How long to charge 18650 batteries from solar....


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gareth

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Apr 5, 2019
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Hello,

My first post, so please go easy on me : ) I'm interested in setting up a power wall with 18650 batteries, nothing massive, just small to start with. And I want to primarily run appliances in my garage from it. I'd like to charge this battery from solar andI'm trying to work out how much solar I need. I know there are loads of variables but basically I'd like some pointers please, on how long this sort of set up would take to charge. I mean, I guess there must be a formula right? X batteries at a capacity of Ywith a supplied input of Z would take approximately X amount of time. Something like that?Let's just assume I want to use the solar only to charge them and not use the power for anything else at the same time. Any suggestions on how to calculate this please?

Thanks,

Gareth.
 

Crimp Daddy

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Feb 21, 2018
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It really is pretty simple if you have 100 watts of solar, and you plan on charging a 1000 watt hour pack, it would take 10 hours to charge.

If you have 200 watts of solar, in the same size pack, if would take 5 hours to charge.

If your appliances use 500 watts in operation, and it runs for 1 hour, then you deplete 50% of your battery.

As long as the solar can catch up and recharge the pack in a reasonable time frame, then you can have high intermittent loads with less solar provided the battery can be charged within a window of usable sun hours.

Without knowing your actual loads and consumption figures, it would be impossible to assist with anything other than example math. There are some other finer points to consider like the depth of discharge, power conversion losses / efficiency, but the above should get you moving in the right direction.
 

gareth

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Apr 5, 2019
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Excellent - thanks. That's all I needed, a rough worked example to go on. I'm sure Google had the answer somewhere, but I just couldn't phrase it right!

Thanks again!
 

friedpenguin

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Apr 1, 2019
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Your charge controller needs to know when to cut off and should have a dump load option or at least a big heatsink if it's dumping the load itself. You don't want to size your solar too large if you aren't utilizing it. Wasted energy at that point. I've added a relay to mine to dump load when the batteries hit 27.5v (max charge 29v) and cut load at 27v. This way less overall available solar is wasted. On sad pathetic PNW days like today my 600w worth of panels are struggling to see 100w total output which means I don't see a full charge but my draw is low enough to not be a concern right now.
 

Sean

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Oct 8, 2016
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friedpenguin said:
Your charge controller needs to know when to cut off and should have a dump load option or at least a big heatsink if it's dumping the load itself. You don't want to size your solar too large if you aren't utilizing it.

PV doesnt need a dump load.

The size of solar arrays are often considerably over-sized to counter seasonal variations, seasonal surplus can be either curtailed, or productively diverted, theres no need to dump it.
 

daromer

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Oct 8, 2016
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Normal is to have solar 20% larger than inverter size in Sweden. No dump load needed for solar. Its often more cost worthy that way for Winter use as Sean said
 

Redpacket

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Feb 28, 2018
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Make sure you do have a solar charge controller as it's very easy to fry your batteries if they get over charged.

If you oversize the solar array a lot, the charge current might be too high for the batteries sometimes - you can use a charge controller with current limiting or switch off part of the array to reduce current.
 
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