Electric Bikes - different voltage in series

I can't seem to find the answer to this question so I will ask here. I have a 36 volt Lifepo battery with BMS from Yesa and a 6 volt SLA battery, both are 10ah, can they be wired in series to get 42 volt, but charged seperately without damage to the batteries, I've actually done this to get 42 volts on my Golden Motor set-up and it seems to work great, the extra speed and torque makes the bike a lot more fun. But I've only gone a about 13km so far, and after checking the motor, controller and batteries they remain cool to the touch, my worry, is reverse charging the 6 volt from the 36 volt by accident.

Thanks

It's OK and causes no problem. Batteries are wired in series to increase the supplied voltage all the time. Should be no reverse charging problem if the 6Volt battery is not more discharged than the 36 volt one. Make sure they are both fully charged before use.

Thanks for the reply - that's what I was hoping - both batteries are 10ah so the capacity is the same, and I always keep them charged up before use.

The amp hour ratings on LIFEPO4 and SLA batteries aren't measured the same way.

A YESA LiFePO4 10Ah battery will probably deliver 10Ah if discharged in one hour.

A 10Ah (rated capacity) SLA battery will deliver only 50%-60% of that amount if you discharge it in one hour, but will deliver its full 10Ah if you discharge it steadily over 24 hours.

So for an e-bike you need about twice as many amp hours of SLA to match up usable amp hours nickel or lithium chemistries.

Also be aware that the number of times you can use a SLA battery goes down very quickly if you use more than ~50% of the usable amp hours before recharging.

To some extent the same is true of LiFePO4 but they are longer lasting overall and somewhat more tolerant of deep discharge.

I wondered about that - so I hooked up a voltage meter to the 6 volt battery, so I can watch what happens to it. I did notice that the SLA will drop more quickly than the Lifepo as you suggested, I can go about 8 miles before I need to shut it down, and switch back to 36 volts, but it makes a nice "booster" while it lasts, it's just a $15 battery, so if I wreck it, no big loss, but it will be fun to use while it lasts.

Adding a 6-volt SLA to a 12-volt SLA..
and then "putting the pedal to the metal"..
will serve you less than useing all the same types of CELLS...
..like...(4)6-volt BATTS..
..or...(2)12-volts..even (4)12-volt BATTS..
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I would not do that..
I would build a (3)..6-volts system if that is totally necessary..
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It is recommended by the Manufacturer and the Seller..to always MATCH..your Battery's.

It is recommended by the Manufacturer and the Seller..to always MATCH..your Battery's.

But in practice it only matters if there is a significant difference in the amp hour output of the batteries.
And note that a 12V SLA battery is made up of 2V cells in series, while a 6V SLA battery is also made up of 2V cells in series.

There is alot you can do..
How-ever..
When it all come down to High Discharge..
You do want matched Part#'s/
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If your just going to power a single Light Bulb..
Yeah...just collect your Electric Charge in SLA..
Hook up a Solar Charger--and check your results,,
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Voltage requirements can exceed your Value--
..there is some give&go..
But as you begin to add..complicated devices..you need the Specifications of matched Power Supply..
..you really can't depend on a Zombie-PSU.