There are a lot of questions here.
Are you talking about building it from individual cells
or from smaller voltage batteries?
What Chemistry?
Let's start with the assumption that you are talking about Lithium
cells. The reason I am doing so is that they offer the most interesting
discussion.
Lithium Cells are rated with two numbers, the AH rating (you said 16AH)
and the discharge (C) rating. The C rating is roughly the multiple of the
AH rating that is the maximum continuous discharge current.
If you exceed the max C discharge rate you can damage your cells.
A good rule of thumb is to work around 80% of the max C rating to
extend the life of your cells.
If you are building a pack from 8AH cells, then you need to hook two cells
in parallel to get 16AH from that one cell. This gets problematic if one of the
cells goes bad and the other doesn't. I don't suggest parallel cell architecture.
The open cell voltage for lithium cells varies according to chemistry.
Li-FeS2 has an open circuit voltage of 1.8 Volts, while LiFePO4 is 3.6V
To get to 36V with 3.6 Volt lithium cells you need to connect
(36/3.6) about 10 cells in series.
Now comes the interesting part. Because of variations in manufacturing,
each cell loses potential by generating heat at a different rate.
Every time you discharge and recharge the series array, the cells that are
least efficient go to a lower voltage, while more efficient ones stay higher.
Your charger, on the other hand, only knows to push current in until they
block it, so you end up putting too high a charge on some of the cells and
damaging them.
The solution is a
BMS (battery management system.)
I do not know anything about these guys, I just googled to get a diagram
http://liionbms.com/php/lithiumind.phpThey look really expensive tho.
The
BMS compares the cells to see if they are at different voltages, and
drains some of the charge from the higher one to try to keep them level.
It means you need one more connection to the
BMS than you have cells
in your pack.
Loose cells are not easy to hook together to carry 20 amps or so that
you may need for your motor.
All in all, it usually ends up cheaper buying a pre-assembled pack.
