How many amps does a car battery have not cranking while the auto is on?

Basic idea is to use a car to charge a 12 V 225 Ah battery, I do have a wall charger, but it only charges at 1 A so it wold take 225 hours to fully charge, the car battery I have is rated with 110 RC and according to the internet that's 110 minutes at 25 A, but is that with the engine running?  

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I've dealt with this issue in my battery charging crusades. SLA & Li-Ion batts can use similar chargers. I have a specific one that took me a while to find locally and don't know where to find one for you but I do have a solution using some basic parts.

If you take a universal laptop ac adapter you find that they do all the volts and amps you'll need for a reasonably quick charge. These ac adapters don't always have a voltage detector to stop it from over charging. The reason for that is that the circuit is, usually, in the laptop. This isn't always the case but is common.

The next thing you'll need is a Step up/Step down voltage regulator. This 100w-150w regulator can be tuned to the voltage you want. Car batteries charge at a 14.4v-14.8v and I wouldn't go above 14.8v max without risking damaging the batt or yourself if it pops. In order for this regulator to work effectively as a battery charger you will need to apply one diode to the positive lead between the regulator and the battery. This keeps the power from flowing back into the regulator once the batt is fully charged. Now that you have a solution you can connect them to your battery in parallel and get increased amperage. Here is the list of parts you'll need to get this going:

Universal Charger:


Step-up/Step-down regulator:




**Important note**

This setup will not charge LiFePo batteries. They require specific circuitry to charge although it is worth it considering their performance.

rickharris2 years ago

I think your asking the wrong question.

Reading between the lines you want to know if you can charge a 12 volt 225Ah battery from your car engine.

The 25 Amps comes from the cars alternator.

It is quite possible that you alternator can produce 150 amps or more - BUT -

Running your car engine at a fast idle for hours to charge a battery wouldn't be economic, think of the fuel used.

I suggest your better bet is to buy a suitable charger for the battery you have.

BTW - 225Ah is an exceptionally high rate for a Sealed Lead acid battery.

Exactly !

You do not want to degas the battery...

The battery doesn't supply any current when the engine is running.


And assuming the alternator is working properly, the battery is either fully charged or charging when the engine is running.