Charging amperage - based on voltage levels

[chorky]

Does anyone know, for the high output alternator found on the JTR or other options, what the actual amperage pushed might be for various levels of voltages?

I am curious and would like to know for a particular wire size application. I know the alternator has a potential of 240 amps, and up to 15 volts, but, am curious what amperage it actually pushes when a battery is, say, drained to 12.2 or 12.4 volts.

A shop manual might help in this case if anyone has one.

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[Radio Guy]

It depends on the battery state of charge. A large completely discharged battery, say 10.5 volts will draw a lot of current when presented with 14 volts or so for charging. Perhaps 50-100 amps for a brief time until the state of charge improves then the current will drop off proportionally. Its not what the alternator "pushes" to the battery, its what the battery internal resistance is, what the charge voltage presented across the battery is and how much current is available from the source (alternator in this case) to charge the battery.

 

[ShadowsPapa]

Does anyone know, for the high output alternator found on the JTR or other options, what the actual amperage pushed might be for various levels of voltages?

I am curious and would like to know for a particular wire size application. I know the alternator has a potential of 240 amps, and up to 15 volts, but, am curious what amperage it actually pushes when a battery is, say, drained to 12.2 or 12.4 volts.

A shop manual might help in this case if anyone has one.

Varies with battery size/capacity, age and so on - and the alternator does a lot more. Everything about the vehicle should run from the ALTERNATOR, not the battery. The battery should be for cranking, or the draw of a vehicle that's sitting.
Resistance internally can rise with battery age.
Here's what I recall from the days of old when cars, trucks and tractors had amp meters and not volt meters - never saw one over 60 amps.
HOWEVER - that's considering there wasn't much electrical load except the battery. So figure that the battery isn't taking much at all in most cases - it's the rest of the vehicle.

What the battery can draw is based on so many factors - temperature of the battery, age of the battery, size of the battery, state of charge of the battery and in our cases - what the PCM does based on engine load and more.

OF course we have two batteries in parallel, and there's a IBS in the main battery's circuit which I think is about 1 ohm, so that controls what goes into the main battery along with the list of other factors.

Maybe I need to mount a JT alternator on my test bench, set an AGM battery up next to it and see what happens. I believe my meter will go to 100 amps. Can't recall now LOL.

Why don't you measure what goes in?

I guess I could put my current clamp on and see what the scope says.

 

[ShadowsPapa]

I know the alternator has a potential of 240 amps, and up to 15 volts,

More than 15 volts. The ECM is limiting it to 15. Full field that baby and watch things blow.
Likely the alternator would fall short of 240 amps. It may get there, but typically they are considered good if they reach 90% of advertised output. At least that's how it used to be.

A shop manual might help in this case if anyone has one.

Won't help because it's going to vary. What if someone puts in a battery of larger capacity and lower internal resistance?

What goes into a battery isn't something a manual is likely to say in any case. Too many variables.
What you could do is run a comparable AGM battery down (comparable to what the JT comes with) and put a big battery charger on it and measure what flows.
A shop manual will tell how to test the alternator to see if it's operating properly - using a LOAD like a carbon pile or similar device.

What a battery takes in is dependent on the battery and voltage used, not the alternator.

 

[chorky]

More than 15 volts. The ECM is limiting it to 15. Full field that baby and watch things blow.
Likely the alternator would fall short of 240 amps. It may get there, but typically they are considered good if they reach 90% of advertised output. At least that's how it used to be.


Won't help because it's going to vary. What if someone puts in a battery of larger capacity and lower internal resistance?

What goes into a battery isn't something a manual is likely to say in any case. Too many variables.
What you could do is run a comparable AGM battery down (comparable to what the JT comes with) and put a big battery charger on it and measure what flows.
A shop manual will tell how to test the alternator to see if it's operating properly - using a LOAD like a carbon pile or similar device.

What a battery takes in is dependent on the battery and voltage used, not the alternator.

so how can one calculate or factor what amperage might be like for a 110ah battery at say 12.2v so about 50-60% soc? So I think you bith know I have the genesis system. Primary battery and auz battery are both 64ah. I cant remember what the internal resistance is but full river does post that spec

so if i wanted to add another larger 110ah of the same chemistry in the bed and connect it to the aux under the hood using my existing 4ga wire which is fused at 100a. NOT for purposes of pulling winch. Purely for camping.
is there a way to calculate what the amp potential is? I am hoping it would be under 100

my previously planned 1 night camping trips just turned into 3 or 4 trips at 5 days each this summer. and trying ti stay away from going lithium which would entirely negate the gensis system, add significant cost, and not work well in winter.

