22 replies to this topic

[rodomo]

My understanding is that regulators "regulate" the charge rate depending on the charge/condition of the battery. A reg. charging a "flat" battery will charge at a higher voltage until the battery starts to charge. The reg. then adjusts to charge at a lower rate and continues to "regulate" the charge rate until the battery if fully charged to whatever the reg. is "set" to. I have seen late cars e.g. 2001 Rover, charge at 15+ volts initially and drop below 15v within 20 secs running and continue to decline as the battery charges. I have seen alternators continually pump out more than 15v feeding a shorted battery. Disconnect that battery and have the same alternator charge a good battery and it calms down to around 13.8ish instantly. Stop the engine and the static charge is usually around 12.6ish even though the alt. is pumping 13+ These comments are based on practical observations.

Edited by rodomo, 28 June 2006 - 01:52 AM.

[Dr Terry]

Hi devils.

My understanding of the alternator/battery relationship is very similar to yours.

The part I always have trouble undestanding is, if the voltage regulator, 'regulates' or 'limits' the peak output of the alternator, why does the voltage go skyward (up to & over 20V) if the battery is disconnected whilst the motor is running.

Dr Terry.

[Dr Terry]

Hi devils.

This is a common problem I've seen over many years on many cars.

The usual scenario is when a guy doesn't have a battery fitted to the car, for whatever reason, then jump starts the car using another car, disconnects the jumper leads & then attempts to drive the car home. By the time he gets home he has blown several globes & maybe even damaged some other hardware (car radio, engine ECU etc). Upon inspection the charge rate has gone ballistic (20V & over). Fit a good battery & all is OK.

The 2nd scenario is even more common, where the guy jumps starts the car because he thinks the battery has just 'gone flat', but in fact it has gone open circuit, so when the jumper leads are removed, the previously described disaster occurrs. This is the basis for many of the myths regarding the 'spiking' of engine ECUs.

Dr Terry.

[Dr Terry]

Hi Guys.

To my knowledge all automotive alternators are 3 phase with full wave rectification, so their output is quite smooth for something that's not filtered. In a fashion, the battery acts as a large capacitor to filter out the remaining 'spikes'.

I know when you drive a car with the battery disconnected, as described above, the 12v noise on the car radio is really bad.

Dr Terry.

[Dangerous]

I think the higher charging voltages are possible as modern batteries may be more tolerant to high temperatures caused by higher level charging than older battery designs. That's just supposition though - I have nothing to back that up. Also, remember that it's the alternator output voltage too, not the voltage that appears at the battery +ve. Maybe the larger electrical load of modern cars causes more of a voltage drop between the alternator and battery, or accountants responsible for new car costs have figured out that it's cheaper to uprate the regulator set voltage and skimp on the copper cables to provide or allow a voltage drop ... or am I just being cynical??

DA, I pretty much agree with your thoughts on higher battery charging voltages. 14.7 volts with no loss works out at about 2.45 vpc, which is at the upper limit, but not deadly to batteries.

Rodomo, if an alternator is continually pumping out 15 volts, it's more likely that the battery is open circuit or high resistance on a cell. 15 volts constantly into an otherwise healthy battery which would have an extremely low internal resistance equates to a bloody lot of power and heat. It would probably boil the battery pretty quickly, unless the alternator or regulator fried itself beforehand. Higher charge voltages when the engine is first started can be a good way to initiate charging for the battery though, but I am not aware of any alternators that are specifically designed to do this - it can happen by default though straight after the engine fires, as the battery voltage is down due to cranking load, and the alternator gets spun up, sees a very low battery voltage (which equates to a low resistance load for the alternator), and then pumps out max output until the battery voltage comes back up to a reasonable level. I wouldn't have thought it would be as long as 15 to 20 seconds though.

Yella, think of the alternator as just another engine - your engine might be capable of 300 hp, but how much it actually puts out depends on the throttle setting - the regulator is the throttle setting for the alternator (engine).

Dr Terry, the battery acts as both a smoothing load (capacitor) as well as a reference voltage to the alternator. If the alternator loses the load (battery), it can't reference its output voltage, and it tends to head to the maximum voltage it can make. This is limited and lessened by the electrical load on the alternator. In my (unfortunate) experience, this is around 18 to 20 volts with a car driving when a battery lead came off, but I think it can get as high as 30 volts with no load, which is probably enough to pop all the diodes in the alternator. DA, you're right, once started by battery voltage on the stator, the alternator can feed itself, which is why a car engine will still run if you just isolate the battery +ve cable.

ToranaSS, I think maybe you're wrong on the alternators, and right on the cheap chargers. All recent alternators (including those of the Torana era) are multi phase and full rectified. Some cheap chargers are only half wave rectified though, in a really cheap and nasty way to regulate charge current. The peak voltage of the half wave rectification is above what it should be to charge the battery (eg the peak voltage may be 16 volts, but the battery only wants to see ~14 volts), so for a flat battery, the cheap and nasty charger still puts out 4ish amps to charge the battery - it still charges OK. As the battery becomes charged, the peak voltage from the charger still exceeds what the battery likes to see, but it's only on for a short time on the peaks of the half wave regulated waveform, where the voltage exceeds the battery voltage. As the battery voltage comes up, there is less power (area under the half wave peaks down to the DC voltage of the battery), so it sort of self regulates ..... sort of. Definitely a good idea not to leave a cheapo non charge state monitoring charger on charge for longer than you have to.

Toranr amore, can't get to that link, but I'll check back later - might be some good reading.