Lawnmower Battery mystery

[LittleJoe]

Hi,
I have a mystery with my lawnmower Battery which I am hoping someone can clarify for me please. I have had a Hyundai Petrol mower for a few years with an electric start from a 12 volt 5Ah sealed lead acid battery. It has worked perfectly and I have always put the battery on charge using the supplied charger. Started every time over the years until I got it out yesterday and instead of turning the motor I just got some clicks and after a few tries nothing. Suspecting the obvious - is the battery flat? Took the battery out and it measured 13 volts so seemed to be fully charged. I have a stand alone Motorcycle auto battery charger so I thought I would put it on charge for a while. After connecting the charger to the battery it showed that the voltage was 14.4v and fully charged !!!?? I connected a 12v bulb across it which lit up brightly. I then did something that I would never normally do but I touched a heavy current cable across the negative and positive terminals briefly and there was no spark at all !!!!! This I do not understand?? Before I think about buying a new battery I thought I would get some comments hopefully from this Forum.
The actual reason that the lawnmower won't turn the engine could be for other reasons such as faulty solenoid, microswitches or starter motor yet to be determined but I need to try and understand the battery mystery first. Anyone got any ideas please?

 

[Jurassicspark]

A battery can fail even with it reading the required voltage. The cells may have gone bad and cannot supply enough current as it used to. You need to get a battery load tester or test with a few resistors.

. I then did something that I would never normally do but I touched a heavy current cable across the negative and positive terminals briefly and there was no spark at all !!!!! This I do not understand?? Before I think about buying a new battery I thought I would get some comments hopefully from this Forum.

A bad battery by the looks of it but not a valid test. The battery might have protection that limits the current on a direct short.

 

[jj4091]

Take it to your local garage and ask them to test it for you.

 

[robinbanks]

Get 8 AA batteries and tape them together, so + on one touches - on the next, in one big long line. You now have a 12v battery that will make a car bulb shine brightly, but when you short it there will only be a tiny spark, if at all. It won't start a mower either

Just because you have 12v doesn't mean you have a battery that can flow a large current; indeed your mower battery can flow much less than your diesel car battery. The 8 AA batteries have a much higher internal resistance and this limits current flow even though collectively they develop 12v across that resistance

Your battery charger will only really tell you how many volts a battery is pushing back with, against the voltage it's using to try and push current into the battery. Voltage alone is not an indicator of battery health. Lead acid batteries can "corrode" inside which limits their capacity to store charge, but the fragments that remain can still work collectively; they way they work is by having plates of lead in sulphuric acid. When the batteries powering something the sulphur from the sulphuric acid starts sticking to the plates, the surface of which slowly begins to turn to lead sulphate as the acid is used up. A charger pushes the sulphur back, recharging the battery. Now imagine you damage the plates by reducing their surface area, or you permanently stick the sulphur to the plate in 95% of the area; this massively diminishes the amount of current the battery can supply even though the 5% of area that is still left can develop a similar voltage, it just doesn't have the same amount of surface area to have the same level of sulphur-sticking-to-plates going on, which is what generates the current

Now, it could also be that the terminals of your lead acid battery have corroded and that's where a large amount of current-limited (big spark limiting) resistance is coming from; a better test here of whether the battery is goosed would be to:

* grasp the mower blades and check you can turn them by hand/the engine isn't seized or hydraulic locked due to excess oil in the cylinder (happens when people hang 4 stroke lowers on a wall)
* let go the mower blades after youre happy the motor does rotate
* clean the battery terminals
* connect the battery up
* either connect your charger or a multimeter so you have a reading of the battery voltage
* attempt to start the lower and watch the voltage
* if it drops significantly, it's highly likely your battery is goosed

When buying another, you might consider a lithium ion one; they can recover from very deep discharges (down to 20%) without ill effect whereas with a lead acid cell, discharging them below 80% starts to stick the sulphur permanently to the plates, ruining their capacity pretty much irreversibly. Lithium ion are a better choice for batteries that will have a seasonal usage pattern like a mower

 

[ericmark]

When my 12 Ah jump start battery pack failed, it would still run the light, but not the tyre inflator, so it does seem likely the AGM or VRLA battery has failed.