# AAA NiMH Backup Battery Pack for 12V Automotive Use

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#### JDW1

##### Member
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 [email protected] 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 [email protected] 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.

##### Active Member
TL;DR. AAA is too small/low capacity for your described load. The vehicle charging system is designed to charge lead-acid so use a small sealed lead acid battery with it's own trickle charger.

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#### JDW1

##### Member
Thank you for your reply, but I don't see why 700mAh would be insufficient and exclude use of 10pcs of AAA. Consider this Battery Drain Calculator. Type in 700mAh, then 1000mA for Device Consumption, then chose a Consumption Rate of 1 (Amps), and you get an estimate of 0.7 hours or 40 some odd minutes. Don't trust that calculator? Try this calculator. Same result of 0.7 hour. Half of that value (about 20 minutes) is my target for keeping the 1A siren alive and blasting.

I also don't understand why you would suggest a battery type to match the vehicle's battery type. The vehicle battery is installed in the engine compartment. The battery pack I am taking about will be used inside the cabin. Also, the car charges the car's battery using the alternator. My device would need it's own power supply in order to supply the appropriate voltage to each of the 10 AAA cells and as such the car's charging system would not be directly used, nor would it need to be.

Another reason NOT to use a lead acid battery is because the battery pack device will not ship with any batteries. It will just be the battery holders and electronics and wire harness, all in a suitable plastic case. The user who purchases the battery pack kit will need to acquire their own batteries, which is why it is most prudent to select a battery type that is most convenient for anyone to buy pretty much anywhere, and AAA NiMH batteries fit the bill. They are also lighter and will fit inside the product whereas a lead acid battery would not.

#### MikeMl

##### Well-Known Member
The only reason to have a battery back up on a car alarm is because the car thief is smart enough to cut the car battery cable, or otherwise disable it. That means your alarm can be powered by the car battery 99.999% of the life of the car. The trick is to use a chargeable power source, charging it from the car's alternator/battery for the full duration from when the alarm is installed until the thief cuts the battery cable (could be years). Then rely on the chargable power source for the few minutes it takes to sound the alarm, or until a pissed-off third party comes along and smashes the $%^$# alarm!

I have some experience with battery power that has to be dormant for years, and then be able to deliver a last gasp. Alkaline batteries are the most reliable for this application. NiCd or NiMh suck! Next best is SLA (provided it is continuously trickle-charged up to just before it is needed.

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#### JDW1

##### Member
Mike, I appreciate your reply but your assumption is incorrect and it seems you did not fully read all of what I wrote in my opening post. I specifically said that when cranking, car battery voltage can temporarily fall to 4 volts and that can reset some security systems and cause problems. A battery back up that is dedicated to the purpose of keeping the alarm voltage close to 12V will prevent that from happening even during cold cranking. So the battery backup device is not simply just to thwart thieves who disconnect the main car battery.

The questions put forth in my opening post stand unanswered. I would appreciate hearing your specific thoughts on what I asked, Especially regarding NiMH battery technology and car interior temperatures. Thank you.

#### JimB

##### Super Moderator
A few thoughts...

The idea of using a commonly available NiMH cell sounds good.

While 700mAH is the capacity when new, after 2 or 3 years many of the cells will have lost capacity due to the normal wear out mechanisms, and may not drive the alarm sounder correctly.

How is the owner of the vehicle equipped with the alarm to know that one or more cells is failing?

The cost of replacing ten cells may not appeal to a number of the owners.
They will try to get by with just replacing the weak cells, only to go through the same exercise in a couple of months time.

Ten replaceable cells means twenty contacts which may be subject to corrosion. A bit of a reliability issue.

Is this security system intended as an OEM item for installation in new vehicles, or an add-on aftermarket item?
I was under the impression that most (all?) new vehicles had a reasonable security system already built in.

I seem to have presented a load of potential problems, but no solutions.
I try to be helpful when I can, but sadly I am failing in this case.

JimB

#### Nigel Goodwin

##### Super Moderator
Mike, I appreciate your reply but your assumption is incorrect and it seems you did not fully read all of what I wrote in my opening post. I specifically said that when cranking, car battery voltage can temporarily fall to 4 volts and that can reset some security systems and cause problems.
If the security system is caused problems by starting the engine, then there's something seriously wrong with it - in any case, during engine starting the security system should be disabled anyway.

As for using NiMh you're always going to have problems with short battery life, and in your application I would suggest they should be changed yearly when the car is serviced - although I see little point in it anyway.

