# Lifestyles & Discussion > Freedom Living >  Calculating Time For Solar Panel to Charge Battery

## Rael

I have a solar battery charger which puts out 15 watts an hour @ 15 volts. It's charging a 12 volt battery with a 125 amp hour capacity.

Assume I run the battery down 50%, leaving it with about 62 amp hours remaining. The solar panel manual states that in a typical week the panel will generate 735 watts (49 amps) in a seven day period. Based on this, how long would it take to charge this battery from 50% to 100%?

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## Danke

Is this from your take home exam?

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## Kludge

What kind of battery? Lead-acid, I'd guess since you aren't discharging beyond 50% of capacity and it has that high of a Ah rating. Make sure you aren't charging the battery at temps outside the range of 35F-120F (if it's lead-acid).

Charging @ 15v is probably dangerous (they do explode) and will reduce cycle life. Ideally, you want to charge @  below 14v for 6 cells (12v lead-acid battery). If you charge above that, you probably want a room temperature as low as possible above 35F to stop some of the damage from overcharging. You really should look into a special lead-acid battery charger (~$10), but you'll probably need an inverter to connect it to if you still wish to use the solar panel. At that point, you're further lowering overall efficiency, too.

Okay - now, the battery effectively has a 1,500 Wh capacity (voltage x Ah capacity). I think in the manual, they probably wrote that the panel produces 735Wh worth of energy in the typical week - but that figure depends on the area you're in and the time of year (being it's Fall, you're probably looking at something like 20% reduced output from the yearly average). Let's take 20% off, and assume the resulting weekly Wh production is 588. 750/588= 1.255 weeks, or about 9 days to charge from 50% to 100%. The solar panel has a fairly low watt output, so you probably don't need to worry much about reducing the charge rate as the battery charges.

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## Danke

> Charging @ 15v is probably dangerous (they do explode) and will reduce cycle life. Ideally, you want to charge @  below 14v for 6 cells (12v lead-acid battery).


Typical solar panel set ups have a controller to clamp down the voltage going to the batteries.

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## Kludge

> Typical solar panel set ups have a controller to clamp down the voltage going to the batteries.


Oh yeah. I totally disregarded he has some type of system specifically for charging batteries. 13.6v-13.8v is allegedly ideal for charging a 6-cell lead-acid battery. Idunno how efficient his system is, so that could change things.

Edit: Reading more, you should be able to charge @ 14.4v until the battery's @ 90% capacity, then 13.6v after until it's completely charged. You want to cut off charging immediately after the battery's been charged, or reduce the voltage input to the battery to 13.2v if you're storing the battery. Lead-acid self-discharges fairly quickly (~4%/week, allegedly). Allegedly, you want to charge the battery @ 14.4v every 10 discharges or so to prevent sulfur crystallization ("sulfation," where the battery will have trouble accepting a charge) and stratification (when the lead & water in the battery separates, with the water sinking to the bottom -- heavy use [charging/discharging] stirs the mixture up to prevent corrosion).

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## Rael

You must spread some reputation around before giving it to Kludge again.

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## Rael

Yep I have a controller on it so it will not over charge. This is connected to a deep cycle lead acid battery on a travel trailer that is off grid.

Would it be possible to top off the battery by charging it from my car? I have a portable battery charger and a 400 watt inverter but for some reason the charger will not work with the inverter. Is it safe to charge the battery from my car using jumper cables?

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## Kludge

> Yep I have a controller on it so it will not over charge. This is connected to a deep cycle lead acid battery on a travel trailer that is off grid.
> 
> Would it be possible to top off the battery by charging it from my car? I have a portable battery charger and a 400 watt inverter but for some reason the charger will not work with the inverter. Is it safe to charge the battery from my car using jumper cables?


Maybe? Your car probably only has a crank motor? They're designed a bit differently from deep cycle batteries. Crank motors to start cars generally have a pretty low Ah rating and output a higher current. Maybe the crank motor could be charged faster, which your car's alternator would be designed for? I'm not sure.

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## Danke

> Yep I have a controller on it so it will not over charge. This is connected to a deep cycle lead acid battery on a travel trailer that is off grid.
> 
> Would it be possible to top off the battery by charging it from my car? I have a portable battery charger and a 400 watt inverter but for some reason the charger will not work with the inverter. Is it safe to charge the battery from my car using jumper cables?


Yes, you can charge the batteries from a car with jumper cables.  But it is better with a trickle charge you would get from a battery charger, especially for Deep Cycle batteries.

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## XNavyNuke

Everything you wanted to know about lead-acid batteries. Its a DOE pdf.

DOE-NDBK-1084-95

Similar to the Nav Nuc (yeah, there's a few batteries on subs) version only Non-Confidential. It doesn't matter your recharge rate if you are overheating your electrolyte. All kinds of bad things happen.

XNN

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## Rael

> Everything you wanted to know about lead-acid batteries. Its a DOE pdf.
> 
> DOE-NDBK-1084-95
> 
> Similar to the Nav Nuc (yeah, there's a few batteries on subs) version only Non-Confidential. It doesn't matter your recharge rate if you are overheating your electrolyte. All kinds of bad things happen.
> 
> XNN


Thanks, looks interesting

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