18v battery repair

[Sharonjh]

Hi all, I have a 18v cordless leaf blower which works for about 15 seconds then turns off. Battery is fully charged and if I leave it for 10 min and turn it back on It goes for another 15 seconds then turns off. Assuming it must be a problem with the circuit board inside the battery? Any ideas?

 

[alec_t]

One or more cells of the battery could be dead. Check with a multi-meter.

 

[Sharonjh]

each of the cells are giving a reading of 3.3v.

 

[jpanhalt]

How long was the pack off charger before you tested it? Have you measured the voltages after a few hours?

 

[debe]

I have a similar board to what is on that battery pack, but its 12v. Those boards eqilize the charge to each cell. But it can also shut down the battery pack electronicly if too mutch current is drawn. Check the current draw of the blower.

 

[Sharonjh]

It's been over a day since being on the charger when I tested it. The blower works fine with another battery.

 

[Colin]

You have use the blower and THEN immediately check the voltage across each cell.

 

[Sharonjh]

I did this and now the readings i'm getting for the 5 cells are 3.17, 3.25, 3.28, 3.18, 3.23

 

[Colin]

Connect your voltmeter across one cell and try again. Try all 5. See if the voltage dips during use.
It might be one cell dropping below a detection voltage.
Also find out the actual voltage of each cell when they are NEW. Some cells are up to 3.7v
It could be the cells are 3.6v to 3.7v and when they are 3.17, the circuit "trips"
Put another cell across the 3.17 and try again.

 

[Sharonjh]

Yes that is it. When under load the voltage of one cell drops very low. Guess its irreparable then?

 

[Colin]

No. Simply change the cell for a $2.00 cell on eBay. But don't get the 6,000mA-Hr cells because they are really only 1A-Hr

 

[Sharonjh]

Great, thank you to everyone for their help.

 

[jpanhalt]

Changing a cell may not be all that easy. They are spot welded. Presumably you do not have access to a correct spot welder for that. If you do have access, it is easier than even soldering.

Soldering is another option but there are precautions. Soldering NiCd and NiMH is not difficult, but soldering Li-chemistry is more tricky. You will have to be very quick and not put much heat into the cell.

 

[Sharonjh]

No access to a spot welder. I'll google it and see if its worth my while getting one.
Don't want to risk blowing the battery up.

 

[jpanhalt]

There are some expensive commercial welders and numerous DIY ones described on the internet. Here is one version I can recommend:
https://www.philpem.me.uk/elec/welder/
I made a modified one about 10 years ago. If this is a one or two time deal, just buy a new battery pack or try repairing with solder. If you are quick (hot iron, large tip, thin solder, good flux) it may work, and if it doesn't, what have you lost?

If you want to get a welder, "tatus1969" has made one available as a kickstarter. Here are links to it:
Buy it here:
**broken link removed**
Development and discussion:
https://www.eevblog.com/forum/projects/guesses-on-what-i-am-attempting-here/msg1232857/#msg1232857

I like his design but am a little skeptical of his choice of power source (LiPo batteries). A selected brand and model has worked for demonstration. But if you look carefully at his later pictures, even those cells have "puffed" a little. Again, that battery pack has probably been through a lot more than you are likely to put it through.

For a full DIY capacitor-discharge version, I suspect you could do it for less than half of the kichstarter price, but you will be spending time to get it to work. Been there done that. However in my case, I was relatively frequently making up battery packs for RC models, and tatus1969's version was not available.

John

 

[Sharonjh]

Thanks for the info. Watched the videos of it in use on kicksurfer. Does look easy to use, however as you said don't know if it's worth the expense for a one off use. Might give the soldering a go. As long as it wont explode i'm happy to give it a try.

 

[jpanhalt]

The major danger with lithium cells that I am aware of is when charging. Solder to the nickle (?) strip. Charge, and see if it keeps the charge. The soldered cell will be a little warmer than the rest immediately after soldering. If it continues to get warmer, that is a bad sign. Let everything cool, then charge and see how it works.

When I have any doubt, I charge cells on my concrete driveway -- definitively not on a wood floor in the house. If it catches on fire, just let it burn.

 

[Sharonjh]

Does it matter if you use a different mAh than the other 4 cells. The current one is 1300mAh and i can only find replacements with a higher mAh.

 

[jpanhalt]

Do you know the chemistry? Li-ion, LiFePO4 ("LIFE") or something else? That info may be in the owner's manual. I would first match the chemistry, then the capacity. If necessary, go to a higher capacity, but same physical size.

 

[Sharonjh]

its a li-ion EAST 3.6v 18650s. Looks like in Aus its going to cost me $20 for a replacement battery. Not sure if it's worth it.