> I've been doing a little reading on NiMH, Li-ion, and Li-po batteries.
> Some of the information is a bit confusing.

Maybe so. But it's all well covered at a simple level 'on the webn'.
But

> 1. What is energy density anyway? Is it energy/mass or energy/volume?

Both are entirely legitimate and both are used.
Depends on what is important to you.
Often it's both.

Mass affects how much you battery weighs for a give energy content.
Volume affects how big it is for a given energy content.

> 2. For both cases (energy/mass and energy/volume), which battery type
> is best (NiMH/Li-ion/Li-po)?

Best is such a strong word :-).

Within those categories there are several subcategories so answers are
general only.

LiIon MAY have an energy density edge (M or V) and loves to vent with
flame and worse.
It has both a "cycle life" 300-500 typically and a "calendar life"
(Think Blade Runner and replicants).
Battery will die all by itself as time progresses.

LiPoly is a variant of LiIon. Generally safer. May allow faster to
much faster charge and discharged.

NimH may have superior cycle life wrt Lixxx depending on how used.
Has NO calendar life.
Has worse charging efficiency and is harder to manage charging.
Self discharge rate is higher than Lixxx.
Overwhelming advantage of NimH is availability world wide in standard
form factors and high volume of standard cells.
NimH much safer than Lixxx.

A Lithium different is LiFePO4 - Lithium Ferro Phosphate.
MUCH higher cycle life. Uncertainty re calendar life if any.
Energy densities similar to NimH.
Easy to charge and manage.
Good temperature range.
The battery of the future for everyday use IMO unless they come up
with something better - which usually happens.
Too non standard so far.
A Goodenough battery (Wikipedia knows)

R
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