12V battery measurements after 44 months in service. Replace or not yet?

Hi Ed,

No. When a 12V flooded lead-acid battery shows voltage of 11.89V or less, that is considered discharged or 0% SOC.

12.06V = 25% SOC
12.24V = 50% SOC
12.45V = 75% SOC
12.65V = 100% SOC

(Add ~0.2V to the above numbers, for AGM batteries.)

See the table which is halfway down the webpage here:
Charging the lead-acid battery

 

Hi Ed. +1 To what Patrick said. Here's some more info on the state of charge versus voltage of a 12V lead acid battery.

Note that there is some small variation in this depending on the design of the battery. (For example the electrolyte concentration may be "tweaked" for one battery application or another. Eg to optimize the battery for slow self discharge versus large cranking amps capability etc.) This will slightly change the characteristics but most lead acid batteries will be very similar to that shown below.

That first attachment shows the SOC versus voltage for the battery, at rest, at C10 charge and at C10 discharge. I kept this graph simple to emphasize the difference between the "at rest" state versus the charging and discharging states. Note that "C10" refers to a current equal to one tenth of the rated amp-hour capacity. For example with a 35 AH Prius battery C10 corresponds to 3.5 amps whereas C5 would correspond to 7.0 amps and C100 would be only 0.35 amps etc.

The second attachment shows the same thing but at a number of different charge/discharge rates.

 

Measure the voltage with headlights on and then with both headlights and rear defogger on. Post the result here and see what people say. Good luck!

By the way I assume you have had enough driving time to charge the battery prior to the 24 hour rest?

 

Don't bother having them test it. It will only be a waste of money because their tests will definitely say it's ok.

Well that's quite a lot of load (amps) so yeah you've got to expect the voltage to drop a bit. The low temperature is only going to make the voltage drop worse so there's not necessarily anything wrong with the battery.

I know that you'd like to be seeing higher voltages but it's only a small battery so if you put a 25+ Amp drain on it then you're going to see it go well under 12 volts, especially with the temperatures you've got at the moment.

A lot of people only look at car battery voltages when they're charging (or immediately after) and they get the false idea that they should always be like 13+ volts or that anything under 12 means the battery is just about dead, but it's not true. Did you know that "cranking amps" are actually "officially" measured at only 7.2 volts (for a 12 volt a battery). In other words they take into account that to supply a real lot of current that you've got to let the terminal voltage get dragged down.

Look at the second of the two attachments I posted at #42 above. It only goes up to "C3" (about 12 amps for your battery) and look at how much the voltage drops at that rate. You were probably measuring at somewhere near "C1" (off the scale on my graph) so around 11 volts is not all that unreasonable.

I wouldn't stress about it. When the temperature gets a bit warmer I'm sure your battery will give "Ig-On" readings over 12 volts, which in my opinion makes it alright. Lets see what others think.

 

20 minutes is probably not long enough to fully charge the battery. This morning after sitting overnight my (maybe somewhat weak?) battery also read 10.9V with low beam and rear defogger on. Not as large a load as yours though. Did you turn the A/C on manually and verify the compressor was running?

Another thing you could do is to monitor the 12V bus voltage with the car in READY mode. I remember someone reported seeing it drop significantly after some time and attributed to that the system thinks the battery is fully charged. I have never seen such large drop and am guessing that's a sign of a weak battery.

 

The A/C compressor is run off the HV system.

 

:doh: So I guess just the blower is on 12V?

 

That, as well as the electric PTC heaters.

 

Well that sucks. Based on this I have been driving around on this battery for over a year with a 0 SOC with no problem. Strange, as it takes a charge like a very healthy battery. Goes from 3-4 amps draw to 0 amps draw in 20 minutes on my charger.

 

Hi Ed,

It's hard to evaluate whether a 20 minute charging duration is good or bad, without knowing what SOC you started at. In general, a marginal battery has limited capacity (compared to a new battery), hence a marginal battery can be recharged more quickly than a new battery (but will also discharge more quickly, given a certain load.)

Note that the voltage measurements that I cited are open circuit, and need to be taken 8 hours or so after the battery was last charged so that surface charge is not an issue. You need to use a digital multimeter to measure those voltages as the Prius should be IG-OFF.

Also, you need to consider the possibility that your battery charger is cutting off current flow before the battery is actually fully-charged. I have an 18-year-old Schumacher automatic battery charger (2/10/50A). I found that the charger turns off before the battery is fully charged. Hence I prefer to charge my 12V batteries using a lab power supply so that I can precisely monitor both voltage and current, and adjust the charging voltage to follow the charging pattern cited at the beginning of the link that I provided in post #41. Since ambient temps are moderate right now, I am using voltages at the higher end of the range to speed up charging time.