Rescuing an 80 volt battery...

[julianthegypsy]

We have a Mountfield battery lawnmower. Sat for a few months and the battery is dead. Red light flashes on the charger. Put a meter across it and it has 62 volts in it. It seems the problem is that the voltage is too low for the charger to recognise it, so I need to get some charge into it to get it to the point where the charger will take over. The usual solution is to hook it to a good battery for ten minutes...but they are £300 a pop and I don't need two. Anyone got any bright ideas?

 

[Robotstar5]

Could you put a 12 volt car battery in series with it? would show the charger 74 volts.

 

[Nomad]

Is it a sealed unit or is its string of cells with connection points?

If you can see the individual cells, try to find a group that are low and use a 12v charger on them?
Repeat for the other groups until the 80v charger can take over?
The bms should balance the cells after a few charge / discharge cycles.

 

[finyuk2005]

what type of battery is it? lead acid or lithium of some kind?
if its lead you could use a dumb charger to bumb the voltage up enough to fool the electronic charger to kick in,
if its lithium of any kind the BMS might be stopping play

 

[julianthegypsy]

what type of battery is it? lead acid or lithium of some kind?
if its lead you could use a dumb charger to bumb the voltage up enough to fool the electronic charger to kick in,
if its lithium of any kind the BMS might be stopping play

I haven't opened it up yet. Photo attached. @mods, thanks for moving it, I wasn't sure if it should be I. Brew room or electrics!
As far as I know it's strings of 18650s, if that's the one I'm thinking of, a bit bigger than a AA battery. I did wonder about finding groups and charging them individually but I'm not sure where to begin!

 

[J1nx]

9x PP3’s connected in series? Can get a pack of ten for not much over a tenner online. Clip together easily.

 

[Keithmac]

Just go carefully as it's not good when a Lithium battery goes up!, that's main function of the BMS really, to prevent fire..

If the cells are way below the minimum charge threshold they won't perform very well even if you do manage to get some charge into the pack.

 

[tigdlo]

The usual solution is to hook it to a good battery for ten minutes

How do you limit the current? This may not always be the best idea, if the batteries don't have internal over current protection it is also potentially dangerous.

A CC/CV bench power supply is a handy thing to have for these king of situations, I have a £35 chinese thing, works well enough for me.

Take photos of the inside of the battery and try to measure the voltage of the individual cells to see if you have any low ones.

 

[knighty]

open it up, if it is a bunch of 18650's (and it probably is)

it'll probably be something like 22 series packs of 3 cells in parallel... (depends how good the cells are and how cheap they were making them, when they were made etc... more modern cells would only need 3 in parallel)

anyway... check the voltage of each parallel pack to see if you have any dead cells

if not, and they're all just low you could use a dumb car charger to charge up 3 parallel packs at once - don't leave it on for ages, 3 in series is about 12.6v max

keep checking them, put it somewhere safe when you do it, in a big old metal pot/tray or something like that

careful not to short out any of the cells or to put your charger on backwards!

it you take it apart and post photos I'm happy to give better directions :-)

 

[julianthegypsy]

open it up, if it is a bunch of 18650's (and it probably is)

it'll probably be something like 22 series packs of 3 cells in parallel... (depends how good the cells are and how cheap they were making them, when they were made etc... more modern cells would only need 3 in parallel)

anyway... check the voltage of each parallel pack to see if you have any dead cells

if not, and they're all just low you could use a dumb car charger to charge up 3 parallel packs at once - don't leave it on for ages, 3 in series is about 12.6v max

keep checking them, put it somewhere safe when you do it, in a big old metal pot/tray or something like that

careful not to short out any of the cells or to put your charger on backwards!

it you take it apart and post photos I'm happy to give better directions :-)

Thanks, will do.

 

[julianthegypsy]

How do you limit the current? This may not always be the best idea, if the batteries don't have internal over current protection it is also potentially dangerous.

A CC/CV bench power supply is a handy thing to have for these king of situations, I have a £35 chinese thing, works well enough for me.

Take photos of the inside of the battery and try to measure the voltage of the individual cells to see if you have any low ones.

I have a bench power supply, but it doesn't go anywhere near 80 volts. It would work for charging the parallel packs one at a time though.

