Using a solar charger to trickle charge battery

Jbob · 2025-06-19T00:31:42+00:00

My 2nd vehicle rarely gets driven anymore so about once a week I have to plug a battery charger in to top off the battery. I decided to get one of those solar charger panels to see if I could connect via one of the cigarette lighter type receptacles and keep it charged. So far it doesn’t seem to be working. Not sure if the solar panel is not rated high enough or just doesn’t work. It’s a 7.5 watt panel and shows green meaning there is current flowing to something. I’ve measured the voltage at the battery but don’t see any higher voltage at the battery terminals so kinda assume it must not be working correctly.

egeezer

real_aurgathor
Yeah, as I recall we had to remove the caps and check the water level(and not smoke or use matches around the battery). Some places used tap water to top up when fast charging, we used distilled. Now that batteries are sealed and chargers are “smarter”, such stuff must not be needed.

real_aurgathor

egeezer Yeah, as I recall we had to remove the caps and check the water level(and not smoke or use matches around the battery). Some places used tap water to top up when fast charging, we used distilled

Yes, water should be checked when fast charging. Since hydrogen is very light and tend to dissipate very quickly, accidents are more likely in places where it can’t easily escape such as rooms, submarines, etc.

egeezer Now that batteries are sealed and chargers are “smarter”, such stuff must not be needed.

I know that there are some sealed automotive batteries, but I do not have any, and not planning on getting one either. For one, I normally get 10+ years on regular batteries. Even though the hydrogen generated in sealed batteries can recombine with oxygen to make water, drying out is still a very common failure mode in them since some of the hydrogen will escape over time, no matter what.

One person on DSLR claimed that he was able to open some sealed batteries and add water, but I have not seen any tangible evidence of that.

Jbob

To update this topic for those interested……..this solar charger does in fact seem to be working okay. I let the vehicle sit for around 3 weeks without touching it. Engine cranked right up without a seemingly weak battery. I may have to eventually move the panel from the front dash, which faces north as the vehicles sits in driveway, to a rear windows so the panel gets better sunlight as the sun eventually does it’s seasonal move southward.

egeezer

Jbob

Jbob
Based on this and your real world testing and experience, a 65 Watt charger would have been way overkill for your purposes;

Jbob I’ve only got around .04 mA of current draw.

Splitpair

egeezer Based on this and your real world testing and experience, a 65 Watt charger would have been way overkill for your purposes;

There are too many variable to make such a assumption.

Is the task to maintain a battery or charge one is the primary one, what is the Ah of the battery and how many solar hours are the at the location where the charge is required.

For example to charge a battery a 50 Ah battery at 13.9 VDC in a Northern area would require something like the following.

50Ah × 13.8V = 690 watt-hours (Wh)

The next step is to consider your charging time requirements. If you want to fully charge the battery in 4 to 6 hours, you’ll divide the total watt-hours by the desired charging time * .85 derate factor.

690Wh / 4 hours = 187 watt panel.

Head down to AZ and it comes out quite differently.

690Wh / 6 hours = 124.8 watt panel.

Now those numbers only hold true if the sun shines every day of the year which it doesn’t, for that information a search of a areas annual climatological summary and factor that in as well.

A good rule of thumb to prevent any problems in most areas of the US with a fixed position PV panel is at a minimum double the wattage needed to cover any weather and sun angle loss’s.

This is also a real handy calculator which factors in solar hours by location.

https://pvwatts.nrel.gov/pvwatts.php

Somewhat-Reticent

Solar hours in what weather?

Splitpair

Somewhat-Reticent Solar hours in what weather?

That’s one of the huge variables, I have a graph somewhere around here from a 12 MW farm that shows almost zero generation over a number of days due to cloud cover.

And that sort of generation variability is why grid managers are demanding PV farms install alternate forms of generation such as NG peakers or be fined for not making the energy they agreed by contract to be interconnected with the grid.

It is also why a lot of developers are building what is known as Virtual Power Plants, which is a development where every home built has rooftop solar allowing homeowners to run their homes from, solar when possible and still be connected to the grid.

It works well for the developers as they can command a higher price for the homes they build, it works well for the homeowners as they have lowered electric bills, however it doesn’t work well for electric utilities and grid managers, as the distributed generation (virtual) is unregulated, forcing them to handle inherent variability of solar.

Now someday the utilities/grid managers may figure out a way to force developers into interconnect agreements, but I doubt that is possible as the generation is not owned by the developer, it is owned by each homeowner and getting them to sign into a interconnect agreement taint happening any time soon.

egeezer

Splitpair
The 7.5 watt trickle charger/maintainer works for the OP even when his panel was pointed north. No calculations needed.

Splitpair

egeezer The 7.5 watt trickle charger/maintainer works for the OP even when his panel was pointed north. No calculations needed.

As long as he doesn’t need to charge the battery and the vampire draw isn’t too high.

I have a dual battery system in my truck and to make that work and fully topped off, or recharged if I left some of the radios on required a 75 watt panel to work properly.

A 7.5 watt panel would never recharge the batteries, nor less keep up with the vampire draw,

egeezer

Splitpair

Jbob To update this topic for those interested……..this solar charger does in fact seem to be working okay. I let the vehicle sit for around 3 weeks without touching it. Engine cranked right up without a seemingly weak battery.

Which was the OP’s goal.
Bumblebees can fly despite past assertions to the contrary 😃

real_aurgathor

Splitpair For example to charge a battery a 50 Ah battery at 13.9 VDC in a Northern area would require something like the following.

50Ah × 13.8V = 690 watt-hours (Wh)

Irrelevant. Solar, or even regular trickle chargers don’t need to charge batteries in the way regular chargers do – they only need to counteract the parasitic draw and self discharge (which is usually much less than the parasitic draw) to prevent, or at least slow down the discharge of the battery.

real_aurgathor

egeezer Based on this and your real world testing and experience, a 65 Watt charger would have been way overkill for your purposes;
Jbob I’ve only got around .04 mA of current draw.

We discussed that current a bit later – that 0.04 mA must’ve been an error, and the real rate should be 40 mA.

real_aurgathor

Splitpair A 7.5 watt panel would never recharge the batteries, nor less keep up with the vampire draw,

Most certainly rubbish on the second part, and possible rubbish on the first part.

A 7.5W charger should have a rated current of 625 mA at 12V. That obviously need to be derated, but given cars with relatively low parasitic draw such as 28 mA, it is still more than 20 times more than the current draw it needs to counteract, so even with a derating factor of 20, that is still over 31 mA.

Splitpair

real_aurgathor Irrelevant. Solar, or even regular trickle chargers don’t need to charge batteries in the way regular chargers do.

So says you, but in the real world people make mistakes and it is a lot easier to let a hour or two of sun charge up a battery to the point a vehicle can be started versus dragging out jumper cables or a extension cord to power a battery charger.

More power is always better than needing more power.

egeezer

It’s really fun watching the contortions 😃

Somewhat-Reticent

As long as they’re civil.

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