Mixing Batteries

[lfairban]

I remember a standard caveat about not mixing batteries of different types in most devices. I am not real sure about why this is not a good idea in all cases, but I can guess as some of the concerns. Anyway, it would be really advantageous to use on an ebike. For instance, one could mount an expensive light weight Lithium battery that would work with the more often traveled range, and then add on some cheap SLAs to extend the range for occasional long trips.

Since there is a slight difference in voltage, 27V for Li and 24V for SLAs, one could use a diode on the output of the auxiliary SLAs. That way, when the Li battery drops to 24V the SLA will start suppling current.

Or, if you had an SLA as a main battery, you could add a diode isolated 18V NiCad drill pack that would prevent deep discharge of the SLA until the NiCad was depleted.

To be really on the safe side, you could isolate both batteries with a diode, but is this necessary.

Could this work?

 

[fechter]

Since there is a slight difference in voltage, 27V for Li and 24V for SLAs, one could use a diode on the output of the auxiliary SLAs. That way, when the Li battery drops to 24V the SLA will start suppling current.

Or, if you had an SLA as a main battery, you could add a diode isolated 18V NiCad drill pack that would prevent deep discharge of the SLA until the NiCad was depleted.

To be really on the safe side, you could isolate both batteries with a diode, but is this necessary.

Could this work?

As long as you use diodes to isolate things it will work. In some cases it may be possible to make direct connections without using diodes, but not usually recommended.

You need the diode to prevent the higher voltage battery from discharging into the lower voltage battery. In a case where one battery is a higher nominal voltage than the other one, then perhaps only one diode would be OK.

 

[dogman dan]

Or you could use one battery for awhile, and then the other. For some reason a lot of people think stopping for 30 seconds to plug and unplug a battery would be too much hassle.

 

[billvon]

Since there is a slight difference in voltage, 27V for Li and 24V for SLAs, one could use a diode on the output of the auxiliary SLAs. That way, when the Li battery drops to 24V the SLA will start suppling current.

Depends on the chemistry and cell arrangement. 30 ni-mh cells paralleled with 10 li-ions gives you the same max-charge voltage (42 volts) and thus will parallel pretty well without any diodes at all. That's what I've been doing.

Also -

Or you could use one battery for awhile, and then the other. For some reason a lot of people think stopping for 30 seconds to plug and unplug a battery would be too much hassle.

I don't recommend this. You hit each battery twice as hard (4x the I2R losses) and thus your range declines and you reduce the life of each of the batteries compared to parallel operation.