Battery charge rate question

Steven LakeSteven Lake Solar Expert Posts: 402 ✭✭
Dumb question of the day. I've been looking all over for an answer to this question and haven't found anything that has properly satisfied my curiosity. Here's the question.

If given a whole day to charge (sunrise to sunset) under both optimal summer and winter charging conditions, would a 20w panel fully recharge a 9ah battery in one full day, assuming it never went below 40% capacity? Also, as an addendum to this question, as a bit of general information, my area gets 15 hours of usable light (sun above the horizon) during the summer and 9 hours in the dead of winter. I don't know what percentage of the day would be considered "peak hours" given those numbers, but I figured to offer them in case it might help me find answer to this question.

My guess is yes, based on the 1w to 1ah ratio I saw posted on here in another thread. However I figure it'd be better to ask and sound stupid than build the system wrong and remove all doubt. :P


  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Battery charge rate question

    my guess is it's not good. it does firstly depend on your solar insolation during the worst times of the year. i did not transfer all of my favorite and useful sites into this new pc of mine as of yet so somebody could chime in with specifics for your general location, but i'm going to guess at an equivalent of 3 full sun hours during that time. the 9ah battery at 40% capacity will have used 5.4ah leaving a capacity in the battery of 3.6ah. (btw we don't recommend going below 50%) 5.4ah/3hrs=1.8a. you have a 20w pv and if the voltage is about 17v (check for your specified vmp here) on the pv this gives you an output of 20w/17v=1.176a through a pwm cc making this a no go as 3x1.176a=3.53ah and you need 5.4ah. if you have 4hrs of insolation then 5.4ah/4hrs=1.35a needed. this falls short again as you can only supply about 1.176a.

    you may need to change the depth of discharge on the battery (recommended) and possibly up the pv power you have to have it work out alright and we didn't even get into the weather aspects.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery charge rate question

    A couple of issues with long day charging. First is keeping the panel aimed at the sun so it stands a chance of producing current. We get 16 hour days in Summer here, but with fixed panels most of the time not enough light falls on them to cause any real power production. As such it is still down to about 5 or 6 hours maximum despite the length of the day.

    The second issue is that early and late sun has to travel through more atmosphere, so the power available from it even with the panels tracking is less. So we come down to that elusive number known as "hours of equivalent good sun" which is a bit hard to predict without an actual install.

    But you can work the calculation backward and determine how many hours of equivalent good sun you'd need. First problem, you state "never went below 40% capacity". You do not want to drain a battery to 40% capacity. I think you've got your reciprocal in place: 60% capacity is 40% discharged. Let's look at max DOD/SOC of 50%.

    For 9 Amo hours that's 4.5 Amp hours. Then you have the charge efficiency of the battery to consider; it will always need more power in than out, usually 20%. So you're looking to harvest 5.4 Amp hours (and we will assume a 12 VDC system considering the small battery and panel). This is about 65 Watt hours.

    A 20 Watt panel will produce about 1 Amp maximum, so you'd need the equivalent of 5.4 hours of good sun to recharge the battery from 50%. With 15 hours of daylight available this is certainly possible even with a fixed panel.

    Another rule-of-thumb borrowed from the world of photography: 2 hours after sunrise and 2 hours before sunset the light will be closest to "daylight temperature" meaning at its normal intensity. So if you subtract those 4 substandard intensity hours you still have 11 hours to work with. If you could aim the panel directly at the sun during those hours you'd get 11 Amp hours out of it. That is about 132 Watt hours.

    All things considered it should work.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery charge rate question

    It occurred to me I'd better add some clarification:

    If the DOD is only 40% and if the panel can be kept aimed at the sun it should work even in Winter.

    If the DOD is really 60% as stated it will not.

    If the panel can not be kept aimed at the sun in Winter it will not.

    If anything else is not as expected in the example it will not.

    And of course it assumes a sunny day.
  • Steven LakeSteven Lake Solar Expert Posts: 402 ✭✭
    Re: Battery charge rate question

    Thanks guys. So, attacking this in the way I understand it, if I have 20w of panel I have a 50/50 chance of fully recharging from 50% within one day. But if I had 40w total (two 20's since that's what I have) then I absolutely would be able to do it in one day, as 20w would be the borderline in a best case scenario, meaning 40w would then become a sure bet for the most part. I know I did state that 40% was the lowest it would go, but that's one of those "outside extremes" I like to include in my calculations because, while I try not to go below 50%, there's gonna be that one time at some point that I will inadvertently take it below 40%, most likely by accident. But even if I don't it's good to insert that into my calculations just in case. It's the boy scout in me saying "it may not happen, but plan for it anyways." :D
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Battery charge rate question

    yea you needed that extra clarification coot. in fact in my reply i didn't account for losses making the 1.176a estimate unrealistically high. it really should be on average about 23% worse on the current output from the pv.

    40w should do it, but weather is unpredictable as a string of bad days could ruin your plans of keeping it charged.

    roughly calculated-
    40w/17vmp=2.35a. at 77% eff this is 1.81a. for 3hrs this gives 5.43ah. just made it if you need 5.4ah replenished.
  • Steven LakeSteven Lake Solar Expert Posts: 402 ✭✭
    Re: Battery charge rate question

    Thanks Niel, Cariboocoot! You guys are always great about providing answers to questions I can't fully get answered on my own! :D
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