This page is not yet finalized, and more discussions are still to happen.
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The NiMH battery packs (both styles) seemed to be giving us lots of issues with draining too quickly (since we often powered the car from the battery during troubleshooting), and seemed to cut out while attempting to close the relays too early (at 12.9V or 1.29V/cell) with still a lot of capacity and useable energy left. Not all of that is due to the cells, but we can definitely improve it.
Aux Batteries Used in MSXII (and spare)
| Battery Image | Chemistry | Capacity (rated) | Weight | Link | Note |
|---|---|---|---|---|---|
| SLA-AGM | 7.2Ah | 2462g | Standard battery - canadian tire, amazon, etc | Spare battery used to 'jump start' the car |
| NiMH 10S1P | 5Ah | 743g | https://hobbyking.com/en_us/turnigy-sub-c-1-2v-5000mah-high-power-series-nimh-single-cell.html?___store=en_us | Battery installed just before FSGP2019 |
 | NiMH 10S 1P | 2.1Ah | 271g | https://www.embeddedarm.com/documentation/third-party/ts-bat3_panasonic_hhr210aab.pdf | Battery used for FSGP2018 (not the exact one pictured) |
The page will go over the use case for our AUX battery, and go over the different battery types that we can use.
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Internal resistance for eneloops, is around 100mOhm (www.stefanv.com/electronics/sanyo_eneloop.html), but can be as low as 18mOhm for the Maha Powerex Precharged D cells (https://mahaenergy.com/powerex-precharged-d-10-000mah-2-pack/). The Turnigy NiMH pack has around 18mOhm/cell (NiMH IR Testing)
To achieve the required voltage, a battery pack with 10 cells in series would be constructed. This does not add any additional complications as NiMH cells tend to self-balance, and thus charging can be done as if it is one high voltage cell. See Charging Algorithms for NiMH Batteries for more details on the charging procedures.
NiCD
This is now an older chemistry and is no longer relevant. From my research, NiMH has surpassed it in every category.
Li-Ion
No Li-ion Lithium based secondary batteries are allowed per regs.
LiFEPO4
No Li-ion Lithium based secondary batteries are allowed per regs. I'm really hoping that ASC changes their minds on this, as the main issue with Lithium batteries is thermal runaway, and LiFEPO4 batteries are not susceptible to thermal runaway. For now, this may be out of the question.
Other Strategies to eliminate AUX issues
There are several other approaches we are also analyzing to avoid all the AUX issues we have been having as well.
- Properly analyze the minimum voltage for switchover on the DC-DC power path IC. Choose the resistor divider accordingly.
- Add a time delay filter on the voltage measurement circuit if it does not impact any other switchover time characteristics significantly.
- Use thicker wires and nickel strips in the construction of the AUX battery (not the cause of the issue, but it can certainly help).
- Shorter wires for the AUX current path (don't go from battery to driver AUX Switch then back to battery - too much resistance and voltage drop).
Final Decision
We are going to be using the NiMH 10S 1P battery made from Turnigy cells bought from Hobbyking.
This AUX battery will sit inside the battery box.
Capacity - 5000 mAh
The max discharge rate of 8A has pushed us to go with larger cells that support this current, as well as have lower internal resistance to keep a low voltage drop during higher load currents (such as the lights, fans, and display).
We aim to be able to power the car during debugging. 5000mAh cells give us around 70Wh of capacity, so with our LV target power consumption of 80W, around 1h of debugging is possible which I think is reasonable.
Chemistry - NiMH
Reason: Lightweight, allowed by regs, high safe discharge rate, and readily available.
We will also be using a similar auxiliary battery to power the horn and driver ventilation fans, possibly with increased capacity.