amps into batteries

[wotayottie]

trying to remember A level physics of 50 years ago and not sure I am succeeding.

I have a 60 amp alternator controlled by a sterling regulator and into ( at the moment) two 110 aH batteries. But the current flowing into the two batteries is a mere 32 amps and I've never seen it higher than 42. Now if I remember correctly the current is a function of the difference between the alternator output voltage and the back emf of the batteries divided by the circuit resisitance. So buying a 90 amp alternator wouldnt make any difference to current flow with the voltage being controlled the Sterling and the back emf being a function of battery charge state.

Am I right?

 

[noelex]

The amps you are getting are typical of your sort of set up.
An easy way to tell if a larger alternator would put more in is to monitor the battery voltage.
Once the battery voltage reaches the set point on the Sterling regulator(14.7v ?) it will start reducing the alternators output and a bigger alternator would not put in any more charge.
Typically you will find for the first few minutes the alternator output is limiting the system ( so a bigger alternator would put in slightly more at this stage), but the voltage quickly rises to the Sterling set point and it is then limited by the batteries acceptance and a bigger alternator will not help.
Alternators generally will not puta out as much as their specifications suggest especially at lower revs.

 

[halcyon]

trying to remember A level physics of 50 years ago and not sure I am succeeding.

I have a 60 amp alternator controlled by a sterling regulator and into ( at the moment) two 110 aH batteries. But the current flowing into the two batteries is a mere 32 amps and I've never seen it higher than 42. Now if I remember correctly the current is a function of the difference between the alternator output voltage and the back emf of the batteries divided by the circuit resisitance. So buying a 90 amp alternator wouldnt make any difference to current flow with the voltage being controlled the Sterling and the back emf being a function of battery charge state.

Am I right?

Your alternator does not produce amps, it produces watts at normally a nominal 14.0 volt into a load. It is designed rating is to supply power to your car, not charge batteries.

As the voltage rises above 14 volt, the amperage will fall, watts is constant due to stator windings. The amps are also effected by any loads, such as lights, fridge, navigation equipment, this will reduce amps available for charging. It can also limit top end voltage the alternator can achieve, a large resistive load can hold down voltage.

Brian

 

[vyv_cox]

I have a 50 (55?) amp alternator and three 110 Ah batteries. Until recently these were all open cell batteries for which I used the highest rate setting on the Sterling. Last year these batteries failed and were replaced by closed cell ones, same rating. I reduced the Sterling rate accordingly.

When I start the engine first thing in the morning, before the solar panels have had much chance to replace the current consumed by the fridge all night, the alternator delivers its full output, 50 - 55 A for a while, dependent upon how discharged the batteries are. The characteristics are similar for both types of battery. The current then ramps down for the next hour or so.

If I don't start the engine until later in the day the maximum current can sometimes be below 20 A, reflecting the much better battery state brought about by 7 amps of solar current pouring into them. Is it possible that your batteries are well charged on each occasion that you start up?

 

[ianj99]

trying to remember A level physics of 50 years ago and not sure I am succeeding.

I have a 60 amp alternator controlled by a sterling regulator and into ( at the moment) two 110 aH batteries. But the current flowing into the two batteries is a mere 32 amps and I've never seen it higher than 42. Now if I remember correctly the current is a function of the difference between the alternator output voltage and the back emf of the batteries divided by the circuit resisitance. So buying a 90 amp alternator wouldnt make any difference to current flow with the voltage being controlled the Sterling and the back emf being a function of battery charge state.

Am I right?[/QUOTE]

Sort of.

To get more charge into batteries which are not fully discharged, you'd need to increase the charging voltage so, fitting a larger alternator would not be the answer.

However, what effect does engine rpm have on your charging current?
Have you tried increasing the rpm to see if the current rises beyond 32amps?

In many cases, its more advantageous to fit a smaller pulley on the alternator (or a larger one on the flywheel) to increase alternator output for a given engine rpm.

