12v Battery - All You Ever Wanted To Know

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Staff member
Dec 15, 2011
Hills above Silicon Valley, California
Having just answered yet another question (outside our forum) pertaining to our i-MiEV's (and Insight's) 12v battery, I thought I'd make this a stand-alone topic that could be linked to, and I've extracted the following from posts on different threads (with a few embelishments) -

Let's review our 12v battery - it lives a very benign and happy life, never has the stress of having to provide hundreds of amps starting current as for an ICE. Thermally, it is not sitting next to a high-temperature ICE; however, it is indeed exposed to brutally-cold northern climes.

The 12v battery is fed by a dc-dc converter which provides a constant voltage. I suspect (but have not measured) that this voltage is temperature-compensated. For example, I just measured 14.48vdc when I turned the car on in my 45degF (7.2degC) garage.

Before this little morsel gets lost, from the EvBatMon thread,
PV1 said:
…I use IGCTPower to monitor my 12 volt system. I can verify that the i-MiEV does indeed have temperature compensation for the 12 volt battery. In colder weather (we just had a mini-blizzard), the voltage goes up to 14.4 volts when first starting out. Up until now in warmer weather, it goes to 14.2 volts and stays there.
The 12v battery only provides three lightweight functions:

1) Provides the current to activate the car's systems when the key is turned ON (Ready).

2) Provides current to run the various 12v accessories with the key turned to the first notch: radio, fan, dome lights, etc. I measured 9.6A current draw with the fan in the midway position on my SE Premium with nav unit and radio and hard drive running. YMMV.

3) Provides a tiny trickle current to keep the alarm system active when the car is OFF. I couldn't measure this with my clamp-on ammeter, but suspect it's under 50ma. Let's see, 30 days idle at 50ma is 30*24*.05= 36Ah … oops, dead battery, so it must be much less than that. I'll measure power-off current draw when I go to replace mine. Note: I still need to measure both the OFF current draw and voltage-temperature extremes when ON.

The battery is BCI Group Size 151R. Mitsubishi calls it a 34B19L(S). From the Yuasa website, this is shown as 33Ah at the standard 20-hour rate (i.e., a constant 33/20=1.65A) before it drops to 10.5vdc, whereas all the BCI ratings use CCA and Reserve Capacity about which we don't care.

For completeness, for the 34B19L(S), Yuasa shows L = 185mm (7.3"), W = 125mm(4.9"), H = 202mm(8"), Overall H = 224mm(8.8")

All right, so what does all this mean? The stock battery size is already excessively-large, IMO, but your usage determines the battery capacity you need. For example, you may wish a higher capacity -

1. If you go away for weeks at a time not using the car and don't plug in a float charger.

2. If you spend a fair amount of time sitting in the car listening to music or with your laptop or other gadgetry plugged into the accessory socket.

I consider the most stressful thing you can do to our 12v battery is to simply leave the car sitting untouched for a few weeks - the gradual battery depletion results in sulphation of the plates which, in turn, never get a chance to recover because the battery never gets an "equalizing" (higher-voltage) charge to clean the plates. Auto dealers murder 12v batteries when the cars sit on their lots untouched for weeks at a time.

Choices -

The car comes with a flooded-lead-acid (FLA) battery. It's advantage is that it is cheap, but the fluid level should be periodically checked ( a PIA as it is difficult to get a mirror and light in there) and eventually it will result in terminal corrosion and maybe a bit of rust around it due to the acid fumes. Alternative is a 'maintenance free' FLA - I don't even know if one is available in this size range. I personally will not replace my FLA battery with another FLA.

AGM (Absorbed Glass Mat) - this is a sealed battery that is very suitable for this application. Usually, AGMs don't like to see more than 14.4v at room ambient (exceptions being Optima and Odyssey AGMs), but it seems that our dc-dc puts out voltage right at this limit so I don't see a problem.

