This is a bit of a Public Service Announcement regarding Lithium Polymer (LiPo) batteries. Over the past year I’ve purchased a couple of used RC cars that came with LiPo batteries. I didn’t need the batteries per-se, but the condition of the batteries when they arrived has made me wonder whether people really know how to properly care for them.
On the beginners page of this very website we have a section on batteries, including LiPos. It gives some basic information but I thought it’d be worth highlighting some of the critical aspects, as LiPo fires are certainly possible without the correct care.
And since the detail below is a bit wordy, here’s a quick summary:
- DISCHARGING ANY INDIVIDUAL CELL BELOW 3v WILL DAMAGE IT
- CHARGING ANY INDIVIDUAL CELL ABOVE 4.2v WILL DAMAGE IT.
- ALWAYS BALANCE CHARGE
- CHARGE AT 1C (the capacity of the battery in Amp)
- CHARGE IN A LIPO SACK/BAG AND NEVER LEAVE CHARGING BATTERIES UNATTENDED
- SET YOUR ESCs LOW VOLTAGE CUTOFF (LVC) >3v/cell
- (Optionally use a Low Voltage Alarm (LVA) plugged into the balance port set to >3v/cell)
- RECHARGE TO 3.85v/CELL FOR STORAGE or
- DISCHARGE A CHARGED PACK DOWN TO 3.85v/CELL FOR STORAGE
- IF A BATTERY PUFFS UP, STOP USING IT
Overview of a LiPo
A LiPo pack is one built from ‘cells’. A single cell is nominally rated at 3.7v (volts) but operates between 3v and 4.2v depending upon state-of-charge. A LiPo pack made of a single cell is referred to as 1S. Therefore a 2S pack, is a battery, made of two 1S 3.7v cells totaling a battery pack of 7.4v. Likewise a 3S pack is made of three cells giving a total voltage of 11.1v.
The cables coming from the battery will include the main Positive (+ve) and negative (-ve) wires, but also a balance lead that links to the +ve and -ve terminals of each individual cell inside. For a 2S pack, that balance lead has 3 wires allowing you (or your charger) to measure the voltage of each cell.
Charging you LiPo
There’s is one mantra here – ALWAYS ALWAYS BALANCE CHARGE. That is when the voltage of each cell within a pack is monitored while charging. You’ll need to set your charger to the number of cells (1S-6S is typical) and to connect both the main connections, and the balance connector to your charger. The majority of the ‘juice’ is fed back into the battery via the main cables, but nearing the end of the charge, the charger carefully tops up each cell separately. A LiPo is fully charged when each of the cells within it reach 4.2v. CHARGING ANY INDIVIDUAL LIPO CELL ABOVE 4.2v WILL DAMAGE IT.
As far as the charging rate goes, unless you’re totally running out of time (i.e. between heats at an important event) then it’s always best to CHARGE YOUR LIPOs AT 1C. C is a measure of capacity so if your pack is a 5000mAh (milli Amp Hour) then you charge at 5000mA = 5A. The charger begins charging at that rate until the cell voltages get nearer 4.2v and then the charging rate slows to a crawl when the charger smartly charges only the cell that needs the power. Charging is completed when each cell in the battery pack has a voltage of 4.2v. CHARGING ANY INDIVIDUAL LIPO CELL ABOVE 4.2v WILL DAMAGE IT.
Pit Tip: Charge on a non-combustible/non-meltable surface. I use an old porcelain floor tile I had lying around. Saves your workbench/window-sill if something goes wrong.
Using your LiPo
DISCHARGING ANY INDIVIDUAL LIPO CELL BELOW 3v WILL DAMAGE IT.
You should always check that your Electronic Speed Control (ESC) has it’s LiPo Low Voltage Cutoff (LVC) enabled, and that it is set to some reasonable value exceeding 3v/cell if user-switchable. Although the ESC manufacturers are trying to be helpful by letting you set it to a value of a particular voltage/cell, the LVC is actually fairly dumb since it can only MEASURE VOLTAGE ACROSS THE WHOLE BATTERY (the balance leads aren’t connected to the ESC). So even though you set the LVC to 3.0v/cell, the ESC actually decides whether you’ve plugged a 2S, 3S or whatever pack into it, and then determines the multiple of 3.0v that it’s going to cutoff at. So when you connect a 2S battery, the ESC thinks “this battery has a voltage somewhere between 6 and 8.4v so it must be a 2S and I’ll therefore cut-off when the voltage drops to 2*the user-selected value = 6v (in this case)”. It’s trying to be smart, but you can imagine that if one of the cells in this 2S pack is going bad, then the ESC won’t see a problem even if one cell was dangerously low at 2.6v and the other was totally fine at 3.5v. The ESC just sees 2.6+3.5 = 6.1v and thinks everything is fine. It’s much safer to set the LVC to something like 3.4v/cell, where a 2S pack would be cutoff at 6.8v.
There’s also a small circuit board available for around $5 called a Low Voltage Alarm (LVA). It’s connected only into the balance lead on a battery pack and is small enough to leave in your vehicle while running. Basically it’s a voltmeter with a bloody loud alarm on it, and it’s CONTINUOUSLY MEASURING THE VOLTAGE OF EACH CELL. The voltage at which the alarm sounds it user selectable, but since you’re measuring the voltage of each cell you can set it to just over 3v/cell. It’s a great fail-safe and one that could save your battery.
Storing your LiPo
If you need to leave your LiPo unused for a few days, weeks or months (you know, because it’s wet out!) then putting them safely in a storage condition will keep them lasting longer. This is something lots of people overlook – myself included when I first started using LiPo batteries. STORAGE is when each cell in the battery pack is at 3.85v. It’s just above mid-way between a full charge of 4.2v/cell, and a used battery at 3v/cell. If you’ve run your battery down to 3v/cell then you’ll need to STORAGE CHARGE TO 3.85v/cell. Most chargers have a STORAGE option, which is just like balance charging but it stops at 3.85v/cell not 4.2v/cell.
If for some reason, you didn’t fully use a charged battery, say because you smashed the front corner off your car on a rock. Then you’ll need to DISCHARGE TO THE STORAGE VOLTAGE OF 3.85v/cell. Most chargers had a discharge setting, but it takes a really long time. The single most useful piece of equipment I’ve added to my battery maintenance is a cheap ($30max) 150watt DISCHARGER/BALANCER that takes your battery pack down to 3.85v/cell, or balances mismatched cells if you forgot to balance charge. There’s lots of similar looking devices out there on eBay/Amazon, but make sure you get the one with the light-bulbs on as that’s what gives you a reasonably quick discharge rate.
Store your LiPos disconnected from your vehicles, and in a sturdy container, so they can’t get damaged or punctured.
Pit Tip: I store my LiPos inside an ammo can, with some foam washing-up sponges cut to stop the batteries moving around and banging into one another. However, do take the rubber seal out of the lid to avoid creating a pressurized bomb if one of the batteries leaks/explodes inside the can.
Taking care of your LiPos will make them last longer and keep you safe
If you have any questions, or corrections then feel free to get in touch.