Has anyone considered a lightweight 12v battery?

[ElectricTireShredder]

Day 34 & still fine on tiny $20 12V, but I've been disconnecting it whenever I won't be driving the next day, SO...:

I realized I should test to see how long it can really park & stay above the supposedly ill-advised 11V (when the drive batt might try to recharge it too fast), so I broke down & got a proper voltage gauge.

First test started last night in my garage, unlocked, so the alarm is off. That's how it normally is when I'm not driving for a few days. I will test later locked for public parking.

Battery Tender.jpg

[ElectricTireShredder]

Already reporting much better results*, even after longer than my first test:

Over 36 hours parked unplugged & still at 12.6V! That's garaged, unlocked. At this rate it could still be above 11.2V after over a week!

It will be interesting to see any difference when I test it locked with the alarm armed. If that goes even half as well, I'll want to try it with daily unlocking, driver door open/close (activating contactors), & relocking.


*Voltages that I reported earlier were measured with OBD, which requires the 12v to be holding the contacts closed, which I later realized reduces the voltage below what it would be simply resting, which is what I am now recording & reporting.

[ElectricTireShredder]

Conclusion so far: 3Lb batt is fine even parked unplugged for over ½ week (with the alarm off) & is on track to be fine for over a week. To be extra-cautious, you could get DOUBLE the longevity at 4.4Lb for only $29*. That’s still WAY lighter & cheaper than OEM, even counting the one-time cost of the adaptors**. You could even QUADRUPLE it to 12Ah & 10Lb for only $40.

Starting in a few days I should be able to test parked unplugged with the alarm armed for a few days, & extrapolate a week from that.


Details: At 3 days + 14 hours with the alarm off, I happened to be watching as it dropped to 12.1V (about 50%)

• Voltage drop has been very linear, so based on that, if you only drive once per week it should stay above 11.7v, which is about 25%.
• Discharging these Absorbent Glass Mat batteries even to 20% lasts at least 100 cycles, according to BatteryUniversity.com That’s 100 weeks, nearly 2 years, or about $1/month.

* https://www.amazon.com/stores/page/3...3-EC3DD2CBD466
** https://www.amazon.com/XS-Power-580-...FSRX0VZ582AARJ

[ElectricTireShredder]

$20 3Lb BATTERY 2nd, HARSHER TEST SUCCESS!

Parked unplugged with the alarm on nearly 3 full non-stop days testing so far, & as expected for lead-acid, the very slow voltage drop is VERY steady, putting it on track to park 11.5 days & still start fine!

It also should last about 2 years, being on track to go 5 days before it hits 20% (11.6v). AGMs like this are supposed to be able to cycle that deep 100-120 times. So even if you only drive weekends & let it deplete 5 straight days a week, 52 weeks a year, 2 years is 104 cycles.

HOWEVER, it's also on track for 7.5 days to hit 11.0v, at which time the drive battery reportedly starts recharging it, possibly with too much current. I'll try to take it that low to check, but even IF it's the case, one could simply get the $29 4.4Lb double-capacity battery, for 15 days parked unplugged & probably 4 years lifespan.

[ElectricTireShredder]

Now actually tested fine for a weekday-driver parked unplugged with the alarm on from 6pm Fri til 8am Tues (long weekend) with 11.9v left.

EDIT: The real limit with THIS small a 12V is 4 1/2 days parked unplugged with the alarm on, & 5 3/4 days with the alarm off.

It likely really would have stayed above 11.0v for 7 days, but even though it reportedly starts with only 10v in the OEM, THIS small battery requires 11.7v to start the car. The rest of my original post stands...

HOWEVER...:

Parked several days & opening the driver's door frequently to load/unload stuff depletes the tiny 3Ah battery a bit. The hatch or passenger door are fine, or with the driver's door closed ONLY 1 click. It still locks, but only with the metal key, so no alarm...

Fully opening/closing the driver's door connects the 12V to the contactors ("clunk" sound), & usually also the brake pump*. I accidentally activated my pump once on this test & it lowered the tiny battery's projected max parking time by nearly 6 hours.

* It seems to stay off if it had just stopped running right before shutdown.

Test result approximations, to estimate parking time for any battery size or type, including Li-ion 12V replacement:

• 0.016A drain when parked unplugged without alarm (avg of 93 hrs).
• 0.02A drain when parked unplugged with alarm on (avg of 103 hrs).
• 0.12Ah total drain for each activation of contactors AND brake pump (open/close driver's door).

[ElectricTireShredder]

Please see the important update added in red above.