Gel - also a sealed battery and actually would be very good in this application as Gels do well in deep-cycling which would be the case if the car was left unused for weeks at a time; unfortunately, the traditional Gels don't like to see a voltage much over 13.8v, so I wouldn't put one there. Don't even know if there's anything available in that form factor.

Lithium - sure, you could stick four LiFePO4 cells in there - for example, four Headways strapped together or even higher capacity like the 3P4S configuration LOWRACER did (http://myimiev.com/forum/viewtopic.php?p=4035#p4035), and it would work BUT I wouldn't put those in there without an individual-cell BMS. Lead-acid is very abuse-tolerant, whereas Lithium is not. Also, 14.4v from the dc-dc results in 3.6v per cell, which is close to the LiFePO4 upper limit of 3.65v - not good for longevity to keep it that high continuously.

For myself, since I usually put a float charger on my i-MiEV's 12v battery when we go away for a few weeks and I can't recall ever sitting in the car for any amount of time while not charging (the dc-dc is activated when the car charges, so you can run the accessories indefinitely while charging), I'll install a small (much less than 33Ah) inexpensive motorcycle AGM when the time comes. I did that on my Honda Insight a decade ago and it's still going strong.

As an aside, my Tesla has a very high (>1A) current draw (even when asleep), but compensates for this by monitoring the 12v battery voltage and, when it gets too low, it turns on the dc-dc converter to recharge the 12v from the traction pack (which causes it to lose about 1% capacity per day). Still unhealthy, as the 12v battery is being continuously cycled and this has led to premature 12v battery failures (covered by warranty). Tesla now installs a very high quality AGM battery in its too-small 33Ah dedicated space (same capacity as our i-MiEV). For myself, I've added a 12v float charger to my Tesla which has completely stopped the cycling and the traction pack no longer loses any capacity (thank you, siai47).
Thanks Joe, this is good information here. I found out the hard way about using a gel cell batteries for a starter battery. I found one that was a pretty good fit for my 2007 Prius and it was only 2 years old but more importantly, it was free! Then six months later the check engine light came on with a couple codes for "low battery voltage" and bad ecu skid control computer. That one got my attention. :oops: Fortunately, a new starter battery took care of everything.
I have a 12V LiFePO4 battery in my C-Zero. I've relocated it inside the car, under the right hand rear seat, where the tire repair kit used to be. This way I made space for a fuel heater. So far I haven't had any issues. I've had it there over a year now, I think. LiFePO4 is very safe and if the cells are well balanced you can get away without a BMS, but YMMV, of course. Headways aren't very reliable. This is what I have:


Battery location:

jsantala, interesting that there are commercial Lithium batteries out there without balancing circuitry, or at least a high- or low-voltage disconnect built-in capability. Do you have access to the individual cells to check to see if they're all the same voltage? Agree that LiFePO4 is quite safe and that's what I use on my boat.

I've actually had pretty good luck with my small 4-cell 12Ah Headway LiFePO4 pack without BMS, and only balance-charge it about once a year with my PowerLab8 while the rest of the time it's grossly float-charged through a Schottky diode and resistor from the i-MiEV's 12v system. I use it to keep my Android tablet with CaniOn alive during trips.

The two paralleled Enderdel 144v Li(NMC) packs (similar chemistry to our i-MiEV traction pack?) in my Sparrow have been running for 2-1/2 years without a BMS, and in all that time only one cell went low (detected by an occasional manual voltage test of each of the six modules) and I simply recharged that one cell to match the oters over a year ago and they're all still going strong and balanced - but I only operate between 25% and 90% SoC. Two parallel strings of 2P36S Enerdel cells, broken down into three 2P12S modules in series. I had midpoint-balanced them for a couple of weeks before installing into the Sparrow, as I take issue with bottom-balancing since most of the time the batteries operate at the upper end and I simply don't get down to the low end.

