12v Battery Voltage Usable Lower Limit

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Staff member
Dec 15, 2011
Hills above Silicon Valley, California
I'm starting this new thread in an attempt to solicit inputs to determine what the minimum usable voltage is from our i-MiEV 12v battery.

We know when the battery voltage gets too low that the results can be the car simply refusing to start and/or displaying all sorts of random error lights and codes.

We also know that in actual operation that, as long as the car goes into READY, the battery voltage does not matter because the car's regulated dc-dc On-Board Charger (OBC) takes over, keeping that voltage somewhere in the 13.7-14.4vdc range.

What prompted me is that while performing some LiFePO4 battery tests I was shocked to find out that my 2-1/2-year-old Mazda Miata AGM Battery https://www.batterymart.com/p-12v-mazda-miata-battery.html was sitting at 12.2vdc, the car having been driven a few hours earlier and presumably charged up by the dc-dc to ~14.4vdc. This battery is only 25Ah, compared to our OEM Yuasa at 33Ah, each presumably a 20-hour rating.

I subsequently fully charged up the AGM battery and discharged it using my West Mountain CBA. It yielded a capacity of 18.9Ah (it took 19.3Ah to recharge it). Here's the graph (the voltage had already climbed back up to 11.46v after the test when I took this screenshot). I had incorrectly entered 32Ah instead of 25Ah as the battery capacity but that doesn't matter since I forced the discharge current to be 2A):


In contrast, here's a snapshot of the 7.8Ah NOCO LiFePO4 test which yielded a capacity of 7.86Ah , but for a low voltage test cutoff of 12.0v (sorry for this poor quality photo of the screen):


Note that the voltage of the freshly-charged AGM (to 14.4v but settled down to 12.8v) dropped almost immediately to below 12v as soon as the minimal 2A load was applied. Contrast that with the LiFePO4 which retains its higher voltage throughout its discharge cycle at the same discharge current level.

What is prompting this is to call into question the conventional wisdom of specifying battery capacity low voltage cutoff being 10.5vdc, which is a totally unusable number for our i-MiEV. I'm looking for a realistic low voltage cutoff number to specify for any future testing and 12v battery discussions. Anyone have any thoughts on what that number should be?
Well it may be an interesting question to answer, but it could also be an expensive experience.

i think you made a bad assumption that only benign problems could result from a low starter battery voltage, and that the DC Converter will be able to fill in the gap.

If the 12V lets go during charging, then OBC damage will likely occur.

If the 12V lets go during READY, there is no guarantee that the DCC will be able to hold up the system. Plus this could be fatal if you are making a left hand turn across oncoming traffic, or crossing railroad tracks, or making a right hand turn to pull out into traffic with a tractor trailer bearing down.

The price of freedom is eternal vigilance--and a fully charged starter battery. :lol:
kiev said:
If the 12V lets go during charging, then OBC damage will likely occur.
If the 12V lets go during READY, there is no guarantee that the DCC will be able to hold up the system. Plus this could be fatal ...
By "letting go", I believe Kenny (kiev ) means a sudden open circuit.

IMO, not too likely for lead acid, but I worry about the BMS of a LiFePO4 battery opening up because the OBC output voltage has exceeded the upper voltage limit (either while charging or driving), for example due to cold (discussed in the 12v battery thread).

I don't believe the lower limit is in danger of being exceeded (downwards) in any scenario other than the car being parked and the battery being slowly bled to death.

Edit: Kenny, thank you for this, and I'm continuing this discussion of open-circuit dangers over on the 12v Battery thread.

Edit: As seen on the graph above, I set the LiFePO4 battery lower voltage test limit to 12.00vdc (3.00 volts per cell). Had I done the same with the AGM, it would have flunked the test immediately and shown zero Ah. Yet my AGM has never shown any problem in 2-1/2 years, suggesting that the i-MiEV will go into READY at a lower voltage. So, the question stands: how low can it go without a hiccup?