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

My 12volt battery sees 12.3 v wether travelling 2hrs plugged in to the battery tender daily or charged via house current
It’s checked with a digital multimeter

 

Roy Peterson
You are an excellent communicator. Thanks for your posts.

 

huh... I have a 2017 Prime... last I checked, charging traction does charge the 12v... hence why Ive been trying to get ahold of a ~150aH Lithium Group 24 battery... the whole thought of trying to put a second 12v in the back Ive been holding off on... versus buying a standard 12V my plan was to build one that's more flat battery than the standard sizes.

are you saying the only time the 12v charges is when the vehicle is in READY mode?

 

wait.. sorry if this is new news to me but.. in the Gen 5, they put the 12v in the back?

 

Yes. The M20A engine is a bit bigger than the 2ZR, so the battery has been relocated to the back again. Gen5s with the 2ZR(like the Mexican Prius), have the battery in the front.

 

interesting... also interesting, I have 3D scans of both engines and I tell yah, the size difference isn't by much! I mean, you can bore a 2ZR out to be basically the exact same 1,987 cc and the block walls would still be strong enough to support it. but the size... worked out and that's how I was able to install the M20A into my 2013 Plugin, just getting it to run in EV mode up to 80mph like my 2017 Prime... I need to find a company with the machines to make the sprauge clutch for me based on my files... hence the delay in finishing it all up. but still.. interesting on the battery placement. safe assumption gen 5 its in the back passenger side?

 

I think it's less about the total size, and more about where that size is. I think the M20A has an intake manifold that projects further forward. That means there's no room for the air intake box in front of the engine like in the 2ZR gen5. The solution was to move the air intake box to the front left corner of the engine bay, which is where the 12V battery sits in the 2ZR gen5. It's also possible that the more powerful M20A needs a bigger air intake box or filter compared to the 2ZR, and that's part of the reason for the relocation.

And the new battery location is indeed in the back passenger side, just like the gen3. I went from a 2010 gen3 to a 2023 gen5, so there's been no change for me.

2ZR



M20A

 

I like the way you think!

 

I would also see 14.3 volts during summer months. Since installing the BM, I have always seen 14.X volts on initial charge voltage. The length of time it stays at 14.X volts has varied. I have not been able to correlate what the variable is.

Cheers

 

I had a voltage monitor in my vehicle since day 1. Just a cig port usb adapter that gives a voltage readout. Was within a negligible voltage difference compared to direct battery terminal in my old car so I feel it's accurate.

Bought the car in winter AND because it was new I didn't pay much attention to voltage changes early on but I do remember it was very often in 14.3v. In fact in winter I can't be certain it dropped below that value much if at all.

Spring, summer, and fall showed the same charging pattern as many have posted. Intermittent 12.7/13v states. I'd even see it drop to those values with my aftermarket audio upgrades going. I don't blast crazy loud so I wasn't hitting peak RMS values but I was definitely pulling more than factory.

Around mid November when it was getting notably cold I noticed it was no longer going into 12/13v modes. Just staying in 14.3. Over 3 weeks of glancing at my usb outlet while driving I saw it briefly drop to 13v twice. Nothing changed in those situations. I also noticed the lead acid bat, based on temps taken with a laser temp device and its resting overnight voltage, was reporting around 75-85% SOC and not able to bring it above that general value. On one extremely cold morning (our first real morning frost) it was reporting as low as 11.9v. Car started fine in all situations though.

What I can guess at is that the 12v charging system is not taking temperature into account and is trying to bring up the battery to an optimal voltage so it stays in 14.3 mode trying to reach that.

Remember that just because we see 12/13/14v, we are not being told how much amperage is being delivered. There very well could be more "HV-->12v" states based on combinations of voltage/amperage.

I'm too lazy and unmotivated to throw my clamp multimeter on it and find out what's actually being pulled and personally I think if someone's going to measure total current it should be done by inline measurement for an accurate reading. But if someone wants to, more power to you. These are just my observations.

Honestly though none of this information is really useful for any practical purpose. We know the limits of the DC/DC system is around 1000W (remember that the dealer added JBL package is rated for 800W RMS so the 12v charging system SHOULD be rated higher than that). Knowing HOW the car charges the 12v system really makes no difference in anyone's day-to-day usage of the vehicle. For those in colder temps, or who desire reliability def ditch the lead acid and it looks like from the other thread avoid lifepo due to BMS issues in cold weather. AGM seems like a solid choice for replacement in most applications here.

Anyone wanting to do crazy stuff will likely be looking into buck/boost converters from the HV system directly as past prius owners have done.

 

I have monitored multiple trips recording a data stream of the PIDs for aux battery system with my scan tool. I can record aux battery voltage, current, temperature, SOC along with multiple other parameters. For SOC, the value I see with the AGM is 46AH. The AGM is rated 50AH. The value for SOC I saw when I recorded a trip with the OEM was 31AH. The OEM rating is 45AH. That was early fall with temps in 70’s here. From that SOC for the AGM, I surmise the hybrid system ECU sees the battery at full SOC.

Several days ago for a 26 minute trip, battery temp was 46 F. For the entire trip, the aux battery voltage was 14.4 volts. The charging current into the battery was pretty constant at 2.4 amps which it was at the end of the trip. The DC-DC converter output was 31.2 amps for most of the trip. It was higher in the earlier part of trip at 50+ amps output due to seat and steering wheel heat being on. I turned both off mid trip.

From this summer, when the battery voltage would read 12.8 volts, the aux battery current on a similar trip was essentially nil. It would pulse plus/minus 0.1 or 0.2 amps, i.e., the aux battery was not being charged.

Earler this year with the OEM battery installed, I measured parasitic drain using an ammeter hooked up in series in the battery negative cable. With the car “asleep”, the current would pulse slowly between 20 and 50 milliamperes. So I use a value of 40 milliamperes for parasitic draw on my car.

The past 2 weeks with temps outside in the 30’s, the battery voltage per my plug in voltmeter in the 12V port has been a steady 14.4 volts for every trip. This has resulted in the BM voltage for first trip of the day being 12.5X - 12.6X volts.

I’m pleased so far with the AGM battery performance in my 2024 Prime.

Cheers

 

I have been monitoring the 12V charging in my 2019 Prime.

Here is the waveform of leaving the vehicle in Park in EV-City mode.

as you can see it stayed at 14.37V for the hour that I monitored it.

Here is a waveform of a trip in EV-City mode that I took the other day. This waveform is of the whole journey from start to stop in Park. All the 14V spikes are where I stopped at traffic lights and put it in Park. The longer 14V spike just after I started the trip was when I stopped at a friend house to pick something up and left it ON in Park.

I now put it in Park as often as possible to charge that 12V battery and I don't leave the vehicle plugged in overnight. The 12V seems to stay between 12.5 - 12.65V at rest now.
It's great to have the battery monitor as it make checking the battery so easy.

 

No aftermarket modification or accessory but it is a New Zealand model which has all the gizmos of reading speed signs, navigation, PSC, LDA, Dynamic cruise control, etc. The heated seats were off but the climate control was on Auto-Eco, so my guess the air conditioner would be on and the refrigerant pump and hybrid cooling pump. our ambient temperature here are 10C - 28C /50F-82F. However I see this even in the morning when it's 10C.