Ooops, drifting off-topic. Despite all that, I would only install Lithiums as a primary 12v battery into the i-MiEV if the pack incorporates an individual-cell BMS, and both an overvoltage and undervoltage disconnect. Gotta make it idiot-proof. :evil:
JoeS, the one that I have doesn't have any BMS or other electronics in it or connected to it. It's just hand balanced once and that's it. You can open the plastic case to check voltages. If you don't have anything connected between the cells they won't get unbalanced by themselves, provided that they are good quality cells and none of them actually break. It would be immediately visible if any of the cells had an issue by looking at the 12V battery voltage before turning the car on. My fuel heater shows th voltage. If it's not 13.3 volts, there is a problem. If you've bottom balanced the cells you can't really cause any serious issues by draining them either. I've broken a few LiFePO4s by forgetting a load on them and they bloated, nothing else. Some of them even came back and shrunk back in.

I've got LiFePO4 cells just bottom balanced in my DIY motorcycle and/or car with no issues as well.

They do sell Lithium 12V batteries meant for motorcycles which do have monitoring electronics in them. I suppose they could be used just as well.
jsantala said:
...If you don't have anything connected between the cells they won't get unbalanced by themselves, provided that they are good quality cells and none of them actually break.
Agree that brand-new good-quality cells from the same lot should truly be identical, and will track throughout their SoC range, no matter where you balance them - at least, for a good portion of their life. Wouldn't be surprised if your pack lasts for 10-20 years.
jsantala said:
It would be immediately visible if any of the cells had an issue by looking at the 12V battery voltage before turning the car on. My fuel heater shows th voltage. If it's not 13.3 volts, there is a problem...
Hmm, does that voltmeter have an offset as the i-MiEV dc-dc runs at around 14.3v, or do the cells sag overnight down to a stable 13.3v? One thing I like about Li(NMC) is that they sit where you left them, with no downward drift to a stable no-load voltage.
the dc-dc is activated when the car charges, so you can run the accessories indefinitely while charging

I proved this to be correct the other weekend, when our EV club was displaying cars at Biosphere 2. I had the mine plugged into 120V all day, to replenish the main pack for the trip back home. But I took a chance and ran the CD player with background music all the time I was doing this. I figured the 12V cell was getting juice from the main pack while charging, but I had one of our other guys stick around until I unplugged and attempted to start up. All was well and the car started up without incident.

I gave some thought to the health of the 12V battery the next day, though, and thought it prudent to top it off with distilled water. While picking up a bottle of that from the grocery store, I found a basting juice injector syringe (the sort of thing you'd have for cooking turkeys or hams) and this proved to be the ideal tool for carefully metering the distilled water into the 6 entry ports.

On a related topic, I'm the lucky owner of one of these . . .



. . . an A123 cell (4S2P configuration / 3.3V per cell = 13.2V total) 4.6aH LiFePO4 motorcycle battery, complete with BMS, assembled by a Chinese firm called Reduce Carbon Energy. I found it on eBay a few years ago and was able to snag it for the bargain price of around $130. This was a direct purchase from the manufacturer and they were a rather unusual crew to deal with. Their web site (I'm unable to find it now) swore up and down that they were to only retail company to use A123 cells in their assembled packs and everyone else out there was lying if they declared their packs were wired up with the genuine A123 article. Further, their eBay selling style was unconventional. While I got this particular battery for the bargain price, they would have identical listings up for the exact same battery at double that price. Both were allegedly new/unused products. So, I chose the cheaper listing and crossed my fingers. A couple months later, I figured that I better get a charger and went back to them, only to find that they wanted to sell me their 5A LiFePO4 unit for double the price I could find elsewhere . Needless to say, I picked up a charger for around $60 from a competitor, instead of $120 with them.

But the battery itself . . . I dig the clear enclosure and it's a beauty to look at. In terms of a good functioning unit, it's appears to be the real deal. I was flying model airplanes back when I purchased it and was wanting to use it for field use, but never seemed to find a consistent application for it. It once spent about a week keeping our house security system up and running, when the much smaller 12V pack inside that box no longer worked. Mostly, I take it to electric car events to show people what a small scale wired lithium pack looks like.

