Li-Ion mower batter charging problem

SiliconChip

I posted this on the Greenfingered board first in case anyone had experienced a similar issue there but the suggestion there was that it's more likely to be a technical issue so this may be a better place to get answers.

https://forums.moneysavingexpert.com/discussion/6345268/bosch-cordless-mower-battery 

To recap, the battery for a Bosch Rotak mower was completely discharged when I came to try it yesterday (no lights showing when I press the button to check the charge), and when I put it on the charger the green light blinks (indicating charging) for about 30 seconds then goes solid green again (which would normally indicate fully charged).

Any thoughts on what the cause might be and how to fix it (if it can be) are welcome.

GDB2222

Can you open it up and have a peek inside? 

[Deleted User]

Battery has probably gone below safe threshold for charging.

Normally each Li-ion cell ranges from 3.4 volt discharged to about 4.2 volt fully charged.

When they go below about 3 volt they will undergo unwanted chemical reactions that aren't reversible and will cause their internal resistance to increase, most charging devices or battery control modules will have an under-volt protection set around 2.5 - 3.0 volts at which point it will not allow charging.

Forcing a charge when it is below 2.5 volts can be dangerous so do not attempt it.

Safest thing to do is replace the battery pack with a new one.

Money saving option, which involves risk, is to dismantle, test each of the cells individually and then decide what to do. You can probably replace individual cells in a Bosch power pack but again some risk involved but will be the cheapest option.

To avoid this, always store Li-Ion cells partially charged (long term), about 40% is ideal but anything up to 80% is ok. When they are empty, always recharge and never store in a discharged state.

SiliconChip

GDB2222 said:

Can you open it up and have a peek inside? 

I don't think I'll be doing that, and I'm not sure what there would be to see anyway.

SiliconChip

[Deleted User] said:

Battery has probably gone below safe threshold for charging.

Normally each Li-ion cell ranges from 3.4 volt discharged to about 4.2 volt fully charged.

When they go below about 3 volt they will undergo unwanted chemical reactions that aren't reversible and will cause their internal resistance to increase, most charging devices or battery control modules will have an under-volt protection set around 2.5 - 3.0 volts at which point it will not allow charging.

Forcing a charge when it is below 2.5 volts can be dangerous so do not attempt it.

Safest thing to do is replace the battery pack with a new one.

Money saving option, which involves risk, is to dismantle, test each of the cells individually and then decide what to do. You can probably replace individual cells in a Bosch power pack but again some risk involved but will be the cheapest option.

To avoid this, always store Li-Ion cells partially charged (long term), about 40% is ideal but anything up to 80% is ok. When they are empty, always recharge and never store in a discharged state.

This seems to me likely to be at least part of the answer, I'm unsure how much charge it had when it was last used before being put away for the winter but it's likely that it was partly discharged. It does appear that the charger is deciding not to continue to attempt to charge the battery after a short time so it probably is goosed. The question now is whether to buy a new battery (ridiculously expensive from Bosch, over £200, pretty expensive from third parties at around £100) or just buy a new mower with warranty and the knowledge that the whole package will have a decent length of life, cost £200 - £250.

GDB2222

SiliconChip said:

GDB2222 said:

Can you open it up and have a peek inside? 

I don't think I'll be doing that, and I'm not sure what there would be to see anyway.

You could test the individual cells, etc.  

These batteries are more expensive than the equivalent Bosch drill batteries, allowing for the voltage difference. Anyone know why?

matelodave

GDB2222 said:

SiliconChip said:

GDB2222 said:

Can you open it up and have a peek inside? 

I don't think I'll be doing that, and I'm not sure what there would be to see anyway.

You could test the individual cells, etc.  

These batteries are more expensive than the equivalent Bosch drill batteries, allowing for the voltage difference. Anyone know why?

because they can

[Deleted User]

Personally I’d be repairing the battery, depending on history of course. If the battery relatively new and only had a few dozen charge cycles, it is probably just one or 2 cells that have failed.

They contain 20 of the 18650 batteries that can be bought for a few quid each, just need to match the spec in terms of capacity and max discharge current. 

Stripping it down will take minutes, testing the batteries properly will take a bit longer. A basic volt meter test on each battery should reveal the dud cell but you can discharge / charge each one individually with a digital charger to check the capacity and replace any that are significantly below spec. 

Even if you replace all 20 batteries it will still work out cheaper than buying new.

The risk comes from having a big imbalance of capacity on the cells which could cause individual ones to over heat during charge / discharge but with a bit of research and a few you tube videos you should be able to understand how to avoid any issues. 

GDB2222

How Are the individual cells joined to each other? Some sort of hard solder? Welds?

[Deleted User]

GDB2222 said:

How Are the individual cells joined to each other? Some sort of hard solder? Welds?

Spot welds, looks easy enough to do. You can get a spot welder for about £15 for a basic one and a strip of tabs for about £5 or buy 18650's with tabs already on them.

https://www.youtube.com/watch?v=PVaWW2TzMVk

But if all batteries need replacing, it's getting over the £100 mark at £5 a battery for decent ones so buying a replacement form a third party might be more economical depending on the capacity.

getmore4less

if buying a new one look for a product where you can battery share.

I have gone for the Worx range for power and garden tools.but not go the mower yet.

they do a mower strimmer pack that is often £200 

WORX WG927E

homebase have stock
https://www.homebase.co.uk/worx-wg927e-cordless-34cm-lawn-mower-trimmer-twin-pack/13280772.html