#### ronsimpson

##### Well-Known Member
Years ago I designed a engine computer that had to run during cranking.
During cranking; during compression the voltage might drop to 4 volts but after the piston goes past center the voltage goes back to near 12 volts. What I saw was a 4 to 12 volt supply at a frequency related to the engine speed. I used a diode and a large capacitor. I used a 1 second hold up capacitor at 1 to 30mA load. (using your numbers) The 1A load stayed on the battery and just acted strange during cranking.

In my case the battery normally was at 12 to 14.5 volts. This got regulated down to 5 volts. The linear regulator needed 1 volt of headroom so the voltage on the large capacitor needed to stay above 6 volts. Even during slow crank the battery voltage stays above 6V many time(s) and keeps the cap charged.

I don't know your electronics. If the voltage really needs to stay at 12 to 14 volts then I see the need for a battery but many "computers" will work down to 6V and a capacitor will work. (assuming you can get rid of the 1A load)

##### Active Member
What NiMH batteries do you have in mind that would charge, discharge and supply the required current at 70° C?

Panasonic doesn't recommend charging above 40° and the graphed data for their "high temperature" models stop at 60°.

#### rjenkinsgb

##### Well-Known Member
As the charge can continue 24/7 except when the engine is being started, you don't need any kind of complex charge controller.

A simple resistor to give a trickle charge should do the job, with three schottky diodes to provide the bypass and charge/discharge routing.
Diode 1: Car battery to alarm.
Diode 2: Backup battery to alarm.
Diode 3: from car battery to a series resistor to backup battery positive.

A 10 cell NiMH pack has near enough identical charge voltage to a car battery.
Whilst the vehicle is running the alternator should hold it at 15V - the same as a the full charge voltage of ten cells.
As long as the current at full charge is somewhat lower than 10-hour rate the cells should last an extremely long time.
I'd aim for 20 hour rate or near that. You may only need something like a ten ohm (power) resistor, mainly to limit the peak current if the backup is low after starting, when the main battery is at 15V.

Unlike lithium cells, a low trickle charge is good for NiMH and NiCD cells. They do not need individual monitoring and only need temperature control in "fast charge" conditions.

#### JonSea

##### Well-Known Member
Add a series diode between car battery + and backup battery +. This prevents the backup battery from trying to charge the car battery if it's flat.

#### JDW1

##### Member

Jim, your thoughts are very helpful. The battery backup pack would obviously need a feature that determines when the batteries should be changed and alter the user somehow (perhaps by audible beeping via tiny buzzer). Since the charger would need to be intelligent and deal with each of the 10 AAA NiMH batteries individually, it should easily be able to know when one or more batteries is charging substantially differently than the other batteries, and during discharging, the battery backup electronics should intelligently know when one battery has been depleted and then stop discharging at that point to avoid a reverse polarity situation. Based on that knowledge, the electronics should be able to determine if the same battery is repeatedly getting depleted faster than the others, and that should help in the electronics making the decision to alert the user for a battery change (via buzzer) or not. This is just a concept at this stage, but is what I am thinking.

Battery leakage is of course a valid caveat and hopefully the electronics can alert the user to the need for battery changes before that leakage occurs. The best than can be done is to gold plate the battery contacts and ensure any battery leakage would not seep outside the plastic case of the product.

The security system is aftermarket.

I appreciate your speculation about problems, but as I have just mentioned, there ought to be solutions. In the event we foresee a big problem with no solution, then I would need to make the call to terminate the project. And consulting with fellow engineers both in person and online is part of my way to fleshing out potential problem areas, especially those that pose a safety risk.

Also, it goes without saying that this electronic device will need over-voltage protection (TVS diode) and reverse polarity protection and protection against load dumps.

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Nigel, thank you for your thoughts. Whether a security system is bad or not due to memory corruption on occasion during cold-cranking is somewhat irrelevant insofar as the battery back project is intended to eliminate that from being an issue by providing a stable 12V power source to the connected security system. But for what it's worth, if I were designing the power supply for the security system, I would use a SEPIC switching regulator (both buck and boost) to address times of low voltage (i.e., cold cranking). But again, a battery backup can prove helpful during temporary voltage dropouts.

Whether "there will always be a problem with SHORT battery life" or not is not the issue, really. The vast majority of the time, the battery backup will supply only temporary power to the security system during cold cranking. Even when the batteries get old, there will still be more than enough power to satisfy that need. I would suspect that the case of a thief cutting the main vehicle battery power is quite rare, and I doubt it would happen more than once during the life of a given vehicle, unless the car in which this system is installed is a rare Italian sports car costing a fortune.

As to battery changes, yes, I would agree that about once a year should be a prudent choice. So to determine that, the electronics should not only consider the charging and discharging state of individual batteries, but there should be a timer as well, such that even if no charging or discharging problems are detected during the course of 1 year (or 2 years, or whatever fixed time frame is to be programmed into the system), the system will start beeping after that timer expires to alert the user to change ALL the batteries. And of course the documentation will make it clear that warranties are null and void if instructions set forth in the documentation are not strictly followed with regard to changing ALL the batteries, and changing them with NiMH (not Alkaline or NiCd).