Finally, most batteries should not be charged at more than about 25% of their capacity for long periods or unless a temp sensor is fitted (such as comes with the Sterling Regulators) to avoid overheating. (consult the battery manufacturer)

 

[ianj99]

Your alternator does not produce amps, it produces watts at normally a nominal 14.0 volt into a load. It is designed rating is to supply power to your car, not charge batteries.

As the voltage rises above 14 volt, the amperage will fall, watts is constant due to stator windings. The amps are also effected by any loads, such as lights, fridge, navigation equipment, this will reduce amps available for charging. It can also limit top end voltage the alternator can achieve, a large resistive load can hold down voltage.

Brian

Sorry, not correct.
Watts, like horsepower is a calculated value and neither exist in reality.

The alternator current is limited ultimately by the thickness of the stator windings. Multiply the current by the voltage across the output and you will arrive at the watts.
If you want more watts at the same rpm, then increase the current.

For horsepower, its the torque that produces the motive force, multiply by the rpm and divide by 5250 and you have the bhp. If you want more power at the same rpm, you need to increase the torque.

If you want to quote watts or bhp, you need to specify one of the other parameters that have been used to calculate the watts or bhp otherwise the figure is meaningless.

 

[snowleopard]

Sorry guys, forget Ohm's law, wattage etc.

It's a Sterling regulator that adjusts the output voltage to produce a constant current (during the bulk charging phase).

You will never get more current than that set by the alternator unless, with a totally flat battery, the alternator's internal regulator takes charge and boosts the voltage higher than that demanded by the Sterling. There is no value in having a bigger alternator beyond the capacity of the Sterling. Depending on the model of the Sterling you may be able to adjust its bulk charging current setting. The maximum current is 1/3 of the battery capacity, i.e. for 110 AH max current should be set for 36A or if the 2 x 110 AH are charged in parallel, 73A.

 

[ianj99]

Sorry guys, forget Ohm's law, wattage etc.

It's a Sterling regulator that adjusts the output voltage to produce a constant current (during the bulk charging phase).

You will never get more current than that set by the alternator unless, with a totally flat battery, the alternator's internal regulator takes charge and boosts the voltage higher than that demanded by the Sterling. There is no value in having a bigger alternator beyond the capacity of the Sterling. Depending on the model of the Sterling you may be able to adjust its bulk charging current setting. The maximum current is 1/3 of the battery capacity, i.e. for 110 AH max current should be set for 36A or if the 2 x 110 AH are charged in parallel, 73A.

There is no way the Sterling regulator can adjust the current - it doesn't measure it so how can it?
It will drive the alternator to maintain the charging voltage, which is set by dip switches according to battery type.

The internal regulator is only there as a backup - it plays no part when a Sterling regulator is fitted. In fact I removed the internal reg from my 70amp alternator when I converted it to insulated return so the Sterling reg does all the work.

 

[cimo]

trying to remember A level physics of 50 years ago and not sure I am succeeding.

I have a 60 amp alternator controlled by a sterling regulator and into ( at the moment) two 110 aH batteries. But the current flowing into the two batteries is a mere 32 amps and I've never seen it higher than 42. Now if I remember correctly the current is a function of the difference between the alternator output voltage and the back emf of the batteries divided by the circuit resisitance. So buying a 90 amp alternator wouldnt make any difference to current flow with the voltage being controlled the Sterling and the back emf being a function of battery charge state.

Am I right?

alternater o/p currrent will never exceed the charge acceptance rate of your battery bank, which as we know decreases exponentially as the battery bank charges. Your sterling regulator attempts to override this natural exponential drop off in charging current (the so called smart bulk charge phase).
As others have already said - typical estimate for maximum charge acceptance rate (& therfore max. alternator o/p) is around 25% of total battery bank capacity, which is 55Amps in your case. This is the highest current your ever likely to see with a 220AH battery bank (at around 50% discharged & less as the bank charges up). So I wouldn't expect any huge advantage in replacing the alternator.

 

[VicMallows]

There is no way the Sterling regulator can adjust the current - it doesn't measure it so how can it?

