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Old 05-23-2011, 07:56 AM   #1
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volts and amps

Its a cloudy day, and my solar panels are only putting out about 2.3 amps. Let's say my panel in the MH reads 10 volts in the house batteries. How long will it take to recharge the batteries to full capacity?
How about bright sun light and the panels are putting out 6 amps for say three hours.... what will that get me?
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Old 05-23-2011, 08:11 AM   #2
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You get so many 'amps' at some voltage. If you're getting 2.3 amp, then you basically have a 2.3 ampere-hr charging rate (that's 2.3 amps at 1 hour). As long as the current is flowing INTO the battery you are charging. Depending on your battery size/type it could take MANY hours or days to get a full charge. That's the best answer I'll give based on your question.


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Old 05-23-2011, 08:14 AM   #3
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You need to know the amp-hour capacity of your house battery system. For example, if you have house batteries with 360 amp-hour capacity and they are 50% drained (probably about right for 10 V output), you will need 180 amp-hours to fully recharge them. Using your 6 amp bright sun light example, that means you will need 30 hours for a full recharge (6 amps X 30 hours = 180 amp-hours). This is a best case scenario because it assumes everything is working at 100% efficiency.
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Old 05-23-2011, 10:11 AM   #4
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If your batteries are at 10 volts, you have drained them well past any 'safe' cycle depth to a completely flat state. Discharges to such a level are a sure path to very short battery life.

For a hypothetical, consider a discharge state which is as deep as you should allow your batteries to go which is 50% for optimum cost effectiveness (this is 12.2 volts or at least greater than 12.0v measured after resting with no significant charge or discharge for at least a half hour). Consider you have a typical 2 battery bank that weighs maybe 150 pounds. Assume your solar system is nothing fancy.

For this hypothetical: at 12 usable watt hours per pound to the 50% state, you have drained about 12*150 or 1800 watt hours from your battery. Recharge efficiency is maybe 80% so you'll need 2300 watt hours to go back into the battery to get them charged. The solar output of 2.3 amps is probably at about 14 volts so that's about 30 watts. It'd take 75 hours for 30 watts to supply 2300 watt hours.

This isn't far from Glenn's calculation and the difference just illustrates the variability inherent in the situation. His calculation method and mine also provide a good example of the different ways to approach the problem which should help you understand what is going on and how measurements are used.

This case also illustrates why solar in RV's is usually insufficient for healthy battery charging. The rule of thumb is that you need at least a watt of solar for each pound of battery. This will only begin to approach a good charging rate under optimum conditions. A good charging rate is sufficient current to get the electrolyte circulating. Even with optimum charging, it takes lead acid batteries more than 8 hours to become completely charged.
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Old 05-24-2011, 05:21 AM   #5
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Don't be discouraged by what you are hearing here. One thing you didn't say was how many panels (i.e., what is your total panel current capacity?) you have. Also, you won't have 100% cloudy days forever. I've had solar panels on our coach since 2004. We've boondocked off and on every year since then. The capacity of my solar panels is around 19 amps on a sunny day. On cloudy days it's still anywhere from 5 to 10 amps, depending on the time of day. It doesn't take long to re-charge to full capacity. However, as mentioned above you should never let your batteries get that far down, as they're completely dead at the 10 volt level and may never recharge. A deep cycle battery should not be discharged more then 50% which according to the manufacturer's is 12.2 volts, or 6.1 volt on a single 6-volt deep cycle. Also keep in mind even when you think you're not using anything in the coach to draw current, no lights on, etc. there is still current being drawn probably pulling at a minimum an amp or two. Which in your case could means your not putting anything into the batteries at 2.3 amps.
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Old 05-24-2011, 05:31 AM   #6
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Originally Posted by Bob (WA0MQE) View Post
Also, you won't have 100% cloudy days forever.
You have never been to NY Bob. Have you?
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Old 05-24-2011, 09:10 AM   #7
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A bit off topic but, i'm not a big fan of solar panels. IMO, they are far from cost effective. Generators are standard equipment on most MHs and that is what I use, even when boondocking. Solar panel technology is just not there.

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Old 05-24-2011, 11:28 AM   #8
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Originally Posted by Pairajays View Post
A bit off topic but, i'm not a big fan of solar panels. IMO, they are far from cost effective. Generators are standard equipment on most MHs and that is what I use, even when boondocking. Solar panel technology is just not there.

Jim E
Not that I disagree with you, but there are those willing and able to pay the price for "silence"
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Old 05-24-2011, 02:57 PM   #9
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Oh you are getting 2.3 amps and the batteries are very dead (less than 10% charge)

So the answer is 1.2 times the total amp hour capacity of your batteries divided by the amps plus 4 hours.

So if you have a 100 amp hour battery (My reference number to make the math easier) that would be 120/2.3 + 3 or around 56 hours.

If you have 200 mp hours double that

And since it gets dark at night double it

It is way more complex than that, but basically here is what you need.

1: The state of charge, in percentage
2: The amp hour capacity of the bank

From this you can determine how many amp hours need to be replaced. Example

If batteries are at 75% you need to add 25% so your starting figure is 25%

there is around 10-20 percent loss when charging so multiply by 1.2 (To be safe)

And the last 10 percent takes a long time no matter what the current your panels can provide so add 4 hours.

NOTE that as the batteries approach full the current may drop.
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Old 05-24-2011, 03:22 PM   #10
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the MH reads 10 volts in the house batteries
That is less then 5% charged. Otherwise very, very dead.

You will be lucky if you can get them to recharge to full again.
Might as well buy new ones and never let them get that discharged again.
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Old 05-24-2011, 04:05 PM   #11
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If your battery voltage reads 10 volts your batteries are flat dead as can be seen here. As one of the poster said their life has probably been shortened. Don't further shorten their life by trying to charge them up as quickly as possible. Batteries like a low slow charge. If you have a 200 amp hour battery and charge them at the rate of 6 amps/hour it will take about 30 hours to charge. However the last 10% of charge can take as much as 20% longer to reach full charge which boost the time to close to 37 hours. The rate of charge decreases as the battery approaches its full charge. Charging batteries is always a trade off between preserving battery life and charging them in a reasonable amount of time just as discharging them to 50% of full charge is a trade off getting the most power from the batteries and preserving their life.
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Old 05-24-2011, 05:31 PM   #12
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Originally Posted by JimM68 View Post
Not that I disagree with you, but there are those willing and able to pay the price for "silence"
I agree somewhat. The OP has a Winnebago so I would assume he is not exactly flush. There may be those who have spent a small fortune on solar panels and never run their generator. However I bet they are few and far between. I guess I like my creature comforts more than they do.

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Old 05-24-2011, 05:58 PM   #13
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Gen auto start. Solar is as dependable as all them lights she stuck in the ground.
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