I am working on a project that requires 12VDC and at least 20 Amps - for maybe 20 seconds, three times a day. The idea of using a used car battery occurred to me. If I look to by a used one, how should I test it ?
If I go this route, would it be fine to trickle charge it continuously - maybe connecting a power supply set to about ~13 or 14 V ? Would it be ok to just put a diode in series with the + lead so as to prevent battery current from flowing back into the power supply ?
Don't rely on a 'voltage' to trickle charge a battery.
Batteries are charged with a 'current', now... the relationship between the two is a direct relationship... so a higher voltage will be a higher current... but if the batteries voltage is too low, then the voltage you connect to it would result in a higher than desired current flowing.
That said, it is perfectly safe to set, and leave a trickle charger on a Lead Acid battery for prolonged periods of time.
A diode would be helpful... but only when the power supply voltage is lower than the battery and this could happen in one of two ways:
Someone unplugs the power supply (oops!)
Or whatever you are powering with the Battery feeds back with a voltage spike.
As far as testing is concerned... It may be best if you can find a family or friend in a car shop. They have a battery tester that will load the battery to see how it responds.
If you test it yourself, you will need to actually pull 10-20A out of it while you measure the voltage. If the voltage drops too low, the battery is no good. Shop are already setup with this.. See if you can call in a favour.
