How to manually control charge amperage?
 

OK, I have what SHOULD be a really basic question for some of you guys.

How do I MANUALLY control the rate of amperage while charging batteries.

This is specifically about "War-Charging" 12V batteries.

I have packs of 4 lithium cells pulled from my Mitsubishi i-MIEV salvage project. They are bundled together to basically be 12V batteries.

They are all dead dead dead. If I try to charge them with a "smart" 12V charger, the charger says there is nothing connected and won't start a charge. If I connect a "dumb" charger, the battery will try to draw more than the 10 amps the charger can provide and blow a fuse.

I HAVE been able to do some charging by more or less "jump-starting" the battery by connecting it with jumper cables to a plain 12V deep cycle battery. It's just like jump-starting a car. The good battery brings up the voltage of the bad battery.

At that point, I have enough voltage reading in the 12V lithium pack that I can start charging individual cells on a CellPro Powerlab 6 charger, which is designed for specifically smart charging lithium.

By war-charging these batteries, I limit the maximum voltage, but I have NO CONTROL over the current, which can be high enough to make jumper cables rather warm.

So how do I MANUALLY control the current?

I do have some 12V/50 watt light bulbs around. I can easily run the 12V flooded battery through the light bulb, then through the lithium and back to the floodie. However, doesn't the bulb drop the voltage by 12, leaving only a tiny amount of voltage dropped by the lithium pack? That gives me something like 4.3 amps of current, but only charging at .2V or something super-low like that.

The lithium won't EVER charge if it's getting .2 volt, will it? Can I expect the internal resistance of the lithium to change as it gets that small of a difference in voltage, and have it slowly come up?

You advice wanted!

A lightbulb in series sounds just what the doctor ordered.

And unless a battery has failed shorted, it's EMF should start to rise pretty soon.

Hi bennelson,
For this situation you want a "carbon pile" battery load tester. Its a big variable resistor with mammoth power dissipation.
http://ecomodder.com/forum/attachmen...1&d=1367708168
New from $100 to about $1000, used, maybe ebay?
But you won't use it as intended. Use the carbon load as a way to reduce the current going into the dead batteries. You can probably figure out how to make it work for you.

-mort

Try a Varrac.

The light bulb will act as the current limiter that you are looking for, try it with the "war charger" that you spoke of and check the voltage at the battery to see for sure, but light bulbs were a common current limiter in the past, partly because they allowed you to see what was going on.
10 amps at 12v is 120 watts, so try using a 100 watt bulb and see what happens.

If you want constant amperage a lightbulb is ideal for what you want to do.

Basically what you will have occuring is voltage division.

In my case I use a diode and a lightbulb run right off 110v mains, because 12v is so much smaller than 110v you end up with your current basically equaling the regular current of the bulb cut in half. A Kilawatt meter can allow you to change your lightbulb wattage to select the current you want and you can visually see the current on the killawat meter.

Understand however that voltage will climb until current flows so you can blow a battery with the full 110v if it doesn't want to take charge, in the case of Bad FLA's (sulphated) this is great for lithium, not so much.

Remember there is no reason you cant do this with a 12v lead acid battery and a bulb, then your current will vary a lot but your voltage will be controlled.

If you need a carbon pile battery testing thingy go to harbor freight, they have a variable 500amp unit (which is more like a 200amp but I digress) that has voltage and amperage. It costs between 25-50 depending on the sale of the week, you can use it to dial in the current you want, just understand you need to redial it often.

Like I said no right or wrong here.

Good Luck
Ryan

Yes, but the devil is in the details.
A light bulb will act as a current limiter, until it blows out. The resistance of a light bulb varies with it's power consumption - so that the ratio of off to on resistance is about 15:1 Ben has some 50 watt 12 volt bulbs. The on resistance is about 3 ohms, the off resistance is about 0.2 ohms.
Ben:
I assume you want to charge the lithium cells at a high rate of charge, perhaps 50 amps initially, but also limit the voltage to 12 volts or so maximum, and when the lithium cells show 12 volts the charger output should reduce to a trickle of around 1 or 2 amps. A resistance of around 0.24 ohms initially, gradually increasing is what you want. In addition the initial charging current may be applied for several minutes, so the 0.24 ohm resistance will be dissipating 600 watts.
You will need a dozen (12) 50 watt 12 volt bulbs in parallel to limit the current to 50 amps. Assuming the initial lithium cell voltage is 0 and the supply is a 12 volt car battery. (Fewer light bulbs for a lower current limit.)
But as the lithium cell's voltage comes up and the charging current drops the light bulb resistance will fall (we'd like it to rise).

How many light bulbs do you have? What maximum charge rate are you aiming for?

-mort

FYI, the older style filament bulbs also work as heating for use in pump houses too to keep them from freezing.

The dollar store is a good source unless ypu want to go with a heat or halogen or spot/flood light.

Mort's info in post #7 is the kind of stuff I'm looking for.

I wish I had a higher amperage ammeter handy. My multimeter only goes up to 10A. I think I have a 100A shunt around, with no matching ammeter, but I should be able to put the multimeter on it to make it work. If I can get that working, at least I can see how many amps are going through on a war-charge.

So far in my experiments, jumpercabling a 4-pack of the lithium cells to a 12V flooded battery causes fairly high current to flow, and does indeed raise the lithium pack voltage.

HOWEVER, each group of four cells is in better or worse condition than other groups, and I have no way to tell what it is by looking at it.

So far, one or two packs seem to draw a considerable amount of current. I've only caused the jumper cables to start smoking once, but that was enough to go "Yipes! Maybe I should figure out a way to control that!?"

Right now, I have a pack of lithium charging from a 12V flooded battery running through a 50watt 12V bulb. It looks like it's charging, but at about a rate of .05V per day. That's a little slow for my likes.

OK, here's a weird question for you solar guys...
PV is current-limited, right? I've seen where guys intentionally short a panel (plugging in its -&+ right into each other) to keep the connections out of the way what the panels are being mounted.

I have about 6 amps of Harbor Freight solar panels handy. If I used those to direct charge the lithium, the battery could only suck so much amperage. That would control the amps, as the panels can only produce so much.

Any reason NOT to try this? (Like destroying my panels?) Maybe I would need a big diode in there?

I believe you sort of agree with what I said, because Ben is using a 12v source to charge a (theoritical) 12v set of batteries the lightbulb behavior is EXACTLY what I would want to happen assuming the voltage curves are matched.

So as the light bulb resistance falls the amount of the difference in voltages is also falling meaning current is still tapering, which in my mind is fine since he has a controlled voltage (battery)

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

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 :(