Scenario is a power supply charging a battery.

If the current flow is greater than desired, how would I go about determining the resistance to be added to reduce the current flow by a given number of amperes?

Alternator charging a battery at 14.5 volts. Current flow is say 20 amps. How do I go about reducing that to say 10 amps?

Thanks.

 

There is no exact calculation for this because there are some unknowns. But let us do some ballpark calculations.
Assume that the charging voltage Vs = 14.5 V.
Assume that the battery voltage VB = 12.5 V.
There is an overhead of 2 V.
If the charging current is 20 A, then the total resistance in the circuit is

R = (Vs - VB) / I = 2 V / 20 A = 0.1 Ω
There is power loss somewhere in the circuit of 2 V x 20 A = 40 W.

You can increase the series resistance with a resistor or a diode. Either would have to be rated at about 50 W.
The resistor would have to be 0.1 Ω rated at 50 W and mounted on a very large heat sink or fan or water cooled.

Or you can use a 20 A diode mounted on a heat sink and cooled as per the above.

 

Thanks Mr. Chips for the reply. Your answer begats another question or two.

The charging voltage is known. It is 14.5 volts.

The battery voltage pre charger connection is measured at 13.2 volts. However, after being connected to the charging source it quickly rises to the same voltage as the charger. 14.5 volts. But this must be the charger voltage not the true battery voltage. Given that there won't be any current flow between power sources of the same voltage, I have to presume that the true battery voltage must be less than 14.5 volts. How much is TBD.

By changing power resistor values to see how the current changes, should I be able to back door into an assumption as to what the true battery voltage may be?

I have ordered a number of 50 watt 1 ohm resistors to experiment with. Thinking that with 5 of them in parallel I can start with a total resistance of .2 ohms. Then see to what extent the current flow varies pre and post resistor installation.

Also thinking that with 5 50 watt resistors the current from each should be on the low side and not require any external cooling.

Thanks again.

 

Scenario is a power supply charging a battery.

If the current flow is greater than desired, how would I go about determining the resistance to be added to reduce the current flow by a given number of amperes?

Alternator charging a battery at 14.5 volts. Current flow is say 20 amps. How do I go about reducing that to say 10 amps?

Thanks.

If the alternator is doing a boost charge which terminates at V=2450mV/cell then adding resistance isn't going to change it as it is in constant current mode. Can't you just leave it alone, as it will reduce by itself when it reaches the target voltage?

 

If the alternator is doing a boost charge which terminates at V=2450mV/cell then adding resistance isn't going to change it as it is in constant current mode.

Never heard of a "boost charge" constant-current mode for an alternator,
It just charges at a constant voltage and the battery condition determines the current, so a resistor in series will certainly change the charging current.

 

Never heard of a "boost charge" constant-current mode for an alternator,
It just charges at a constant voltage and the battery condition determines the current, so a resistor in series will certainly change the charging current.

What do you think happens if it is presented with a flat battery? If it charges at constant voltage it will burn out, so it charges at constant current until it reaches absorption voltage, then at constant voltage.

 

What do you think happens if it is presented with a flat battery? If it charges at constant voltage it will burn out, so it charges at constant current until it reaches absorption voltage, then at constant voltage.

If course.
But that doesn't mean a series resistance won't reduce the charge current for a typical partially discharged battery as you implied.

 

If course.
But that doesn't mean a series resistance won't reduce the charge current for a typical partially discharged battery as you implied.

It depends. If (I*R+Vt)<Vabs where I is the constant current output of the charger, R is the added resistance, Vt is the terminal voltage and Vabs is absorption voltage then it won't.

 

I always use an old car headlight as a current limiter. Bulbs are built to handle and dissipate heat and of course they give visible feedback.

A headlight draws roughly 5A at full auto system voltage. Well under that at lower voltages.

If you're careful and you're sure of a lower voltage drop, you could use a bulb rate at a lower voltage. But it'll be much harder to find than an old headlight.

 

First I have heard of a typical automotive or marine alternator being anything other than a constant voltage type of device.

 

First I have heard of a typical automotive or marine alternator being anything other than a constant voltage type of device.

Has it never occurred to you that a battery might have a maximum charging current? (Generally C/5, or 20A for every 100Ah of capacity)

 

I always use an old car headlight as a current limiter. Bulbs are built to handle and dissipate heat and of course they give visible feedback.

A headlight draws roughly 5A at full auto system voltage. Well under that at lower voltages.

If you're careful and you're sure of a lower voltage drop, you could use a bulb rate at a lower voltage. But it'll be much harder to find than an old headlight.

So far I have used an automotive bulb with 3 ohms of resistance. It reduced the current from 15 or so amps to less than 2 amps. Then I tried two in parallel and got the current flow to around 3.5 amps. Have a few high power 1 amp resistors on their way from Amazon. Will play with them to see if I can get the current to around 10 amps.

 

Has it never occurred to you that a battery might have a maximum charging current? (Generally C/5, or 20A for every 100Ah of capacity)

The battery handling the current flow is not of concern.

The current flow in my application will be well below what the battery can handle. My experiment is to reduce the current so as to not cause harm to the alternator.

Say the battery will accept a current flow of 100 amps. But the alternator is rated at 85 amps. Running the alternator with an output of 85 amps could result in an early failure. Willing to reduce the current flow, taking longer to charge the battery, while helping the alternator.

 

The battery handling the current flow is not of concern.

The current flow in my application will be well below what the battery can handle. My experiment is to reduce the current so as to not cause harm to the alternator.

Say the battery will accept a current flow of 100 amps. But the alternator is rated at 85 amps. Running the alternator with an output of 85 amps could result in an early failure. Willing to reduce the current flow, taking longer to charge the battery, while helping the alternator.

The alternator will have an over-current limiter.

 

It depends. If (I*R+Vt)<Vabs where I is the constant current output of the charger, R is the added resistance, Vt is the terminal voltage and Vabs is absorption voltage then it won't.

All that means is, if you crank through the numbers, the alternator current limit is unlikely to be a factor for the conditions stated in the TS's post.

 

The alternator will have an over-current limiter.

How does that work? I know the old alternators I've worked on had no such protection but I haven't cracked one open in decades.

 

How does that work? I know the old alternators I've worked on had no such protection but I haven't cracked one open in decades.

By reducing the field current until the main output current was within spec.
I think the old ones used to limit because if there was too much load on it the belt slipped.
The new ones are complicated enough - you’d hope they could current limit to protect themselves, especially for how much they cost.

 

The battery handling the current flow is not of concern....My experiment is to reduce the current so as to not cause harm to the alternator.

I have always been impressed by this forum usually asking to describe the problem first, then helping to come up with a solution. Rather than helping someone build a wrong solution.

 

The most important details have been left out: What type of battery is it? And, assuming it is rechargeable, what is its capacity? That is why phones need the manufacturers' recommended / provided chargers.
14.5 volts sound like a backup power for a PC or something?

 

The most important details have been left out: What type of battery is it? And, assuming it is rechargeable, what is its capacity? That is why phones need the manufacturers' recommended / provided chargers.
14.5 volts sound like a backup power for a PC or something?

Lithium starting battery for an outboard motor. 100 AH.

It can be charged using most any constant voltage power source. With the voltages varying.