54 an iPace might charge at 3 C – crazy fast. And, in a 0-60 sprint, peak discharge on them is going to be, say, 4 C. So, in our situation, they’re just tickled, they’re not under any stress, so they last a lot longer.” Battery packs are linked together in pairs using a series connection to achieve the desired voltage, and also as a protective measure against the effects of different levels and rates of degradation. “At no point do we link two batteries in parallel,” he emphasises. “If you have one that’s badly degrading one that’s not, normally you’ll find that the stronger battery will buffer the weaker one, and it will degrade the stronger battery really quickly by exceeding its maximum discharge rate.” Hazell explains that the inverter has multiple high-voltage (HV) inputs that are isolated from each other to cope with multiple packs. Within the inverter are multiple HV buses that handle the DC-to-AC conversion. One inverter can handle two pairs of batteries. A Qube with six batteries (three pairs) would have two inverters. “That is the way we have managed it, just to remove that risk of potential issues with paralleling packs,” he says. “Most of the risk in energy storage comes from this, because if you have multiple packs parallel and you get one bad cell, all the good ones can rapidly charge the bad one, degrade really quickly and cause a thermal runaway event.” Fire safety To deal with electrical fires, Qube has a CO2 or powder extinguisher in the roof, which is triggered when the internal temperature rises high enough to expand a heat-sensitive fluid so that it breaks a glass bulb and releases the extinguishing agent. Obviously, other arrangements are needed for battery thermal events and fires. The thermal management system monitors each pack individually, and automatically shuts the systems down if a pack overheats, while the container has vents near the top to clear gases released from a battery in thermal runaway. Fellten works with local fire services and it has obtained a standard fitting for a fire hose. The plan is to install such a fitting at the front of every Charge Qube, which will allow firefighters to clip on a hose and leave it running. The container will eventually overflow through the top vents. Lower down there are more vents with silicone seals designed to let air in and out normally, but these will be forced shut by water pressure as the container fills, Hazell explains. Ironically, it is possible for water to both trigger and suppress thermal runaway in a pack by first shorting out and then taking the heat away. A test by a customer (who will remain nameless) proved this. Battery packs are not sealed unless all the HV connectors are in, and the customer filled the case containing the pack with salt water. “A couple of cells went bad, but because the battery was submerged in cold water at the same time it never went into full thermal runaway,” he says. “The water that came out was warm.” Fellten has taken precautions against this problem by blanking off all the unused connectors on each pack to ensure they are sealed. March/April 2025 | E-Mobility Engineering Rated at 1000 V, this inverter enables the use of standard EV packs, phase balancing and integration with the power grid. The AC distribution box below is from Proteus (Image courtesy of Fellten) Proven off-the-shelf radiators and fans were chosen for their low cost and plentiful supply. They serve battery thermal management and cool the container interior (Image courtesy of Peter Donaldson)
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