accessory battery discharging overnight hatch open

1. The original equipment Prius 12V battery has a capacity of ~35 Ah or ~420 Wh.

2. Typically, for long life it is not deemed advisable to allow the battery to discharge below 50% capacity.

3. Once the battery is discharged, it takes a very long time to be recharged. If you don't leave the car READY at least 8 hours, or hook up a battery charger overnight, then the battery never has a chance to be fully recharged and sulfation starts to form, which reduces battery capacity.

4. When the hatch is open, besides powering whatever cabin lights may be on, the battery also has to power current drain taken by the body ECU and other ECUs that are not sleeping. Some ECUs remain active (which requires power) if a door or hatch is open. One example that you may notice is that the brake actuator pump may run when you open the driver's door. This requires the skid control ECU to be powered up, as well as the pump motor.

5. You can use a digital multimeter to measure quiescent current draw. I think you'll be surprised to see that this current draw may approach ~1A if the hatch is left open, even when you've turned off all the cabin light switches. When the hatch and doors are correctly closed you should measure quiescent current draw at ~0.02A +/- as the security LED flashes on and off.

6. Hence, if you get tired of buying replacement 12V batteries, it would be good to close the hatch and all doors when the car is not in use. Or, hook up a battery charger to the car whenever it is in the garage so that the battery has a chance to catch up on its charging. Good luck.

 

If you leave the hatch open a lot, then try the following. Buy a Battery Minder or Battery Tender float charger. They come with ring terminals that you can attach directly to the battery. The other end has a pigtail, which you can leave peeking out from the floorboard. Then, all you need to do is plug the charger into the pigtail whenever you leave the hatch open. They put out about 1.3 amps so it is enough to feed the hungry computers.

But I am curious how you got an Exide battery to work. Is it SLA or AGM? Did you have to swap the JIS terminals for SAE like in the elearnaid optima kit?

 

Slightly off topic -

We have seen a number of threads regarding mischief caused by critters climbing into various parts of people's cars. That is all by itself to discourage me from leaving hatches, doors or even windows open all night (being from the Midwest myself, I know there are plenty of rodents around Cleveland always looking for shelter). I can't imagine that not needing to close the hatch once in the evening or open it once in the morning is that big a deal from a "convenience" standpoint, as compared to the minor inconvenience of failed batteries or the massive inconvenence of a family of mice shredding your interior for nesting material. IMHO, YMMV, etc.

 

I think you need to redefine "convenience".

I don't mean to sound "snarky" but I don't know of any vehicle Prius or otherwise where it would be recommended one leaves a trunk open or a hatchback open overnight.

There's a lot of reasons in any vehicle why it's not a good idea. Probably more with a Prius.

I can't imagine a scenario where leaving the hatch open would be an absolute necessity...and how "inconvienient" is it to open it when you need to open it?

You are free to do what you wish, but you've already bought a new battery, are already experiencing further problems...and at some point one should come to the realization that the easiest solution to this problem is just to keep your hatch closed when not in use.

 

Do note that the door lights are probably also on when the hatch is open (I can't confirm this as I can't see them under the seats with the front doors closed). If they are, that's more current draw.
I leave the front windows down about 1.5" so the car will vent properly when parked overnight (any water collected in the footwells, plastic outgassing, etc.) if that is what you are trying to accomplish.

Bottom line, DO NOT leave -any- door open when the car is not being used. You can leave it unlocked and then anyone can open it from the rear without needing a FOB. DO keep the garage locked. While the morons can't drive the car away, they CAN rifle through the storage areas.

 

Patrick, After reading this post, I think you may have some answers for me. In the recent three months, I did some "extras" for my '08 Prius: replaced interior bulbs to LEDs; added LED (Bosch) DRLs to the front grill; and also an Optima Yellow Top. Everything was fine, until this past weekend, when I heard a "new" beep(s) when I opened the front door to exit. Of course I would hear these beeps if I left my fob inside the car, but that wasn't the case. Now, two days later, I've been having to jump-start the car a lot, since my trips have only been under 4-5 miles, where I would have to jump-start the car, just to get home.

According to my neighbor, he said that he saw my interior dome light (with new LEDs) was left on overnight, -which sounds like the same story (below) with the open hatch.

So, would a typical battery charger, say from Sears, AutoZone, etc. solve my problem? An alternative could be a day trip from SF to LA and that should fully charge the battery? Since my Yellow Top is so new, I am hoping to save it, vs. buying another. Most important, is that the diagnosis is right, vs. yet a different cause. Thanks in advance!

 

Yes, you could purchase a battery charger, ideally one that has a setting for AGM batteries, and use that to charge your 12V battery overnight.

