This page is not yet finalized, and more discussions are still to happen.
For MSXII our aux battery was an NiMH 10S1P battery pack. Originally, we used Panasonic 2000mAh Rechargeable NiMH 'AA' sized cells. Just before FSGP2019, we switched to Turnigy 5000mAh NiMH cells.
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.
The page will go over the use case for our AUX battery, and go over the different battery types that we can use.
Use Case
The Aux battery is used to power the critical LV systems of the car during the startup and emergency states. It is also used to provide the power required to close the HV battery contactors - this is a current spike on the order of 3-4A, so our cells must be able to handle this.
The Aux battery must also be easy to charge, swappable, and would be nice if it could hold power over a long time as it can be several weeks between turning the car on and off during off-terms.
For simplicity, we would like our Aux battery to be able to power our LV systems directly, without a DC-DC converter between it. Our LV system is 12V, trimmed to 13.5V, so we will need a battery within the same voltage range.
Battery Options
Primary Cells
Going with a primary battery would be a switch to a single use cell, and thus more waste. For this reason, I cannot recommend using them in a solar car.
However, they would not need any voltage monitoring and can provide the required voltage and current. We would have to be careful about how much we use the cells to avoid producing unnecessary waste. This would mean requiring a power supply or separate battery for extended testing sessions.
SLA
SLA batteries are typically thought of as heavy and out-dated when discussing next-generation solar cars. However, in the situation of an auxiliary battery, they are a viable option.
A typical 12V SLA battery fits the voltage window perfectly.
Charging SLA batteries can be done very easily, with a current limited power source and could be left permanently connected when the DC-DC is on, the battery being charged with a float charge.
The battery would have to be carefully selected to minimize the weight while still providing an acceptable capacity and internal resistance.
SLA batteries also have low self discharge, but must be stored fully charged (https://batteryuniversity.com/learn/archive/advancements_in_lead_acid)
NiMH
If we go for an NiMH battery, I do not see a reason not to go with a Low Self Discharge (LSD) battery.
Charging an NiMH battery requires a specialized charger IC that can detect the rate of temperature rise of the battery at the end of charge, or the secondary effect of slight increase in voltage. NiMH batteries must also regularly be conditioned with a full charge and discharge cycle and occasionally also requires a 'refresh' cycle. For this reason, they are a little more complicated to work with, but can perform amazing when treated properly.
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/).
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.
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 batteries are allowed per regs.
LiFEPO4
No Li-ion batteries are allowed per regs.