Tuesday, December 20, 2011

Electrical Question

Assuming that it starts snowing this winter, I'll be putting the Corvette under a car cover for two to three months. Traditionally I have had to jump start the car at the end of winter.  I am told that a float charger is the right tool to use--it keeps the battery charged, but shuts off when the battery is fully charged.

1. I presume that overcharging a car battery is bad for it--and that is why float chargers exist.

2. Is it better to keep the battery fully charged with a float charger than to jump start it at the end of winter?  Does a battery benefit from being kept alive?  I know that the lifetime of some categories of battery are dependent on the number of full discharges that they experience.  Is this the case with lead acid car batteries?


  1. I'm no expert, Clayton, and hopefully you'll get some expert feedback. As a layman who has dealt with the issue for years (motorcycle batteries, riding less during the cold months), my understanding is that lead-acid and similar batteries do much better when fully charged... and in fact letting them getting fully discharged decreases their lifespan. And thus, a "float" or "maintenance" type charger is your best bet.

    (That's not my practice. I try to fire up the ol' hog at least every couple weeks all winter and at least go on a short ride to keep the battery charged and the tires round. That may not be practical for you.)

  2. Once the snow starts to fall where I am, the Corvette goes nowhere.

  3. It is better to keep a battery fully charged (we're talking lead-acid type batteries but this is true for most other rechargeable batteries as well more or less).

    It would be less hard for the battery that isn't being charged if it were kept inside so that it doesn't get cold. The electrolyte in a battery can actually freeze in sub-zero temps if it is low on charge which will ruin the battery.

    The lead plates in a battery can sulfate if allowed to deep cycle (drop to a very low charge) and this will reduce the battery life.

    So I would not let the battery get so low that you have to jump it. A trickle charger that charges at max 2 amps and adjust the voltages to match the state of the battery is the way to go. Float charges are ok, but some of them are of low quality so personally I would get a slightly higher quality charger (Schumacher is a major manufacturer and I believe they make all of the Die Hard chargers sold at Sears and Kmart--and are probably also the ones sold at most of the auto parts stores). The real expensive chargers are mostly for starting cars with dead batteries but probably don't add much to the charging aspect.

    If you put the charger on the battery once a month and let it run till it is fully charged again you will probably be ok and never have to jump start again. Of course you could just leave it on all the time---your call, but don't let it get so low a jump is needed. I am wondering if you have an older battery or the alternator/voltage regulator on your car is working ok as well. That is easy to test with a volt meter.

    I hook up my chargers once a month to my outside vehicles that aren't being driven and let them trickle charge all day long (Saturdays and Sundays are a good day for doing that) till they are full again.


  4. I'd suggest being careful about which charger you employ and the quality of the unit; I've had expensive electronics elsewhere in the circuitry killed by a float charger. When I put it on an oscilloscope I saw a large amount of random stray voltages when it cycled between "charge" and "maintain" modes, and especially going back to charge mode. Easy solution is to disconnect the battery cables before connecting the float charger.

  5. The car sits unstarted for three months. I'm not surprised the battery goes flat.

  6. I don't know where I heard it, but it is my understanding that NiCad batteries like deep cycles, from full to almost complete discharge before recharging, while lead acid batteries of the type used in cars like to be topped off after each use, never really getting low. Marine batteries are an exception to this through some variation in design I do not understand.
    You might look at the regular chargers to see if one turns off at full charge.
    I have a 1970 truck that has no electricals operating when it is off (eg the clock), and it has sat for more than a month and still started. Of course this is in So Cal. Maybe a battery blanket and a block heater would be helpful, too?

  7. How long a battery will hold a charge depends on the charge level before it is taken out of service, the health of the battery (electrolyte and plates) and the temp it is stored at. I could probably let my batteries go till spring and still start the vehicles (but not a smart thing to do). But then I'm in Ada County so we don't get as much continuous sub zero weather....

    I agree with Fred on the quality of units which is why I suggested buying a better quality unit. Those cheap units might damage the car electronics (I would be leery of the Harbor store's "el cheapo" units). The only problem for you using a regular charger may be that regular chargers are typically large enough that you can't put the unit under the hood and then fully close the hood (which I'm guessing you might want to do and not have to clean snow off to hook one up once a month during the winter).

    Whatever you do not letting the battery deep cycle by letting it sit for months without a charge is a must!

  8. The idea is to compensate for the internal discharge that the battery experiences, when allowed to discharge completely, the plates form a layer of lead sulfate which will kill the battery forever. also, as stated earlier, a discharged battery electrolyte has a higher freezing temp than a charged one does. Further the heat generated by this charging will help to prevent the electrolyte from freezing but it isn't completely freeze proof. If possible, bring the battery inside with a quality float charger (which is just a linear voltage regulator set to put out 2.3-2.35V per cell (13.8-14.1V for a car battery)) attached and keep in relatively warm until spring and reinstall the battery.

  9. Clayton,

    I run tractors, atv's, and generators, all battery started. About $100k worth of equipment.

    I also get west central mountain temps including down
    to -30. I only use this brand of battery maintainer; they use a processor algorythmn to optimize the battery state.


  10. I'll chime in here and say much the same thing: fully discharging a battery is bad for the battery due to the chemistry involved. The depth to which you can discharge the battery varies by type/chemistry/construction, but car batteries should never go below 30% or you seriously degrade the capacity and life (and even then, most battery guys will say not less than 40%). If you want to get an idea of the price difference in a battery designed to handle deeper discharge cycles, compare a marine deep-discharge battery with a car battery of similar capacity and you'll be able to tell why folks don't do deep discharge on car batteries.

    I keep the batteries from my "toys" (ATVs, lawn tractor, boat, motorcycles, etc) on float chargers in my unheated garage most winters. It freezes in the attached garage (no heat and -40 outside does that), but I've not had problems with the batteries themselves freezing while being maintained. My chargers are solar panel things I've built from spare parts to show my son how to do electronics so I can't recommend the manufacturer or construction. ;-)

    The better quality float chargers aren't linear regulators, but rather buck regulators since those are more efficient (e.g. http://www.linear.com/product/LT3652). If you get a linear regulator those things will get hot since they burn a ton of power in the pass device to regulate the power down and that's not good for the lifetime of the charger among other things.

  11. One note, Clayton:

    most modern cars have electronic controllers that operate in 'sleep mode' while the car is powered off.

    This is especially true if the car came with a key-fob. The ECU that listens for key-fob commands has to have a standby-mode in which it can be awakened by the fob's radio signal. While this is usually designed to be low-power-usage, it is not a no-power-usage state. The controller draws a small, but measurable, amount of current from the battery.

    Inside the auto industry, this is often called dark current.

    Most cars are designed to last at least a month between ignition cycles. Some last two or three months.

    A few vehicles even have a long-term-storage setting available to the user. This setting will typically shut off remote-key-fob functionality, among other things, and allow the car to start more easily after a long stay in a garage (or airport parking lot).

    However, I don't know if your Vette has such an option...

  12. The only disadvantage to disconnecting a battery is that some car computers reset their settings when so disconnected. This can result in the car computer not running right for a few miles when reconnected and restarted as internal tables compensating for injection pressures etc. get rebuilt. Don't know if that applies to the Corvette motor in yours.