Mixing battery pack technologies

[ITgreybeard]

I am building a battery system for a 24-volt e-bike motor, and am considering mixing a couple of different battery technologies to power it:

24-volt 10ah NIMH (@ ~4KG) and
24-volt 7ah SLA.(@ ~ 8KG).

Obviously this is not a great deal of battery capacity, but it should meet my needs.

The NIMH technology is great for slow steady current draw, if my information is correct. The SLA technology, on the other hand, shines when bursts of power are needed, but is pretty heavy for its capacity.

Presumably, riding at an easy pace on the flats would draw from each technology. Riding up more demanding inclines would presumably draw more upon the SLA than NIMH, if the information that the SLA more readily ramps to high current draw is correct.

I would carry on-board, and use simultaneously, power from each battery technology, which would be connected in parallel. My concern is that the voltage from one technology could exceed that of the other at some point in the ride, and that I should avoid the resulting current flow from one to another.

Should I consider using diodes or mosfets on the positive side of each battery technology to avoid any cross currents? The current draw of the motor could be up to 30amps, but voltage will not exceed 24volts. What specs should I therefore be looking for in selecting these diodes or mosfets, if that is a solution to the problem?

 

[wassabi]

Yes, different voltages will put stress on the lower cell. Is this stress enough to destroy the battery? Probably too expensive to test and find out.

Recommend two voltage regulators, one for each battery.


Does the bike motor need a constant current flow? Probably not. The easiest solution I think would be to have a switch to select between battery 1 and battery 2.

 

[ITgreybeard]

The bike does not require a constant current flow. In fact, on the flats or downhill, the motor is ordinarily not required, and is therefore throttled down or off.

I could manually switch the battery source, but would have to do that at the start of each hill, and then back again after the top.

Pardon my newbie questions here:

What is it that regulators do that would lend themselves toward a solution? Do they in some way prevent backflow of current?

Thanks for having taken the time to look at this question.

 

[wassabi]

There are a few different methods of voltage regulation. Most common use an integrated circuit with an input, output, and ground. Current only flows one way.

Another type would be simply placing a zener diode in parallel with the batteries. Neither battery could exert a voltage greater than the zener's voltage rating on the other battery.