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Ron, thank you for your suggestion of the capacitor. A capacitor/diode kit is actually an optional product currently being sold for people experiencing cold-cranking problems. But the merits of a full battery backup device are that it can sustain 1A to the 12V Siren for an extended period of time which even a compact-sized super-capacitor could not. And the reason a large super-capacitor was not chosen instead of a battery pack in this project concept is because of COST. Remember, this product will ship without batteries, which keep the product cost low and eliminate the need for the seller to warehouse batteries which have a finite life.

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KJ6EAD, thank you for your question about temperatures and charging. Please remember that in my original post I talked about that. The electronics of the battery charger will need TEMPERATURE SENSING, not only of the individual batteries but also ambient temperature. When temps get too high or too low, the electronics must stop Charging or Discharging until temperatures return to an allowable level.

Now you might want to ask, "Well, couldn't a thief just pick a hot day in Arizona to cut power to the car without worry the battery backup may prevent discharge due to the high ambient temps inside the car?" And the answer to that is, "yes, but how would that thief know that particular car had the battery backup system installed?" The answer is, he could not. But the real question then becomes, how often would it be that the battery pack could not be charged or discharged due to temperatures being too cold or too hot? And I would appreciate hearing experienced opinions about that.

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rjenkensgb, thank you for your thoughts about continuous trickle charging. While a case can be made for a simplistic trickle charger, the fact remains that temperatures inside a car can rise to extreme levels (enough to kill people) and so a more complex controller with intelligent temperature sensing would need to be incorporated, for all the aforementioned reasons. Even trickle-charging a set of NiMH batteries when the ambient temps inside a car are 160°F/71°C is asking for trouble, as per the battery datasheets.

By the way, we could recommend specific NiMH batteries to the buyers of the battery backup system, but realistically, most people would want to buy Panasonic Eneloops on Amazon. Except for Panasonic's high temperature AAA batteries sold to OEM's, the Eneloops are about the best NiMH AAA's available as of August 2018 (and yes, we've considered VARTA). Panasonic also has one rather large A-sized NiMH battery (BK1100FHU) that can operate -40 to +85°C, but that technology is not yet available to consumers in smaller battery sizes.

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JonSea, yes, a series-diode (specifically, a good Schottky) will prevent the backup battery pack from flowing back to the car electronics. The battery backup must hold up only the security system and nothing else.

#### rjenkinsgb

##### Well-Known Member
Even trickle-charging a set of NiMH batteries when the ambient temps inside a car are 160°F/71°C is asking for trouble, as per the battery datasheets.
Can you provide links to these datasheets as nothing I can find explicitly shows that.

Everything I have found so far related to temperature effects on the overall functional life of the cells and to limitations on fast charge current as temperatures increase.

With overcharge impossible (due to insufficient voltage) and reduction of charge current as the two battery voltages equalise, it appears trickle charging should not cause massive problems.

Remember that the vehicle battery voltage will start to drop when the engine is off, so no charging will take place until a restart, at which point the in-vehicle temperature should reduce rapidly.

And obviously ensure the backup unit is fitted as low down as possible in a shaded but ventilated position. Heat rises - lower should be cooler.

I'd try temperature probes at different locations in some vehicles to obtain real-world temperature data and also build an "oven" test rig and start experimenting - you will need that anyway.

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#### JDW1

##### Member
rjenkinsgb,

My meaning was merely to say that most AAA NiMH battery datasheets specify the maximum temperature for CHARGING and DISCHARGING, and with the interior of most vehicles in the sun rising to 71°C in the summer, such a temperature is high above what is allowed as per the AAA NiMH datasheets (or technical specifications). For that reason, the battery pack electronics would need to terminate charging and terminate discharging when the vehicle's internal ambient temperature (the air surrounding the battery pack) rises higher than the battery specifications allow. Without such a temperature-controlled charge/discharge feature, the battery pack electronics would continue to charge even if the interior temperature of the car hit 80°C, which most likely would not bode well for those poor NiMH batteries, even using a teensy trickle charge.

Another consideration about "WHEN to charge" and "the METHOD of charge" pertains to car battery drain. While driving, the battery pack electronics could pull several Amps if it wants too (assuming heat dissipation is not too high), but when the engine is switched off the charger circuitry would either need to cease altogether (to avoid draining the car's battery), or charge for a designated short period of time at a rather high charging rate, or for a modest period of time at a lower charging rate. This is an important consideration which has not been decided yet. But it is critically important that the battery charger NOT kill the car's battery when the car's Ignition is switched off.

