Alternator 101 questions

[Saldar]

My concept of battery charging is the at the rudimentary “blowing up a balloon” stage. I’m trying to grasp how an alternator works to charge RV batteries and have some questions.

When you are charging house batteries through the alternator, is the RV converter involved?
I'm guessing the converter is out of the picture because it acts between the shore power input and the 12V buss.

When you are charging through the alternator, what controls the charging voltage?
I guess you get the voltage that the alternator produces — which is probably appropriate for the cab battery type. Is this the cab battery’s bulk charge voltage? Is there any “smart-charging” by the alternator? What if the house batteries and cab batteries are different types (e.g. AGM vs. wet)?

As usual, I’m visualizing a WBO Travato, which is has a 30A electrical system. Could its 180A alternator supply the house batteries more than 30A? I believe there is a 50A fuse in there somewhere. Because the alternator is 180A, why wouldn’t this fuse blow all the time?

What keeps the alternator from overcharging the batteries? Do the batteries get mostly full and stop accepting charge at the “low” bulk voltage, but the continued presence of the voltage doesn’t hurt anything?

At the Interstate Battery blog I read: “An alternator rated at 100 amps requires about 2 horsepower.” Just curious if people think much about the effect of charging on gas mileage when they report values. Charging would require as little as of 3% of the Travato’s total 280HP. But I have no idea how much of a van’s total horsepower is normally in use while it’s moving.

I’m not sure how many alternator misconceptions I have. Please set me straight on these issues. Thanks for all the great info on this blog.

Sally

[markopolo]

I'll give some general answers & maybe others can fill in specifics.

When you are charging house batteries through the alternator, is the RV converter involved?

No.

When you are charging through the alternator, what controls the charging voltage?

The alternator regulator or PCM in newer vehicles.

I guess you get the voltage that the alternator produces — which is probably appropriate for the cab battery type. Is this the cab battery’s bulk charge voltage?

More or less yes. The computer and/or heat will cause a lowering of the output voltage.

Is there any “smart-charging” by the alternator?

Yes and No - even a dumb system is kind of smart as the batteries will only accept what current they can.

What if the house batteries and cab batteries are different types (e.g. AGM vs. wet)?

Not really a problem. AGM & wet cell have similar charge profiles.

As usual, I’m visualizing a WBO Travato, which is has a 30A electrical system. Could its 180A alternator supply the house batteries more than 30A?

Yes, the 30A likely refers to AC voltage. (120V AC x 30A = 3600W) The alternator output is DC.

I believe there is a 50A fuse in there somewhere. Because the alternator is 180A, why wouldn’t this fuse blow all the time?

It would likely be a self resetting breaker. The current Travato's have two batteries. The batteries, if very low, will only accept high current for a short time. If there is a 50A breaker and the batteries are very low then starting the generator and starting the engine would be the fastest method to get the batteries to the point where they are accepting less than 50A.

What keeps the alternator from overcharging the batteries? Do the batteries get mostly full and stop accepting charge at the “low” bulk voltage, but the continued presence of the voltage doesn’t hurt anything?

The batteries can take some abuse. Often the voltage drops below a full charging voltage because of the alternator getting hot.

At the Interstate Battery blog I read: “An alternator rated at 100 amps requires about 2 horsepower.” Just curious if people think much about the effect of charging on gas mileage when they report values. Charging would require as little as of 3% of the Travato’s total 280HP. But I have no idea how much of a van’s total horsepower is normally in use while it’s moving.

It would have an effect on MPG but the load decreases as the the batteries charge.

[Saldar]

The batteries, if very low, will only accept high current for a short time.

Does this mean the batteries accept more than ~50A only when they are very depleted and a 50A limiter would not be an issue when charging up from, say, 50%?

Does anything "turn off" the alternator if you drive when the batteries are fully charged? Is it easy to over-charge?

Can you get near full charge from an alternator? Or would you need to use your converter or solar converter to get the last 10%?

I hadn't thought of the possibility of running the engine and the generator together. I didn't know there were self-resetting fuses! Your answers are very clear. Thank you for sharing your alternator knowledge.

[booster]

If you have AGM batteries, even a single one will draw more than 50 amps for a while, probably even at 50%. The auto resetting breakers will protect things as long as the work correctly. One problem is that they will wear out pretty quickly when cycling on and off all the time.

Nothing shuts the alternator off from the batteries unless there is dedicated switch to do it, and I don't think Travato has one. They can be added, as many of us have.

Yes you can easily overcharge your batteries if you go for a long drive when the batteries are full, like when coming off shore power.

To get the batteries full on the alternator, you need to drive somewhere in the 8 hour range from 50% down, I think. Probably near 6 hours of that would be at low amps, as the last 25% of the charging will take close to 6 hours, if you want to be very full.

[cruising7388]

Originally Posted by booster

Yes you can easily overcharge your batteries if you go for a long drive when the batteries are full, like when coming off shore power.

If so, this will come as an unwelcome news to millions of RV and passenger vehicle owners that regularly start off their lengthy drives with fully charged batteries.

[booster]

Originally Posted by cruising7388

If so, this will come as an unwelcome news to millions of RV and passenger vehicle owners that regularly start off their lengthy drives with fully charged batteries.

