One of the things I’ve had to learn afresh since getting the HPI Bullet is how to manage LiPo (Lithium Polymer) batteries. They are quite a different beast to the old NiCads I was used to.
I’d read a few things about having to condition the batteries and running them in, but it seemed the jury was out as to whether or not that was required. I even got conflicting advice from the store where I bought the car.
I was surprised to find that I was unable to find a “LiPo 101” that wasn’t RC plane-specific, so I wanted to document what I’ve learnt in case it’s of assistance to someone else who’s just starting out with their own LiPo powered RC car — more after the jump.
What I’ve got
I also grabbed a Sparrow EQ charger at the same time.
What the battery specs mean
I wasn’t able to find any especially good resources to make sense of the specs etc. of the battery packs (especially in relation to cars, rather than RC planes), however I found a great intro in a review of Thunder Power LiPos in the January 2011 edition of Racing Lines magazine (which I picked up over the hols). The key passages are reproduced below:
A single LiPo cell has a voltage of 3.7V.
To get the voltage to 7.4V manufacturers put two batteries in series hence the ‘2S’ in the spec and 3S gives you 11.1v and 4S 14.8V.
So ‘4S’ means a pack is made up of four single cells and so on.
The maximum current draw the battery is capable of is measured in multiples of the capacity, so a 5000mAh batter that has a ‘C’ rating of 10 (or 10C) can sustain a maximum current draw of 50amps.
The higher the current rating, the more of the battery’s power that can be delivered at a time — e.g. higher C = higher power. Points 3 and 4 of Jim McPherson’s excellent forum post (focused on RC planes) expands/explains this further.
Using the Sparrow EQ charger
When running in Li-Po mode, the Sparrow EQ always runs in balanced mode. This means you have to connect the balancing lead , in addition to the Dean-style plug , to the charger (as depicted below):
Once you’ve done this, you press the [Enter Start/Stop] option briefly/once to enter the mode to edit the various options. Changing the settings to match the rating information printed on the battery pack was all I needed to do. I’ve read references to setting the charge rate to 1C etc. (which is related to the specs etc. outlined above — e.g. you shouldn’t charge a LiPo at more than 1C = for a 5000mHa LiPo the input rate should not be higher than 5 amps to my understanding), however, I’ve not had to worry about that as the Sparrow does all of this for you automatically once you’ve set these values up correctly.
It took a bit of digging to work out what the different elements of the LCD display meant during charging (something that is missing from the manual). What I’ve been able work out is:
- Charger mode — this will flash between “BNC” and “CHG” when charging, and display the battery type etc. when changing settings (which is covered adequately in the manual).
- Charge time — the amount of time the battery has been on charge (or how long it took once charging is complete)
- Charge input — the mAh input into the battery during + at the end of charging
- Battery voltage — total voltage across all battery cells during + at the end of charging
Running in the battery
As mentioned previously I received differing advice on running in the batteries. One sales assistant tried to explain to me a rather convoluted method of running in the battery, another said just get it out and run with it, being careful to not run it beyond the point where the car felt like it was lacking power.
After doing some research on the net, I couldn’t discern which was the correct advice, so just went and ran with it. This was a mistake — both battery packs ended up displaying faults after the first run: one of the packs “puffed up” slightly (though this was only slight — so it was hard to tell if it was an issue); the other had a run time roughly 50% the time of the first one.
After some further runs I was convinced there was an issue with the second battery and returned both to the store to be checked, only to find that both had issues. Thankfully the store replaced them and then gave me some better instructions for running them in — something I would now recommend!
The basic process for running in a battery is to progressively increase the intensity and length of use, taking note of the various settings from the charger as you go.
So, for example, I first ran the car very lightly for 5 mins, then recharged. The second run I ran a little bit more intensely for about 7 mins. The third run intensely for 9 minutes, and so on…
Each time I kept a log of the readout from the charger — along with notes of intensity etc. An example of the first few runs follows:
|Run details||Time to charge||mHa (amount charged)||Start V||End V|
|5 mins — light||10:45||648||Not recorded||8.41V|
|7 mins — medium||21:56||1489||Not recorded||8.415V|
|9 mins — hard||36:12||2639||7.687V||8.407V|
|11 mins — hard||32:38||2369||7.72V||8.412V|
|14 mins — medium hard||82:59||2554||7.519V||8.395V|
|~20 mins — medium (to auto-cutoff)||57:13||4429||7.421V||8.413V|
In doing so, you can start to get a sense of what sort of time you’ll get from each charge based on how hard you run the car, as well as how much charge will be required. By tracking these values, I was surprised about how much of a difference in time straight out bashing vs. running around a track could have, for example. But also, I was able to verify when I did hit the auto-cutoff that it was calibrated at an appropriately “safe” level (about 3.7V per cell).
So far (touch wood) I haven’t had any issues with the (replaced) batteries and have a pretty good idea how long I can get before the auto-cutoff kicks in based on driving style — but even if I hit the auto-cutoff, I know it’s not going to be so low as to cause battery damage.
A tip for the Bullet: manually set the ESC
As mentioned earlier, a 2S battery should reach a total of 8.4V and not below 6.4V (an estimated “safe” voltage of 3.2V per cell). Even thought I had enabled the auto-cutoff using the default setting of the Bullet’s ESC (as advised in the car’s manual), but the store rep explained to me that sometimes the auto-cutoff features of ESC’s are lower than that recommended, and that I should double-check mine. In doing so I found out that this appears to be 2.6V per cell by default, which is too low!
I have since reset the auto-cutoff to 3.6V, I think — the car’s manual leaves a lot to be desired in working out how to change such settings, but the tracking values (as outlined above) suggest I’ve managed to get it to the right setting.