Auxilliary Battery/ Substitute

Hey Andy, I like the idea of a 7 AH gel cell as a backup battery, and in fact use one this way. With a slightly complicated control circuit, that battery is charged when Prius is "ready", and not discharged when off.

Incidentally this is on a 2001 Prius with 70k miles.

As far as using it as the only 12 V battery, you would have to remember that with only 1/4 or so of the original capacity, your time until complete discharge by parasitic electrical load woud be correspondingly shorter.

I did in fact recently replace the original 12 V battery with a different sealed and tube vented battery, of 31 AH. The lead terminal posts needed minor modification to fit Toyota clamps, details available.

Would also mention that even a completely sealed PbA battery (like the gel cells) will vent hydrogen gas if they are overcharged. Now, we all suppose that the Prius is too clever to allow this to happen, but (opinion) that small risk is exactly why a tube vented battery was OEM.

I must not be too worried about that because I also have a gel cell in that circuit! However it is behind some protection that further limits the chance of an overcharge.

 

That would be 140 hours, not days.
AH stands for Amp-Hours, not Amp-Days. So 7AH will last 7 hours for 1 amp draw, or 14 hours for .5A draw, or 140 hours for .05A draw.
140 hours is 5.8 days.

You would also want the battery to supply power for your radio, lights and other stuff without having to power the car, though I would admit, having the car 'idling' in READY isn't a big deal. They could easily draw at least 1A, so forget about drive-in movies with a 7AH battery. In a drive-in you definitly don't want the ICE to start and spoil the sound and the mood.

 

The approx. 35 milliampere rate is an instantaneous measurement, meaning 35 MAH per hour or 840 MAH per 24 hrs. I have measured such values on my Prius and have seen other similar reports.

Whatever the actual value (and it is somewhere in that range) we read reports of Prius 12V batteries discharged after short periods sometimes, with both versions of Prius. To me, this suggests that I would not want a much smaller battery.

A while back I estimated that the car's emergency 4-way flashers would probably not last an entire night, with the OEM sized battery. This might be another reason for toyota to not specify a smaller battery. They would have saved 16 pounds and many yen using your approach, so I suppose it was considered.

But look, I have little doubt that a 7AH battery would work as the one and only 12v batt, at least with the daily recharges you suggest. In fact I would be very interested in knowing the battery's life in this application.

We do know that the startup sequence includes a peak drain of about 10 A when the brake fluid pump runs. We don't know what the peak charge rate seconds later might be, but it may well be higher. This large recharge spike may limit the life of your gel cell standalone.

But connect it reliably, please. Should a spade lug connector pull loose, the DC/DC converter may be completely unloaded, and probably unhappy. At that moment, a trouble code might be the least of your worries.

 

Yup. It used to be that you could test an alternator by disconnecting the battery with the engine running. If engine still runs, especially with headlights on, alternator good. These days, the alternator would overvoltage, frying electronics. The charging system in the Prius is solid state, but still may put higher voltage than ECU's can take if there were no battery to control it, thus frying ECUs. You DON'T want to do that on a Prius.

 

Yes, we have one or two here in the Tampa Bay area.

 

There is a drive in movie in 29 Palms, CA. Having said that, the backup battery topic:

A previous backup battery system was presented by Peter Rawlinson, in the Yahoo Prius_Technical_Stuff files folder, within “12v batteryâ€, called “Ebatt753.jpg†One must join the group to access the files, but that is no barrier.

Peter connects a 1.3 AH gel cell to the accessory outlet, presumably modified to make it “always hotâ€. When it is charging from the Prius, rate is limited by a 12 ohm (I think) resistor and a diode. The diode prevents any current flow from the backup battery to the Prius, except when a bypass switch is depressed. There is also a 15 amp fuse shown, which may be the one already present in the accessory outlet circuit.

I used a larger 7AH gel cell because, well, I had one. Also, previous attempts to boot the car with alkaline D cells in series, and with 2 zinc carbon 6v lantern batteries, were unsuccessful. I wanted to be sure that this backup would perform even when also facing the “load†of a discharged auxiliary battery, on a dark and stormy night, etc.

My backup battery is stuffed behind the Prius’ 12v, a relatively inaccessible location. It is activated by a momentary pushbutton on the dash, in one of those unused switch holes lower left. The circuit (attached) also has a blocking diode, but I bypassed the charge-current limiting resistor (it was recommended to me to use a 12v light bulb for that purpose) after seeing how low the charge currents actually were. There is also a 10 A slow blow fuse, which is the first thing that the backup battery positive terminal “seesâ€. Can’t recall (nor see) which diode I used, but at least 1 amp maybe 2. The actual charging current is much lower though.

