Supported Battery Types

The first thing to look for in a charger is whether it supports the type of battery you’re going to be using. If you’re buying a brand new charger and batteries, it’s likely to be all LiPo, but if you’re buying used equipment, you might stumble on NiCad/NiMH chargers, for example.

DO NOT use a NiCad/NiMH charger with a LiPo battery! These are liable to over-charge, which can cause the pack to vent flammable electrolyte gas. Here’s a demonstration of what can happen when charging with a NiCad program:

If you take care of your batteries, you’ll be fine, but I just want to impress that it is essential to use a charger designed for LiPo. The good news is that if you still have NiCad/NiMH batteries around, most LiPo chargers have charging programs for them as well, so you can have one charger to do it all.

“C” Rating

You might see things like 1C, 2C, 60C, and so on, and wonder what that’s about. This isn’t Celsius degrees, but in the context of batteries, refers to an amount of current that’s relative to the capacity (hence the “C”) of the battery. So if you have a 5,500 milliamps/hour (5.5 amps/hour) battery, and it says it is rated at 60C of output, it means that it can put out 330 amps of current. Or if they say you should charge it at 1C, it means you should set your charger at 5.5 amps.

Better quality batteries usually have higher “C” ratings for their output, as it means they can feed more power to your motor when you accelerate hard.

Charging currents are also expressed in a similar way. 1C is the “standard” charging current, with LiPo experts saying that 2C or even 3C should be okay with the current hard-cased packs, in good condition (no cracks or other damage), but the BRCA dictates that no more than 1C should be used at their events, for safety (since the danger with LiPo is mainly with over-charging).

But when looking around for chargers and batteries, you might notice that batteries now start at about 4,0000 mA/h, and can go all the way up to almost 8,000 mAh, but some cheaper chargers only go to 2.5A or 3A, which wouldn’t be sufficient to even reach the recommended 1C! There is no danger with that, but charging at a lower current will take proportionally longer, so you might want to make sure your charger can do at least 5 amps, to keep charging times reasonable.

Balance Charging

On the batteries page, I explained that battery packs have some number of cells in them.

If your LiPo pack has more than one cell, it’s important to use a “balance charger”. This is because each cell in the pack might not be performing equally. A charger will send power until the voltage of the battery reaches a certain level (usually 8.4V, in the case of a 2S LiPo), but since this voltage is the sum of the cells voltage (because they are in series, “S”), it’s possible that one cell is slow to charge and has only 4V, and the other has 4.4V. Then, during the race, the weaker cell will run out of power earlier, and your car will run on partial power (not quite fully drained, but clearly slower), and that’s no good!

Also, if you remember the warnings at the top, the main danger with LiPo is over-charging, so if one of the two cells in your pack really gets out of balance, it might become dangerous, as you good cell might get over-charged to tried to make up for the weak one.

A balance charger has extra leads, so that cells can be measured and charged individually. For example, a 2S pack has three connectors, big ones for each ends, and a small one in the middle, which is used for balance charging.

There are some variations with those extra connectors, so make sure your charger has the right leads for the batteries you will be using (ask your hobby shop).

If you use 1S (single-cell) batteries, then you don’t need balance charging, of course, since there is nothing to balance. :-)

Connectors

There’s a few different types of battery connectors, and some that look very similar (like barrel connectors, which come in 4mm and 5mm sizes!). Check that you have the right leads for your battery (again, ask your hobby shop!).

Input Power

It wasn’t obvious to me at first, but some chargers connect to the mains for power, and others use 12V, which can run off a car battery, but needs a power supply to run off mains power.

When I was looking for a charger, I saw one with great specifications and features that seemed like a great deal, but almost did not notice that it was 12V only! It looked like a “great deal”, but I was comparing it to other chargers with built-in mains power supplies, so really, the discount was because it was missing a part I needed!

Storage Program

I explained on the batteries page how to store batteries so that they do not lose capacity over time, by leaving them half-charged. Some chargers have a “storage” program designed to put the battery at the best level for storage. I explained a technique to do this approximately, so this feature isn’t required, but it’s definitely nice to have.

It’s especially handy if you store batteries for a long time, you can simply run them through the storage program every three months to make sure they remain at a good level for storage (batteries slowly leak power, so even if it was at a good level when you stored it, over time, it can go down to damaging levels).