LiPo battery storage: voltage, bags and swollen packs

The storage voltage, per cell and per pack

Every LiPo cell is about 3.7V nominal and 4.2V full. For long-term storage, Gens Ace gives a range of 3.6V to 3.9V per cell, and its FAQ puts the target at roughly 70% charge, about 3.8V per cell. Traxxas says to store its LiPos at 50% capacity. Spektrum Smart batteries default to 3.90V per cell when they drop into auto-storage. The numbers differ slightly, but they all point to the same middle ground: never full, never flat.

The last row is the Gens Ace limit for discharging under load; the manual says never to go below 3V per cell. That is a floor for driving, not a target. A pack measured at rest will read higher than it did under load, so use your ESC's low-voltage cutoff and stop well before the floor. The per-pack figures are simply the per-cell figure times the cell count; LiHV packs charge higher, so follow their label.

How to put a pack into storage

1. Let it cool first. A pack straight out of the car is warm. Gens Ace says never to charge a battery that is hot to the touch, above 100°F (about 38°C). An infrared temp gun takes the guesswork out of it.
2. Look it over. Check for puffing, dents, split wrap and damaged leads or connectors. If anything looks wrong, skip to the swollen packs section.
3. Run the charger's storage function. Many balance chargers have one, and it charges or discharges each cell to the storage voltage. Check what your charger calls it and which voltage it targets in its manual. On a Spektrum Smart pack, the Spektrum S155 G2 charger can set the Auto Storage delay (12 to 240 hours) and the storage voltage.
4. Balance it. Plug in the balance lead so every cell ends up at the same voltage. Gens Ace says to stop using a pack whose cells will not balance.
5. Stay with it. Spektrum's charger manual says never to leave charging batteries unattended and never to charge overnight. A storage discharge is still the charger working on the pack, so treat it the same way.
6. Unplug and label it. Disconnect it from the car and the charger. A strip of tape with the date and "storage" saves guessing later.

How soon? If you will drive again within a day or two, a full pack can wait. Past that, put it into storage. Traxxas warns not to store a pack discharged, because a flat pack left for longer than 7 days can over-discharge. Spektrum Smart packs wait 72 hours by default before they discharge themselves to storage, which is a sensible rule of thumb for any pack.

Where to keep it: bags, boxes and temperature

• A fireproof container. Both Traxxas and Gens Ace say to store LiPos in a fire-retardant or fireproof container. A LiPo safe bag is the usual choice for a few packs; some drivers with bigger collections use a metal ammo-style box instead. We would not seal a box airtight: a failing cell vents gas, and the container needs somewhere to let it out.
• Not loose together. Gens Ace warns that loose batteries can touch terminal to terminal and short. Keep plugs capped or packs separated, and keep keys and tools out of the bag.
• Room temperature, out of the sun. Gens Ace allows storage between -10°C and 45°C (14°F to 113°F), and for more than three months asks for about 23°C (73°F) and 45% to 85% humidity. A car boot in summer is well outside that, so bring packs indoors.
• Away from things that burn. A shelf in a garage or a concrete floor is better than a bedroom carpet or a cardboard box of spare parts.
• Check them. Gens Ace suggests checking stored packs every few months. If a cell has drifted well below storage voltage, top it up to storage; if it has fallen far below the cutoff or the pack has puffed, retire it.

Hard-case packs resist knocks better than soft packs, but the case does not make them fireproof. They still go in the bag.

Swollen or damaged packs

Traxxas is direct about it: never use a LiPo that feels soft or has swollen. Gens Ace gives the procedure if a pack gets hot, balloons or swells while in use or on the charger:

1. Stop charging or discharging and disconnect it.
2. Put it in a fireproof container, such as a LiPo safety bag, in a safe open area away from anything flammable. Gens Ace suggests outside any building or vehicle.
3. Watch it for about an hour. Gens Ace notes a battery can still ignite after that.
4. If it has stabilised, take it out of service for good. Do not keep using it, and do not ship it.

How to dispose of a LiPo

The EPA says lithium batteries should not go in household rubbish or recycling bins. Take them to a battery recycling point or a household hazardous waste collection instead. Its handling advice:

• Put each battery in its own plastic bag.
• Cover the terminals with non-conductive tape such as electrical tape. On an RC pack, that means the main plug and the balance plug.
• If the pack is damaged, contact the battery maker for handling instructions before you move it.

Many hobby shops and electronics stores run battery drop-off points, and your local council or waste service can tell you where the nearest one is.

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