Planning 12V battery experiment
 

Hi,

I've posted this in "Prius Technical Stuff", PriusChat, and here looking for comments and suggestions.

For my airplane project, I need a light-weight, 12V battery, that has 'drop-in' characteristics. There are light-weight, 12V LiON batteries but gosh they are expensive and are sensitive. In contrast, the NiMH are a lot easier to manage and I have experience with our Prius NHW11 modules.

GOAL: Configure NiMH cells as a 12V, drop-in battery

I checked the specifications for D-sized, 10Ahr cells and found:

1. 1.0V minimum charge per cell
2. 1.25V maximum charge per cell
3. 30A maximum "rated" discharge

Typical 12V, lead-acid operating range:

1. ~10.5 to 11.0V minimum lead-acid battery voltage to start Prius
2. 13.8 to 13.9V typical charger limit

So I did the following calculations:

• minimum voltage cell count 11V/(1V/cell) = 11 cells
• maximum voltage cell count 13.9/(1.25V/cell) = 11.12 cells

It looks like 11, D sized, NiMH cells have a voltage range consistent with the minimum and maximum voltage levels of our Prius. The 30A maximum discharge rate (per specs) is in the range that have been reported for Prius operation. The 10Ahr is about 1/3d the capacity of the lead-acid battery BUT I suspect the lead acid battery is sized to survive a month or so with the parasitic loads.

There is a problem of parasitic load, the keyfob receiver and ECU memory which as been reported in the 30-35 ma range. The parasitic load can over-discharge the NiMH battery just as it does a lead-acid battery. Lead-acid cells are a little more tolerant, less instantly killed, by the effects of fully discharge. So I need an active system to protect the NiMH battery from a complete discharge.

To address the parasitic load, I'll install a 12V solar panel with Schottky block and Zener voltage regulator. The Arduino will measure the solar panel, charging current, expected to be in the 100s of ma in direct sunlight. Hopefully the daily solar charge will be larger than the daily discharge losses and parasitic loads. If not enough, I can add a second panel.

PLAN:

I have ordered, 11, D size, NiMH, tabbed cells rated at 10Ahr. When they arrive, I plan to work up a minimum volume package and install it in our 2003 Prius. It will also be instrumented so we can get some metrics.

I will be using an Arduino nano; a 30A, Hall effect current sensor and; a resistor network to monitor the voltage. Temperature will be measured from the Arduino in the middle of the pack. "Heat is the enemy" and the Arduino will be placed perfectly.

From an economic standpoint, the 11 cells are coming in about $7.15/cell, just under $80. I still have to work up the housing and add the 12V, 1.5W, solar panel charger, $15. Add the housing and we're looking at about $100 for a 12V battery that should weigh significantly less than a lead-acid battery. I did not include the instrumentation as this is part of my experiment, getting facts and data.

I'll probably use a semi-log scale to analyze the data:

• 1, 2, 5 days - preliminary (will carry 12V backup, battery)
• 10, 20, 50 days - steady (optional, the 12V backup battery in trunk)
• 60, 60, days for a year - long term

Depending upon how well the storage works, we should have fairly accurate data showing Prius 12V operation with the solar charger.

We are entering the warm seasons, which curiously is the time of maximum heat, the greatest stress on the NiMH cells. As for handling parasitic loads, the system won't get a proper, winter test for at least 9-10 months. But after a year, we'll have metrics on how well the 12V, drop-in, NiMH battery works out 'in real life.'

Considered:

The battery could be built using spring-loaded, cell carriers so a single failed cell could be replaced. But welded tabs have the lowest resistance and are significantly better at handling vibration. Also, 11 cells in series are going to have similar operational characteristics. If one fails early, the others are likely to soon follow.

There are F size, NiMH cells with 15Ahr capacity but I have not found them with welded tabs. Worse, they are significantly more expensive than the D sized cells. Still, they may be needed in higher latitudes where there is less sunlight in the winter. Getting tabs welded on them is not that hard.

In theory, a voltage operated, low-voltage, isolation relay (aka., MOSFET) could keep the NiMH from completely discharging. But there needs to be a 'reset' so you can get in the car and boot it right away. If the initial battery/solar cell works, throwing a light on the solar panel might work. I may reconsider this as part of the initial test.

Anyone see a problem or suggestion?

