Well, while working on the boat’s engine, I was surprised by this little sod, who’d managed to crawl into the air intake skin fitting on the transom, and got very irritated at the engine being fired up! How the little dude avoided getting sucked into one of the cylinders, I have no idea! The wee mouse was recovered from the air intake & released on the towpath.
Here’s something I didn’t think I’d be doing! Here’s a teardown of a BMW 5 Series G30 530E Hybrid Battery pack – a monster 351V, 9.2kWh Lithium pack, obtained for it’s cells to replace the boat’s aging lead acids.
This is something I didn’t have the safety gear to do right of the bat – opening one of these packs is a potentially lethal exercise, with 6 unfused battery modules in series, quite capable of blowing pieces off a nice conductive sack of salt water like a person. Cue the purchase of high-voltage rated gloves for protection, just while I got the pack split into something more manageable.
Needless to say, the combination of current capacity & voltage present in EV or Hybrid vehicle battery packs is nothing short of lethal, and these units should be treated with considerable respect.
Here’s the beast of a battery. Enclosed in an aluminium cast housing, it’s very heavy, and definitely not a one-man lift!
After removing the top cover, secured by combination Torx/10mm hex bolts, the internals of the pack are visible. There’s no sealant on the cover, just a large rubber gasket, so this came off easily. There are 6 individual modules in this pack, all wired in series with massive links. There’s also a cooling system for each battery module, supplied with refrigerant from the car’s AC system – there’s a TXV mounted on the side of the battery pack. I didn’t see any heaters present, but I don’t know if BMW have done any neat reverse-cycle magic to also heat the modules if required using the AC system on the car.
The modules are arranged 3 to a side, double-stacked at the back, then a single module at the front. The pack would normally sit under the rear seats of the vehicle, hence the unusual shape. The refrigerant lines going to the evaporators on this side of the pack can be seen in the bottom right corner.
The main contactor pack is on the left side, just behind the massive DC output connector. I’ll dig into this in another post later on.
The right side of the pack is arranged much the same as the left, the main difference here being the battery ECU is tucked in at the top here, along with the interface connector to the car, and the refrigerant lines to the TXV on the outside, which I’ve already removed. Each module has a cell balance control unit, in this case one is mounted on the top of a module, and on the side of the module in the lower right corner.
Once all the modules have been removed, the evaporator matrix is visible on the bottom, a series of very thin aluminium tubes, designed for the best contact with the aluminium frame of the battery modules.
Popping the plastic insulating cover off the battery module reveals the internal construction. I’ve not been able to find exact data on these cells, but I’m assuming them to be a similar chemistry to the ones used in the BMW i3 packs, so 4.15v Max, 3.68v nominal, 2.7v Minimum. The alloy frame itself is of laser welded construction, and there are 16 cells in series per module, giving about 58.8v per module. These will need to be reconfigured as 4 sets of 4 cells in series for 14.72v.
All the individual cell taps are nicely loomed down the middle of the module to each cell, and there are 3 temperature sensors per module (the red epoxy blobs).
The individual cell links are laser welded to the terminals of the cells, so this does make life a little more difficult when it comes to reconfiguring them. The links appear to be made from Aluminium, so soldering is going to be a bit more tricky than usual.
These days USB powerbanks are very common – ranging in capacity from about 1Ah, to about 20Ah. Internally, they’ve all got much the same format:
- Lithium Ion cylindrical or Lithium polymer pouch cells for energy storage
- DC-DC boost converter
- Microcontroller & LED Battery Gauge Display
- Lithium cell protection & charge control
As the maximum voltage of a lithium cell for common chemistries is 4.2v, there needs to be a DC-DC converter to boost the voltage up to 5v for the USB ports – There are dedicated chipsets designed for powerbank use available everywhere for this part, and this section is going to be the most energy-wasteful part of the system.
To get a handle on the discharge efficiency of these units, I ran some tests with a constant current load, on different powerbanks from different manufacturers. All were in the range from 5Ah-20Ah, and all had ports rated for 2.1A Max output current.
The load was set for a nominal 2A current, and the powerbanks were fully charged before a discharge cycle. All powerbanks were in new condition to ensure that age-related degradation of the cells wasn’t going to be much of a factor.
Without further ado, here’s some test results:
|Nameplate Capacity (Wh @3.7v Cell Nominal)||Nameplate Capacity (Ah)||Measured Capacity (Wh, Calculated @5V Output)||Measured Capacity (Ah)||Ah Efficiency %|
Overall, these efficiency numbers are pretty poor with an average of 59.735% across these 9 samples. I expected at least high 80’s for efficiency on powerbank DC-DC converters, which must be pretty well specialised for the input voltage range by now. I suspect this is mostly to do with keeping costs down in mass production.