solar panel on individual lifepo cells

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colin9876
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solar panel on individual lifepo cells

I have made my 24v scooter run from 7 lifepo cells (10ahr)
I have put a 5v 5w solar panel on each cell.
I like this better than charging a string and worrying about balancing.
I never run it down too far and leave it in the sun 2 or 3 days. Between rides. Luckily it never seems to overcharge the cells as they are all around 3.3 v when ever i measure them.

Complex question, but what is the optimum voltage to charge cells at in theory. Would i be better with a 4v 5w panel?
Im wondering if a cell charges proportionally to the voltage? In theory does a 5v 1amp charger put power in better than a 4v 1.25a charger?

reikiman
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Re: solar panel on individual lifepo cells

Depends on the actual voltage coming out of the panel in the light conditions you have.

5v would overcharge the cells and be a bad choice. 4v is better in that it wouldn't overcharge the cells. But that's assuming the cell is producing 5v or 4v in the light conditions you have.

- David Herron, The Long Tail Pipe, davidherron.com, 7gen.com, What is Reiki
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marylandbob
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Re: solar panel on individual lifepo cells

You would be better off with 8 cells, because the operating voltage would be closer to the voltage of a charged 24 volt lead-acid battery. (28 volts at charge completion)-7 cells would be good for Lithium Polymer cells-- With 8 Li-Fe/PO4 cells, connecting a 15 volt solar array (3 of your 5 volt panels, in series) to each 4 cell group would be about ideal.--Bob

Robert M. Curry

colin9876
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Re: solar panel on individual lifepo cells

@bob, i know what u mean but i dont want to charge any of these cells in series, thats where all this solar stuff goes wrong, theres no need. I think having the cells individually connected to their own solar panel is the most logical and efficient.

The physics i need to know is this
If solar panel produced v volts (today v=4.8)
And s is state of charge of bat (now s=3.3)
Is charge rate proportional to
(1) v Or
(2) v-s

If its (1) then droping the panel voltage will only reduce the charging rate slightly
But if its (2) , i.e. proportional to v-s, then droping the panel voltage will greatly reduce the charge rate

The reason if wonder if its (2) is because i know other physics things, like cooling rates are proportional to the Difference in temps, so i wonder if its the same that batts charge at a rate proportional to voltage difference?

marylandbob
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Re: solar panel on individual lifepo cells

We should really discuss this via telephone, as there are MANY complications in what to you seems a simple solution! Lithium cells/batteries are best charged in a two stage process, the first stage is current and voltage limited, the second stage is constant voltage, untill charging current drops to a low value, where the charge is then terminated, as float charging is generally not advised for lithium. With lithium cells, the maximum charging voltage applied is critical, and many cells should have the charger REMOVED once charging is complete. YES, voltage differential does influence charge rate. Solar cells have significant series resistance as a charger, therefore they are "current limited" devices. Solar cells also have a parallel resistance, which will discharge your battery cells during low loght/darkness, unless series diodes, or other means are used to prevent it. Seldom are solar cells MATCHED in output or illumination throughout the day, thus charge per cell will tend to also be unmatched.--One solution? A big solar array, producing hundreds of watts at perhaps 24 to 60 volts, connected to a capacitor and a group of wide input voltage range D.C. to D.C. converters, each designed to regulate and supply charging voltage to one cell.(4.2 volts for Li-Polymer, or about 3.65 volts for LiFePO4 cells)Many lithium cells will produce internal gassing, with swelling of the case and cell destruction, if charging voltage goes too high! You have been lucky, as your solar cells are too small to fully charge your battery cells, and have not exceeded the voltage limit so far. (If you use a series diode, to prevent discharge during darkness/low light, you must enter its voltage drop into the charging equation)-Bob Tel:301-439-3873

Robert M. Curry

colin9876
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Re: solar panel on individual lifepo cells

Thanks bob for ur number, but im in soon-to-be-bankrupt UK so cant phone US. I know everything u say is true but im aiming for simpler solution. What ive got set up has been working fine for many months, never had to mains charge and had 50 plus 5km rides.
But although it works in practice im not sure it works optimally in theory.

Ur v knowlegable, can u answer this ...
Does a charge voltage of 5v charge a 3.3v cell lots faster, or just a little faster than a 4v charge?
Ie is difference in rate in ratio 4 to 5 or more like ratio .7 to 1.7

So my specific question , is charge rate proportional to charger_voltage OR (charge_voltage- batt_voltage)

marylandbob
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Re: solar panel on individual lifepo cells

A charging voltage of 5 volts would OVER charge your battery cells VERY fast, but lucky for you, you do not have a 5 volt charging voltage!--Why not?- you may say, my solar panel is a 5 volt panel! -The answer is: Your panel fails to remain at 5 volts when charging the battery cell, because it is too low powered to do so, and its internal resistance causes the battery to control the voltage, by loading the cell down. (go measure it, while charging) If your solar panel COULD maintain 5 volts while connected to a discharged battery cell, the charging current would be VERY HIGH, and the battery cell would be destroyed very quickly! A different solar panel, capable of higher amperage, but a different voltage, would charge faster, providing it is capable of at least 3.65 volts under load. You could connect a 48 volt solar panel, rated at 50 watts, directly acccros one battery cell, and it would give results only slightly better, or possibly the same, as what you are getting now, as the available AMPERAGE would be similar, and both solar supplies would have the actual operating voltage controlled by current limiting, imposed by the battery cells lower impedance. (A 50 watt, 5 volt panel would charge your battery cell much faster than a 100 watt 24 volt panel, because of this characteristic of solar panels and batteries.) If left alone, in bright sun, for long enough, it IS possible that any panel capable of producing voltages above 4.2 volts per cell WILL eventually charge the battery cell above the 4.2 volt level, and cause damage, but the time required with low-power solar cells and high power battery cells may be longer than your available sunshine!--I could call YOU, if you desire-not quite bankrupt here at least not YET! (finkers crossed, rubbing lucky charm)-Bob

Robert M. Curry

colin9876
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Re: solar panel on individual lifepo cells

That makes alot of sense, and ur right when solar panel is on batt, then the voltage measured 3.31 cwhen the panel was measuring 4.8 and batt 3.3 seperately), so as u say the batt does determine the voltage.

Ill give u a buzz sunday eve, which i guess will b sun afternoon for u
Thanks

reikiman
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Re: solar panel on individual lifepo cells

uh... skype is free ... http://skype.com

- David Herron, The Long Tail Pipe, davidherron.com, 7gen.com, What is Reiki
- Electrified Electra To

Fourcycle
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Re: solar panel on individual lifepo cells

I think what he's doing might be okay. It's only a 5 watt solar cell at best. Solar cells are basically constant current devices where the current is proportional to the amount of sunlight hitting the panel. As a power source they are pretty soft, especially a little 5 watt unit. If there is a overcharge problem, it could be solved with a little ~1.5 amp shunt regulator set for the maximum battery charge voltage.

If your only tool is a hammer
everything looks like a nail.

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