How long do car batteries last usually

No charge in the battery or dirty posts... smart chargers will not charge a completely dead battery. Also check the fluid level if u have an older batter that has screw on tops. Fill any low screw down top with distilled water only. No tap water .

If the battery is not maintenance free, check the water levels in each cell. If you need to add water be sure to use distilled. Never use tap water unless an absolute necessity to get the vehicle running as it will destroy the battery.
Make sure the charger is set to 12v. Some chargers will have a 6v setting that typically are used in golf carts, boats, off grid solar.
Check the terminals on the clamps for corrosion.
Be certain that the clamps are tight on the battery posts.
If everything checks out, get a new charger.

7 or 8 won't help you, you need 12.5+ volts to start an engine.

If the battery is dead because of age then it's done for. If it's dead beacuse something was left on, stereo, lights, etc. Then yes, jump starting the car will allow the car's alternator to recharge the battery.

The portable charger should NOTE be connected to a battery while the battery is connected to a vehicle! Charge the battery off the car, then disconnect the charger and put the battery into the vehicle to start it! If the charger is not working check the fuse! If that is ok then probably damaged the circuitry!

It is best to "trickle charge" a low/dead car battery using the lowest setting. The "jump start" function will probably not have enough juice to start your car by itself. Refer to the manual of your charger for safety precautions.

It sounds very battery related like the battery is on the way out, but start looking at the easy things first.

Clean the top of the battery and the terminals with a solution of hot water mixed with a tablespoon of bicarbonate of soda.
This will neutralize the acid.

Rinse with clean water and dry the area.

If your radio has a security code obtain this first before proceeding with the next steps.

Remove the negative battery terminal (black) and thoroughly clean the battery post with sandpaper or wire brush, and then the terminal itself.

Repeat the same for the positive terminal (red).

Be careful not to join a circuit between the two terminals with a metal object.
NEVER test a battery's level of charge by jumping the terminals together with a screwdriver or similar to see if it sparks, it will mostly like ignite the Hydrogen gas in the battery and explode.

Check the electrolyte levels in the battery if this can be checked, and top up with distilled water if necessary.

Charge the battery overnight with a borrowed or bought battery charger.

If you have a voltmeter check the voltage with the battery fully charged and disconnected from the battery charger, it should be at least 12 volts DC.

Start the car and check the voltage again, it should be greater than 12 volts, usually around 13 volts DC or a little higher.

If the issue still occurs after this take the car to an auto shop that has a decent battery analysis machine, and they will also be able to test if the alternator is outputting the correct voltage and the regulator.

try tightening battery terminal or remove and clean, check for loose connection, refit connector and tighten properly.

try cleaning the battery cables at the battery post, this will cause this trouble. If this doesn't work then it probably the starter

lead acid (auto) type batteries cannot be left in a discharged state very long without damage ...if they have been discharged awhile then they will recharge but at greatly refuced capacity ... sometimes they can be recovered but usually its perminant damage .. even if the batteries can be saved it would still take several hours to recharge ...so leaving it on a charger overnight will show you if its a lost cause or not ...you can have the batteries "load tested" usuallly at auto parts stores for free .....they should be fully charged first to save time ...lead acid batteries should always be kept in a state of full charge ...never store them in a discharged condition or allow them to selff discharge (a discharged battery will freeze over the winter) ... charging once a month should keep them healthy .. more often in a car since the computer draws current all the time .. if you have a regulated charger then you can leave it connected all the time for storage ...high resistance battery connectios will have the same effect .. so remove and clean the battery terminals to eliminagte that possibility ... you can have your alternator tested at most auto parts stores ..usually free .. if you have maintainable batteries then check the water level . dont over fill .. use distilled water ...if your charging systems are ok then it sounds like you need new batteries ...

Yes to both questions. the charger must sense 41.5 volts to activate the relay inside the charger. Being a repair shop, I have a charger that is rigged to charge even DEAD batteries. I do not recommend this for the novice. A charger set on 12 volts will bring the batteries back up to 8.2-8.6 volts. A cheap trickle charger may not put out enough to charge a battery of this size. Remember that they are used to MAINTAIN a charge. Connect the Voltmeter and watch for voltage rise. If it rises to 8.1 or above, it is charging. Trojan battery company recommends charging at least once a month.