Multi batteries in series?

[SydneyCommuter]

Seeing Gary's multi battery setup gave me an idea. Advice from the pie brains trust appreciated.
I currently have a 36v, 16ah gm battery on my internal controller pie. As I could do with some extra watts for some evil hills on my 30km commute was going to go 48v. Then after seeing Gary's post it occurred to me, what about keep using my 36v and add a second 24v in series giving 60v. I thought I'll make up a small junction box so I can keep the power cable to the std so I can go back to one if wanted.
I've scoured the forum for ref to voltage applied to stock internal pie some ref that 62v limit on the controller.
Do u recon it'll hack the 2 in series? I checked the o/p on my 36v and fully charged with no load its measuring  40v on my avo at the Anderson plug. So add 24 and I'll be well over 62v. I should easy do wk and back on one charge with the extra electrons, so no more carrying battery up to my desk to charge for trn trip any more.
Do you recon try it, ok +or- a few volts or wait and upgrade to t pieiii in the new year, or just be boring and be satisfied with the extra couple hundred watts and 5 km/hr with the 48v?.

[Bikemad]

Putting a 24 and 36V battery in series would be too much voltage for the Pie's controller (and the MPIII) because it would be over 70V (42V +29.4V) fully charged, but two identical 24V packs should be fine.

Basically, the capacitors in the controller are only rated for maximum safe voltage of 63V and any more than 63V would eventually result in their failure.

I carried out some basic tests on my original Magic Pie when I first had it to compare the maximum power from 14Ah SLA batteries and Turnigy 5Ah 20C LiPo packs and I discovered then that the MKI Pie's controller would not even work at only 61.63Volts.
The tests were done by raising the wheel off the ground and then holding the throttle fully open and gradually applying the brake until the wheel was almost completely stopped.

These are the readings from my watt meter:

Battery      No load voltage    Max Amps    Min Volts    Max Wattage
24V SLA       25.76        20.14     23.09        465
25.9V LiPo    27.30        20.29     25.80        525
36V SLA       38.70        20.21     34.79        703
48V SLA       51.50        20.33     44.99        915
48.1V LiPo    50.30        19.85     47.75        949
55.5V LiPo    58.70        20.08     55.51       1115
59.2V LiPo    61.63 voltage was too high to test (the Pie refused to work!)

If you have two or more batteries of identical voltage (and type) you can either connect them in parallel, or simply plug each one in separately when required.
If you had a 48V and 36V pack, you could only use them one at a time by simply swapping the battery lead from one to the other.
Then you could use the 48V pack to get to work quickly, and then return home at a more leisurely pace using the 36V pack.

The current MPIII controller also uses 63V capacitors, so the MPIII is unlikely to support the higher voltages that it was originally expected to use.

Don't forget that when you use a higher voltage pack, you will use up more of your battery's energy fighting against the increased wind resistance at the higher speeds, so a pair of 24V batteries would probably take you much further in parallel than if they were wired in series.

Alan

[e-lmer]

I would like to add extra warning here.

If you hook up two batteries with dissimilar amp-hours in
series, then when one of them drops below the suggested
cut-off voltage, the other may hold the total voltage high
enough to damage the lower power one.

If your batteries have built in cutoff circuitry then you are
safe.

[SydneyCommuter]

That's pretty unequivocal. Many thanks for that comprehensive response Alan!
 I'll get a 48v on order ASAP.
I could go the 36 as a range extender as u suggest, I was also having a wild moment thinking of seeing about repurposing my 36 to a 48 I assumed that the 36 16 ah has the same cells but in different config and diff bms to the 48 12ah, do u think I could get a 48 bms and/or connectors and turn the 36 into a 48?? From the gm guys?

[Bikemad]

The 48V12Ah pack uses thirty nine 3.7V4Ah cells arranged in thirteen groups of three, whereas your 36V16Ah pack uses forty cells arranged in ten groups of four.

Getting hold of a suitable BMS board and balance lead connector assembly might be difficult, as GM have now changed over to LiFePO4 instead of LiMn cells.

Modifying a pack will not be straightforward, because the cell layout is different, and they are spot welded to different metal tags for the different packs.

Your 36V pack will be similar to the 24V pack shown below, but with three more groups of four cells.

And don't forget that you would also need a properly matched 48V charger (54.6V regulated output) to be able to charge the newly modified pack.

Alan
 

[SydneyCommuter]

Many thanks Alan for that comprehensive response yet again.
Looks like I'll have to give this a lot more thought, I'm starting to think that by the time I sort out reconfiguring the cells and a dif BMS it would be much more expeditious and practical to just get another 48v. Given I have a bunch of other more pressing issues to resolve first, like cracking the case and fixing the fault stopping me programming, so I can turn off regen so I don't get the delay starts on my stop start frequent intersection commute. Still I can get on with that now I know, cheers Mark