If you have the need to drive from SF to LA then that would be fine. However it would be just as effective (for battery charging purposes) and your fuel bill would be much less, for you to leave the car READY and in P overnight, parked in a place which is safe and which will allow the exhaust gases to vent.

 

So far as I know, only the Optima YellowTop and the OEM battery Toyota sells, are safe to use in a Prius.

You want a AGM battery so there is less sulfuric acid thrown around the cabin in case of an accident, and you want a battery that is externally vented so Hydrogen gas does not build up.

 

Thanks Patrick,

I purchased a Schumacher (SSC-1500A) battery charger with AGM specs; it will arrive tomorrow aftn, and I'll connect it right away with the hopes that my problem will be solved. BTW, any harm in connecting the charger to the terminal located in the fuse box vs. directly to the battery posts in the trunk?

True, I could take a RT to LA and back, however, if I did, I'd take my quick start jumper along for the ride....

 

Yes I routinely charge as do most of us on the terminals under the hood. Thats what its there for.

But my personal opinion is you probably killed that battery. The Optima is an excellent battery but not immune to sulfation.
And it was discharged just the way sulfation likes it. Nice and slow.
You may try a charger with anti sulfation mode. But I would be suspicious of it from here on out. And the really sucky part is
its new enough that even if it recovered it may be a battery
with minor sulfation damage that never charges properly and kills your mileage. So if you see your mileage tank its may be because the car is struggling to charge that sulfated battery properly. Sorry.

When you get it going routinely use the mfd battery load check
on it to see how its doing especially after sitting all night. Before you start the car in the morning do the mfd test to see how low the battery B+ sank.
Good luck.

 

There's some good info in this thread that I didn't previously know. I often leave both the hatch and the rear doors open (in well aired garage) after I've been to the beach. This to let it air out, as I've usually had lots of wet stuff (towels and wetsuits, dog even) in it.

Sometimes I just leave it an hour or two like this, but I often forget and leave it all night. So far no problems experienced, but after reading the thread I think that in future I'll just leave the windows down.

 

The battery charges just fine from the under-hood fuse box "jump point". I run my "battery tender" to there.

 

LED dome light alone should not kill the battery overnight. Once the 12 V battery is charged for 4~5 miles, even though not fully charged, there should be plenty of charge to start the car, i.e. shouldn't need to jump start. I wonder if there are shorts or leaks introduced when the "extras" were added to the car.

 

Excellent point! While the LED dome/door/license lights are not as much suspect, I did have the LED DRLs hooked-up by a stereo/alarm installer. I didn't have much confidence in myself to do this, and so I hope he does.
Meanwhile, I did charge my battery overnight; it took several hours to go thru the "desulfation mode" (hopefully that's good), and so today I will take my Prius on an all day drive -with my jump-starter in the back seat, just in case...

 

Well, I hope that I've "put this issue to bed..."
After speaking with Bernie Littman (eLearn Aid), he deemed my Yellow Top as "unable to hold a charge." 22-hrs later, I had a replacement at my door. It took way less time to install this one since the retro-work for the connectors were already done. So, I'm back on the road, and yes, my calculated (and predictable) mileage after fill-up was a whole 20.48 MPG. I do expect way better on the next tank, for sure.

I have now added 225 miles to the new battery, and have checked the voltage on the MFD about 5-6 times, and each time the range is a tight 13.8-14.0 VDC.

Thanks to this forum, and some quick reading at the Optima (and other) sites, I have a lot more respect for batteries -that I have mostly taken for granted. Thanks for everybody's input, and yes, my Prius remains a "keeper."

 

That looks like the charging voltage (when the car is ON), not the battery voltage (when the car is OFF). Check the procedure again.

 

Good show! Back on the road. You learned a very expensive lesson
in that a battery does not need to be completely killed for it to be damaged. The majority of battery complaints I see on here are people who have discharged there batteries...jump started them....never bench charged them...then cannot figure out why it goes dead again.

BTW, alot of us routinely charge our new/good batteries either with float chargers or normal chargers.
I catch mine about once a month. Just throw a charger on the jump terminals under the hood and take notice how long it takes for that battery to fully charge up to 0 current draw.
I still have my original battery and its fine. Been charging it since new. It takes about 10 minutes to fully charge it in 2 amp mode. That has not changed since new.
When I see that change its Optima time. Plus routine charges to top off the battery are very good for the battery health and will prolong battery life.

But exactly what Richard said above concerning the mfd check of battery health you need to be aware of all 3 readouts:

Ist button push: Ignition

2nd push: Accessory under load

3rd button push: Car in Ready with battery charge voltage applied.

The most telling mode is the 2nd accessory under load.That will tell you the true battery capacity or its ability to hold voltage under load. Anything below 12 volts on #2 is suspect.

Good luck with your new battery and keep the doors closed.