 

[chorky]

It depends on the battery state of charge. A large completely discharged battery, say 10.5 volts will draw a lot of current when presented with 14 volts or so for charging. Perhaps 50-100 amps for a brief time until the state of charge improves then the current will drop off proportionally. Its not what the alternator "pushes" to the battery, its what the battery internal resistance is, what the charge voltage presented across the battery is and how much current is available from the source (alternator in this case) to charge the battery.

i would not plan to go below 50% SOC at a the most. They spec it is capable of 80% DOD without immediate damage but thats risky. I would attempt to stay at 70% SOC or higher

spec sheet for the primary and aux battery in the genesis kit
https://fullriverbattery.com/wp-content/uploads/FT750-25.pdf

spec for a group 31 i would like to consider putting in the bed and linking into the aux battery system. Wires to the bed are 4ga fused at 100a currently.
https://fullriverbattery.com/wp-content/uploads/FT1100-31ST.pdf

 

[chorky]

I was also reading this
https://fullriverbattery.com/wp-content/uploads/battery-installation-manual-english.pdf


here they give an example of recommended and maximum charge rate for bulk stages. The example given with a 115ah battery would be 40amps for maximum. However I am curious if this is what they just recomended ax the max or if due to internal resistance this is the most it would draw. I am
assuming this is just the max recomended charge rate and that it can draw more depending on alternator output, voltage and temp. any thiughts?

 

[Radio Guy]

I was also reading this
https://fullriverbattery.com/wp-content/uploads/battery-installation-manual-english.pdf


here they give an example of recommended and maximum charge rate for bulk stages. The example given with a 115ah battery would be 40amps for maximum. However I am curious if this is what they just recomended ax the max or if due to internal resistance this is the most it would draw. I am
assuming this is just the max recomended charge rate and that it can draw more depending on alternator output, voltage and temp. any thiughts?

The truck alternator should not be current limited up to its rated output, so if the battery is capable of drawing 100A charge current when discharged then that's what its going to do. There will of course be voltage drop in the wiring and the potential alternator current will drop off at low RPM so there are variables. A deeply discharged battery will usually draw big current for a short amount of time and that will taper off rapidly. I don't believe you can calculate the maximum possible charge current unless you have a lot of data on a specific battery and have very controlled lab type environment.

 

[chorky]

The truck alternator should not be current limited up to its rated output, so if the battery is capable of drawing 100A charge current when discharged then that's what its going to do. There will of course be voltage drop in the wiring and the potential alternator current will drop off at low RPM so there are variables. A deeply discharged battery will usually draw big current for a short amount of time and that will taper off rapidly. I don't believe you can calculate the maximum possible charge current unless you have a lot of data on a specific battery and have very controlled lab type environment.

what would be a ‘short amount of time’ though. Are we talking 200 amps for 30 seconds? Or 150 amps for 20 minutes? Maybe its not really something I can figure on calculating…. Which means a big sad face for figuring out if my plan will work or not…

 

[sharpsicle]

what would be a ‘short amount of time’ though. Are we talking 200 amps for 30 seconds? Or 150 amps for 20 minutes? Maybe its not really something I can figure on calculating…. Which means a big sad face for figuring out if my plan will work or not…

I'm curious, what is the plan that critically hinges on this answer? Are you worried it won't charge an added battery?

I get the feeling you might be over-thinking this one a bit.

 

[chorky]

I'm curious, what is the plan that critically hinges on this answer? Are you worried it won't charge an added battery?

I am worried that with a half drained battery in the bed, it will try to pull over 100 amps to recharge thus blowing the 100a fuse or burning the 4ga wire


I have the genesis system. Under hood is a 64ah aux battery disconnected from the main battery when below 12.7 volts. I have a +/- 4 ga wires going to the bed where it distributes to provide power to the compressor, fridge, etc…. It is fused at 100a under the hood to protect the wire.

