I'm trying to repair a nice portable LED work light--Schumacher SL27G. The lights work, but it doesn't charge correctly.

With a dead battery, when plugged in, the red charging light comes on, but changes to green in about 5 minutes--long before it could be fully charged.
When charging, I measure the voltage at the battery as about 7.84v. If I use the lights as soon as the green charging light comes on, they last just a few minutes. If I leave it plugged in for 4-5 hours (recommended charging time), they last 30-45 minutes.

Battery pack is 18650, 4000mAh, 7.4v. The chip on the PCB (which I've drawn out, if that's helpful) is an XL4001E1.

The 'regular' components in the charging side of the PCB all test ok, and I 'think' the SMDs are ok.

Does this sound like a common chip failure, and would it be worth just replacing it (they're very cheap), or is the problem more likely elsewhere?

Thanks!

 

You get 7.4V from 2 cells in series. Fully charged voltage is 8.4V. Leave the charger on for 24 hours and if the voltage of the battery is only 7.8V while the charger is on then the charger is faulty.

 

The 'regular' components in the charging side of the PCB all test ok, and I 'think' the SMDs are ok.

I'm curious how you know this. Visual inspection? Some sort of measurement?

What you should do is figure out how to monitor the charging voltage while the battery is being charged. Watch the voltage as the charger decides that the charge cycle is complete. It is quite possible that the batteries are bad and not the charger - batteries fail far more often than chargers.

 

I tested each individual resistor, diode, cap--all in circuit, so a few are ? marks.

I have checked the voltage while charging, and just repeated it to be sure. Some new info:

1. When I run the lights until they go out, battery voltage is then zero. Just found this, so now suspect these cells are bad?

2. 4007 diodes at input and output of charger, and XL4001 chip all get too hot to keep finger on, for the several minutes charger takes to get to green LED. More evidence battery is bad, or could be normal?

3. When hooked to charger, 0v battery immed goes to 7.6v, then slowly to 7.8 or so, before jumping to a 9v reading when LED turns green.

IF I need 4 new batteries, suggestions for what kind/where to buy?

Thanks for the help!

 

Your battery pack goes to 0? There might be a protection circuit that's preventing further discharge. Hard to say.

The 9V reading is very odd. I believe it should top out at 8.4V for a 2 serial cell charger. The chips getting hot isn't a huge issue because they are spec'd run at 80C (176F) or higher. Do you see signs of burning on the chips/board/case?

18650s are available everywhere. You could break into a Tesla... but do a search. be aware there are lots of dodgy batteries (and battery sellers).

 

Check out the datasheet and see if their circuit compares to yours...


https://www.google.co.uk/url?sa=t&s...ACbgQFgglMAA&usg=AOvVaw01op-VATxZSyYRP-u67J_6

 

Your battery pack goes to 0? There might be a protection circuit that's preventing further discharge. Hard to say.

The 9V reading is very odd. I believe it should top out at 8.4V for a 2 serial cell charger. The chips getting hot isn't a huge issue because they are spec'd run at 80C (176F) or higher. Do you see signs of burning on the chips/board/case?

18650s are available everywhere. You could break into a Tesla... but do a search. be aware there are lots of dodgy batteries (and battery sellers).

No sign of burning on the PCB. And, as soon as green light came on, components became much cooler.

The 9v is strange. I'd checked charging voltage a couple of times before and never saw it above 7.84, though that was 1, 2, 3, and 4 hours AFTER green light came on. Not sure I'd checked immediately when it changed from red to green.

I'd looked at the data sheet--the circuit showed there is very similar to what I have.

BTW there are 4 batteries in the pack, 2 sets in parallel I presume. If one went bad, might that explain all this?

Thanks.

 

yes, one cell going bad in a 2S2P pack could explain general charge instability. Is there any additional circuitry in the battery pack?

 

yes, one cell going bad in a 2S2P pack could explain general charge instability. Is there any additional circuitry in the battery pack?

no

 

yes, one cell going bad in a 2S2P pack could explain general charge instability. Is there any additional circuitry in the battery pack?

