Watch this if you think its all crud !
But don't get up set by this guys first comments.
https://www.youtube.com/watch?v=SVD1dIGcmXk
Hi AG. Before he turned on the relay his battery was sitting at 13.13V, and
at the end when he turned off the relay switching his battery was reading 13.03V.
In the video when the relay is operating, it is chattering away at a pretty high frequency, and
it is producing big switching spikes and arcing. This high frequency spikey noise it produces can make
digital voltmeters read incorrectly.
Not sure if analog voltmeters will do a better job when subjected to
this kind of spikey noise, but they might. Notice in the video how the voltage on the digital voltmeter
is sometimes reading high and sometimes reading low while the relay is running. The green sparking he is talking
is almost certainly due to the metal in the relay contacts. His contacts looked like brass or similar. I have noticed that
metal with copper in it tends to produce greenish colored sparks.
As soon as you disconnect the battery from the source of this kind of spikey noise (disconnect the relay in this case)
the digital voltmeter can show quite a different voltage reading. The guy said that if he leaves the battery sitting for a long time
its unloaded terminal voltage comes back up to around where he started from, but anyone experienced working with
batteries knows that a battery's unloaded terminal rest voltage can be quite misleading as to what the actual charge is on the battery.
In the video he was using a large capacity battery, and if his relay switching is not drawing too much current, a short run of even
an hour may not discharge the battery very much. This is why I suggested earlier on in this thread that you should probably run these
battery self charging circuits for a day or two steady, and then test to see what the charge is on the battery. A way I test the charge
on batteries is to connect my battery trickle charger to the battery and see how long it takes before the charger indicates the
battery is charged. That is maybe not completely reliable, but it seems to work fairly reliably in determining the true charge on
a battery. The longer the trickle charger takes to charge up the battery, the more the battery was discharged.
I have experimented a fair bit with driving spark gap circuits using a 12V lead acid battery, and I have seen the same thing where
the digital voltmeter doesn't show the battery voltage dropping, but as soon as I turn off the circuit the voltmeter often shows
a lower battery voltage reading. However, in some cases of shorter runs with the sparkgap circuits, after I finished, the battery
did seem to rebound back, and when I put it on the battery trickle charger it showed the battery was fully charged. This was
for shorter runs however. This is why I think you really have to do tests for longer periods of times like at least 24 hours to see
if the battery can really maintain a true charge. Those spikey high frequency high voltage waveforms produced by arcing may
actually put some sort of 'surface charge' on a battery in come cases, but it may well not hold up for much longer runs.
All the best...