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 12v 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”?
• And when you Googled "lead-acid charge algorithms" what did you learn? – WhatRoughBeast Nov 25 '17 at 15:25
• 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. – Tony Stewart Sunnyskyguy EE75 Nov 25 '17 at 15:39
• "long trips" what? Is this for an electric vehicle? – Trevor_G Nov 25 '17 at 16:21
• @Trevor I guess he means long trips away with the car left in the garage. – Steve G Nov 25 '17 at 16:42
• 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. – Solar Mike Nov 25 '17 at 17:16

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.