I've been researching charging flat batteries and I think, if I've understood correctly:
Trickle charging isn't always possible from flat, because:
- Some trickle chargers don't charge at all unless they detect a voltage close to 12v, and completely flat batteries will tend to be lower than this
- Sometimes sulfur build up inside the flat battery prevents charging, and needs high voltages to beat
However, if it is possible, slow charging or trickle charging appears to be preferred over:
- jump starting, which apparently can damage the alternator
- regular charging, which I think can strain the battery if done from flat? I might have misunderstood this
How can I test if a particular charger is charging a particular battery?
- Regarding minimum voltage, I can't find any manufacturer's info, but connecting the charger to a multi meter gives a reading of 19V. I'd imagine if it was the type to require 12V in the battery it would show 0V in the multimeter? (Not sure why it's 19V not 12V though [update] it's because its open circuit voltage is much higher than its voltage under load)
- I have no idea where to start regarding checking if the sulfur issue might be preventing charging.
- I tested the battery before connecting the trickle charger, and it gave a reading of 2 volts, which I believe is typical of a completely flat battery (we're talking "door and window switches don't respond, turning ignition makes no sound whatsoever" level of flat)
- The charger itself gives no information at all.
Charger is a Nikkai Power 2.4W solar charger, so I'm expecting a very slow charge even if everything works. I'd like to know how to find out if it's doing nothing before having waited several days!
Update: one day later, I checked the battery voltage again and it had increased from around 2V to 3.8V. So, it is charging (and yes, it is extremely slow - this thing is more commonly used as a maintainer than a charger).
I also replaced my crappy multimeter than can't measure DC amps and can only measure DC volts to the nearest whole volt (why do they even make these?!?) with a better multimeter that does decimals and DC amps. The amp reading for the battery after a day's charging is (unless I'm reading it wrong) 40 mA. This seems insanely low - certainly explains why the car is unable to do anything, even central locking. When connected in series with the charger, the amp reading stayed pretty constant, when connected on its own with the charger disconnected, the amp reading was dropping quite fast (losing something like 0.4 mA a second). This also looks like additional confirmation that the charger is working. [update] Don't measure the amp reading of the battery without the charger like I did!