Oops, spoke too soon: There are several components on a circuit board between the battery pack + and - terminals. Are you suspicious of them?
Thanks.

 

Not suspicious. It probably is a protection circuit that is causing the 0V reading when the pack is discharged. A Li-whatever cell is considered discharged at around 3V but can actually be discharged further. Depending on the specific chemistry, there is a point below which the battery is damaged. The protection circuit prevents that. It may also prevent overcharging so be sure to keep it in the pack if you replace the cells. Also, it could have temperature measurement that shuts down the charging process if it gets too hot. Keep an eye out for any sensors when replacing the cells.

As I said earlier, the odds are well in favor of the problem being a failing cell. Although, a poorly engineered, cheap Chinese product could be running components out of spec and cause chip failure. Still, my money is on the batteries.

If you can get measurements of the individual cells, that might point you to a bad cell. Or at least which pair has a bad cell.

 

Oops, spoke too soon: There are several components on a circuit board between the battery pack + and - terminals. Are you suspicious of them?
Thanks.

After so many laptops bursting in flames they made batteries manufacturers to add the protection to cells in series. The theory is that if a cell discharged to 0 its voltage reverses because of the voltage from the other cells in series and stays 0V forever. When the battery is charged again to the required voltage the cells don't reach the voltage and they over heat from over charge. This takes them to a thermal runaway and a fire.
The protection circuit checks each cell's voltage and when it finds a faulty cell it declares the battery as dangerous and switches it off forever. Changing one cell doesn't help because when you disconnect one cell the circuit declares the battery dangerous forever and you have to buy a new pack.
These cells connected in series without the protection are like fingers of dynamite.

 

Not suspicious. It probably is a protection circuit that is causing the 0V reading when the pack is discharged. A Li-whatever cell is considered discharged at around 3V but can actually be discharged further. Depending on the specific chemistry, there is a point below which the battery is damaged. The protection circuit prevents that. It may also prevent overcharging so be sure to keep it in the pack if you replace the cells. Also, it could have temperature measurement that shuts down the charging process if it gets too hot. Keep an eye out for any sensors when replacing the cells.

As I said earlier, the odds are well in favor of the problem being a failing cell. Although, a poorly engineered, cheap Chinese product could be running components out of spec and cause chip failure. Still, my money is on the batteries.

If you can get measurements of the individual cells, that might point you to a bad cell. Or at least which pair has a bad cell.

Hmmmm......Isolated the cells and don't see one that's obviously bad. One (paralleled) pair reads 3.3v (each) and the other 4.2v. These readings taken right after work light on long enough to 'turn off' on its own. Will it help to hook them back up, charge as much as it will, unhook, recheck each voltage? Other ideas?
Thanks.

 

They should have the same or very similar voltage.

 

They should have the same or very similar voltage.

Yes, agreed. But I'm not sure what to make of that. Was expecting/hoping to find a single cell different. Seems less likely a pair of cells would fail? I can't just assume the lower voltage pair is bad can I? Does this finding suggest perhaps that the protection circuit might be bad? May be easiest to just buy 4 really cheap cells, hook them up to this protection circuit and see how they do. If all right, then invest in better quality?

Thanks.

 

One cell is probably bad but since they are in parallel, you can't tell which. It's up to you how you want to go - no wrong answer.

 

One cell is probably bad but since they are in parallel, you can't tell which. It's up to you how you want to go - no wrong answer.

They're not in parallel--I've removed one connection on both so each pair is only connected on one end.

 

One cell is probably bad but since they are in parallel, you can't tell which. It's up to you how you want to go - no wrong answer.

I don't mind buying 4 more cheap cells to see if that fixes the problem--if that's the most straightforward approach, but I prefer being a bit more scientific, rather than just throwing parts at a problem. Are there any more tests/measurements I can make to narrow this down?

Thanks.

 

From what you've said, two cells have failed - they don't charge up (or retain a charge for long). Can't get more scientific than that.

 

From what you've said, two cells have failed - they don't charge up (or retain a charge for long). Can't get more scientific than that.

Isn't it possible the protection circuit is the problem, rather than the cells? Not easy to test that?

Thanks.