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Old 05-06-2013, 01:03 PM   #11 (permalink)
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Hi Ben! Yes you absolutely can just use the solar panel. It will sag down to 12v and the current will be whatever it is. It's almost a constant current source, so even shorting the output of the panel will still only give around the same current as at 12v. I've done this a lot with a homemade panel and charging 12v and 3.3v batteries. The open circuit voltage of the panel was over 20v, and the 3.3v battery didn't care, and neither did the panel. The only issue is you aren't maximizing the power output of the panel, but who cares about that. haha

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Old 05-20-2013, 02:46 AM   #12 (permalink)
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Solar panels, bad boy chargers, and variacs

I agree that the solar panels are good as a low current source. It just takes a while.

Make SURE you don't forget about it for a few days and overcharge the cells, or have mismatched cells and overcharge one cell ... the open circuit voltage may be higher than the lithium cells should go. Bad things happen when you overcharge lithium cells.

For the comment on using a variac, that's a good idea as well. The Variac is an auto-transformer that can adjust it's output voltage using a large dial. Limiting the voltage into your bad-boy charger will limit the current.

If I was using the bad-boy, I would:
1 - connect the bad-boy to the variac, and the output of the bad-boy to your DC voltmeter
2 - dial up the variac slowly to get the maximum voltage you want at the output terminals, like maybe 13.2VDC. That should be as high as you go with the battery connected as well. If the meter has a setting for RMS voltage, check that as well since the output voltage without a load could look like AC to the voltmeter. RMS voltage will peak at about 1.4x the value that you read on the meter.
3 - turn off everything and add your current meter and the shunt resistor in series with your batteries (I would not use light bulbs. It makes things complicated).
4 - dial down the variac to 0 and turn it on
5 - raise the variac output voltage slowly, writing down the shunt voltage in mv at 1 amp, 2 amps, 5 amps, etc. If it's linear then you can use your shunt to measure much larger currents than the 10 amps that your meter can.
5A - use only if the ammeter and the DC voltmeter are the same meter. You can set up as in 5 but stay at the 1 amp variac setting, then shut off the variac (leave the dial setting as is). Remove the ammeter from the circuit so you can use it as a voltmeter. Turn on the variac and check the voltage across the shunt. It takes longer, but you can verify voltage per amp by repeating for 2 amps, 5 amps, etc.
6 - turn off the variac
7 - with the badboy connected through the shunt to the batteries, dial the variac down to 0, turn it on, and dial up the variac until you get the current you want (measured in mv across your shunt) or you reach the dial setting in step 2(up to the limit of the fuse on your variac, 15 amps for an 1800W variac like I have).

The current flowing into your batteries should fall as the charge on the batteries rises. Since the batteries have a flat charge curve and you are only trying to reach that flat section so that your smart charger works ... all should be good ... as long as a cell is not bad ... I'd still be paranoid and check them every hour or two.

Jack Rickard over at EVTV has posted something about the voltage on a bad cell rising for a while, then diving as an internal short from copper ... or was it aluminum ... slivers short out the plates internally. I think that there is a lot of heat generated and there was some chance of a fire if you did not catch it in time. You may want to set this up outside, on a patch of gravel so there are no combustibles around? No suggestions on how to keep it sheltered from the weather, though

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