10w solar panel connected to lead acid battery

I have a question about whether a 10w solar panel left connected to a typical car battery-about the size of a Toyota Camry would prolong the life of the battery or actually cause harm by overcharging.
That is connected without a solar regulator direct to the battery. The solar panel has an inbuilt diode to prevent reverse current flow at night.
I have been told that a 20w panel could "cook" the battery however with a reasonable sized battery there is no problem.
Thanks
Peter

Generally, no panel should be connected without a SC. Why would you choose to do that?

Next, the big question is, are there loads connected?

The Ah per day input needs to be higher that the output, by 10-15%

If not, the battery is isolated from all loads,

assuming starting off 100% fully charged as per endAmps, then - for AGM which can get harmfully overcharged - better to just connect for a day every few weeks or monthly.

For FLA, no harm at all getting overcharged, but then the water needs to be more frequently topped up.

But best IMO not to trust any charge circuitry unattended for long, better to just connect for a day every few weeks or monthly.

But all this is a bit fussy overthinking for a cheap Starter batt, an expensive deep cycling House bank maybe worth that level of care.

By (imperfect) memory, it is fine to "float" flooded lead acid batteries at about 13.3V or less. That means you can leave them on indefinite trickle charge at that voltage, to maintain a fully charged state. You can probably verify this figure at the Battery University website. For perspective, a typical voltage to recharge a depleted battery could be 13.8-14.5V, resulting in a charge rate of (very approximately) 5-15A depending on battery size and state of discharge. That is done until full charge is achieved; you don't want tio oump it indefinitely that way, once fully charged, water will evaporate from the electrolyte. Your 10W panel will charge at less than 1A (W/V = A; 10W/12V = <1A). That is a very low charge to apply to the battery, and it is only when the sun is shining. Even a 20W panel would probably be OK, but bear closer monitoring. So I say it is OK.

Get yourself a cheap digital voltmeter ($20 or less) and leave it connected to the battery, and turn it on whenever you want to measure. My guess is that voltage will not get over 13.0V at full panel output of 10W.

Is the panel rated at 12V or what? What is the battery supplying? Unless you are loading it very lightly, you will need to charge it by some other means than a 10W panel.

Best battery health and longevity happenif kept charged to 12.0V-12.6V open circuit (no load and not being charged). Under 12.0V says it is discharged more than ideal. Lead-acid batteries kept fully charged can last many years. Repeated deep discharge greatly shortens life.

The recommended "Float" voltage varies by battery, follow the mfg specs.

Keeping any charge source hooked up all the time is IMO too trusting.

If significant loads are being fed, might be necessary, but then a bigger panel is required.

Best to check regularly anyway, monthly would be my reco minimum.

This sounds like one of those trickle chargers that people leave on car batteries that they don't drive very often. I think it will work. You'll want to make sure the open circuit voltage of the 12V nominal panel minus the diode voltage drop matches the float charge voltage of the battery (normally around 13.5V). If the OCV is too high, perhaps extra diodes or a linear regulator could bring the voltage down.

I think the OCV specs for a panel may not be very exact, so you may need to do some testing. Also dependent on the amount of sunlight the panel gets, so cloudy days may not maintain the battery as well as sunny days. I think 10W or 20W will both be fine. I guess a linear regulator with a slightly oversized panel will maintain charge best through inclement weather. I don't see any risk of "cooking" a battery as long as the output voltage is correct.

Best battery health and longevity happenif kept charged to 12.0V-12.6V open circuit (no load and not being charged). Under 12.0V says it is discharged more than ideal.

Click to expand...

No! 12.6 volts is fine, but anything under 12.4 volts is seriously undercharged. As you correctly note, lead-acid batteries like to be at 100% charge, not 60% or 70% or 45%...

by LeftieBiker » Oct 27 2020 5:17pm

Best battery health and longevity happenif kept charged to 12.0V-12.6V open circuit (no load and not being charged). Under 12.0V says it is discharged more than ideal.
No! 12.6 volts is fine, but anything under 12.4 volts is seriously undercharged. As you correctly note, lead-acid batteries like to be at 100% charge, not 60% or 70% or 45%...

Click to expand...

People forget fully charged 12V is what 12.68 may vary a little not sure but 12.5 is considered 50% and 12V is 0%.
Parked my car for 7 weeks once and put one of them 3w solar charger on my car and it was good when I went to drive it. Did not check SOC to report. Depends how many watts are needed to matain charge but all lead Acid don't last long unless you get into the heavy plated group.

You and LeftieBiker are both correct, thanks. There's a good chart here https://batteryuniversity.com/learn/article/how_to_measure_state_of_charge if you scroll down a little. Note that measurement is best if the battery has been at rest (no load and no charging) for a while, they say 24 hours but that's a little picky.

30 minutes of no load rest will usually suffice.

For real precision, up to 72hrs

and the [V vs SoC] table varies greatly from one type to the next, even models within one maker's lines

and changes over time as SoH declines

and with temperature.

IOW estimating SoC% from voltage alone is very rough, in the best of circumstances.

And those would be using an accurate SoC meter to build a [V at rest vs SoC%] table specific to that battery

maybe one for cold, one for hot

and then redo it when SoH% has fallen by another 2-3%.

Of course if you **own** such a meter, then the lookup table becomes redundant.

______
Battery University is a **great** resource for noobs learning the general basics, jargon etc

For specific details, not so much, lots of inaccuracies and overgeneralisations throughout

Desert Washington Oregon California Nevada Arizona New Mexico they all have deserts.. Go solar paint your house and roof. That one would be one big battery charger.