For starters, there is the big difference between NiMH and LiPo batteries. For NiMH think sort of old-fashion regular batteries--just bigger (like Alkaline "C", but the rechargeable kind). For LiPo think new age technology like you'd find in the newest cameras and power-tools.
There are a ton of different categories of each type on the market and then there is even more kinds within each category. At the top level, LiPo beats NiMH hands down, except in one area--price $. LiPo is more expensive than NiMH. But in terms of usefulness of the actual battery LiPo is cheaper. For example, a decent NiMH battery runs you around $20-$40 whereas a decent Lipo battery starts at around $40+ and goes all the way up into the hundreds of dollars. But overall the LiPo battery will last longer and run harder per charge than the NiMH.
Additionally LiPo's weigh a lot less than NiMH (which may or may not concern you), but does for me as I like speed when I RC; both in terms of top-speed and "punch" when I hit the throttle. LiPo's also last longer if properly maintained. And the maintenance part is where I'm learning a lot about battery technology. If you do invest in LiPo batteries as I am, do yourself a huge favor and spend the extra money on a really good charger like mine from Reedy:
A must-have feature on any LiPo charger you buy is the "balance-charge" feature. That is the purpose of the rectangular circuit board with wires device seen in this pic above. It ensures the cells/segments within the battery are evenly charged. Also you want an audible warning for when the battery reaches max charge. And you want select-able control over how fast you actually charge the battery (to high and you'll damage it). Each LiPo battery comes with a specified charge rate. But as an overall guideline, I just think "the lower the charge rate the safer I am." So it may take several hours to charge my LiPo battery, but that's fine with me if it helps the battery's life expectancy.
Now for internal guts or cells of a battery, both types--LiPo and NiMH--have a "cell" listing on the outside. Each means a similar thing, but are written differently. For NiMH batteries, cell actually means individual round, fat batteries that are enclosed in the battery "pack." It is common to see 6-cell, 7-cell and "hump" 8-cell (called "hump" because the last cell is on top of the pack by itself) NiMH batteries. In LiPo batteries however, it is referred to as "S" and indicates the segments inside the battery pack (LiPo packs come in both soft and hard cases, but the segments are not visible). It is common in LiPos to see 2S, 3S and 4S. In general, the higher the "S" the more voltage the battery can deliver.
For RC surface vehicles (as the flying RC hobby has a whole other set of batteries that hurts my head to think about), voltage is important and is commonly listed on LiPo and NiMH as: 7.2v (NiMH), 7.4v (LiPo), 8.4v (NiMH), 11.1v (LiPo), and 14.8v (LiPo). An easy way to remember it for a "lay person" (not an electrical engineer trained person), the higher the voltage the more "power" that can be delivered to the RC vehicle's motor. Now of course it is not that simple, for there are many factors that I'm only beginning to understand, such as the ability for the Electronic Speed Control (ESC) in the RC truck to handle the voltage and pass it to the motor without frying your vehicle's components.
For both types of batteries another important factor is called the "mAh" rating. This is the milli-Amper hour rating of the battery and denotes how long the battery can hold a charge for. So again for the "lay person or hobbyist" it is good to know that the higher the mAh rating, the longer you can run your RC model per charge. And the higher the mAh, the more expensive it is (makes sense right?). Common ratings seen for mAh are 3200, 4200 and 5000 (there is even 10000 mAh batteries that will cost you a lot money). Typically a 4200 mAh battery, whether it is NiMH or LiPo, will give you around 30-40 mins of RC truck run time.
Another part of battery knowledge is charge and discharge ratings. And related to that, the overcharge rating. Until recent improvements apparently in Lipo technology it was common for fires to occur with LiPos that were overcharged. Also, if you discharge or "use-up" the current charge in the LiPo battery you can ruin it for good. So higher-end (i.e. more expensive) RC models that are designed for LiPos have a low-voltage kill circuit that kicks in when the battery gets to low before it is permanently damaged. For NiMH, the difference is heat; as you can completely discharge a NiMH battery without worry as long as you do so in a way that the battery does not become extremely hot (as you can ruin the battery if you over heat it). That is why you never charge or use a hot-to-the-touch NiMH battery. NiMH chargers tend to be simple wall plug devices that will turn off when the NiMH is fully charged.
Last but certainly not least, and actually next to voltage the most important aspect of LiPo batteries, it is very important to know the actual rate the battery can deliver its power out...and that is know as the "C" rating. Typically, the more affordable LiPo "C" ratings are 20C, 25C and 30C and are perfectly good for the casual RC enthusiast. For more serious RC hobbyist who want maximum performance you want the more expensive 50C, 60C and 100C rated batteries. And for those who actually race their vehicles professionally a high "C" rating, say around 150C and 200C, is a must (and will set you back more than $300); as those are the batteries that can win races and provide top speeds. Now to achieve those higher "C" ratings it means the battery must normally come with a higher voltage, i.e. to achieve an 100C or more output you will need a larger 14.8v 4S LiPo battery.
Connectors are also important elements. Two plus decades ago when I built my very first Tamiya RC buggy, there was really only one type of connector... the standard or "Tamiya" connector. Now the RC hobby marketplace has at least six different types and growing. On just my family of RC SCTs and Jeep alone I have the 4mm Bullet, Tamiya, EC3, Traxx, and Deans (or T) connectors to worry about. It gets confusing, but essentially the newer types--Traxx and Deans--work better to keep a steady rate of current through the wires and connection, which obviously makes for better performance. Luckily on eBay there is a large seller group that make various connector-to-connector setups (e.g. I had to buy a female EC3-to-male Deans wired connector for one of my trucks).A common question I think when it comes to the RC ground hobby and batteries is "...are LiPo and NiMH setups inter-changeable?" The quick & simple answer is "No." But as with most things, there are exceptions; some RC companies prefer LiPo, but allow you to run NiMH instead (hence lowering the price overall) and set-up the internal components to handle both right out of the box. Also it is generally safe to run a "low-level" LiPo in a NiMH setup, say like a 7.4v 2S LiPo. This provides a tad better running experience without the danger of frying your model. The safest measure is to read your documentation and online forums to check your trucks battery needs/allowances. An example is, the crappy budget ECX Torment 2wd 1:10 SCT comes setup and with a "free" terrible 7.2v 6Cell crap-tastic 1800 mAh NiMH battery, but runs far better without any internal changes required with a 7.4v 2S LiPo battery instead. The wife's Axial Jeep shows on the box a picture of a NiMH battery pack as the separately sold element to power the model, however all of the documentation inside the box highly recommends a 7.4v LiPo battery and is even setup that way--as you see, it can be confusing as heck!
