This article will focus primarily on Brushless and LiPo technologies, as these are the most common techs people use for 1/8th scale electric. I will add information pertaining to the use and charging of NiMH packs soon.
Why would I want an electric?
Did I just hear you say electrics are for kids?! I don't know about you but I wouldn't give my kids this to play with:
Now, if that didn't convince you, maybe this will:
(EV = electric vehicle, ICE = internal combustion engine)
Look at the light blue line. That's right. Instant torque starting from 1RPM, and a power curve to die for.
This, gentlemen is why electric is not just for kids.
Advantages of electric
- No dirty oil/nitro
- No glow plugs
- No glow ignitor
- No buying & carting around expensive fuel
- No tuning the engine
- No separate receiver battery
- One less servo to go wrong
- No need to start the engine
- Fairly quiet - suitable for neighbourhoods or night running
- Less maintenance
- Expensive! - charging equipment and batteries alone will cost you ~$500 for high quality products.
- You can only run if you have extra charged batteries. No carrying around a tank of fuel that lasts a whole day.
PRIMER! Lets get started.
There are several different chemistries of battery that are used today in RC.
- NiCd - Nickel Cadmium. Old school, not used so much any more, we won't concentrate on this.
- NiMH - Nickel Metal Hydride. Very popular in a lot of applications. The hump pack in your savage is most likely NiMH. NiMH cells are reasonably safe and offer good performance, whilst having little memory. Nominal cell voltage is 1.2v.
- LiPo - Lithium Polymer. The popularity of this battery has exploded in the RC world lately. Good LiPos offer extreme performance whilst saving weight, and being a nice square cornered form factor. The downside is they are DANGEROUS if not handled properly. More on safety later. The majority of this article will focus on LiPos. Nominal cell voltage is 3.7v.
- LiMn - Lithium Manganese. Very similar to LiPo, but a lot safer. The penalty for this is that they weigh more for the same capacity. Not nearly as popular as LiPo. Nominal cell voltage is 3.7v.
- A123 - Lithium Ion. A123 batteries are a new contender on the scene. The form factor is like large NiMH cells, meaning they are tricky to fit in most trucks or cars. Aircraft owners however are finding great use for these new cells. Nominal cell voltage is 3.3v.
I'll use the analogy of water travelling through a hose for these terms.
This is the amount of electricity travelling through a wire or battery at any given time. Kind of like the difference in between having a fat hose and a thin one. Measured in Amps (A)
This is the "punch" that the electricity has. Kind of like the pressure in that hose. 12v might be like a faucet, 120v might be like a pressure washer! Measured in Volts (V)
Both Voltage AND Current have a bearing on the power your truck will have, and it is a very simple relation:
Current is linearly related to Torque (more current, more torque!)
Voltage is linearly related to RPM (more voltage, faster it goes!)
Like the fuel tank on your savage, the capacity of a battery determines how long it can run your truck for. Capacity is measured in a term Amp-Hours (Ah). This means how many hours the battery can output 1 Amp of current for. For example, a 4Ah battery could power a source sucking 2 Amps for 2 straight hours.
Often capacity is measured in mAh (milliamp-hours). a 4Ah battery is the same as a 4000mAh battery.
You will most likely be able to fit around 5000mAh batteries in the flux battery boxes, although I cannot confirm this yet.
This has the biggest bearing on how fast your motor will run. To give you an example, most 10th scale electric cars use somewhere in the region from 7.2v to 11.1v (depending on the chemistry of the battery and other factors). 8th scale cars and trucks may use anywhere from 14.4v to 22.2v.
When people talk about LiPo battery packs, they don't usually talk about the actual voltage, instead they say "3S", "4S" etc. I will elaborate on the terminology of lipo packs later, but in short this is the same as voltage. A 3S pack is a pack of 3 serial cells, meaning it has 3 x 3.7v cells linked together, adding up to 11.1v. 4S has 14.4v, etc.
You will find in a truck like a savage, 4S, 5S or 6S are all suitable voltages.
For those of you wanting raging beasts... 6S (22.2v) is your voltage!
The final specification of a battery pack is the discharge, measured by something called "C rating". This is how much juice the pack can deliver at any specific time - a bit like how much fuel a fuel system can deliver at once in a car.
For example a 30C pack with a capacity of 4 Ah (4000mAh) could deliver 4 * 30 = 120A at once.
