I recently finished a project in which I installed an additional cigarette lighter port on my car that was always on (the existing one was only on when engine was running), and connected a small solar panel to the new cigarette lighter port, to top-up the battery.

I rarely drive the car, maybe once per 30-60 days, and had found that the battery had discharged a couple of times when I did finally get around to driving it.

I'm sure this will correct the initial problem I installed it for, but came across a question.

I was reading online that sealed-lead-acid batteries should be 'maintained' either monthly or a few times per year with an 'equalizing charge', which I read was specifically over-charging the battery to bring the cells up to the same voltage.

My question is: If the solar panel plugged into the car is delivering a consistent 17.5-18 volts throughout the day, with a small amount of current, would that equate to the same thing as me having an equalizing charge performed on the car? I'm not sure how high the battery's voltage will go with this constant trickle-charging, but to me it seemed like it might stop off around 12.7-13 volts.

  • Welcome to the site. I think it really depends on what the current draw on your battery is when in the car. Every car is going to draw a different rate on the battery depending on how it's setup and if there are any faults in the system. One of the main things which draws is the radio's stay alive function (that part of the radio which retains the memory) as well as the ECM. Not sure why you'd want an 18v one when there are plenty of cheap 12v options available. – Pᴀᴜʟsᴛᴇʀ2 May 21 '17 at 18:07
  • I think it's a nominal 12v, with a max output voltage of 18..... If the highest voltage the panel put out was 12.00v, and your car battery was sitting at 12.00v, there would be no current flow. – schizoid04 May 21 '17 at 18:36
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    You may be right :o) There's a reason why I don't answer some questions ;-) – Pᴀᴜʟsᴛᴇʀ2 May 21 '17 at 18:43
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    @Pᴀᴜʟsᴛᴇʀ2 because the solar cell has a high internal resistance and the 18V open-circuit PD will not be there all the time. IIIRC, the cells don't like revese bias, so I would put in a silicon diode to prevent that. I would proabaly go with 18V too: if the rated output is only 5W, the batttery will not even notice any 'overcharge' :) – Martin James May 21 '17 at 20:07
  • There are no "12V solar panels". Solar cells vary their power output drastically with irradiation, so you must select one which has a maximum output voltage significantly above 12V to get the battery charged; 18V is a common choice. – JimmyB May 22 '17 at 11:37

(Assuming here the panel can provide 18 volts at a significant current; I'm not sure how quickly the voltage goes down with increasing current.)

The voltage is likely too high. You will experience water electrolysis, i.e. water is split to oxygen and hydrogen. Hydrogen is flammable, so do this only in an area where ventilation is good, and avoid any sparks near the terminals of the battery. Disconnecting the solar panel needs to happen far away from the battery terminals.

Because of the electrolysis, you need to add distilled water to the battery back to keep the water level at the same level. If your battery is one of the types where you cannot add distilled water, it will be destroyed by the high 18 volts (example: AGM and gel cell will be destroyed as will some flooded batteries where there is no way to add distilled water -- well, gel cell isn't used in cars typically).

I wouldn't bother. Instead of this, use a continuous trickle charger with proper voltage control to keep the battery healthy. You may find some solution where a charger controller uses a solar panel as it power source, which you may find useful if the car is stored far away from an electrical outlet.

However, if your battery is already destroyed, you may try to resurrect it with the high 18 volts if there's a way to add distilled water. Be careful with the explosive hydrogen and with the corrosive battery acid, though.

If you want to expand your set of tools, consider purchasing an adjustable laboratory power source with current and voltage limits. It allows setting those limits precisesly, so that it will find many users, along with charging and resurrecting batteries.

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