Failing battery - what's going on?

"Multimeter reading shows 6.3v so battery is fine; fuel and spark OK; engine turns but will not start.  Why?"

Many of us will have had our Sevens tucked away for the winter, and thanks to the Covid lockdowns (2020 - 2022) some will have been in suspended animation for much longer than that. A well known casualty of long term storage is the battery, with many purchases prompted by a desire to get the Seven started without excessive handle swinging.

I had an email from a fellow Club member, who has had his battery for a few years and who took the precaution of buying a reputable brand and keeping it topped up on a trickle charger. Despite this, there is still an extreme reluctance to start, even though the starter motor turns the engine over. Having taken what seemed good care of the battery, it still failed after about six years, and he wondered why.


Battery OK 1Six years is indeed a ripe old age for a battery – not just for Sevens but moderns too. My Modern gave up doing stop start at 5 years, although leaving it on the drive during lockdown didn’t help. Then, about a year later, it started with reluctance and a message came up on the dash “ Drive for at least an hour, do not turn off the engine, try to maintain highway speeds”. Next time out, it quit.


A sure fire way of ruining a battery is to store it flat or discharged. The specific gravity of the electrolyte drops as the battery is discharged, as the sulphate in the electrolyte comes out of solution and deposits on the plates and forms a sediment in the bottom of the cell. Recharging the battery causes the sulphate to (mostly) return to the electrolyte. This incomplete dissolving of the sulphate is what causes the battery to fail over time, even if in a perfect state of charge.  



Battery OK 2


Battery OK 3


Batteries prefer to be used, and lead acid batteries have failure modes that mean that cossetting them actually can make things worse. As the battery ages, the sulphate coating builds up on the plates which inhibits the access of the electrolyte to the surface for ion exchange. How this appears electrically is that the internal resistance of the battery starts to rise. This resistance is inside the battery, and good old Ohm’s law applies – the REDUCTION of terminal voltage per ampere drawn equals the internal resistance. Thus even a tenth of an ohm will mean that fifty amps into the starter motor will cause the terminal voltage to drop by five volts, leaving you just about one volt left to fire up the ignition. 

This coating build up is speeded up when the battery has little or no current flowing through it. A “trickle” charger raises the voltage to just add a little charge (or replace the charge lost through self discharge) but at very low currents. Thus the battery, although at full electrical potential (and at the correct specific gravity of electrolyte) will appear fully charged to a voltmeter. However, as the plates are coated (“dirty”), when you draw significant current – such as for the starter motor – the voltage plummets. Being a series wound machine, the starter will carry on cranking down to just two or three volts (assuming it can get enough amps). However, the voltage left to run the ignition is way too small to get anything beyond the feeblest of sparks. Hence no go.  Battery OK 4

There’s a couple of things that can be done to delay this geriatricity in the electricity. True “Trickle” chargers provide a steady low rate of charge; “smart” chargers have a pulse mode, whereby the battery gets a short burst of higher current every once in a while. The aim of this is to raise the charge up to “gassing” levels briefly, and acts like steradent on the plates and fizzes them clean. It’s not 100% effective, but it is much better. 

 If you only have a trickler, then another good practice is occasionally, (every few weeks) disconnect the charger and turn on the headlights. Leave them on for 15 to 20 minutes to give the battery a caning. Yes it’ll go flat, but then put the trickle charger back on. Incidentally, I’ve used this trick on a nearly end of life battery in the car on a cold day – put the headlights on for five minutes and the thrashing warms up the electrolyte and pops off some of the crud. Then turn them off, pull the starter and cross the fingers. It has got me out of a hole.  Another symptom of the crudded up battery is when it’s reluctant to start for the first cranking of the day but then proceeds to behave nicely after the coffee stop, lunch, roadside fettle session etc. But it is giving you a warning that it’s time is becoming nigh.  


This article, by Geoff Hardman, originally appeared in Seven Focus, May 2022 pp 6-9

See also:

Battery Care and Maintenance

The Lead Acid Battery