DIY "Mild" Plug-in Prius Project: Grid-powering the 12v side w/ deep cycle batteries

[orange4boy]

Who needs a DC-DC up converter when you can hack a half destroyed battery to add a cell to a 12V to get 14?

Disclaimer: What I have done here is dangerous. Try this and you will die a horrible and painful death. Or at least you will be blinded, scarred and lead poisoned. You have been warned.

OK now I shall proceed with my story.

This winter I had a bit of an accident with the Elec-Trac in which one of my deep cycle batteries was damaged beyond repair. Or so I thought at the time. At some point I had remembered Darin mentioning a hillbilly battery mod in which a screw is drilled into a battery to tap one or more cells for an unusual application. Distilling hooch perhaps? I can't remember. Anyhoodle, I thought that since the battery still had three good cells why not save them and tap them off for the 14V. Much cheaper and more efficient than a DC-DC converter with enough juice since I had a worthless battery anyhow.

I drained the battery acid from the toast battery into a bucket while wearing a full hazmat suit. I then pried off the cover using a screwdriver and a flat scraper/chisel. I got it half off but then just busted it in half removing the vent side. I then could get at the bars inside to drill and tap them with a 1/4-20 tap. I cut one of the bars so I could split them into 3 separate cells. When I had carefully tapped the bus bars and removed all the lead filings, I hot glue-gunned the top back on. I poured the acid back in, hooked up the leads and, presto! Three 2V cells.

I have a nifty "Silver Beauty" battery charger which is blue. It has settings for 6, 12, 18, 24, 30 and 36V so I charged it up on the 6V setting.

That was last week. Today I finally had a chance to try it out. I loaded a good 12V battery and the hillbilly hack batt into the trunk and connected them together. 14.9V all up. Turned on the Prius and it dropped to 13.9V which is pretty much what it runs on from the DC-DC.

I tried the Silver Beauty and at the highest 12V setting it charges the "14V" battery nicely.

Disconnected the converter and bypassed the 12V battery so as not to boil it and did my regular commute on which I usually get 4.3-4.4 L/100Km. I was never able to beat 4.3.(54.7 MPG) First run I got 3.9 L/100! (60.3 MPG) I managed to end up at the same HV SOC so I wasn't "cheating"

We will see how it averages out but I'm thrilled with the result. It may turn out to be less of a gain because I have a hunch they just switched to summer gas here.

[orange4boy]

3 more runs at 3.8 (62mpg), 3.9 (60.3) and 3.8.

Battery is working great. Now if only I could remove the dead/unused cells. What I really need is a custom 14V battery or a set of Saft Ni-cads or a set of Prius cells. Only cheap.

Time to look up plastic welding.

Later in the week I will do some runs with the DC-DC hooked up again to see if this is a summer gas bump.

Wouldn't it be cool to have a set of these running things? Just string however many you need one cell at a time:

HOPPECKE single cells of the OPzS type series have a live expectancy of up to 20 years and a cycling expectancy of up to 1500 cycles with 80% depth of discharge.

A friend of mine actually has a set of similar giant glass cased lead acid batteries that he got as surplus from BC Hydro. He uses them in his home for his micro hydro and solar power storage.

[orange4boy]

Yee HAW! Hillbilly battery. Does it work? Dern tootin it works!

The DC-DC keeps about 13.8 the HB battery puts out 14.7 fully charged.

So as the battery voltage drops below 13.8, the DC-DC starts to pick up the load just as you predicted, Bob, when you thought of the diode arrangement. This way one avoids all the hassles with diodes though. I just happens that 13.8v is to the 14v battery as 11.8V is to a 12V. That means the battery will never drop into dangerously low discharge. It will still have a little bit of capacity left over but it's a 90% there.

At the end of the drive I measured the 12V for a voltage SOC and it was showing ~12.2V resting. That's about a 60% SOC. 50% is about as low as you would want to go with these batteries. We were driving for a couple of hours today but I don't know how fast it dropped.

I was able to get 3.5 L/100 on our usual highway run. Previous best was 3.8. Totally not scientific. I will have to try to do the a-b-a at some point.

This is sort of what you were talking about right?

I was thinking about this at lunch and realized the deep-cycle battery only has to provide the 90% load over the time, vehicle base load, and let the vehicle inverter with diode handle the peak loads (aka., rear window defroster, windshield wipers, night driving lights and brake.) Anything over that would drop the output voltage and the inverter would then provide the make-up or peak power. For example, you measured ~20 A. in your first test. Add whatever you consider to be additional, standard load plus 5-10% and that would be your deep-cycle battery DC-DC converter design goal << 100 A. <grins>

I'm going to look into some battery combination possibilities that give me that sort of range.

[bwilson4web]

<forehead SLAP>GOSH I wished I'd thought of that!!

Originally Posted by orange4boy

. . . What I really need is a custom 14V battery or a set of Saft Ni-cads or a set of Prius cells. Only cheap.

Time to look up plastic welding.
. . .

I've ordered some 1/8" polyethylene welding rods last night for my NHW11 module refurbishment. I tested polypropylene last night, no good. I have some left over, battery case fragments but I'm not enthused about sacrificial case plastic ... unless I have a bunch of it.

Looking at a pair of NHW11 modules:

• 1.2 V (nominal cell voltage) - 14.4 V

I have been looking for an application for refurbished NHW11 modules not in the traction battery pack. You've provided a perfect answer.