But now that I've got an i-MiEV, I'm thinking about carrying around this 4.6Ah cell in the car for emergencies.
JoeS, fully charged LiFePO4 cells will have a voltage of around 3.3-3.4 volts per cell after you leave them to rest. Thus around 13.3 for the four cells. The heater panel may show a little less than actual due to losses in wires, but it's close enough and the most important thing is that it reads the same every time. I think it shows about 14.6 when the car is running or charging, so 3.65 per cell, which is fine for LiFePO4.

I don't think you could actually use four 18650s directly, because like the i-MiEV main pack itself, they use LMO/NMC chemistry and thus would require a 4.1-4.2 charging voltage which just doesn't add up. There would have to be some trickery involved. But yes, it's easier to keep track of their voltage since it hardly drops after charging.
I've wanted to use Li-NMC 18650 cells, but couldn't get the voltage range right. 4 cells would be continuously under-charged, but 3 cells would be way over-charged and likely go thermal.

Although, Lithium Titanite (LTO) batteries appear to have a voltage range suitable for use as a 12 volt battery. Max voltage is 14.5 volts for a 5s (5 cells in series) pack, nominal is 12 volts. Each cells is 2.4 volts nominal. Among other advantages, LTO has:

1. Longer cycle life (3,000-7,000 at 70% of original capacity)
2. Higher power input and output (up to 10C for both charging and discharging).
3. Quite stable

Disadvantage is lack of energy density compared to other lithium chemistries due to the lower voltage (2.4 volts vs. 3.2 or 3.7).

For my off-grid solar, I used to use 10 NiMH cells in series to act as a 12 volt battery. The voltage range is perfect, and NiMH has tolerance to misbalance and slight overcharge. 14.5 volts is just about fully charged, and the cells are empty when pack voltage reaches 10 volts. They settle to about 13.5 volts after a charge, and can sit anywhere in their SoC window with minimal harm.

I still have only experienced battery memory on NiCd, but everyone seems to think that all batteries have memory. As for their rated cycle life, I have AA batteries from over 13 years ago that still work well, not to mention the original RAV4-EVs still running around on their original NiMH packs.
Except DIY 12V replacement info is missing:

"A 10 mm socket and wrench is the only tool necessary to remove and replace the battery, although a terminal cleaner would help make a clean connection."

DIY replacement discussion

From the manual: Type 34B19L
Internet Search i-miev battery = Group 151R

manual screenshot image fail!




Edit by Moderator - here are the links for the above images -


OK from 2012 manual...

warning Do not disconnect the negative terminal of auxiliary battery for 1 minute after turning the electric motor switch to LOCK (OFF).

Do not keep the auxiliary battery removed for a long period of time (approximately 1 month or more). (The BMU will not be able to comprehend the main battery status.)
If the auxiliary battery has necessarily been removed for a long period of time (approximately 1 month or more), perform "BMU Learning Value Manual Writing." (Refer to GROUP 00 - Precautions before Service - Procedures for Writing BMU Learning Value .)

Removal steps
1. Dash pad clip
2. Auxiliary battery cover
3. Connection of the auxiliary battery harness [negative auxiliary battery terminal]
4. Connection of the fusible link box, engine harness, front harness assembly [positive auxiliary battery terminal]
5. Auxiliary battery holder
6. Auxiliary battery bolt
7. Auxiliary battery
8. Auxiliary battery tray
9. Auxiliary battery holder bracket
10. Fusible link box cover
11. Connection of the auxiliary battery harness [positive auxiliary battery terminal]
12. Connection of the auxiliary battery harness [positive auxiliary battery terminal]
13. Fusible link box bracket
14. Fusible link
15. Fusible link box (1P)
16. Fusible link box (6P)
17. Auxiliary battery terminal
GdB, thank you for adding the information to this thread. I took the liberty of editing your post to make the links visible even though you have the text nicely covered. The fourth link shows the related hardware.
With many of us sequestered and cars sitting idle, I'd just like to remind everyone to remember their 12v battery and activate the car every few days or else attach a float charger directly onto the 12v battery. For the last six weeks my i-MiEV hasn't left home except to drive the garbage cans down to the main road once a week, and my wife goes down in her i-MiEV to retrieve mail from the post office, also once a week. We're very fortunate that our next-door neighbor with a large family (and an i-MiEV) includes us on his shopping list. Stay safe, everyone.
I've been keeping my fleet moving. Since my home internet is a joke, I've been bopping into town to use free Wi-Fi while sitting in the car. (Thanks, Panera Bread. Promise I'll make it up to you. ;) ) Haven't had to make many supply runs since we normally keep a few weeks' stock on hand anyway, and we doubled up before everything was shut down. One of the benefits of living outside of town; the distance discourages daily trips.

