are you sure you have all that right?
If your using Li-on batts. then use two their light weight and the energy is easy to recharge.
lithium-ion batteries are notoriously difficult to recharge. if you buy an off the shelf battery charger meant for this battery type you should be all right but it's kind of complicated to build your own.
The issue is that in series the first unit will probably starve the second for voltage.
what do you mean here? connecting batteries in series is how you build a battery pack.
i'm presuming you mean it is not a good idea to connect cells of different value together in series as when the lower rated one runs out of power current will be forced through it backwards, destroying it.
as long as you are using batteries of the same type and charge level, connecting them in series is the standard way of building a battery pack of higher voltage than a single cell.
If your using Metal Hydride try for one, you need to bleed them dry to prevent the 'memory effect'
while early nickel metal hydride batteries had a few issues with memory effect, any you buy today are really easy to look after. i have never found any of my packs to need any special care.
while it may be a good for single cells, "bleed[ing] them dry" is a very risky tactic if you have several of these cells connected in serial in a battery pack. if one runs out of power before the rest it will be damaged by the other cells. in general it is not good practice to completely drain any battery that is made of multiple cells.
as for lead acid batteries, they are perfectly safe as long as you don't over charge them. there are probably not the best choice for a bot though unless you need to be able to suck all the power out of your battery in a short space of time. (this is why they are still used to start cars. you can draw a huge amount of current from lead acid batteries without doing them damage.)
sorry to disagree.
batteries is one of these areas that every one has their own opinions about.
I though that if you had 2 batteries in parallel and the current draw stays the same, they would last longer.
yup. that's what ComputerGeek's saying, as the voltage stays the same, your equipment will draw the same current.
what does change is the number of Ah (Amp hours) goes up. in this case you could draw the same current (Amps) for twice as long (hours).