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# Thread: Question about minimum volts and watts to charge a small 12v 2-battery system

1. ## Question about minimum volts and watts to charge a small 12v 2-battery system

Hi! This is my first post on the forum. I looked around here and found info that helped me get to the point where I am, so it made sense to post here when I got stuck.

I'm stuck because I can't figure out if there is a point at which there will be too much internal resistance for my charger to charge my batteries.

Basically, my question is, is there a limit to how many 12v batteries wired in parallel I can charge with a single 50w solar panel? If so, how do I calculate it, and does it depend on how many amp-hrs the batteries are rated for?

The panel has the following ratings: Maximum Power Voltage: 17.5V; Open Circuit Voltage: 22V; Maximum Power Current: 2.86A. An Amazon comment from another purchaser said he measured 18.9v DC from this panel in an area on the olympic peninsula and measured 18.9vdc in "a shaded area on overcast days".

Based on my calculations for location and necessary reserve time, the bare minimum I can get by with is probably around a 110 amp-hour battery, but to be safe, I want to use a bank of either two 115 amp-hr batteries, or two 80 amp-hr batteries.

More specific details about my system in case it's relevant:

I have a system I want to deploy for about 3-5 years with minimal maintenance. My conservative estimate is that it will draw about 1 amp continuously, but the real average draw is about 0.5 amps, and I can make it shut off at night if I have to. It's being deployed on the North Olympic Peninsula (Washington State, in the United? States), where there's pretty crappy insolation in the winter, so I want a good margin of safety.

I am building a small long-term timelapse system that connects a canon camera running CHDK with an Eye-Fi card to a Verizon mifi 2200 cellular router. I've got a little microcontroller that cycles power periodically, and a board that splits 12v down to the voltage required for the camera and the router.

Andy

2. Whoops. get out pencil and paper - back to the drawing board.

Lead Acid batteries need at a minimum, to recharge, 3% of their capacity. So if you had a 30A battery, a 3 amp charger is needed.

A 50 w panel, can realistically produce 80% of spec, in full bright sun - leaves you with 40watts @ 14V recharge voltage, = 2.8 amps.

If you go with a larger battery, the effect of chronic undercharge will eventually kill it.

Like using a small boat as a tugboat. It might move the big ship, but the motor will soon die.

3. The generic formula is really easy math a 5th grader can do. For each watt hour of battery capacity you have (Battery Voltage x Amp Hour Capacity @ 20 Hours), you need approximately .17 watt panel voltage.

So if you have a 12 volt @ 10 AH battery you need a 20 watt panel to use the battery to its maximum potential. In reality the panel wattage will vary deepening on your location and the amount of INSOLATION, and the type of charge controller you use.

For example if you live in Phoenix AZ using a 12 volt 10 AH battery using 20% of the battery capacity per day year round you will need a 8 watt panel.

If you live in Seattle WS using a PWM controller you will need a 25 watt panel to do the same job.

4. Originally Posted by Sunking
The generic formula is really easy math a 5th grader can do. For each watt hour of battery capacity you have (Battery Voltage x Amp Hour Capacity @ 20 Hours), you need approximately .17 watt panel voltage.

So if you have a 12 volt @ 10 AH battery you need a 20 watt panel to use the battery to its maximum potential. In reality the panel wattage will vary deepening on your location and the amount of INSOLATION, and the type of charge controller you use.

For example if you live in Phoenix AZ using a 12 volt 10 AH battery using 20% of the battery capacity per day year round you will need a 8 watt panel.

If you live in Seattle WS using a PWM controller you will need a 25 watt panel to do the same job.

Can this 5th grade math be used in reverse to see how many battery's you can charge if you have 5- 129 watt 12V panels?

5. ## I'm confused.. is trickle-or float-charging bad for batteries if they're not full?

The above replies make it sound like (1) I should get a charge controller that stops charging the battery (or batteries) if the amperage drops below ~3 amps, and (2) I needt two 50 watt solar cells to charge a single 115 amp-hr battery that's got a 0.5 amp constant draw.

Really?!

That sounds like overkill, especially since all sorts of little solar panels are sold to keep rarely-used batteries charged.

At this draw rate, I'll be drawing the battery down about 10% over a 24/hr period if there's no input voltage. Since the solar panel is feeding it for a good chunk of the day, it seems like really the battery is going to mostly be within the upper range of it's chargedness. Won't I essentially be trickle-charging and float-charging the battery for much of the time? It's not like I'll regularly be drawing it down that much.

But I am not that experienced with this.

I can find all sorts of information on the internets about the maximum amperage to apply but very little on how low-amperage charges make your battery die. I thought I just had to overcome internal resistance...

So... how fast will I kill a 12v battery if I'm only charging it with a 50-watt panel? What if I have two in parallel - will I kill them faster? what will happen?

And could you refer me to a resource that gives me more information on the why as well as the how to calculate?

Thanks for your time (and patience),

Andy

6. ## oh wait... math...

I just read the above posts more carefully... If I have a single 12v 115 AH deep-cycle flooded battery (\$89 at home depot), then by the above, using fifth grade math...

I would need 12*115*0.17 = 234 watts of panel voltage... so... I'd need FIVE 50-watt panels to keep it properly charged so I could run a 6-watt timelapse camera?!

Wow.

Call me silly for being skeptical, but I really could use a reference for those calculations.

7. Originally Posted by andyroo
At this draw rate, I'll be drawing the battery down about 10% over a 24/hr period if there's no input voltage.
More like 20%/day if designed properly. go more than that with lead acid technology and and it is like smoking, you shorten the battery life.

There is a huge difference between a Trickle Charge and Daily Recharge. A Trickle Charge is only to overcome the self discharge rate of the battery in question. A daily charge is to replace what is used daily plus enough to make up for self discharge and some over usage.

To trickle charge say a 12 volt 100 AH battery all you need is a 8 to 25 watt panel. To replace 20% of the battery capacity in a day which is normal design discharge is 80 to 250 watts depending on your location and Insolation.

8. Here is the existing long version of the math

http://www.solarpaneltalk.com/showth...-and-PV-Sizing

The last post in the thread (#13) is a bit of a re-write into english vs geek .

9. ## So I need 300 watts of solar panels to keep up with a 6-watt drain... yipes

Ok, I did the math and according to the solar calculators, my drain would be 6W x 24H x 1.5inefficiencyfactor / 0.9H insolation (seattle winter) = 240Watts of panels. So I need five 50-watt panels for my little 6 watt timelapse camera... bummer.

Better work on my efficiency if I want to get down to a 50W panel. Hmm. Looks like I need to start by shutting the whole shebang down at night. That gets me somewhere better than halfway to my goal. Now I'm down to two-ish panels.

let's see.

Now my battery should be 6W x 24H x 1.5inefficiencyfactor /12v x 5daysAutonomy = 90AH.

So I have the battery part right. But if I halve everything, I want to use a smaller battery?

(I never did find any information on why it's bad to slowly charge my battery, or why I shouldn't buy two 110AH batteries...)

10. Originally Posted by andyroo
Ok, I did the math and according to the solar calculators, my drain would be 6W x 24H x 1.5inefficiencyfactor / 0.9H insolation (seattle winter) = 240Watts of panels. S
Move to Tuscon AZ and you only need a 40 watt panel to do the same job.
Originally Posted by andyroo
I need five 50-watt panels for my little 6 watt timelapse camera... bummer.
Why would you use 5-50 watt panels. I would just use 1-230 watt panel or 2-120 watt panels.

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