Yes it is, and probably one of the reasons many batteries don't last all the long. There is also a lot of variation in how long folks get out of their batteries, which likely has a lot to do with their camping and charging patterns. There a several variables that can shorten battery quite a bit, so they can add up.

[markopolo]

It used to be that there'd be considerable voltage drop from small-ish gauge wire, long runs to the batteries & diode type isolators. It might be 14V under the hood but only 13.5V back at the house batteries.

I'd even say a fully charged battery was a rarity. The older converters only put out 13.6V or 13.8V.

That's all changed or changing with the better chargers and better wiring and relays instead of diode type isolators etc.

[markopolo]

Originally Posted by Saldar

............................... I didn't know there were self-resetting fuses.......................

Yes, they're used for things like this. I don't know if Winnebago uses them on the house battery run though.

It's not clear to me what limitation is on the wire from the relay to the house batteries on a Travato. It would be best to contact Winnebago and ask them.

Forum member Wincrasher confirmed that it was 1 gauge wire in his Travato on that wire run.

[booster]

If it is 1 gauge, it will likely easily handle whatever a factory 180 amp alternator will put out hot. It could have a breaker of fuse of 150 amps or higher.

[Saldar]

Nothing shuts the alternator off from the batteries unless there is dedicated switch to do it, and I don't think Travato has one. They can be added, as many of us have.

The Travato has some kind of battery switch:"The coach battery is also protected by a disconnect switch." Do you know what this is meant for?

If you add a switch would it go between the alternator and the rest of the 12V system? Would you use your battery monitor (e.g. Trimetric) to decide when to disconnect?

[booster]

Originally Posted by Saldar

Nothing shuts the alternator off from the batteries unless there is dedicated switch to do it, and I don't think Travato has one. They can be added, as many of us have.

The Travato has some kind of battery switch:"The coach battery is also protected by a disconnect switch." Do you know what this is meant for?

If you add a switch would it go between the alternator and the rest of the 12V system? Would you use your battery monitor (e.g. Trimetric) to decide when to disconnect?

What the call a "disconnect switch usually is a switch or relay that shuts off the batteries themselves from the coach. It could also keep the alternator from charging the batteries, depending on where it is in the wiring, but you would have no coach power when it is off. It would be used normally to isolate the batteries when not being used to eliminate parasitic losses.

Yes, the switch would go between the alternator and the 12v system of the coach. I think Winnebago uses a standard relay (at least in some models), so the switch could be a small switch in the wiring to the coil of the relay, so you could shut it off.

Yes, you would use the battery monitor to determine when to turn off the alternator charging to the coach.

[booster]

Originally Posted by markopolo

It used to be that there'd be considerable voltage drop from small-ish gauge wire, long runs to the batteries & diode type isolators. It might be 14V under the hood but only 13.5V back at the house batteries.

I'd even say a fully charged battery was a rarity. The older converters only put out 13.6V or 13.8V.

That's all changed or changing with the better chargers and better wiring and relays instead of diode type isolators etc.

The voltage drop certainly is there, even in big wiring systems. We have 3/0 wiring and show a voltage drop of .59v at our max 280 amp limit. At our more common limit of 180 amps, it is .38v. In our case, I intentionally allowed the drop to be seen by the batteries, by sensing alternator voltage at the separator, because I didn't want the starting battery to see upwards of 15v for long periods of time because of the regulator upping the voltage to compensate for the voltage drop to the batteries if I sensed at them. For us, it is really almost no consequence to do it that way, as we can also adjust the amps to get what we want, as we have excess capacity. All we lose is the difference in watts due to the difference between 14.3 and 13.7 or 13.8 volts at fixed amps. Since the watts are what determine how much the batteries heat up, watts is the limiting factor, so we could balance out the watts by increasing the amps by the same % as the the voltage drop.

I think in the past, with the fixed voltage (13.8v) chargers, batteries were nearly always undercharged while camping, as Marko says. I also think that they were often severely overcharged if the charger was left on for days/weeks/months once you got home. The classic "boiled dry" that we so often heard. The voltage is too low for decent overnight charging, and too high for float, so no win there. The new chargers are getting better, it appears, but most likely still leave the batteries undercharged most of the time. More of them are moving away from algorithm charging to using amps to control stages, but they are not measuring battery amps and are using a % of the charger output to control things. If the the battery banks size and the charger settings happen to match perfectly, you could get full, but that can be tough to do. The chargers usually switch to float at abot 10% of charger capacity, so a 200ah bank that needs to get to get to 1 amp to the batteries would only have a 10 amp charger which is way too small. I expect to see more chargers controlled off of batteries monitors or shunts, as they seem to be drifting that way, and some currently exist. Even making the % setting adjustable would be a big plus, and at least one charger does that.

Again as Marko said, diode isolators are on the way out. The .7v drop through them keeps the coach batteries from charging well, although it does help prevent severe overcharging. A relay type separator with a way to shut it off is a way better setup in every way.

Of course, all of this could get to be obsolete information in the not so far future. As we have seen in many of the lithium battery systems, the charge control has been moved to the batteries themselves, making the solar, alternator, and shore charger just sources of amps and volts.