Alternator: Voltage or current limited with high load?

[ayates]

I should have been able to figure this out myself, but...

If a high load (like a partially discharged large battery bank) is placed on an alternator (with a standard OEM regulator), will the alternator be voltage or current limited? That is, will the voltage stay at say 13.8V and the current be limited to the capacity of the alternator, or will the voltage drop?


Thanks,

Allan.

[Wotname]

The alternator is voltage regulated.

Max current is either limited by the capacity of the alternator or the acceptance charge of the battery - whichever is lower.

[smac999]

It will be current limited.

If the battery bank is big and depleted. The voltage will be low. Possibly in the 13v range. The amps will be max. The voltage will climb as the batteries charge. Once the voltage hits it’s set point Say 14.4v. The current will start to drop and the voltage will hold. That is the transition from bulk to absorption

[john61ct]

With a very high-CAR low resistance chemistry like lithium, a not even that large bank can demand / pull a higher current rate than the alt/stock VR is able to safely produce.

Most setups effectively stop charging by dropping voltage to protect the alt. Some will actually let the alt burn out.

That is why advanced external VRs, like MC-614 are used, besides a Float stage and adjustable voltage setpoints,

their ability to limit current to the maximum that alt can produce (in those ambient temps) while keeping voltage striving for the setpoint during Bulk / CC and holding that voltage at Absorb / CV

protects both the alt, and the LI bank, which you want to charge at well below 0.5C, for longevity.

So yes, until the CV (Absorb) setpoint is reached, we say the charge is in CC (Bulk) mode.

But only those better VRs actually limit the maximum current early on keeping voltage high, and also later - while holding voltage later in CV stage from climbing past the setpoint, possibly **also** limiting current then!

[john61ct]

With a lead bank, even relatively high CAR chemistries like AGM, the resistance is self-limiting, current usually can't climb high enough to cause damage to the bank

as long as the VR is properly limiting voltage.

But for powered boats running all day, a VR that eventually drops to Float will help prevent overcharging sealed batteries.

While with lower CAR more robust FLA, you just need to keep their water topped up.

[thomm225]

If you've ever had cars with the Alternator Gauge/Battery Gauge they can tell you the answer to this question quickly and simply.

The newer gauges say mid 1990's and up show voltage so once you start the car if everything is working right they will read in the center or near about 13.5 - 14 volts or so.

On the old current gauges, especially if the car has been off a long while (days) you will see the gauge go way passed center maybe 3/4 fluctuation due to the higher than normal current flow then slowly come back to center as the battery charges

[waterman46]

A proper multistage regulator will try to set the voltage to achieve bulk charge current as programmed, with an upper voltage limit of about 14.5V. Actual output voltage will be whatever it takes to push that current into battery. If alternator is not capable of supplying the programmed bulk current even as low as about 10 V, a good regulator will sense an overcurrent fault and shut down the alternator or limit it's output current. Without that protection or a temperature sensor circuit, the alternator may overheat and be damaged.

A standard OEM regulator does not have a bulk charge mode, so in a way it is a bit safer for the alternator. But damage to alternator could occur if it's output was shorted to ground, or to a huge battery bank.

[Stu Jackson]

Allan, wotname and smac are correct. Others are close but with the possibility of confusing you.

I suggest you go to Balmar | Marine Charging Systems | Battery Monitors | Multi-Stage Voltage Regulators | High Power Alternators | Serpentine Pulley Conversions and download any of their external regulator manuals. Each of them have a very good description of how their regulators work.


The only differences between an internal and external regulator are the programmability and temperature sensors. An internal regulator can't do anything but set a voltage limit and run the max current the alternator can make. In fact, internal regulators can ONLY do bulk and absorption (i.e., rising voltage at constant current - the alternator's rating - until the voltage set point is reached, and then tapering current).

If you read and understand the Balmar info you'll see the distractions.

The Ample Power Primer is also good on what batteries need:

The Ample Power Primer is still available on the Wayback Machine at https://web.archive.org/web/20160323...mer/primer.pdf
It remains one of the best...
Many thanks to Bill Murdoch

[thomm225]

Go with my vehicle alternator example.

Many that are just learning like some of the posts above want to make understanding current/voltage harder than it is.

Another way to understand is to think the old hose routine. The pressure you turn on at the tap is the voltage, and it will stay constant. The flow is the current!

[Wotname]

Originally Posted by thomm225

Go with my vehicle alternator example.

Many that are just learning like some of the posts above want to make understanding current/voltage harder than it is.

Another way to understand is to think the old hose routine. The pressure you turn on at the tap is the voltage, and it will stay constant. The flow is the current!

Unfortunately this doesn't help answer the OP's question which was

If a high load (like a partially discharged large battery bank) is placed on an alternator (with a standard OEM regulator), will the alternator be voltage or current limited? That is, will the voltage stay at say 13.8V and the current be limited to the capacity of the alternator, or will the voltage drop?

