I have been asked to play a role in the design of a 12V backup battery pack that would be used to backup a vehicle security system and siren in exclusively AUTOMOTIVE applications. I have general electronics experience but no experience with the design of a battery backup system, and the organization calling for the design of this system also lacks design experience in battery backup and battery charging devices. Note that the battery backup device would NOT be used to backup the entire vehicle -- only a backup of specific electronics, which again, in this cases is a 12V vehicle security system and its 12V siren, the maximum current draw being 1A@12V. I am writing this post to garner opinions on how realistic this design is, and to determine potential caveats.
When cold-cranking, vehicle battery voltage can temporarily fall to 4V, and the aim is to have this battery backup device sustain the security system alone in that low voltage case so the security system continues functioning as normal. Also, when the security system is Armed/ON, the battery backup would engage if a thief removed one of the vehicle battery terminals or otherwise cut power to the vehicle, in which case the alarm system Siren may trigger. Most alarm system Sirens draw 1A@12Vdc average, whereas the alarm system control unit which draws anywhere from 1mA to 30mA (average, at 12V). The most taxing load on the battery pack would be an active siren blast, which again draws roughly 1A (average). It is a design goal to maintain the siren blast for roughly 20 minutes, during which time some decrease in siren volume is expected and allowable.
The battery pack under consideration should be as safe as possible to use over the life of the vehicle, and it goes without saying that batteries would need to be replaced during that time. Safety is an utmost concern with batteries in a car. No one wants to be responsible for an fire in a vehicle caused by a faulty battery pack. For this reason a design goal is for the pack to use user-replaceable NiMH AAA cells, rather Lithium Ion cells. The reasoning for AAA-sized NiMH batteries is that they are small and easily replaceable and not too expensive (can be purchased from Amazon in most countries) and can supply the required current. Ten pieces of AAA NiMH batteries would yield roughly a 12V output, at 700mAh (depending on the AAA NiMH battery manufacturer). And although most alarm systems would run fine in 7 or 8pcs of 1.2V AAA NiMH batteries, the siren would achieve greater volume when powered by roughly 12V. Whether the battery pack should function if only 8 out of 10 cells is an important consideration. It is also highly desirable if damage to the battery pack electronics can be prevented in the event the end user (vehicle owner) ignores warnings in the printed documentation and accidentally inserts the wrong type of batteries in the battery pack. For example, if they insert Alkaline AAA or NiCd AAA batteries rather than NiMH AAA's, damage should be prevented, but is this safety feature achievable?
The charging circuit would need to be intelligent and temperature sensing. It would need to charge the NiMH batteries individually and avoid overcharging, and disable charging when outside a predetermined ambient temperature threshold. To ensure the longest life possible from the batteries, there should be an Automatic Recondition feature that would engage every few months to drain all the batteries and then charge them, repeating that sequence TWICE. When discharging, the controller would need to ensure that no one AAA battery falls below a predetermined voltage threshold (prevent over-discharge) so as to avoid the battery from going reverse polarity. In light of this, are there any off-the-shelf battery charging controllers that would achieve this? Or would the charge/discharge controller circuit need to be a 100% custom design? (A low overall price point is important.)
This battery pack will be mounted inside the car, usually under the dash and above the foot pedals, where most car alarms are mounted. In the summer, vehicle cabin temperatures can rise to above 160°F/71°C, although the temperature in a shaded area under the dash should be lower. Precisely HOW much lower the temperatures are in shaded areas of a vehicle cabin is really the main question here and the basis for this entire post. Is this design concept realistic in light of potentially high vehicle cabin temperatures, even with an intelligent controller that senses ambient temperatures and prevents charging or discharging based on those temperatures?
What other caveats to this design concept should be considered?
Lastly, do any of you have experience designing NiMH battery backup devices for 12V automotive use?
Thank you.
When cold-cranking, vehicle battery voltage can temporarily fall to 4V, and the aim is to have this battery backup device sustain the security system alone in that low voltage case so the security system continues functioning as normal. Also, when the security system is Armed/ON, the battery backup would engage if a thief removed one of the vehicle battery terminals or otherwise cut power to the vehicle, in which case the alarm system Siren may trigger. Most alarm system Sirens draw 1A@12Vdc average, whereas the alarm system control unit which draws anywhere from 1mA to 30mA (average, at 12V). The most taxing load on the battery pack would be an active siren blast, which again draws roughly 1A (average). It is a design goal to maintain the siren blast for roughly 20 minutes, during which time some decrease in siren volume is expected and allowable.
The battery pack under consideration should be as safe as possible to use over the life of the vehicle, and it goes without saying that batteries would need to be replaced during that time. Safety is an utmost concern with batteries in a car. No one wants to be responsible for an fire in a vehicle caused by a faulty battery pack. For this reason a design goal is for the pack to use user-replaceable NiMH AAA cells, rather Lithium Ion cells. The reasoning for AAA-sized NiMH batteries is that they are small and easily replaceable and not too expensive (can be purchased from Amazon in most countries) and can supply the required current. Ten pieces of AAA NiMH batteries would yield roughly a 12V output, at 700mAh (depending on the AAA NiMH battery manufacturer). And although most alarm systems would run fine in 7 or 8pcs of 1.2V AAA NiMH batteries, the siren would achieve greater volume when powered by roughly 12V. Whether the battery pack should function if only 8 out of 10 cells is an important consideration. It is also highly desirable if damage to the battery pack electronics can be prevented in the event the end user (vehicle owner) ignores warnings in the printed documentation and accidentally inserts the wrong type of batteries in the battery pack. For example, if they insert Alkaline AAA or NiCd AAA batteries rather than NiMH AAA's, damage should be prevented, but is this safety feature achievable?
The charging circuit would need to be intelligent and temperature sensing. It would need to charge the NiMH batteries individually and avoid overcharging, and disable charging when outside a predetermined ambient temperature threshold. To ensure the longest life possible from the batteries, there should be an Automatic Recondition feature that would engage every few months to drain all the batteries and then charge them, repeating that sequence TWICE. When discharging, the controller would need to ensure that no one AAA battery falls below a predetermined voltage threshold (prevent over-discharge) so as to avoid the battery from going reverse polarity. In light of this, are there any off-the-shelf battery charging controllers that would achieve this? Or would the charge/discharge controller circuit need to be a 100% custom design? (A low overall price point is important.)
This battery pack will be mounted inside the car, usually under the dash and above the foot pedals, where most car alarms are mounted. In the summer, vehicle cabin temperatures can rise to above 160°F/71°C, although the temperature in a shaded area under the dash should be lower. Precisely HOW much lower the temperatures are in shaded areas of a vehicle cabin is really the main question here and the basis for this entire post. Is this design concept realistic in light of potentially high vehicle cabin temperatures, even with an intelligent controller that senses ambient temperatures and prevents charging or discharging based on those temperatures?
What other caveats to this design concept should be considered?
Lastly, do any of you have experience designing NiMH battery backup devices for 12V automotive use?
Thank you.
