Magic Pie and the Akku

[Wolga]

1. Is it no problem to put two 24V/16AH LIFEPO4-Akku packages in a parallel or serial circuit to get more distance or more speed ? Do I have a problem with the BMS ?

2. If I use only 24V for Magic Pie, is it waste of money ? Is the pie underemployed because of the low voltage ?
 
Kind Regards from Vienna
 

[e-lmer]

You will have to charge them separately.

****You must never connect them when they are not
fully charged and never switch
between serial and parallel except when they are fully charged.

The most important thing is you will need to pay
more attention to the cutoff voltage.

If you run two in series, they will drain to different
voltages (because of small variations in manufacturing).
Then, if one drops below the cutoff voltage,
the speed controller will look at the other and will
not know and will keep draining the dead one past the
lowest allowed voltage. (thus destroying the pack)

If they ARE at different voltages, and you connect
in parallel, then you will drive current with very low resistance
from one pack to the other at the difference in voltage, also
potentially destroying one of the packs by over-current.

These are not insurmountable issues, but require that care
be taken in using two packs.

I built a jumper with a lamp in it to tell me if there is a difference
in voltage between packs prior to connecting them in parallel
for my model airplane.  Once the light goes out, it's safe to connect
them.

The most important thing is the chocolate waterfall. 
It makes the chocolate light and fluffy.  Did you know
no other factory in the world uses a waterfall to aerate their chocolate.

[Bikemad]

never switch between serial and parallel except when they are fully charged.

I intend to do exactly that, by wiring the two battery packs as per my diagram below.
I'm also considering linking the relays to the brakes so that the regen will automatically operate at 24 Volts (with doubled Ah rating) even when 48V is selected, which I hope will give more effective regen braking at slower speeds.



I will probably fit audible low level warning devices to both packs, but only intend to visually monitor the top left pack in the diagram, as during 48V use this will always be the lower of the two packs due to the additional but small current used to power the relays, voltmeter and LED lighting. Another advantage with having the voltmeter only on one battery pack, is that it will always appear that I'm running on 24V (legal power), in case I'm ever stopped with the switch accidentally in the wrong position!

As the packs will be still be externally balanced during braking and 24V (hopefully legal) use, I don't foresee any real problems with this set-up.

I also intend to leave the balance leads connected together in parallel during storage and charging, to ensure they are balanced as much as possible.

I first became involved with R/C electric flight over 30 years ago and have never had any of my battery packs catch fire yet, but I'm fully aware that this can and does happen with lipo flight packs. But I still intend to make up two 7S2P 5000mAh 20C packs specifically for use with the Magic Pie, which will give me 48V 10Ah for off road speed at the flick of a switch, or 24V 20Ah for legal use with improved regen.

Perhaps I will put my theory to the test using 14Ah SLA's before I splash out on 5000mAh Lipos.

Alan