Your extra cell on the Pb battery is a brilliant solution. I've always hated the diode drop and have been thinking about more efficient buck power supplies. The diode was more 'proof of concept' and a permanent version would look at using Schottky to reduce the forward drop. But I like your solution better and I especially like the idea of using refurbished NiMH batteries to handle the 'plug-in' load. This is the type of application perfect for my purposes.

My only concern had been whether or not the inverter voltage control would 'suck' electrons out of a higher voltage battery. Have you tried using a current sense resistor to see if there is any reverse flow into the inverter? That will be my first test. <grins>

Thanks,
Bob Wilson

[orange4boy]

<forehead SLAP>GOSH I wished I'd thought of that!!

Collaborative solutions. That's why we are here.

I love it because it's plug and play. I was expecting a lot of wiring, relays switches and voltage monitoring. Now all I have to do is charge and go. It's partner friendly!

Incidentally this happened by accident. I was supposed to disconnect the DC-DC converter but I forgot. I realized later while we were driving. When I checked the 12V (for easy SCO reference) battery and it showed 11.5 volts under load, I kind of freaked because I thought I had fried the DC-DC with the 14.7 volts. I thought it should read 13.8V. I later tested it by disconnecting the 14V battery and everything worked fine. I then realized that the 12V battery would not read 13.8V with the DC-DC on because of the extra cell. WHEW!

What are the voltage / SOC numbers for those Prius modules? They have a tighter band don't they? I wonder what their SOC will be when they hit 13.8V.

Your extra cell on the Pb battery is a brilliant solution.

Thanks but it's half mine and half Darin who suggested tapping a battery with a screw to tap single cells. I then used this idea to charge one low cell on one of my 12v deep cycles which made me think of salvaging the broken one and tapping/adding one cell.

The one hitch here is the charging. I have a charger that will do the trick slowly but it may not have enough juice to really finish the charge. I still have to test this. Otherwise I will need a 14V battery charger. I wonder if there is any way to adjust an existing charger. Some have pots inside to trim the peak voltage. Hmmm.

My only concern had been whether or not the inverter voltage control would 'suck' electrons out of a higher voltage battery. Have you tried using a current sense resistor to see if there is any reverse flow into the inverter? That will be my first test. <grins>

No. Didn't check this. Don't have any simple way. I guess could install an ammeter. I guess a big diode would solve that if it was back feeding. I don't know enough about electronics to even guess.

I still have to test the FE savings but that will be a simple matter.

[bwilson4web]

Well maybe I can help a little:

Originally Posted by orange4boy

. . .
What are the voltage / SOC numbers for those Prius modules? They have a tighter band don't they? I wonder what their SOC will be when they hit 13.8V.

They have a steeper drop-off but they are essentially a flat line on discharge.

Originally Posted by orange4boy

. . .
The one hitch here is the charging. I have a charger that will do the trick slowly but it may not have enough juice to really finish the charge. I still have to test this. Otherwise I will need a 14V battery charger. I wonder if there is any way to adjust an existing charger. Some have pots inside to trim the peak voltage. Hmmm.
. . .

The traditional, off-the-shelf approach is an autotransformer but these can be a little pricy.

Perhaps see if the primary winding is exposed, taking off turns of the primary will increase the output voltage. Usually the secondary is the outer, exposed winding and though adding secondary windings increases output, there usually is not enough space. More of a hack, another approach is using Schottky diodes to cut the forward voltage drop by at least 0.5 V.

Another idea would be to see if the output of the transformer is center tapped with the outer ends feeding two diodes. Use a full-wave bridge fed by the outer windings. This will double the output voltage but risk over charging. Add a power choke to the primary winding to reduce the input voltage to reduce the primary voltage and/or use a light dimmer switch.

Just some ideas.

Bob Wilson

[2009Prius]

Great info! Could orange4boy please post a circuit diagram of the final mod? Thanks!

[unixrocks]

I just joined the forums after reading this and I dont get what the goal is here.

Is the goal to turn off the DC to DC converter for short periods and run a 14V battery pack?

Does the DC-DC converter constantly pump 14V into the system or will it stop if the battery is providing the 14v?

I *think* I understand what your doing but I am missing a few critical pieces of info about how the DC-DC converter works and when and why the heck if the system runs at 14V they have a 12V battery installed?!?!?!?!

[bwilson4web]

Originally Posted by unixrocks

I just joined the forums after reading this and I dont get what the goal is here.

Is the goal to turn off the DC to DC converter for short periods and run a 14V battery pack?

Does the DC-DC converter constantly pump 14V into the system or will it stop if the battery is providing the 14v?

I *think* I understand what your doing but I am missing a few critical pieces of info about how the DC-DC converter works and when and why the heck if the system runs at 14V they have a 12V battery installed?!?!?!?!

The Prius has half a dozen control computers, electric assisted brakes and steering. The NHW11 also runs daylight running lights. So the background power runs just under 500 W. This is a significant load so using a charged battery instead of taxing the drive system improves road mileage. Overall efficiency is not changed as the battery charge has to come from another source. But as a way to avoid buying gas, it works.

Bob Wilson

[Daox]

Originally Posted by unixrocks

I just joined the forums after reading this and I dont get what the goal is here.

Is the goal to turn off the DC to DC converter for short periods and run a 14V battery pack?

Does the DC-DC converter constantly pump 14V into the system or will it stop if the battery is providing the 14v?

I *think* I understand what your doing but I am missing a few critical pieces of info about how the DC-DC converter works and when and why the heck if the system runs at 14V they have a 12V battery installed?!?!?!?!

All chargers charge 12V batteries at ~14V. Your regular alternators put out this voltage and thats what the car runs on since its constantly running.