Testing of the 3Lb 3Ah $20 battery seems complete, now that I took it all the way until it wouldn't start the car.
THIS tiny battery will start it at 11.7v (27%) but not at 11.6v (20%).
Slightly larger sizes MIGHT work at lower voltages, but to be safe, I'll assume they can't:

Lead-acid 12V* parked unplugged:

3Ah can go 4 1/2 days with the alarm on, or 5 3/4 with it off. (3Lb $20)
6Ah would go 9 days with the alarm, or 11 1/2 without. (4.4Lb $29)
10Ah would go 15 days with alarm, or 15 without (8Lb $30)
12Ah would go 18 days with alarm, 23 without (10Lb $40)

*A Li-ion 12V replacement should go much longer.
They have a much flatter discharge curve:
Attachment 38316

[ElectricTireShredder]

Another important update:

After seeing the gauge at 11.6v above, I opened the driver's door to try to start the car, & the brake pump came on, which seems to drop the voltage quite a bit. At one point while hooking up my 12v charger the car seemed to be trying to run the pump, & the gauge showed something around 5v! This may have half-killed the tiny battery, because the next time I parked for only 73 hours with the alarm off, & it was already down to 11.65v! (gauge alternating between 11.6 & 11.7).

I realized I should have started it from the passenger seat, to avoid activating the brake pump first, so this time I did that & it started fine, even with the brake pump activating immediately upon startup. I didn't have the nerve to try only one key click to see if that was okay. I just turned it quickly all the way, but it's at least worth trying, if it ever won't start from the driver's side where the pump drops the voltage as soon as you open the door.

The pump doesn't always come on, even at startup. It seems to stay off if you pump the brakes until it activates right before shutting down. I will try to see how hard it is to add that habit to my twice-a-week driving schedule. Doing so (or starting from the passenger seat) should allow parking longer until the voltage dropped lower, since it seems to avoid the big voltage drop from the pump.

I now wonder if that's the reason our cars sometimes start even after the 12V warning shows, & sometimes WON'T start even when it HASN'T shown yet: Maybe the warning activates somewhere below 11.65v, at which time the car will only start if the brake pump has not already been activated.

[ElectricTireShredder]

Short Update: The tiny $20 3Lb 3Ah still seems fine for weekday commutes & even 3-day weekends left alone parked unplugged, although I recommend the GEL model. However I often park 4 days with no plug, so I ordered a $35 4.4Lb 6Ah GEL direct from MightyMaxBattery.com.

Long Version: I left my hood ajar last night, & the alarm off, to check the voltage before & after unlocking the car today. 12.6v before, & 12.5 after, even using the key so no lights flashed! It might have been at 12.55 & only dropped to 12.54, but the point is it does take a bit of a bite out. In the 73 hour test 4 days ago I had unlocked & relocked at least twice, so I still believe that for daily weekday driving the 3Ah is fine, & even okay for a long weekend IF you don't unlock & relock it! I'm also not 100% certain that I had turned off my OBD dongle (I often forget to), however...:

- Nearly every week I'm parked for 3-5 days with no plug access at all,
- Sometimes I need to unlock/relock to load/unload things,
- Sometimes I forget to turn off my OBD dongle,
- I don't want to wear out my contactors, turning the key one click for a few minutes to recharge the 12V,
- Doubling it to 6Ah is still very light & cheap.

Compared to AGM lead-acid batteries, GEL is supposedly more tolerant of:
- heat from our onboard Li-ion charger,
- slight overvoltage from our 12V charger,
- deeper discharge from parking unplugged for several days.
Gel's primary drawback is higher cost, but with such a small size there's little difference. I would have got the 3Ah in gel but it didn't show on Amazon at the time.

Amazon again didn't show all models of their top-rated Mighty Max, but MightyMaxBattery.com did, for the same price, free shipping, & less tax. Tip: If you put one in your cart & wait a while after entering your email, you might get a 5% coupon! (like I got AFTER quickly placing my order).

[ElectricTireShredder]

I learned/realized a few more things from a final 6-day parked-unplugged test of the now-abused* 3Ah battery:

- For ANY size 12v battery, a fix for its faulty warning** would be an inside mount of the add-on volt gauge shown in post #71 above. Way before it's too low to start from the right side, it flashes red non-stop, without even touching it. It starts flashing at 11.6V (20%) which is a 10% safety buffer:

- The tiny 3Ah was on track to go nearly 7 full days & still start: It started with only 11.3v (10%!) with the left door closed to avoid pre-activating the contactors & brake pump. It wouldn't start from only 11.2v though (7%).

- If yours fails to start, wait 15 minutes with it off & try again from the right because that long after my failed start it went back up 0.7v.

- Having completed all the software updates, my drive battery never self-connected to charge the starter battery. I tested all the way down to 4.0V, so there's no worry about it catching fire if left parked too long unplugged. If you DID let it get so low it needed a jump, it would still be wise to check it for heat after starting, although it has always been stone-cold when charging, even from 20% (11.6v).

* I drained it all the way down to 0% once, & then all the way to 20%, before this test.
** No warning showed before or after failing to start at 11.2v