Packs generally have "constant" discharge ratings and "burst" discharge ratings. The burst rating is what the pack can deliver in short bursts, for example to aid in acceleration, but it can not maintain this current for long.
So what happens when your discharge is not good enough? Generally the motor system will "cog", during acceleration, which is where it stutters and hesitates, a bit like a car running out of gas. Cogging is a sign that your battery is not up to the task!
Look for batteries with a constant rating of 20-30C if possible. The higher the better.
Lithium pack terminology
You may find some packs referred to with the numbers and the letters "S" and "P". For example a 3S pack, a 4P pack, or perhaps a 2S2P pack.
"S" stands for Serial. When cells are stacked in a serial configuration, their voltage adds up. For example a 2S pack is 2 3.7v cells in serial, adding up to 7.4v total. The capacity remains the same as each individual cell.
"P" stands for parallel. When cells are stacked in a parallel configuration, their capacity adds up. For example a 4000mAh 2P pack would create a 3.7v 8000mAh pack.
The terms can be combined, so for example a 2S2P pack is 2 packs of 2 serial cells stacked in parallel. So if using 4000mAh cells, a 2S2P pack would afford you 7.4v at 8000mAh!
Running multiple packs in one vehicle
Some vehicles take a single pack (like the FLM savage brushless conversion). Using a single 4S/5S/6S pack in these is suitable. Other vehicles like the Flux, e-maxx, e-revo, e-savage have two battery compartments. Having two compartments is better for balance and usually can accomodate a higher overall capacity.
In the Flux, if you wanted to run a 5000mAh 6S pack, you would need to divide it up between the packs. So for example you would run two 5000mAh 3S packs - one in each battery box. The Flux is pre-wired to accept two packs like this. It combines them in the same way as mentioned in the section above. These two separate 3S packs essentially become one 6S pack.
Keeping your pack in good health
LiPo cells will be damaged and die if they fall below a certain voltage. The safety level is 3v per cell. Most good ESCs have built in devices called LVCs (Low voltage cut-off). This stops the truck when the voltage falls to this critical level, letting you know it's time to stop or recharge the pack. If you were to run a LiPo with no LVC until the battery died, it would really be dead - no recharging would bring it back (and would be indeed hazardous).
The flux's RTR ESC has an LVC, so no need to worry about that. You must however, make sure the LVC is set correctly in the settings to match the battery. a 6S battery needs the LVC set to 18v (6 * 3v).
Don't leave LiPo packs in your truck. Always remove them and store them in a safe fireproof place. I use a small metal lock box. They only cost a few dollars and will keep your batteries safe. The added bonus is you can lock the box to stop any young children you may have from playing with them.
If you won't be using your pack for a long time, charge it half way, so the voltage does not drop naturally below the critical level mentioned above.
This is my lockbox, to give you an idea.
Similar containers like ammo boxes are equally useful.
Dangers of Lithium Chemistries
LiPo and other lithium chemistries have a lot of power stored inside them. Unfortunately this power can be released in the form of fire. LiPo packs are delicate and should be handled with care. A punctured cell can very quickly catch fire and will quickly turn the pack in to a raging fireball. One of the biggest dangers comes from improper charging. I speak below about proper charging practices. NEVER exceed the stated charging rate/voltage. NEVER let the pack become overcharged. NEVER leave a pack unattended on charge. ALWAYS store your packs in a fireproof container. I want to make myself very clear on this, because people have lost homes due to lithium packs. You don't want your savage, car or home to look like this:
(These are all real results of LiPo accidents)
This is a video of an over-charged LiPo:
Now that I've scared you a little - don't be scared away! LiPos are perfectly safe as long as you treat them with respect, charge them correctly and store them safely.
To charge LiPo, you need a specifically designed LiPo charger. You cannot charge them with any old charger, and doing so is very dangerous.
So what current do you charge LiPos at? [s]There is only one answer to this. ALWAYS 1C or lower.[/s] Times have changed and whilst it used to only be safe to charge your packs at 1C, many packs accept higher charge rates, for example 2C or 3C. The battery should state on it. If in doubt ALWAYS revert to 1C. This "1C" is relative to the pack you have, so the current will be dependent on the capacity of pack you own.
This means if you have a 4000mAh (remember, 4Ah) pack, you must charge at 4A. If you have a 6000mAh pack, charge at 6A. If you have a 6250mAh pack, charge at 6.25A.