 

[J1nx]

Doesn’t get much easier

 

[Turbo]

If it has 80v it could be 20 18650's in series, depending on what the manufacturer quotes as the nominal voltage of each cell *.

If its reading 62 volts now that means each cell is down to 3.1 volts. Normal working range of a 18650 is 3.7 to 4.1 volts, common minimum cutoff on many chargers is 3.3 volts.

The good news is that you should be able to save this battery as they are usually ok down to 2.9-3 volts. If they go below 2.5v they are generally toast!

I would not recommend that you use a 12v battery charger as they are capable of dumping a few amps of charge across a lead acid battery, not a good idea on a li-ion unless you like explosions!

Its best to trickle charge them slowly using a few hundred milliamps so a current limited bench power supply is a good suggestion.

What I use is a mobile phone charger with a pair of crocodile clips fitted, clearly marked + & -. Conveniently it puts out 3.7v max and its only 150ma so it can't overcharge the cell unless you're a Norbert!
I leave it on the cell, checking the voltage every 5 minutes. Once the cell gets to 3.3v leave it another 5 minutes then remove it. Continue checking the cell voltage to see if it drops quickly, if it does its not a good sign.

I have rescued quite a few li-ion battery packs using this method, its slow but fairly safe. All you need is to get the voltage high enough to let the proper charger do its thing.


* Most power tool makers quote a 5 cell battery as 18v = 5 x 3.6v, the nominal voltage of each cell.
Others like Lidl use the charged voltage of each cell, 5 x 4.1v = 20.5v, a sneaky way to make their tools sound better than most others!

 

[knighty]

I charged up hundreds of 18650 cells from a 40amp power supply set to 4.2v

if a cell tries to charge at crazy amps it's dead anyway

a normal car battery charger will be fine for a short supervised charge while you pull the cell voltage up a bit

 

[Pete.]

If it's a lithium battery be bloody careful. I tried to revive a 40v one which I thought ws discharged by measuring cell voltage and applying a charge from a normal battery charger. Turned out it had physical damage on the cell-cell connections and I gave up, left it on my bench. A couple of weeks later I went back out to the workshop, cleared some stuff off my bench and found the thing pumped up like a football. I'll never know how close it came to destroying my entire workshop and machine collection, but I bet it wasn't far off.

 

[Kram]

Extreme care required, first I'd open up and check each cell, and the type of cells. Then I'd likely use an rc lithium type charger to balance charge 6 cells at a time. May aswell do a full charge too.

Be aware that lithium tech now has cells with a nominal of 3.85v and a max of 4.4v, and I have some of these, they are out in the market to be aware of, tho unlikely on a power tool.

RC chargers are common and cheap on ebay/amazon, they take care to use, and should not be used unattended, but its the best tool for the job. Generally they wont do more than 6 cells series at a time tho.

 

[julianthegypsy]

Update! Took it to the repair cafe this morning as I wasn't quite confident enough to start trying to charge individual clusters in case I blew it, literally. Anyway, it had eighty batteries in groups of 2 in parallel and 2 in series.
Thirty of them were fully charged at 3.9 volts and 10 were dead as doornails at about 0.2 volts. Peeled all the rubber off etc and charged the five pairs from a bench power supply, limiting the current, and we have success. The guy doing it knew a lot about electronics but had never seen one. He was reluctant to pull it apart but I wasn't and he was more confident than I about charging it. Result! Battery got up to 75 volts and the charger accepted it and it's now fully charged. A morning well spent.

 

[Anguz]

as an electrician i would rather mess around with large vsd's with 690v feeds than a battery pack like that. they scare me

 

[julianthegypsy]

as an electrician i would rather mess around with large vsd's with 690v feeds than a battery pack like that. they scare me

80 volts dc is kind of scary!

 

[Kram]

Nice to see success

volts and the charger accepted it and it's now fully charged. A morning well spent.

Please note the charger accepting it, does not make it safe, you essentially bypassed a safety feature. Extreme care required, keep it quarantined while charging and after charging in a fire proof place.

It is likely the bad cells will discharge quicker or result in the pack becoming out of balance. I would be giving it some use and checking again in a few weeks time.

Example, a waterproof outdoor container, filled with sand, well away from building or vegetation. If it goes up in the night, no lasting damage will be caused.