There is too much voltage drop in long wire round-trip to the dash, so the circuit includes a 12 volt relay. A small circuit board is glued to the backup battery and impact-protected with some aluminum sheet metal epoxied to the battery case. I admit that by that point I was going a bit overboard! It is, by the way, very easy to pass wiring along the “kick plate†channels below the doors.

Anyway, it will boot the car at the push of the button when a deeply discharged 12v stands in for the Prius’ aux batt. I might have made it more convenient to monitor the voltage on the backup battery, to determine when it has reached end of life.

As far as replacing the 12v aux batt, I use the Westco/MK Battery 12V31M. Purchased in person at 1645 S. Sinclair St. Anaheim CA 92806 for $96.33 incl tax. Tel 800-372-9253 url www.mkbattery.com. How fine it would be to receive commissions for further Prius sales! Well, I’d have to split them with Dave Dorrance, who led us there. This battery has 2 vents so you’d need to fashion your own vent line (not hard). The lead terminal posts are too large so you’d have to cut them down (as I did) or use different connectors (as Dorrance did).

 

Good work!

Here's a suggestion that might simplify things a bit. If you put the bypass enable switch on the ground side, then you would only need one wire to the dash, rather than 2. You can pick up the ground for the switch from the chasssis at the dash.

 

Right you are Dan. Here's 10 pts.

DAS

 

Andy, a few things you probably don't realize.

First, the outlet in the console is not connected when the car is off, so you couldn't use it to connect your gel cell to the 12V to jump start (Jump power?) the prius. The outlet on the dash is the same way, but can be altered to be full time on with a small mod.

In Ready, the inverter has a converter that changes 206 from the HV battery to 12-14V at around 80A. It practically powers the car in Ready; the aux battery is practically unused in this mode, other than to keep the voltage constant.

Now you could use the cosole outlet to keep the gel cell charged while you are driving or in ready. I use it to charge my electric razor using a 12v/120V low wattage inverter originally intended for my laptop. But if your aux battery dies, you will have to find an alternate means to connect the gel cell to the 12V line.

How do you get 53MPG while parked? Some sort of warp drive? You aren't putting on any miles, so any fuel you use to recharge the HV battery would be 0MPG, which would average in to your existing MPG and lower it.

 

Safety wasn't as much of a concern back then.

 

Refereing to the schematic - there could be conditions under which more than 10 amps will flow between the batteries, so the circuit should have a current limiting device in series, like the 12v light bulb suggested. Of course the fuse is current limiting, but I'm suggesting something that would survive on the dark and rainy night and allow some of the charge from the fresh battery to be transferred to the dead battery so you can get on with starting the car. Replacing a fuse in those circumstances would result in another blown fuse - eventually, by replacing and blowing fuses, you'd get the battery voltage down to where it wouldn't blow the fuse, but at that point it wouldn't be much good for starting the car either. Better to use a current limiting device to slowly bring the batteries closer in charge, or switch the dead battery out of the circuit, or bypass the fuse altogether, as you do when you use jumper cables to connect the fresh battery the way the manual says to.

I keep a charged 7 Ah gell-cell in the center console and will use it with clip leads, on the post under the hood that's the designated point for applying a 'jump start' battery. I know it's tempting to just plug something into the 12 volt outlet, but that's probably fused too - you could end up blowing fuses and being no closer to starting the car. And if all you have is the plug for the 12 volt outlet, then you wouldn't have a way to apply the battery to the jump start point either - a very unhappy scenario that could be avoided by just opening the hood and doing it the way Toyota said to do it.

The voltage difference was .5 Volts, not .5 Amps, and the number of amp-hours lost isn't mentioned - amp-hours used might be a more useful measure here, although I agree that the state of charge of a lead-acid battery, given the voltage, is an indication of relative charge:

State of Charge of 12 volt lead-acid batteries

Electrical charge is usually measured in Coulombs. A Coulomb per second is an Ampere. One Volt is the electromotive force required to push a Columb per second of charge through an ohm of resistance. Actually, the fundamental unit of charge would be the electron, and there are 6.41418*10^18 electrons/coulomb.

Okay, now I'm happy :lol: