Wonky Battery

Have had 5 batteries this summer with similar behaviour. On a tractor, a wheel loader, a lawn mower, a car and a motorcycle. Only explanation I can figure out is that the internal resistance has become so high that although voltage without  load seems enough and a smart charger indicates a fully charged battery, when under load the voltage drops so much that engine don't start. Of course more obvious on machines with electric start.

Yes, high internal resistance so volts are good but no amps hence the true battery test is a load test not just voltage.

Use a moving coil (analogue) meter.  The coil resistance will be  sufficient to draw current if its available.  A  multi-meter (Digital) has high internal resistance and will not allow current to flow but  still display a voltage level.  A test bulb is by far a better indicator of the battery status.

Jon

I had this happen on a cheapish motorcycle battery which had not had much use at all, it read at least 12V but had no power. Took it back to the supplier who gave it to his assistant to test in the workshop at the back. A voice from the back called out "It's got nowt in it!". They gave me a replacement battery of the same type and make and that one lasted OK.

The above is nominally correct, you need a load. Digital meters-do not trust them. In future read the 12.7V and while you are reading it apply a 21W stop lamp! Bingo voltage should disappear. Any testing on bikes to find a missing voltage a 2W instrument lamp a couple wires and maybe a crock clip on one of them. Do not rely on smart chargers either.

I have a number of batteries needing regular charge. sometimes I forget and a battery  refuses  to achieve full voltage even with a "smart" charger, Sometimes they need a jolt with an old fashioned  charger to get them up to a voltage for the smart charger to  even accept them.  I have a marine de-sulphator  that I attach to the tardy battery while its on a charger , can take a few days or a week but eventually the battery  voltage will climb to 14.4 volts (on charge ) and gives a usefull service again. Sometime a quick inversion is needed to mix up the acid  The marine unit gives the battery a series of rapid  high voltage shocks (low current !)  that encourages the sulphation to disperse . Sounds  a bit suss  but has worked lots of times for me. 

Read some time ago of a chap bought a dirt-cheap battery for his car. After not very long, it was too flat to start the engine. Not having a battery charger of his own, took it to a tyre & battery service station to have it recharged. The chap who took it in was surprised at how light it was, but put it on a booster charge anyway. Even more surprised when checking up on it a couple of hours later, to see smoke coming out of the filler caps.Long story short, with owners permission (it was scrap anyway) took a hacksaw to it after draining the acid and, as he had surmised, half the "lead" plates were plywood.....

I find that AGM batteries are worth the (nowadays) small extra cost. They retain their charge for an extremely long time without needing boosting as long as the vehicle does not have a current leak.

Maybe a bit off-topic. The rate that lead batteries discharge when not in use can differ considerably. Most bike batteries I have needs one or two charges during winter to be usable next year. Modern bikes with a lot of electronics often draws current when not used so they need more frequent charging. On the contrary a WD16H fitted with a cheap 6V battery from a low price stores chain, known for dubious quality batteries, stood unused for 5 years and still able to sound the horn. On some bikes fitting of a quick connector makes charging easier. My Venom needs removing the carb to get access to the battery.

Nearly all my motorcycles have easy direct (via fuse) battery connections for charging purposes, mostly DIN sockets. Exception is the Commando where I use the standard 2- pin socket.