Another 12V Battery Thread

FAILURE SYMPTOMS

This morning, I was able to confirm my four year old, Odyssey, PC925 has failed:

• Starting temperature 15.8-16.3F measured at terminals
• Ending temperature 23.8F at both terminals after 20 minute commute
• Cold-start battery 9.8-9.6V (Graham miniscanner read)
• Warm-start battery 11.2->11.1V (normal charging voltage 13.8 during commute)

I have always associated heat with a failed cell and seeing an 7F rise in 20 minutes for this 24 lbs of mostly lead, says it has a significant, resistive load, a dead cell. At just under $200 (including tax), it ran me $50/year. I was hoping to see more than 4 years in the claimed "3 to 8 year service life."

One possibly related symptom, the electric doors no longer lock/unlock from the keyless entry fob. The lights flash indicating the signal is received and one door seems to work but the other three remain inert.

REPLACEMENT BATTERY OPTIONS

It so happens, I have a spare, PC925 that I was planning to install in my wife's ZVW30 as part of my 1kW inverter experiment. Now, I'll probably just use it but as I start my replacement battery plans, I've also had one suggestion for:

• UB12350 - a U1 garden tractor battery, estimated $60, 23.2 lbs, 35 Ahr

I am open to other suggestions for 'low cost' batteries.

There are also 'wild card' options such as a lighter battery but at 24 lbs, it is 0.8% of the 3,000 lb weight. This is in the marginal range of 'worth it.' I also have a bunch of 7.2V used NHW11 modules but in series, the voltage is too high to take a charge and single, they are too low to power the car. Worse, their measured 4.5-5.5 Ahr is off by an order of magnitude.

DIAGNOSTIC HYPOTHESIS

Float Life vs Ambient Temperature

Source: Technical Manual, seventh edition, April 2011, US-ODY-TM-001, pp 12

This makes a lot of sense as living in North Alabama, our summer temperatures frequently run to over 90F and in the closed trunk, even in the shade, higher.

Mitigation:

A 'fan' might help with the solar heating of the trunk. However, a fan needs power and a source of less-hot air (it is Alabama.)

Charge Voltage Temperature Compensation

Source: ibid, pp 17

Our NHW11 has no temperature sensor on the 12V battery, much less a known way to mitigate the 13.94V charging voltage. According to the graph in the manual, at 41C (106F), the ideal charging voltage is ~13.35V and the 'cycling' voltage maximum should be 14.40V. The 13.94V is not excessive but lower would be better for longer, 12V battery life. We know the ZVW30 has a temperature sensor built into the battery holder.

Mitigation:

A temperature compensation circuit for the charging path that reduces the 12V battery voltage proportional to the temperature.

Parasitic Loads

Source: ibid, pp 14

Depending upon source, there are reports the parasitic current drain is 30-38mA. It is not uncommon to find an NHW11 left parked for three or more weeks will have a dead battery and require a jump.

Mitigation:

1. Install a tickle charger and plugging the car into the grid when home.
2. A solar panel, tickle charger.

Bob Wilson

 

I appreciate the suggestion and would add I've had other $60, lead-acid batteries suggested, UB12350 and UB12500. But I no longer think the battery purchase price is as important as maximizing the service life.

The PC925 has about the same replacement cost as the Toyota replacement battery. When including the water I added to the Toyota battery in 2007, both the Toyota and PC925 lasted the same four years. But there are other, uncompensated costs:

• Time lost replacing 12V battery - even if we hire a Sears or other service technician, it takes labor to replace the battery.
• Another lead-acid recycling challenge - sure these costs are part of battery service life but it may be avoidable.
• It is the Prius way
  • We pay the same price per gallon at the pump but get nearly twice as many miles.
  • We pay the same for tires but fully inflated, get all of the miles.
  • Perhaps we can pay whatever others pay and get twice the service life.

I think we can stretch the 12V battery life regardless of the battery cost.

Friday morning and evening:

• After an overnight, cold-soak, the battery showed 9.8V and still started.
• Battery terminal bolt, temperature increase, battery weight ~24 lbs:
  • +7F in the morning, 20 minute commute
  • +8F in the afternoon, 25 minute commute
• Operating voltage 13.8V which corresponds to an additional ~500W load (from earlier UPS tests)
• Warm-start, the battery instantly drops to no more than 11.3V suggesting one cell is dead

Bob Wilson