12V battery at 12.5 volts. Should I worry about sulfation?

Crazy not to.

That said, 6 years old and 12.5 volts, it might be on the way out. An electronic load tester will give a better idea. I'm using a Solar BA5 for that (current iteration is BA9). You connect it directly to the battery posts, set a few variables and run the test. It'll tell you the as-tested Cold Cranking Amps, and give a verdict: Pass, Pass-but-Recharge, or Fail.

 

Another option is to hook up a charger once a week instead of daily to a float charger.

 

Yeah then it's overkill. We're using ours sometimes as little as once a week, albeit good drives usually, so my perspective is coloured by that.

Batteries are batteries. Just because Prius doesn't crank the engine, it doesn't negate the significance of low Cold Cranking Amps. A battery in good shape will invariably have tested CCA higher than spec.When your battery has lowish volts and CCA less than spec, it's time to replace.

 

Your Gen 3 does not have a BMS. The SOC the Gen 4 and Gen 5 BMS keeps the 12-V battery at is actually as low as 75%, which is 12.35 V for a flooded cell and 12.58 V for an AGM cell.

You are right about the importance of occasionally using a battery maintainer (not a battery tender) in Gen 4 and Gen 5.

Also, Gen 2 and Gen 3 have AGM batteries, which have a voltage about 0.25 V higher than the OEM flooded-cell battery has in Gen 4 and Gen 5.

 

I don't know how accurate your measurement was and if the battery was rested for at least 16 hours, but if both are true, 12.50 V is 87.5% for a flooded cell, which is very good for Gen 4 and Gen 5. One caveat: Sulfated batteries (degraded due to sulfation) may have an artificially raised open-circuit voltage, which would not correspond to a good charge capacity.

The Gen 4 and Gen 5 battery-management system (BMS) is very miserly and likes to keep the 12-V-battery SOC at as low as 75% (12.35 V for a flooded cell and 12.58 V for an AGM cell).

Yes, occasionally using a good battery maintainer like Noco Genuis or such (not a battery tender) in Gen 4 and Gen 5 is a great idea to bring the SOC to 100% and reduce sulfation, as the BMS hardly ever charges to a 100% SOC.

 

I wouldn't worry about it; especially since it's a daily driver. I check my battery quarterly or semiannually; whenever I reminder to. A couple of times I've found it down to 11.75 Vdc. It still got the car into 'ready mode' without assistance. I put a charger on it @ 6A for a couple of hours and it's back up to 12.75vdc. The rumor is that a gen4 will go into ready mode, all the way down to 10.75vdc. Just check your electrolyte levels every year and you should be good. I normally get at least 10 years out of my batteries by just maintaining them properly. Don't know about this Prime yet, it's only been 4 years on this OEM battery - but I haven't been stranded yet....

 

For what it’s worth my 20 Prime has charged several days to 100% recently, if the night temps drop into the forties. Miles and hours had nothing to do with it.
Other times it charges nothing on a three hour trip. The only way I can see to reliably ward off dead battery is a battery monitor. And to keep the 12 volt healthy as much as possible.

 

Amazon.com : bluetooth battery monitor

 

The BM2 Bluetooth battery monitor is pretty good, and mine turned out to be perfectly accurate to 0.01 V.

 

The Ancel bm200 works fine, have two plus a bm2. I bought on price. Next stop maybe hall effect to measure amps, if I want to waste money for that amps reading. Not really going to change much what the car does. I think Toyota needs to reprogram the charging.

 

I wouldn't bother much with the current, as you can guestimate it, but it could be fun, albeit more obsession.

 

Car charging again today like a champ. Cold night for my area, forties. This afternoon, evening, it will be a wimp at it. If it would charge like it does when it’s a champ it would be normal.

 

What I meant was that use a smart charger/maintainer, not a trickle charger.

 

That system may be imposing an additional load on the car when it’s sitting. A phantom load test is relatively easy to do. In a nutshell: place ammeter in series between car body and neg post, let car sit, off, for at least half an hour, all doors closed, fobs well away, and measure. Should be around 20 milliamperes.

 

It is more than 20 mA in Gen 4 and Gen 5, which are a lot more advanced than your Gen 3.

These are my observations in my 2021 Prius Prime Limited:

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I got a more accurate number on the parasitic drain:

• It is a 0.03-V drop in 24 hours for the 50-Ah AGM battery,
• which is a 2.1% SOC drop in 24 hours,
• corresponding to an average current of 45 mA.

It seems typical for a modern car. I doubt it makes a difference with our without Toyota Connect subscription.

Considering that the battery can already be at as low as a 70% SOC, you do not want to leave the car undriven without a battery maintainer for longer than two weeks.

Note that even if you do not drive the car and do not have a battery maintainer, you can turn it on to the READY mode for half hour or, even better, for one hour once a week to recharge the battery. It will unlikely go over an 85% SOC, but it will prevent a battery failure and/or serious battery degradation.

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