12V battery low-voltage auto-disconnect

Background:

We all are aware Toyota has dropped the ball when it comes to saving the 12V from a total discharge. This has caused many premature replacements of the 12V, either because they were ruined by the total-discharge followed by a suspected high current charge cycle or on a routine basis every 3 or 4 years in anticipation of failure. It is really a shame that for such a high-tech and GREEN car as the Prius there should be such a thing happening. I heard there are many cars on the market that automatically turn off the accessories and such as the 12V drops in voltage.

So currently:

1. No advance warning of a 12V on its last legs.
2. No drain protection for the 12V from accessories left on inadvertently such as a visor light and such or from being left parked for several weeks at an airport lot for example.

Ideal scenario:

The Prius would proactively and automatically disconnect accessories including the SKS if there's a drop in voltage beyond a threshold. The SKS computer is already running so, it could have perhaps performed this job as well? Now when one finds the SKS not working, they would have the first clue and when they do start the car the HSD can display a warning about the 12V just like it does for oil changes etc., but still start the car and let it be driven.

Alternatives:

1. Get a jump start from a different car: even when this option is available, one MUST make sure to connect the jumper cables correctly keeping in mind the polarity, etc. We have heard of instances where connecting the cables wrongly fried the electronics in the car. It has also been suspected that either the act of the jumpstart or the subsequent high-current charge by the Prius charging system has caused the 12V battery to be ruined.

2. Separate jump battery in the trunk. Now one has to remember to periodically charge the jump battery and contend with other cons same as in one.

3. Replace 12V routinely every 3 or 4 years: Not a Green option so there's the enviormental cost and it still doesn't save one from accidentally leaving a visor light on and such or if left at an airport parking lot for a few weeks or more.

4. Install a low-voltage auto-disconnect switch which is what I have done this past weekend. The jury is still out on how well this works since I have had it for only a couple of days now. I am now looking at these 2 possibilities:

a. The switch disconnects the battery when (not an if) the voltage reaches 11.8 or below and doesn't recover in 30 seconds. This may happen next year or 3 or more years from now, I wouldn't know. But when it does happen, I would simply reconnect the battery using the bypass switch and start the car and go wherever I need to, but now

i. I have an advance warning.
ii. I am not stranded in the middle of a park and such.
iii. The car will always start with a voltage above around 11.8V or so, so the computers will not run into a weird state like they have when people tried to start with a 12V that was deeply discharged.

So, in this case this is what I would do:

i. unless I can attribute the drain to a visor light or such, I will simply replace the battery at the earliest convenience.

ii. otherwise, I can put the 12V on a charger overnight and you can bet the chances are great that it will come up close to its former charge level since it wasn't allowed to discharge much below 11.8V in the first place.

b. The switch misbehaves such as it doesn't disconnect at 11.8V like it's supposed to, then the POS will come off of the car. But I have already tested the switch outside of the car using a battery holder consisting of 10 AA batteries and it did work (disconnected when the voltage reached and stayed below 11.8 V for 30 seconds), so the possibility of the switch not doing its job inside the car is pretty low, like any other electronics item in the car failing.

I have done the due diligence look thru PriusChat for history on these switches and have come across some from circa 2008 when a dealer in CA installed a bunch with one called "Starter Guard" which doesn't seem to be in business any longer. There have been complaints of the switch actually disconnecting the 12V overnight and people having to use the override switch to start the car, but it's not clear if they actually ever removed a possibly faulty/oversensitive/mis-programmed switch (like many have promised they would) or if the problems were caused by bad 12V batteries losing their charge overnight. I have PM'd a few of them to see if they still had the switches installed in the car and if so what their later experience has been, but haven't heard back from them.

There's also a concern/possibility that the switch might disconnect the 12V, if it senses that voltage has reached 11.8 V and stayed there or under for 30 seconds while the car is being driven but this I would not expect to occur normally since the 12V bus has at least >13V being supplied by the inverter/converter/HV-battery. Even so, I didn't want to take the risk of an inadvertent disconnect by a malfunctioning switch and so I actually made sure my car runs normal even when the 12V is disconnected after starting.

There are not many choices for these switches, at least in the US and I got one off of ebay for $25. It's the Battery Brain brand that has the manual reconnect switch. I didn't want to try the more expensive ones with remote switches as I wanted to keep it simple.

I found the installation to be surprisingly simple since the 12V positive cable is actually in 2 sections (the cable section attached to the end section with a bolt and nut) and I simply introduced the switch in between those 2 sections.

0. Wear non-conducting gloves.
1. Disconnected the negative cable at the terminal and moved it aside.
2. Disconnected the positive cable at the terminal, removed the screw to disconnect the end section from the cable section, introduced the switch in between the cable section and the end section, and tightened the end connections on the switch.
3. Installed the other end of the end section back on the positive battery terminal.
4. Reconnected the negative cable from the car body to the negative battery terminal along with the negative cable from the switch.

NOTE: Actually the positive cable need not be disconnected at the terminal. It can be left as it is and the section behind it can be disconnected and the switch introduced in between the 2 sections.

MAKE SURE TO

 

[Sorry somehow the above got published before I completed it]

MAKE SURE THE ENDS OF THE SWITCH ARE ORIENTED PROPERLY. THE BATTERY END TO THE BATTERY TERMINAL AND THE OTHER END (I don't remember what this is labeled) TO THE SYSTEM SIDE.

Here are the pictures. I will update here as to my experience with the switch.