I reckon Stu got it right in post #8

[thomm225]

Originally Posted by Wotname

Unfortunately this doesn't help answer the OP's question which was

If a high load (like a partially discharged large battery bank) is placed on an alternator (with a standard OEM regulator), will the alternator be voltage or current limited? That is, will the voltage stay at say 13.8V and the current be limited to the capacity of the alternator, or will the voltage drop?

I reckon Stu got it right in post #8

The voltage will be constant and the current will increase to max allowed.

Pretty simple stuff.

Again, you can make it seem complicated or as simple as it actually is, but I understand how newbies at electronics like to make it seem hard

[a64pilot]

Alternators are voltage regulated, but current limited.
You cannot reduce amperage and maintain voltage without reducing the load, they are interrelated.

In other words if your charging a battery bank that will accept 100 amps to reach absorption voltage. But your alternator is limited at 80 amps, then you alternator will put out 80 amps at a voltage less than absorption voltage it will maintain 80 amps until it can climb to absorption voltage from that point on current will be reduced and voltage remain stable.

People that don’t understand how they work often make the comment that voltage can remain constant but current reduced. That can’t happen.
You can only control the voltage, but that also drives current, increase voltage, current goes up, reduce voltage current decreases. But you can’t reduce current and maintain voltage.

In other words if my bank will accept 100 amps and climb to absorption voltage, if I reduce the amperage, the voltage of the bank will decrease. Because it takes 100 amps to get to absorption voltage.

I believe that the confusion comes from power supplies that can both the amperage and voltage can be set, but in that case the amperage you set is a limit, the max but it’s not maintained, it’s dependent on load, voltage can be set and maintained, you can maintain a set voltage from zero load up to the max amperage, but not beyond.

If you set a power supply to absorption voltage and the amps to 10 and hooked it to that bank that takes 100 amps to get to absorption voltage, the power supply will limit out at 10 amps and the voltage will be well below absorption voltage, voltage will slowly climb over time as the bank charges.

[Pete7]

Originally Posted by thomm225

Many that are just learning like some of the posts above want to make understanding current/voltage harder than it is.

Agreed, drifting a thread by discussing lithium and filling a post full of abbreviations without first writing them in full just confuses someone trying to understand more about the subject.

Also not helped when Allan clearly said an "an alternator (with a standard OEM regulator)" to then answer about external voltage regulators (VR).

As Stu said, "In fact, internal regulators can ONLY do bulk and absorption" to then imply that a VR can drop the voltage to float is wrong for an OEM alternator. Some folk would be better off giving personal examples.

So my experience with a 60A Valeo alternator is that when the engine is started it will ramp up to 14.2v quickly, probably the alternator's regulator limit. Current will be about 40-42 A, probably the batteries limit, never seen more into a 220AH bank. As the batteries charge up and the alternator gets hot (there are no free lunches, small alternators charging house banks rather than a car battery get hot) then a battery monitor will show the current dropping down. We can be down to 20A in 30-60 minutes and it will continue to drop to low single figures.

We could improve the charging regime with an external regulator, but choose not to because we use solar instead for most of our charging needs rather than run an ordinary alternator long and hot.

Pete

[a64pilot]

Only thing I would add to that is that internally regulated alternators do not usually make absorption voltage, but usually some number above float, but below absorption.
This means that on average it takes them longer to fully charge a bank, often much longer

They are essentially the same as an automobiles alternator, which is not meant or designed to charge batteries, that may sound strange but it’s true, they are meant to supply current to run the cars electrical system, that’s why most all battery manufacturers recommend charging their batteries with a charger and don’t tell you to just drive the car around for awhile.
Automobiles and power boats only discharge the battery a very small amount by starting the engine, yes it’s huge amps, but for so short of a time not much energy is lost, so not much charging is needed.

So if your planning to use your alternator as a primary charge source for your battery bank, you need a system designed to charge batteries, and that’s a programmable, temperature regulated external regulator.

[Wotname]

In the interests of keeping it simple and accurate and to help the OP with his understanding of his equipment [i.e. a high load (like a partially discharged large battery bank) is placed on an alternator (with a standard OEM regulator)] and to correct various incorrect statements already posted, please consider the following.

His alternator will be voltage regulated - not to be confused to constant voltage source. It will also have a maximum current rating. Put another way, the alternator will try to reach a set voltage as determined by it's internal voltage regulator. It will never go above this voltage but may be below this voltage if it's maximum current limitation is exceeded or with some internal regulators, if it gets too hot.

Additionally the battery will only accept so much current as determined by the battery chemistry and it's actual state of charge (SOC). If this acceptance current at any one time is lower than the alternator's rated current output, the alternator will simply stay at it's regulated voltage point while the battery receives it's acceptance current. As the battery increases it's SOC, the acceptance decreases. This is an instance where the voltage is maintained and the current decreases.

If the battery's acceptance current is higher than the alternator's rated current, then the voltage will decrease below the regulated point as the alternator struggles to supply the current the battery will accept. This will continue until battery's SOC increases to a point where the acceptance current is again below the alternator's rated current. Then the voltage remains at the regulated point. Unless of course you burn up the alternator first .

That's it in a nutshell