AGM battery for Gen 4/Gen 5 Prius/Prius Prime and observations on the 12-V charging system

I don't think the charging system distinguishes the battery type. What I have been observing is that the charging voltage and behavior is almost random. Sometimes it will float at 12.89 V, and at other times, it will float at 13.05 V.

What is worrisome now is that lately, the charging system has stopped charging the battery no matter how/when I drive the car. Curiously, it was charging well for the first two weeks, but as of Week 3, the charging has stopped. As a result, the rested open-circuit voltage has now started dropping everyday. This could be exactly why Gen 4/Gen 5 Prius/Prius Prime batteries are failing—the charging system simply refuses to charge the battery to save a tiny bit of fuel, as explicitly stated in the Toyota new-car features manual.

Uplus does not recommend the open-circuit voltage to go below 12.70 V. So, if that happens, we will have to periodically manually charge our batteries.

 

As long as it remains above 12.70 V for the AGM battery, it should be OK. Note that the AGM-battery open-circuit voltage is about 0.25-V-higher than the flooded-cell-battery open-circuit voltage.

 

This is the rested-battery open-circuit voltage so far for the Uplus EN LN1/DIN H4/BCI 140R AGM battery:

day voltage

18 13.05 V
19 12.98 V
20 12.95 V
21 12.92 V
22 12.90 V
23 12.87 V
24 12.87 V
25 12.87 V
26 12.87 V
27 12.87 V
28 12.87 V
29 12.87 V
30 12.86 V
01 12.85 V
02 12.84 V
03 12.87 V
04 12.84 V
05 12.86 V
06 12.85 V
07 12.80 V
08 12.83 V
09 12.80 V
10 12.82 V
11 12.84 V
12 12.79 V
13 12.82 V

For an AGM battery, about 12.78 V is 90% SOC; so, it looks like the rested open-circuit voltage has stayed above 90% SOC so far.

One thing I have learned is that the reason for the 12-V-battery problems on the Gen 4 and Gen 5 Prius/Prius Prime cars is not parasitic drain, the way the car is driven, or how long it is plugged in. While these could all contribute to problems, there is only one main reason for the battery problems: the way the DC–DC converter charges (or does not charge for a better phrase) the 12-V battery. The DC–DC-converter voltage is often minimized to about 12.89 V to improve the fuel efficiency, and it rarely switches on to the charging voltage, which is about 14.10 V. (Note that this is explicitly stated in the Gen 5 new-car features manual.) This results in 12-V-battery problems when things don't go right. As the battery ages or things go unexpectedly, I am guessing that the system tends to think the battery SOC is OK (sulfation and stratification both increase the open-circuit voltage) and further reduces the charging time, which leads to an avalanche effect and 12-V-battery failure.

 

@Roy Peterson can take some trunk-installation pics.

 

I will take a pic when I get a chance. I should also tape the battery monitor on the battery when I get a chance.

 

It is an AGM battery, not an EFB. EFBs are middle-tier lead–acid batteries, whereas AGM batteries are top-tier batteries, with standard batteries like the OEM battery being low-tier. Start–stop cars require AGM batteries (better but more expensive) or EFBs (cheaper). HEVs, PHEVs, and BEVs require AGM or LFP 12-V batteries, but Toyota used a standard battery to reduce the costs.