Both of my i-MiEVs got new 12 volt batteries in the last year or so, and haven't had a dead battery since I discovered the trick of starting the car in the middle of a door unlock-lock cycle. The Bolt got a big AGM battery in it, so it'll sit for a long time without needing a charge.

Both of my i-MiEVs have this quirk where if the car is locked, and I go to retrieve something out of it with the simple process of unlock doors with key fob, open a door, close it, and lock the car with the fob, I'll have a dead 12 volt battery within a week. If I unlock the doors with the fob, open a door, start the car, shut it off, close the door, and lock it with the fob, the car can sit for a month+ and be able to start without a jump. Whatever drains the battery, having the car fully cycled before locking the doors lets it power down successfully.
I put Miata AGM batteries in two of my three cars - One of them has been in use now for 4 years or more and both are working fine. No issues ever with a dead battery, even if the car sits for a month or so

We discovered the "quirk" early on, that the 12v gets drained if locked by the fob. Since then, I have into the habit of locking by pushing the lock button on the door on the way out, making sure key is in hand.

Replaced the 12v with a sealed 12v from Harbor Freight a year ago, seems to be holding up fine.
GdB said:
Do not keep the auxiliary battery removed for a long period of time (approximately 1 month or more). (The BMU will not be able to comprehend the main battery status.)
If the auxiliary battery has necessarily been removed for a long period of time (approximately 1 month or more), perform "BMU Learning Value Manual Writing." (Refer to GROUP 00 - Precautions before Service - Procedures for Writing BMU Learning Value .)

"BMU Learning Value Manual Writing." Why is this necessary and what does it do? Is this a MUT3 function or from a different diagnostic tool?
OK, I am not a car person, even though I owned my first Electric car in 1965 (long story).
Anyway, I am having some awful problems with my 2012 I-Miev. It will not do anything, even the key will not turn beyond the 'ACC' position.
This started two days ago, when the temperature went over 104 Fahrenheit here in S. California.
I'm hoping the problem is simply the 12v battery, which is what I am looking at first anyway.
However, as I said, I am not a car person, and have never had any interest in cars.
So this is probably a dumb question, but I cannot find any answer to it anywhere online, so I am asking here.
I can see on the battery where the high and low lines are for the necessary amount of water, but I do not see anyway of telling how much water is currently in the battery. How do you tell how much water there currently is?
RonSmithJr said:
... how much water is currently in the battery. How do you tell how much water there currently is?

I would be surprised if it needs topping up but maybe your climate demands a design of 12V aux battery that requires it...? I haven't seen a proper 'flooded'-type car battery (that needs regular maintenance ie topping up) for decades.

In any event, the key question that needs answering is; what voltage has the 12V aux battery? In my experience (of European and Japanese i-MiEVs) it needs to be a good 10V to get the drive-sytem to turn on. Even so, at less than this (down to below 9V) *something* should happen at the 2nd key position ie dash lights etc. If you aren't even getting that then it would seem the aux battery is completely stuffed or it isn't connected!
If the key is physically restricted from moving past the ACC position, then the gear shift lever is not fully engaged in the Park position or the cable has become dislodged. There is a mechanical interlock between the key switch and the gear shifter.