May I add my two-pennyworth !
Assuming the confirmed health/efficiency of the starter motor I would first of all ensure all ignition components are fit, then check the actual ignition timing : a couple of degrees retarded and sluggish turning over and a reluctance to fire could be mistakenly blamed on the battery.
The traditional hand-held "Load" tester comprises a heavy metal spring or coil resistor mounted between two "Prongs" with insulated handles between which a voltmeter is also fixed. Pressing the two probes onto the battery terminals simulates the load that the starter motor imposes, the voltmeter indicating the reduced power available to energise the ignition circuit.
Two points about the load tester : The voltmeters are usually of a " Centre Zero " type, the needle swinging away from the prong on the positive pole of the battery ( Handy to check terminal polarity !) and most "Load" or "Drop" ( ie Voltage Drop" ) testers are stated to be suitable for use on batteries up to SIXTY ampere/hour capacity, lower than the capacities of six-volt batteries for VW's ! Accordingly, a low or poor test on a 6-volt VW battery is more-ominous than a similar reading on a smaller six-volt. This type of tester should only be applied for a similar time to that during which you'd expect the vehicle to have started.If the voltage reading continues to "Sink" after three or four seconds on a fully-charged battery then watch for a cell to "Short" or "Gas" ( And Smell !)
The best check for a battery's state of charge is to use a small volume/capacity hydrometer
which indicates the acid's specific gravity ( Small hydrometer because there's not too much acid in each cell from which to draw a test sample!). Avoid contamination by trying not to transfer residual amounts of acid from one cell to another.
At normal room temperatures ( 20 C to 25 C ) a healthy gravity between 1.250 and 1.280 is ideal. Readings between 1.200 and 1.240 indicate partial discharge and a charge-up needed but lower than 1.200 point to a flat battery and longer charge needed. Some hydrometers have colour-coded zones indication acid strength ( High, Medium or Low)
A battery will give a false load-test result if the gravity is below 1.240. If you have a variable charge RATE ( Not 6 or 12 volt ) charger ( ie High or Low settings ) it is advisable, especially in cold weather, to commence charge at a low amperage
There being only three cells to compare on a six-volt battery, variations in gravities are harder to detect than on six-cell 12-volts. Variation between cells of 0.1 s.g. or more after charging are usually indicative of a suspect cell which will "Gas" visibly when load-tested. After charging and prior to load test, remove the three cell filler/vent caps and blow across the battery lid to dissipate the explosive hydrogen given off in the charge process.
Always abide by the golden rule to prevent sparking an explosion ! Starting charge : Crocodile clips on to battery then switch on charger. When finished : Switch OFF charger then remove crocodile clips ! No unexpected sparking nor loud, corrosive bangs !
Off on a tangent, many modern sealed batteries boast a "Magic Eye" charge indicator. This is a simple optical float hydrometer device calibrated to float green when OK, usually yellow when part discharged and clear with a red circle or plain black when discharged. Bear in mind that this device only indicates the state of charge of that ONE cell, not the whole battery.
It was a widely accepted " rule of thumb" in the days of only moulded hard rubber batteries that for every degree centigrade that the temperature drops below freezing point, batteries lose Five per cent efficiency. Minus five degrees robs a battery of a quarter of its power potential, 60 amp/hr down to 45 and pro-rata onwards, minus ten equals half power !.
I personally have revived batteries from motorway-stranded lorries so flat that the electrolyte froze ! (It had reverted to water !). FLAT is not always FAULTY ! But there again, you can recharge a flat one but can't re-incarnate the dead !
In an emergency to get started in very cold spells you might consider taking the battery indoors for a warm ! Standing a cold battery in a bowl of warm and then progressively warmer water works wonders. I would not, however, recommend standing your battery on the AGA nor in front of your log fire - A spell in hot water within an inch of the lid or deck and an occasional agitation enabling thorough warming of the acid works wonders !
Hope my contribution is of some help . John aka Jollychuffy