#### Nigel Goodwin

##### Super Moderator
Nigel, thank you for your thoughts. Whether a security system is bad or not due to memory corruption on occasion during cold-cranking is somewhat irrelevant insofar as the battery back project is intended to eliminate that from being an issue by providing a stable 12V power source to the connected security system. But for what it's worth, if I were designing the power supply for the security system, I would use a SEPIC switching regulator (both buck and boost) to address times of low voltage (i.e., cold cranking). But again, a battery backup can prove helpful during temporary voltage dropouts..
I just can't see the need at all - there are MILLIONS of perfectly working car alarm systems out there - are you saying you've managed to find one that's so poorly designed it doesn't work properly?.

#### rjenkinsgb

##### Well-Known Member
AAA NiMH battery datasheets specify the maximum temperature for CHARGING and DISCHARGING
I repeat: Can you provide links to these datasheets as nothing I can find explicitly shows that.

There is no additional battery drain imposed by that system; the only continuous drain is the idle current of the security system itself, which will be drawn from whichever battery has higher voltage at any instant.

Charging would only occur while the vehicle is running.

#### JDW1

##### Member
Nigel, I certainly appreciate your feedback and please don't take this the wrong way, but "the need" or lack thereof is irrelevant to this discussion as is talking of inferior and superior car alarm designs. The topic of this thread centers on a battery backup system for 12V vehicles that incorporates AAA NiMH batteries. It is in the design concept stage and the focus is whether the concept is viable in light of temperatures we find commonly in most vehicles, especially during hot summer months. If you have extensive experience using AA or AA NiMH batteries in vehicle cabins, I would certainly appreciate hearing your detailed words on that. Nevertheless, in response to what you wrote, and regarding "poorly designed car alarms," one must consider there are markets outside the US for such products, and in fact that is where this battery backup design will end up being used. I've been in the vehicle security aftermarket since 1994, and I can tell you that many cars themselves are "poorly designed" when it comes to their electrical systems, even the most recent Toyota HiAce vehicles sold in Japan are rather notorious for their lackluster electrical design. And simply throwing another \$15 of "better electronics" at a low cost car alarm isn't always prudent or acceptable. Not all electronics work well on dirty power. Now that I've said that, it once again would be greatly appreciated if we could focus more exclusively on the battery backup design concept, especially with regard to expected temperatures in a vehicle cabin throughout the year and that impact on charging and discharging. Thanks!

rjenkinsgb, I am not sure why you cannot Google up any NiMH battery datasheets, but here is one of many that are conveniently available, which shows exactly what I said previously. More specifically, there are temperature ranges for Charging and Discharging, storage, etc. on the following datasheet. Keep in mind this datasheet is for the INDUSTRIAL spec (OE sales) of the Panasonic NiMH battery line, which has wider acceptable temperature specifications than your commonly available consumer-grade Eneloops.

https://industrial.panasonic.com/cdbs/www-data/pdf2/ACG4000/ACG4000CE246.pdf

In response to what you wrote, whether "charging occurs when the vehicle is running" depends completely on the design of whatever it is we are talking about. Indeed, I could design a system that charges with the ignition ON but the engine off, or I could design a system that charges with both the Ignition and engine off, although in that case I would need to take care to keep average current consumption low, or charge at a few hundred milliamperes for only a short duration and then stop.

#### JonSea

##### Well-Known Member
No piece of advice is ever as good as that which you want to hear.

#### JDW1

##### Member
No piece of advice is ever as good as that which you want to hear.
Jon, I'm afraid I don't fully understand the intended meaning of your statement above. I am simply trying to keep on topic, and indeed in other forums, keeping on topic is considered to be something almost "sacred." Yet if I read your words correctly, it would appear that straying from the main topic in this forum seems to be the equivalent of "embracing good advice." Please note, however, that I politely replied to that "advice" in my previous post, also citing my own experience in the automotive security aftermarket and mentioning the fact that even big car makers like Toyota can produce cars with "poor electrical design." If that isn't an eye-opening, I don't know what is. Now if anyone in this forum has aftermarket vehicle security experience (not speculation or simply good engineering experience, but practical experience in that industry), then I all the more seek to hear your opinions because you should be aware of the same things I am. But again, I seek opinions on the device mentioned in my opening post. I can chat with you all day about car alarms and whether they are good or bad, but at the end of the day, I posted my original post to center on a battery backup system for in car use. Perhaps I should have hidden the fact it is to support car alarms, as that would have avoided strong opinions for or against car alarms.

I am thankful for all the replies received, regardless of the topic. I greatly appreciate the kind replies of everyone to date.