Bob Wilson

A 12Vdc drop-in for "model" airplane or "real" passenger-carrying airplane?

Schottky diode voltage drop is ~ 0.3Vdc, is that acceptable? Either a contact relay or FET would be much lower, but relay is vibration prone.



P.S. - You have mail.

Do a Google search for N19WT.

The Schottky is just to prevent reverse flow from the battery through the solar array. In the dark, the ones I've tested have a small but significant current flow. But in bright sunlight, the 12V panel I tested easily generated over 20V.

I would prefer to use a buck regulator so when the solar panel generated excess power, we could get a closer match to the battery load. But we are talking about 1.5W. For now, KISS makes more sense and if I need more power, adding a second panel is cheap. But let me get some metrics, first.

For the battery isolation, I'm open to a suggested part:

• 30-60A - bidirectional
• default OFF - an asserted gate is needed to go to a conductive state
• minimum voltage drop

But this is an option to look at with some operational metrics. Disconnecting the 12V NiMH battery wipes out the ECU memory and there is significant in-rush.

The primary issue is the solar panel generating enough power in daylight to handle the 24 hour, parasitic load.

Bob Wilson

Hi Bob. I'm a big fan of keeping it simple, so I'm scratching my head why a $20, 8lb 12v 12ah battery (which is trivial to charge) won't suffice.

Mighty Max ML12 12 12V 12AH F2 Battery for APC UPS Computer Back Up Power | eBay

I mean you want it to be reliable it seems, but getting experimental w/it appears to be counter to that goal.

Hi,

Normally I would agree unless another goal supersedes:
I will have to measure the weight of the assembled NiMH pack but the back-of-the-envelope:

• 8.38 lbs - SLA/AGM, 12 AH
• 3.98 lbs - 11, D cell, NiMH, 10 AH

Saving 4.39 lbs helps mitigate the addition 9.2 lbs from the inflight, adjustable prop. In my rebuilding effort, I am concentrating on losing weight and reducing drag while addressing risks:

• 172.6 Lbs HAPI VW, 60 hp -> 112 lbs Hirth 3502, 60 hp
  • 3,200 rpm prop -> 2315 rpm geared prop, lower drag
  • wet oil sump -> oil injection, no oil changes or filters
  • carburetor -> fuel injection, electric, no mixture, no carb heat
  • flywheel alternator @3200 rpm -> flywheel alternator @5000 rpm, 20A
  • 3.5 gal/hr @60 hp -> 3.9 gal/hr @60 hp
  • 60-140 mph, fixed air cooling -> 60-140 mph, adjustable liquid cooling, lower drag
• 52" fixed pitch prop, 5 Lbs -> 52" adjustable pitch, 14.2 Lbs
  • no electrical load -> 30A peak to 4A sustained, risks fuel injection ECU
  • 0-60 mph lugs engine -> 0-60 mph full power, takeoff
  • 60-100 mph full power -> 60-100 mph full power, climb
  • 100-140 mph over-revs engine -> 100-140 mph full power, cruise
• previous VW battery -> Odyssey PC625, 13.2 lbs, 18Ahr
  • single 12V electrical puts fuel injection ECU at risk
  • 'soft-start' may allow use of lighter battery
  • independent 12V for fuel injection ECU, fuel pumps to complete flight
  • all other loads on the primary 12V including starter

The desire for 'simple' has been one of the chief complaints about our Prius because it has an independent, electrical, motive power system. The Prius transmission is very heavy due to the electric motors, traction battery, and power inverter. But the Prius complexity gives us nearly a doubling of fuel fuel efficiency, 25 MPG -> 50 MPG. Fuel efficiency trumps simpler but inefficient cars.

Today we have options that were not possible decades ago. So we have choices IF we pay attention to the requirements, design, and throughly test what we about to do. In this case, I'm looking at NiMH batteries which offer a significant weight savings without the complex battery management needed for LiON.

Thanks to our Prius, I feel comfortable with NiMH batteries and confident in my ability to manage the charge/discharge. In contrast, I don't share that confidence with LiON because of the flatter SOC vs voltage curve and more abrupt transitions at the charge limits. As for lead-acid, weight is the problem, and though somewhat more tolerant of abuse, they still suffer the same problems as NiMH and LiON.

