switched off the larger battery during the ride

Did another long test ride: now the motor draws more or less evenly from both batteries.