I would like to add a group 31 110 ah AGM in the bed and connect it directly to the 4ga wire that comes from
the 64ah battery under the hood to increase camping time without recharging.

But I dont know if the battery is drained to 50-70% what amp draw will occur for that battery to recharge with the engine running. If it is under 100 amps thats great, if over 100a then I need to reconsider some things.





Well I attempted to contact full river and the guy couldnt comprehend my question. Tried transferring me to someone else but I got disconnected. So are there any electrical engineers out there that can answer this question? If a 110ah group 31 battery is at 50% SOC, what would the potential amp draw of that battery be when the engine is running in order to recharge?

 

[Radio Guy]

what would be a ‘short amount of time’ though. Are we talking 200 amps for 30 seconds? Or 150 amps for 20 minutes? Maybe its not really something I can figure on calculating…. Which means a big sad face for figuring out if my plan will work or not…

My experience with automotive size batteries is they can hog a lot of current when first connected to a charger with unlimited current capability and they taper down pretty fast. If a battery draws 50 amps when first connected to a charger you can watch the ammeter slowly go down, maybe 40A after 30 seconds and after a few minutes it might be 20A and stay in that range for awhile as it very slowly tapers down from that point. This is with a constant supply voltage around 14.2V Its been awhile since I've charged a really dead battery but that's what I remember.

Your not going to burn up 4ga wire with 100A. It might get a little warm over time but at very high current it will drop the voltage some at the battery. You can easily calculate your length of 4ga wire resistance then the voltage drop over XX feet at different current levels like 20A, 50A, 100A, etc.

If a battery wants to draw 50A when connected directly to 14.2 volts add even .5 ohms in the circuit and that will decrease the current a lot. If you simply put a .2 ohm resistor across a 14.2V source it will draw 71 amps. Across a .5 ohm resistor it will be 28.4 amps, so your wiring and your fuse resistance is going to limit some current for you.

 

[chorky]

My experience with automotive size batteries is they can hog a lot of current when first connected to a charger with unlimited current capability and they taper down pretty fast. If a battery draws 50 amps when first connected to a charger you can watch the ammeter slowly go down, maybe 40A after 30 seconds and after a few minutes it might be 20A and stay in that range for awhile as it very slowly tapers down from that point. This is with a constant supply voltage around 14.2V Its been awhile since I've charged a really dead battery but that's what I remember.

Your not going to burn up 4ga wire with 100A. It might get a little warm over time but at very high current it will drop the voltage some at the battery. You can easily calculate your length of 4ga wire resistance then the voltage drop over XX feet at different current levels like 20A, 50A, 100A, etc.

If a battery wants to draw 50A when connected directly to 14.2 volts add even .5 ohms in the circuit and that will decrease the current a lot. If you simply put a .2 ohm resistor across a 14.2V source it will draw 71 amps. Across a .5 ohm resistor it will be 28.4 amps, so your wiring and your fuse resistance is going to limit some current for you.

Awesome! This is exactly the type of info I was looking for!

the internal resistance for this particular battery is speced at 2.3mH

I would have to test the resistance from the battery underhood through to the bed but I doubt its very much.

The 4ga wire is based on Blue Sea Systems chart. I use them as they add in a safety margin being mostly marine applications. And with the compressor running plus a fridge and possibly charging a battery I expected it to get pretty close to 75 amps. Which is why I am curious of potential draw from the battery.

 

[chorky]

A folllowup scenario I am looking to figure out is what sort of damage is actually done by charging two or 3 batteries of same chemistry at different rates with different capacities. Obviously one with the least capacity and most charge would routinely be overcharged but I am looking to see if there is a way to quantify this. I doubt there is though.

for example the main 64ah starting would be mostly charged. Maybe down to 12.7 when the combiner disconnects them. The aux 64ah would be down to 50-70% while the in bed 110ah would also be 50-70%.

 

[chorky]

Here is an interesting resource that explains a few things I have seen mentioned but not explained. A full river 60ah for example can receive a max charge rate of 21a

https://workshoppist.com/agm-battery-charging-rates/


however! All of this still pertains to the optimum charging rates of the battery itself. It does not necessarily discuss what amperage pull a drained battery can have. So using the full river 60ah example above, it is unclear to me if that 21a spec is what the battery can handle without damage, or what that battery will pull with a unrestricted amperage amount (high amp alternator)

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