If your charger cannot charge at the exact current you need, set it to the next lowest. Never higher. So with a 6250mAh pack, if your charger can do 1, 4 or 8A, you would want to charge that pack at 4A.
The charger will require knowing what voltage your pack is. Simply make sure it is set to the same voltage as your pack. Typically chargers will refer to the voltage as cell count, like I mentioned above - 2S, 3S, 4S, etc.
Charging the pack
When charging, check your pack, make sure you know what the specifications are. Set the charger. Check the specs again and make sure the settings match. Double check. Triple check. Thankfully most good chargers these days come with safety features which make it hard to accidentally overcharge or charge on the wrong voltage. But this is no reason not to triple check your settings before starting a charge.
Don't leave the pack unattended whilst charging. It is recommended to also charge it in a fireproof place - a stone counter or a ceramic surface of some kind would be ideal. Several shops sell something called a Lipo Sack which is a fireproof bag you can place the battery in whilst charging, to drastically reduce any chance of an accident burning your house down.
Personally I use a small metal lock box to store and charge my LiPos. And importantly, I never leave them unattended. If you need to leave to go and do something, ask a family member to keep an eye on it, or stop the charge.
Because of the sensitivity of cells to voltage drops (mentioned above in the health section), they must be kept in line with each other's voltage. This is achieved by "balancing" the pack. This is done with a device similar to a charger called a balancer. Some LiPo chargers have balancers built in, and some balancers are devices in their own right. The battery has a special connector called a balance tap/balance connector. You simply plug it in to the balancer and wait till the balancer tells you it is finished. This should be done periodically, preferably after a full charge.
The tricky part is that there are several brands of balancer connectors. Hyperion/Apache/Method taps are the most common, and for that reason I recommend you order batteries with those taps if you are unsure.
[align=center]Brushless Power Systems[/align]
Brushless motors are a revolution in electric motor technology. They only have one moving part and no mechanical contact anywhere apart from the bearings. This makes them extremely efficient and reliable. They need zero maintenance (apart from replacing worn bearings). This is a far cry from the older hot mod brushed motors where brushes and commutators frequently wore out and needed replacing or re-cutting.
Brushless motors require Brushless specific ESCs. Regular ESCs will not work.
(ESC stands for Electronic Speed Controller, and is the brain and power provider for the motor)
Sensored and unsensored systems
There are two main types of brushless motor - sensored and unsensored. Sensored systems have dedicated sensors to detect what position the rotor is at and use this information can make the spinning of the motor much smoother at low RPMs. Unsensored systems infer the rotor position based on something called back emf, which is less precise.
Most systems are unsensored, as it is fairly unnecessary and adds expense. A good ESC can run a unsensored motor very smoothly. The standard RTR system on the Flux is unsensored.
ESC - Continuous Current
This is the maximum current the controller can handle. The current that flows through the ESC is determined by what the motor demands at that situation. Acceleration is the hardest on the ESC - it draws the most current at once. If the motor demands more current then the ESC can handle, and the battery is capable of providing that much current (discharge rating on the battery), the ESC will simply go "pop", you'll see little dollar signs floating out of your truck, and have an expensive paperweight.
With the Flux, you have little reason to worry. The RTR ESC is rated for 120A continuous (and significantly more for burst periods), which is more than enough for the demands of a 1/8th scale truck.
Motor - Kv
Brushless motors are rated by something called Kv. This is a simple term which means the number of RPM the motor can do per volt applied to it. So for example, a 2200Kv motor on a 6S pack (22.2v) would spin at 48,840rpm.
Higher Kv motors spin faster and use more power, and generally lower Kv motors have more torque.
The stock Flux comes with a 2200Kv motor.
Powering your receiver
An advantage of electric powered cars is the fact you can forget about receiver packs. No need for a hump pack any more, because the main battery can power your receiver. This is done through the ESC, by a device called a BEC (Battery Eliminator Circuit). The wire that runs from the ESC to the throttle out on the RX not only gets the throttle position from the RX, but it doubles by powering it. So you don't need to plug anything in to the power port on your receiver.
Hope this helps anyone out, I know it's kinda of daunting for those who have never touched electric before, but I assure you that its easy to get to grips with. Brushless is an exploding market and will get very big in the next few years, so if even if you aren't sure about electrics, I suggest you try one! It might just change your mind.
I'm pretty exhausted after writing all this, so I'm sure there is stuff I have got wrong/typo'd/missed out. Please reply with whatever and I'll update the thread with it.