0
\$\begingroup\$

I am thinking about designing a car battery charge maintainer for long trips/car storage, and use a solar panel as an energy source. All other things aside, a few questions:

  1. Can you use standard CC/CV charging methods? I have experience designing chargers for Li-Ion - wondering if the same principles can be applied.
  2. On days without much sun, boosting output voltage up might not leave me with substantial current to play with. I’m curious what the average drain rate due to internal losses is for a 12 V car battery?
  3. If anyone has designed a charger before, at what current do you cut off the charge cycle and finish the “top off”?
\$\endgroup\$
9
  • 2
    \$\begingroup\$ And when you Googled "lead-acid charge algorithms" what did you learn? \$\endgroup\$ Commented Nov 25, 2017 at 15:25
  • 2
    \$\begingroup\$ Before designing anything you 1st have to understand how it works. How does a car alternator charge it's battery? What is the maximum CV? (14.2) What is the Voc? How does MPPT work? I know , but you need to learn everything not just a superficial question. \$\endgroup\$ Commented Nov 25, 2017 at 15:39
  • \$\begingroup\$ "long trips" what? Is this for an electric vehicle? \$\endgroup\$
    – Trevor_G
    Commented Nov 25, 2017 at 16:21
  • \$\begingroup\$ @Trevor I guess he means long trips away with the car left in the garage. \$\endgroup\$
    – Steve G
    Commented Nov 25, 2017 at 16:42
  • 1
    \$\begingroup\$ They exist already : solar panel with suction cups or magnetic strips, put them on the roof of the car and plug into the cigarette lighter... As for the losses a constant draw of 40mA is about average for some cars ie the anti-theft , but only after a set time to allow other circuits to shut down. \$\endgroup\$
    – Solar Mike
    Commented Nov 25, 2017 at 17:16

1 Answer 1

4
\$\begingroup\$

Can you use standard CC/CV charging methods? I have experience designing chargers for Li-Ion - wondering if the same principles can be applied.

Yes.

what [is] the average drain rate due to internal losses is for a 12v car battery?

Battery datasheets I have looked at quoted self-discharge rates of 3-6% per month at 20°C. What this doesn't say is that it is initially much higher, is strongly temperature dependent, and often increases as the battery ages (due to contamination and shedded plate material). At 40°C a 'typical' car battery might loose 5%-10% in the first 24 hours after charging.

what current do you cut off the charge cycle and finish the “top off”?

I set the charge current to C/5 or less, and 'top-off' voltage to 13.8V. At this voltage the battery can be 'floated' continuously. Theoretically the float voltage should be adjusted according to temperature, but I just make sure the battery is at room temperature (20-25°C) during charging. To ensure that a 'flat' battery gets fully charged I leave it on charge for at least 24 hours.

For a 'wet-cell' car battery I occasionally let the voltage go up to 14.4V. This equalizes the charge in each cell, but the battery gasses and needs topping up with de-ionized water.

Lead Acid Battery Charging

enter image description here

\$\endgroup\$
2
  • 1
    \$\begingroup\$ If you figure a "typical" car battery at 50Ah, then 6%/month = about a 4mA average self-discharge rate. The standby loads from the car electronics (keeping the clock running, the ECU happy, the cell modem awake if you have one of those) are probably greater than that. \$\endgroup\$
    – hobbs
    Commented Nov 17, 2021 at 13:36
  • \$\begingroup\$ @hobbs good point. I answered the question asked ("what [is] the average drain rate due to internal losses") but of course external drain should also be considered. The car I owned in 2017 had no fancy electronics apart from an LCD clock, which probably only drew a few uA. \$\endgroup\$ Commented Nov 17, 2021 at 19:04

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.