I'm running my Prius off a cheap wheelchair battery

[eljefino]

Disclaimer: Some would call this butchery.

Prius #3 was giving me grief, telling me it had issues being in Park if I sat there with the radio on for a few minutes. Classic symptoms of a failing 12v battery. New ones are $150 (potentially $110 at pep boys after $40 rebate) but money's tight and I was bored.

So I got a 35 Ah AGM battery (This guy) and went at it.

First note is the different terminal style, which needs fiddling. Second is its reduced height, which required shimming. Thirdly, there's no port for it to offgas. Since it's an AGM battery, in the hatch area, and behind some trim with a vent to the outside, I'm not worried about hydrogen.

For the positive terminal, one removes the post clamp, bends the tab over, and bolts it up with the through-bolt provided. I added a wider washer on the back-side so it wouldn't dig into the lead of the terminal.

For the negative, I simply spread the existing post clamp slightly then used the battery's hardware to bolt through.

On my first attempt at installation, I shimmed the battery with both wood blocks underneath. This had the positive terminal sticking up so far it could have been bumped by something heavy on the cargo floor, transferring energy through the passenger side floor/trim piece. So I revised it with one block below, one up top giving the clamp adequate tension.

Worsely, the HV battery output vent tube doesn't fit around this mess. I live in the cold and have never heard the HV battery fan even run aside from a fault condition, so am not worried. This fan's output will now bounce around in the no-man's land until it finds its way out the body exit flap.

So, it's a kludge! Works good though.

 

[eljefino]

This is the positive terminal (and fuse blob) un-bent, stock-- and bent, installed. Also shows negative terminal adaptation.

This is version #1, where things were too high and had potential interference.

 

[eljefino]

At final height. I can take no credit for this-- it's been done many times over on PriusChat. The truly thrifty use U1 lawnmower batteries that are $15-20 at malwart, but those have more hydrogen offgassing dangers.

This size AGM battery is pretty popular with powered wheelchairs and solar charging. HF sells a similar one for $69.99 minus 20% coupon if a Prius owner ever finds themselves with an immediate need. Since the car doesn't really need cold cranking amps, a deep discharge style AGM is actually better suited for use. It gives enough current to boot a few computers, close a few relays, then the inverter and HV system provide a steady 14.0 volts for the car's low-side bus.

 

[maverickfhs]

Alright if I may, that's just GENIUS

I hope it works for sometime...

 

[CR94]

I installed a very similar battery almost 4 years ago, using similar methods, except that I did not need to modify the terminals. It's getting a little tired by now, but even if it lasts only 4 years, that's more years per dollar than you're likely to get from a "proper" Prius battery, which typically costs around $180---even more at most dealers. The modest life span could be blamed on it being a cheap ($65, including shipping) Chinese AGM, or the fact I left a door ajar about 20 hours (=14 ampere-hours) once, or chronic undercharging due to the system voltage cutting back to ~13.7 volts too soon. Belatedly, I moved the battery compartment temperature sensor lower in the compartment, so the voltage will hang longer at ~14.7 volts.

I slightly modified the battery so any gas venting (which shouldn't normally happen with an AGM) will be directed outside through the original vent tube, instead of into the car interior.

I liked taking that 7 pounds of dead weight out of the tail of the car.

 

[eljefino]

Originally Posted by CR94

I liked taking that 7 pounds of dead weight out of the tail of the car.


You can take that emergency tow ring/screw thing next to your spare tire out too, noone is going to know or remember it's there.

 

[CR94]

Originally Posted by eljefino
Originally Posted by CR94

I liked taking that 7 pounds of dead weight out of the tail of the car.
You can take that emergency tow ring/screw thing next to your spare tire out too, noone is going to know or remember it's there.
I did, although it's only about 1 pound. The original jack handle and lug wrench came out too, because they're redundant weight (because other tools can substitute).

The problem you had listening to your radio in accessory mode was aggravated by the fact that merely putting the POWER switch into that mode, even with nothing turned on, sucks roughly six times as much current from the battery as the basic radio itself does. Therefore I don't torture the battery by listening to the radio with the car off, which I liked to do occasionally in my Mazda. It was too old to have that annoying modern fault.

 

[JHZR2]

I like it! Ive seen weird voltages on my 12v bus in my accord. Figure when the time comes I may go AGM.. perhaps this is a good bet for that car too...

No point in replacing the 12v battery in that car though since like the Prius it cranks off the high side.

 

[ekpolk]

No butchery at all! An elegant and creative solution to a couple of problems. Most notably the death of the old 12v, which is then compounded by the predatory pricing of the replacement at the dealers. Grrrrr.

As the OP and JHZR2 both noted that whatever goes in there is not burdened with the need to actually crank the engine. All the 12v battery in a Toyota hybrid has to do is boot the system and power 12v items while the system is in the accessory mode. When my 12v died, I got an Optima Yellow-Top installed. It was performing beautifully until I had to trade the car.

Back to the OP's work, installs like this are much more creative and interesting. I'm not sure what absolute minimum amount of current is necessary, at 12v, to get the system up and running, but I've thought about contriving various "MacGyver" setups in a severe pinch. For example, eight D-cell (theoretically any size 1.5v battery) batteries linked in series would give you 12v, though I strongly suspect that the amps might fall a bit short. You could do eight parallel-linked "clusters," in series with one another. Somewhere between this "really long flashlight" and a small wheel chair battery, there must be a line. It would be fun to find out where.

Incidentally, in the Gen-4 cars (that's the 2016 Prius through present models) Toyota finally moved the 12v battery back under the hood, a much more user-friendly location, and if the 12v does die, it's very easy to jump-start, again under the hood, just like any other car with a traditionally located battery.

Good work!

 

[CR94]

Another "creative solution" would be a Group 51 AGM, with terminal adapters. In several brands, the 51 is internally and externally identical to the same brand's Prius-specific model except the size of the terminals, yet tends to be less expensive and easier to find.

I don't see how the rear location is significantly less user-friendly. Not subjecting the battery to high under-hood temperatures should theoretically allow longer life.