With luck, I'll bring the empty weight of N19WT from 695 lbs to 650 Lbs. This means faster acceleration on takeoff (i.e., shorter runways,) faster climb (i.e., avoiding trees, power lines, and reaching cruise altitude sooner,) and more payload (i.e., ~8 gal or 2 hours.)

Other Dragonfly owners have put larger engines, O-200, rotary, Corvair, 2100cc VWs, and rediscovered the problems of higher weight and fuel consumption. Instead of landing at 60 mph, they are at 70 mph. Instead of flying nearly 600 miles, they plan 400 mile legs. They fly solo or two small people with no overnight bags (my second passenger is my wife.)

Bob Lutz calls losing vehicle weight 'the virtuous cycle.' It leads to less drag, lower fuel consumption, and better performance. So that is why I'm using the technologies that are available since 1980, the year the first Dragonfly flew. These are technologies not available 35 years ago.

Bob Wilson

ps. I use "lbs" for specs or paper numbers but "Lbs" for those I have measured. I fly 'in the real world.'

They have lots of "drop in" lithium batteries, i.e. 2 lbs, 12v, 14ah:
Amazon.com: Shorai LFX14L2-BS12 Extreme Rate Lithium Iron Powersports Battery: Automotive

If you are bent on 30 year old technology then I don't know what to tell ya. Also I'm not sure what ECU you are referring to since you are talking about a plane one minute then a prius the next. I don't know why the ECU can't be disconnected (or the whole battery) in either event, or if it is custom arduino why data can't be stashed on eeprom on shutdown.

Thanks,

The last set of LiON specs didn't look that good to me and their prices were excessive. But it is good to see the technology getting more affordable.

Sorry it wasn't clear but hopefully this will help:

• LiON - still evolving as prices are changing on a day-by-day basis. In contrast, the NiMH is fairly well bottomed out and have relatively benign characteristics. I'm confident about using the NiMH but would prefer to let others explore the LiON replacement batteries . . . for now. Still, I need to research the vendor's specs, not just the Amazon sales page.
• Prius vs airplane - it has been insights gained from the Prius that have made it easier to adopt new technology in my plane. Aviation is a curious mix of advanced and anchient and having the Prius experience has made it easier to adopt 1990s and later technology into the plane.
• Disconnecting the fuel injection ECU and/or fuel pump turns off the engine. It is in effect a critical part of the engine.
• Arduino nano has significantly improved characteristics over the MSP430 I was originally planning to use . . . five years of Moore's Law.

Since it looks like LiON is approaching a commodity, at least in the motorcycle world, I'm interested. But I have this project and need solutions I can trust in a timely fashion.

I went to the Shorai web site and started reading their FAQ. Sure enough, I found risks I don't want to tackle:

• 13.1V maximum charge - over charging a LiON battery is bad news, very bad, as some of the laptop battery fires attest. In contrast, the Hirth alternator will be using a standard, lead-acid regulator and they typically charge at 13.8-13.9 V. The 11-cell, NiMH battery will be within this limit and has some tolerance for overcharge by generating excess H{2} that recombines within the sealed cells.
• -18-32C temperature range - HUH! This is not even automotive grade temperature ranges. Other source put LiFeP0{4} operating temperature maximum at 55C. Still too heat sensitive for my Alabama climate.
• cold cranking - they recognize the same thing I've seen that non-spinning, motor inrush current is the enemy BUT it turns out 'soft-start' is not that hard today with power semiconductors and micro-controllers. This is a solvable problem if the battery and load are considered to be parts of a system and NOT treated as separate parts.
• charger regulator table - their voltage ranges are about 1V higher than a traditional 12V, lead-acid battery system. It is one of the reasons why I am not using two, 6-cell, Prius battery modules. The 3.2V of this battery pretty well puts it out of the range I'm comfortable with handling.

Depending upon how my NiMH experiment turns out, I may revisit this approach. But it is good to know LiON batteries are starting to show up in off-the-shelf solutions at reasonable prices.

Bob Wilson

FWIW, LiFePO4 is generally used these days vs li-ion, better temperature range, more robust and immune to overcharge, cycle life, etc. They are in very widespread use, so it is probably worth ensuring that is the base chemistry if you are looking into 12v drop-ins.

Edit: here is a "typical" lifepO4 cell in an EV these days, -25C to 75C
http://liionbms.com/pdf/thundersky/TS-LFP100.pdf