# How can a car battery be charged by dynamo (or is it an alternator?) at the same time it is being used by the car components?

note and question

* Is it an alternator or dynamo?


I am not an electrical engineer and neither have any core knowledge but, this is a simple procedure of car battery charging using a dynamo, which is present in all cars and bicycles. But what i don't understand is how can a battery be utilised and charged at the same time?

* That is, head lights drawing current.
* But dynamo is giving current.
* Everything done by using same battery terminals.
* So BHOOM must be a explosion!!

• but this method, is bad right? as it charges when car running discharges when car at stand still (assuming that all components of car are in a running state). so battery charge and discharge, charge and discharge, bad bad? – user2830 Jan 31 '11 at 17:11
• Cars have alternators. Alternators need an electrical field to start making electricity (can't charge a completely dead battery) but how much power they provide doesn't vary a lot with rpm. Because of their relatively stable power output they work well in cars. Also, when you stop a car, the engine is still rotating (unless it's a hybrid) and you still generate electricity. – Andrey Feb 1 '11 at 0:00
• Have a bank of smaller batteries instead of one big battery. Better yet a bank of capacitors. – user43232 May 22 '14 at 8:32
• Older cars (e.g. pre 1970 Land Rover) have dynamos. – Pete Kirkham May 22 '14 at 10:43

In short, it can't. If the battery is being charged then current is flowing into it, so it can't be powering anything. It is the charger/dynamo/alternator that is powering the components whilst charging. In the case of a dynamo or alternator, if the output drops below the open-circuit terminal voltage of the battery, the battery takes over powering the components and is therefore no longer charging.

• but this method, is bad right? as it charges when car running discharges when car at stand still (assuming that all components of car are in a running state). so battery charge and discharge, charge and discharge, bad bad? – user2830 Jan 31 '11 at 17:10
• Ancedotal Evidence: If it was seriously detrimental to the performance of the battery, your car wouldn't be using lead acid batteries. Different chemistries of batteries behave differently. – W5VO Jan 31 '11 at 18:38
• If the battery is completely discharged repeatedly, that can eventually damage it. But in normal use, it is only partially discharged (unless you leave your headlights on all day with the engine off, or keep trying to crank an engine that won't start). – Jeanne Pindar Jan 31 '11 at 20:01
• If you ever have an alternator die, you'll find out that the battery isn't capable of supplying everything for very long... – endolith Feb 10 '11 at 18:59
• As far as I know, lead-acid batteries like being fully charged all the time. They also like slow charging. These are exactly the conditions that the comment of user2830 describes. Source: batteryuniversity.com/learn/article/… – Vorac Jul 3 '12 at 8:22

The battery is only being used when the alternator is not running. While the alternator is running (delivering power), the battery is essentially just another load like lights, radio, etc. For most of the time, the alternator is just maintaining the battery charge and thus supply little actual current into the battery.

• That's usually the case... During peak electrical loads (staring up a large fan motor or similar) at low engine idle (therefore, low alternator output) power is pulled from battery. As soon as the load goes down and or the RPM's come back up, the alternator then produces enough to meet the car's electrical needs and the battery starts getting charged again. – RQDQ Jan 31 '11 at 21:30

The same way a laptop battery can be used and be charging. And if you have more current from your dynamo than is needed to power the lights, etc. those can be powered while charging the battery.