Front Auxiliary Battery Pack

The following page is structured to define the auxiliary battery pack for MSXIV. This page builds off of the page created by Micah Black found here Aux Battery - MSXIV

At the moment, the front auxiliary battery pack is designed to be used for the horn and driver fans. It needs to provide at least 12V and needs to be able to supply a current of up to 10A.

Our previous battery setup of 10S 1P Turnigy 5000mAh NiMH cells (shown below) seemed to work well however it was mentioned that it was draining too quickly as we powered the car using the aux. battery during troubleshooting. If possible, evaluate the possibility of using a spare SLA 12V (or lithium-ion at non competition settings) battery to power any troubleshooting power needs.

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According to the guidelines, our aux. battery falls under the “Supplemental Batteries” section. According to 8.2.B, other battery technologies will need to be evaluated by Headquarters and details must be submitted before the 4.3.A.2 due date. If approved, use of the SLA will significantly increase the usage time of the aux. battery pack as well as the time between charge.

Maybe a discussion here on SLA discharge rates and the types of SLA batteries would be useful - there are high CCA batteries for engine starters that don't like to be drained, and deep discharge SLA batteries that typically do not support high discharge currents.

Another issue was the battery pack seemed to cut out while attempting to close the relays too early. See the MSXIV aux battery page for details. This could be an indication that the cells in series do not posses equal amounts of charge, but another more likely possibilty is that the ESR of the whole circuit was too high, generating an undervoltage fault while the relays were pulling in with 4A as discussed on that page. LSD (Low Self Discharge) cells hold charge better, many have very little capacity but the Powerex Precharged D supplies 10,000mAh per cell. Link: Unfortunately, due to NiMH’s high discharge rate, this is not controllable without active balancing but it can be managed. @Hanqi Zhao What is 'this' in the above sentence?

For our purposes, it’s important to note that active balancing is not really beneficial as it leads to a higher quiescent current. This is the amount of current consumed by a circuit when it is not performing any work. To maximize battery life, quiescent current consumption should be as low as possible.

Because the pack will have all the cells in series, they carry the same current during charging. One way to address the charge issue is to provide a full charge (charge to capacity) to the batteries even if it means overcharging some cells and dissipating the extra energy as heat. Overcharging occurs when the some of the cells in the series need less charge while the others receive a full charge.

@Hanqi Zhao Please add: is overcharging safe for NiMH cells?

Decision:

Upon review of the NiMH 10S 1P battery made from Turnigy cells, it was determined that this is still the best choice. The batteries are:

• Easy to recharge

• Is allowed by current regulations to be an aux. battery (Li-ion not allowed)

• Allows for 45 mins of debugging time at full LV load (80W). 80W is our target LV load, not yet measured.

  • 5000mAh X 12V = 60Wh

  • 60Wh / 80W = 0.75h = 45mins

• Allows for ~ 208 mins of operating time for the horn (5mins) and the fans

  • 5000mAh X 12V = 60Wh

  • 60Wh - 10Wh (fan) = 50Wh

  • 50Wh / 14.4W = 3.472h = 208.3mins

Aux Battery Capacity

Required loads:
4 fans, 8 hours driving = 0.3A * 12V * 4 fans * 8 hours = 115.2Wh
Horn, 5 minutes = 4.5A * 12V * 0.0833 hours = 4.5Wh

Total Energy stored in battery: 115.2Wh + 4.5Wh = roughly 120Wh

We should give some headroom here, so aim for at least 150Wh of energy storage, or limit the fans to a lower current draw. We should check how much room we have for the fans, and also check with Adrian to see if we should be using smaller ones. We have asked Adrian about the number and specs / size of the fan because we might downsize them. This would mean requiring a smaller battery.

This means we will need at least a 10S2P of the NiMH C size cells, but 3P would be preferable.

Product Specifications:

Link for Purchase:

Other sources for cells: Amazon