How to manually control charge amperage?

[bennelson]

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!

[jakobnev]

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.

[mort]

Originally Posted by bennelson

How do I MANUALLY control the rate of amperage while charging batteries.
...
You advice wanted!

Hi bennelson,
For this situation you want a "carbon pile" battery load tester. Its a big variable resistor with mammoth power dissipation.

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

[oil pan 4]

Try a Varrac.

[Ryland]

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.

[rmay635703]

Originally Posted by Ryland

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

[mort]

Originally Posted by ryan

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

[Cobb]

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.

[bennelson]

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?

[rmay635703]

Originally Posted by mort

Yes, but the devil is in the details.
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.

(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).

-mort

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)