Agree completely. Indeed the 'bulk charge' voltage (14.8v when set for open flooded; etc) is purely maintained for a fixed, timed period (aprox 1hr), after which charge voltage drops to 13.8v ..... or higher if the original internal regulator is kept and it's setting is higher.

Sometimes, if I don't consider that sufficient Ahrs have been replaced after the one hour ....based on Battery Monitor cumulative Ahr reading ...... I 'retrigger' the Sterling for an additional hour.

A REALLY smart regulator would know all such variables .... but imagine the cost! In practice the Sterling does a perfectly adequate job most of the time.


Edit: Comments refer to the Sterling REGULATOR. The Sterling Alternator-to-Battery CHARGER may well be able to monitor the actual charge current and respond accordingly. (whether it does I have no idea).

 

[halcyon]

A REALLY smart regulator would know all such variables .... but imagine the cost! In practice the Sterling does a perfectly adequate job most of the time.

Just a designer's little rant, no, a smart regulator that can do more than that need not, and does cost a lot, it's just how you design the operating system.


Brian

 

[VicMallows]

Just a designer's little rant, no, a smart regulator that can do more than that need not, and does cost a lot, it's just how you design the operating system.


Brian

Agree .... I should have phrased it 'imagine how much more they would feel they could get away with charging for such a "premium" (!) product'

 

[cimo]

Agree completely. Indeed the 'bulk charge' voltage (14.8v when set for open flooded; etc) is purely maintained for a fixed, timed period (aprox 1hr), after which charge voltage drops to 13.8v ..... or higher if the original internal regulator is kept and it's setting is higher.

Sometimes, if I don't consider that sufficient Ahrs have been replaced after the one hour ....based on Battery Monitor cumulative Ahr reading ...... I 'retrigger' the Sterling for an additional hour.

A REALLY smart regulator would know all such variables .... but imagine the cost! In practice the Sterling does a perfectly adequate job most of the time.


Edit: Comments refer to the Sterling REGULATOR. The Sterling Alternator-to-Battery CHARGER may well be able to monitor the actual charge current and respond accordingly. (whether it does I have no idea).

if the OP doesn't mind - I'll go off on a tangent - as this may be of interest to some.

A very cheap and easy way to provide a manual "bulk charge" feature to a normal alternator/regulator/battery setup is to place a signal diode in the voltage sense line (configuration must be battery sensed rather than machine sensed). The alternator now sees the battery voltage as being -> actual battery voltage minus 0.6 diode forward voltage drop, & the higher alternator output current will be maintained. A switch in parrallel to the diode will allow the diode to be bypassed, returning the charging current to normal levels. It's a very cost effective, simple & reliable alternative to smart regulators. The downside - is its a manual system. Leaving the diode in circuit for long periods wouldn't be too healthy for the batteries.

 

[bonny]

Would not disagree with most of the replies. The current taken by the batterie(s) is a function of their internal resistance (not back emf which only applies to inductive circuits), their current voltage and the output voltage of the charger (14.2 for standard alternator, 14.6ish for sterling).

What I will add is that charging batteries at anything above 14.2 (which the sterling does) will cause gassing. Gassing causes electrolyte loss (actually just pure water - not the acid).
As a result you will find that the level will drop more rapidly than if you had not used the Sterling. Provided you keep this topped up (by adding distilled or de-ionised water) you should be OK. Don't let the level drop below the plates or you will finish up with a prematurely defunct battery.

We recently moved a narrow boat for some friends with 6 110AH batteries 3 years old which had a sterling fitted with a 90A alternator. The batteries were jiggered and took 2.5 Litres each to top up!!! - Effectively bone dry.

The message is check your battery electrolyte level more frequently (and top up) if using a rapid charge system such as the Sterling.

If your batteries are a bit older they could be suffering from sulphation (build up of sulpher on the plates) this effectively reduces the plate area and hence the AH rating of your battery.
There is a product "battery desulphator" which is supposed to reverse this but can take a few months. Can be bought for around £30-£40 I think but is the money better spent on a new bat.
I've built my own which I've had fitted since 8th May and have yet to see any noticeable improvement - but then it only cost me a few quid to build. (2005 140AH).