Another 12V battery thread

I wanted to bring a conversation to the forum so those who have a similar car might be able to suggest things that I might have missed. I can address generic Prius problems but since I don't have a 2016 Prius on my driveway, I can't replicate the instructions and measurements discussed.

BACKGROUND

You can check by monitoring the 12V battery voltage:

• above 13V - car is charging battery
• below 12.85V - battery is not being charged

I've had a Gen-1, Gen-3, and now a Prius Prime. So far, the 12V is charged when the car is in "READY". I have not looked at other states and assume the battery is not being charged.

Ok, I see three hypothesis:

1. Increased parasitic load - obviously the amplifier is suspect. There is an 'ignition switch' operated 12V supply and an 'always on' buss. I would always recommend using the 'ignition switch' operated buss which can be tapped at the cigarette lighter. This ensures any amplifier draw even when off is truly off. However, there could be a light that is staying on instead of going out.
2. 12V battery is dying - a cell could have failed and that could leave the car 'dead' each morning.
3. 12V charger is weak - it is possible (very unlikely if the car runs OK after going READY.)

A lot of these potential problems can be understood if you can read the 12V battery voltage:

• In morning before using the jumper box. This tells us how discharged the battery is.
• In morning after using the jumper box and car is in READY. This tells us how well the DC-to-DC converter is working as it tries to charge the 12V battery.
• End of day and car is in Park and READY. This tells us how well charging has worked during the day. Feel the battery and see if it is unusually warm to the touch as this could indicate a failed or failing 12V battery.
• Immediately after turning off the car. This is the best voltage the 12V battery might hold but we don't know until a second measurement.
• About 30-60 minutes later. This shows how fast the 12V battery is discharging or has failed.

So these are the voltages we are interested in:

1. 13.8-14.0V - the DC-to-DC inverter is charging the battery.
2. 12.85V - a new, fully charged, 12V battery will hold this voltage.
3. Less than 12V - the battery has reached its 'end of life.'
4. Less than 11V - the battery had a failed or shorted cell, it must be replaced.

Now if you have a Volt-Ohm Meter (VOM) that has a 10A scale, I would measure the 'parasitic' drain on the 12V battery. Turn off the car and close all doors and hatch. Raise the hood and measure the current between the 12V battery negative and the frame. This is very tricky because there a capacitors that will impose a substantial in-rush current flow that could blow the fuse in the VOM.

To safely measure the current, use a clamp-on current attachment. The alternative is to run a jumper from the ground wire and the battery negative post that can be opened (i.e., two pieces of wire twisted together in the middle would work.) Then put the 10A scale, amp meter across the battery post and ground reference and open the jumper. All current will flow through the meter and we want to see a current in the 30-40 ma range. Anything higher means some load is draining the battery.

What I don't know is if raising the hood also turns on a light for the engine compartment. If so, I would try to remove the bulb and do the test.

Any suggestions or changes?

Bob Wilson