 

I would appreciate all suggestions, including specifically in regard to the following:

1. I need a way to insulate the positive terminal. What would you recommend? I am thinking of electrical tape, but would it be compatible with the lead/terminal and such? I don't want the tape to leave a liquidish residue after a while...

2. Right now, if the disconnect ever happens, I would have to flip down the rear seat backs, remove the black storage holder (or alternatively disengage the latch on the hatch and open the hatch with the other hand) and the battery cover itself to get to the reconnect button on the switch. Instead, I would like to have a bypass switch that I can bring up to be in the area above the battery cover so all that I would have to do would be to flip down the rear seat back and simply engage the bypass switch (switch it on). Any suggestions here? I believe the bypass wiring and the switch should be capable of handling the momentary burst of 60Amps that's used when the car starts?

 

This you can still do if you regularly keep up on the health of the battery and want to proactively replace it.

The volt meter obviously won't cover me if my daughter leaves the visor light on or if I leave the car parked at an airport for an extended period of time.

It's about building as many defenses/layers as are feasible and reasonable for one's given situation.

Already covered in the original post. I am not disciplined enough to

a. Keep the charge up on the small jump pack by recharging it periodically.
b. Get the polarities right while jumpstarting while I am possibly in a hurry or under some sort of duress.

Not to mention, in this case, I would also have to very likely deal with a ruined/unrecoverable 12V battery which won't be fun to deal with if I am out touring, even when I may have a jump pack with me.

 

Also, as mentioned in my original post, I would have liked Toyota to have managed the 12V discharge in such a way that the settings could have been stored before the disconnect and then restored as part of the boot up. But left to my own devices (as I have been), I will any day take loss of settings (not a big deal at least for me, just some radio presets) over possibly getting stranded or possibly damaging the car with a wrong-polarity jumpstart.

 

My concerns in a and b above can be allayed (at least for me) if there's a jump pack out there that:

a. I can plug into one of the 12V cigarette adapter sockets and forget, meaning it will not result in an overcharge nor get drained when the car is off.
b. has polarity-protection built-in, meaning it won't supply the charge if I connect the pack wrongly.

If any one knows of such a pack, I would appreciate info on it (a couple of these I looked on line seemed to need charging every 3 months or so from a 110V outlet). I might be willing to deal with a possibly unrecoverable 12V battery and the fun that goes with it ...

ADDITION: for the sake of the radio presets and the odometer histories and ECU learned settings and the dealer objecting to this mod (read warranty preservation).

 

If one is really concerned about losing the presets, one idea that comes to mind is a small rechargeable 9V battery that can be connected in parallel to the 12V (but bypassing the auto-disconnect switch) with something like a diode that will allow current to go from the 9V to the system and in the reverse only allow current above 13V to reach it. This way, the 9V won't deplete the 12V and will be recharged only when the car is in Ready. Is this workable?

 

Another feature I would be looking for in a jump pack.

c. Enough capacity so it's not sucked dry by the deep-discharged 12V battery, before I have a chance to get into the car and switch it on and then is able to both supply the 60Amp burst for getting the car to ready while contending with the 12V recharge demand.

Some in other threads suggested carrying a wrench and a small replacement 12V, which is essentially what the usage of the disconnect switch accomplishes, without the hassle of the wrench, a charged up replacement battery, etc...

 

I am not an electrical engineer, so I am just throwing some likely very dumb ideas around here. Another one being, a 12V charger that I would plug into the 12V adapter in the front which will keep on charging a little 12V over there while the car is on and I would take another set of wires from the little 12V in the front and feed them to the 12V bus via the OBDII port with a diode that will only allow current to flow from the battery into the OBDII. This way, even if the main 12V is disconnected, the little 12V will keep some output in the bus to keep the presets alive till it dies and before I would reconnect the main 12V, I would disconnect the little 12V from the 12V adapter but still leave it connected to the OBDII. This way, the little 12V will not suck any of the power coming out of the main 12V when I reconnect the main 12V.

 

I see there are many memory savers available that use the OBDII port. See:

http://www.amazon.com/Schumacher-INC-12V-OBD-6-Cable-Assembly/dp/B002J8LMZS

What would be nice would be, a battery/charger system that I can leave connected to the 12V aux/cigarette outlet (which, on the Prius, is turned off when the car is off) for charging its battery and also has a 12V outlet that I can feed a cable (such as the above) from, to the OBDII port. This way this battery can keep feeding the bus even after the main 12V is disconnected by the auto-disconnect switch I have installed and can possibly help save the presets...

Please alert me if anyone sees such a system. TIA.

 

Thanks for the caution. I didn't know there was a fuse in the positive cable end.

The instructions call for it to be on the positive lead. But I would like to do what you suggest if it's possible.

Below is a picture of a second one I have that I am planning to put in my Camry Hybrid. It basically has the following:

1. One end labeled "BATT +"
2. The other end labeled "+ CABLE"
3. A black wire protruding from the side that needs to be connected to the negative battery terminal.
4. A push down switch under the yellow cap to switch it back on if it ever disconnects due to low voltage being noticed continuously for 30 seconds.

As can be seen in the photos in my OP, the above are connected as asked for by the instructions.

I would appreciate guidance on what ends need to be connected to what on the negative lead. Specifically, if I connect the ends 1 and 2 above between the negative battery terminal and the negative cable, what do I do with the black wire? Does it go to the positive battery terminal? TIA.