Not VMX related but an electric question

[Doc]

Picture this...a 12v car battery, it worked fine in the morning after a charge the night before. Drove around all day like a beauty then last night after it sat for about 5 hours I hopped in to drive to the shop and the dashlights were nothing but a feint glow I pulled out the battery and connected it to a charger and the needle went straight of the dial at over 8amps and the unit begins to cycle on and off  bugger me the chargers broke so I dig out the spare connect it up and turn it on and it does exactly the same thing  what's going on?? could an earth in the battery cause this and how did it happen..I'm at a loss as I have never seen this before 

[cyclegod]

Over charging can cause the internal plates to warp and short out (a 12 volt battery is normally six 2 volt cells connected in series internally) using a hygrometer to measure the specific gravity of each cell will reveal which is at fault. Lead/acid batteries should be charged at no more than 5% of the stated ampere/hour rating of the battery. When fully charged a 12v car battery should read 13.8 volts free standing (unconnected) for at least 1 hour after charging. Most chargers have an over-current protection built in using (typically) a positive temperature coefficient current limiter, when it gets hot with too much current its resistance increases.

[Doc]

okies, thanks cyclegod, I have a feeling this problem is an overcharge in which case I'll have to do some testing to see what's at fault. Hopefully something as simple as the reg (if it's not built into the alternator that is) it could be the batteries too, all of them (3x)are $36 supercheap items and they are all about 3-4 year old but it seems to coincidental that they would all stop holding a charge at much the same time. It's only the one that causes the cycling but coincidence again says, up till this point it was my best battery. Better go dig out the manuel and get some batteries for my pocket tester   

edit: I just tried to charge the sus battery again after I left it sit overnight and now it is taking a charge at around 3.75v    wha tha? I noticed yesterday when I disconnected the battery while the motor was running it removed a little drag on the engine and it idled a few rpm higher..it was as if the battery was dead flat and trying to charge but infact it was fully charged..buggered if I know..I'm off to hack some bushes so I'll get stuck into it later this arvo..did I ever mention I hate cars! 

oh and incase you are wondering,
1 battery is for the mower starter (total loss system but a charge lasts for months)
1 battery for the shitbox van
1 battery for a spare cause ya just never know when shite like this will hit the fan 

[cyclegod]

A dead flat battery (less than 8 volts) should trickle charge until it gets to about 10.6 volts then switch to full charging current before switching back to trickle when the battery is about 13.5 volts. Most older style lead/acid battery (not sealed maintainence free) last about 3 years before starting to give up the ghost, mineral deposits form on the plates usually from inpurities in the water added to each cell (USE DISTILLED WATER ONLY!!!) reducing the effective area for ions to travel between the plates and ultimately reducing the capacity of the battery.

[mx250]

Doc, the behaviour of your batt is pretty normal in my experience. Works fine then kaput; takes a charge and won't hold a charge.

I've known a wet cell batt to last 15 years, but it was pampered. What 'kills' them is overcharging/overheating, lack of water and virbration. And it pretty hard to protect a batt absolute from all of these in operation.

18mths marginal, 3 years is good, 5 years excellent, 15 years improbable.

[Doc]

yeah Graeme, I am probably best served by going and buying new batteries and take it from there. 3 years is good especially as they were el' cheapo's. I've had batteries go before on lots of occassions but never had one spin the charger off it's tits like this one did when I went to charge it. My guess is along the lines of cyclegods comments, I feel a warped plate may be the issue as I touch the 2 terminals of the charger together (which 1 shouldn't) and it gives the exact same on/off cycling result. I'm thinking the battery when warm is expanded and the internal plates may touch but when it cools it contracts and the plates separate enough to trickle charge again but in the meantime when the plates were touching is when it was discharging itself. This is my theory now everything else seems to test out ok. Clear as mud!  I thought so..