Alternator vs solar charging

[BillTheHiker]

I have found that my solar panel will charge the camper battery (1 year old Exide AGM) up to 14.4 volts, according to my multimeter. If I run the refrigerator at night the battery will drop to a low 12 number, typically 12.1 - 12-3 volts. On my last day of camping it was cloudy and raining on the way home and after a three hour drive the battery was at 12.58 volts. I put the camper in the garage and plugged in my new "smart" battery charger, which is a 4 stage charger. The next moring the charger showed 100% charge and 12.9 volts. The multimeter also showed 12.9 volts. I let the charger run in trickle mode a couple days and still showed 12.9 volts.

I took the camper to a shop that specializes in auto-electric. With the refrigerator running and the truck engine running and the solar panel unplugged they measured 6 amps going to the camper battery. they said the wire from the alternator to the camper is 16 gauge and said replacing it with a heavier gage would charge the battery faster, but likely would not take it past 12.9 volts. they also said the alternator and smart charger have a voltage regulator that prevents them from charging more than what they think the battery needs.

So why does my Morningstar controller allow the battery to be charged more than the alternator or charger?

Does all this sound reasonable to you folks?

 

[Mighty Dodge Ram]

I can’t speak to the solar as I haven’t installed my Renogy unit yet. But I just returned from a desert trip with temps in the 100s and my ARB running solely off of the alternator charge into a marine battery. I can get from 12.5 to 13.5 volts (measured at the fridge) running to the back where the battery is located in my shell. Key: I’m using a Lance Camper system that has 8 gauge wiring. I never got less than 12.3 volts in the morning when I checked. And, I run the fridge very cold so that I can keep ice cubes and frozen foods in the bottom of the unit. I feel confident that once I install my solar that I’ll have my bases covered with plenty to spare. Seems to me that perhaps your first step might be to run a larger gauge charge wire as suggested by the shop to see if you can increase the charge coming from the alternator. I would have expected more of a charge after a 3-hour drive.

Looking forward to replies from those with much more knowledge than I and how you address this issue.

 

[Vic Harder]

Welcome to world of battery charging. Thanks for providing lots of good info on your system, it makes commenting/advising a whole lot easier!

Battery voltage is not a great way to measure SOC (State of Charge) but it is a start. 12.2V in an AGM is about 50% charged. And this implies NO Load on the battery, and that it has been "just sitting" for a while. That means no load and no charging on it for a few hours. Since this rarely happens when we are looking at our batteries, you can see why the voltage measurement isn't that useful. And that is why we use battery monitors with shunts that measure all the current going in and out of our batteries to get a much better SOC reading. Victron BMV 712 or Trimetric 2030 monitors come to mind.

Next, your alternator. Different manufacturers spec different voltages for their trucks. My 2002 GMC puts out 15+ volts, many Toyota's are far lower. Given enough volts at the source, a bigger wire does make a huge difference. So, first, find out what the alternator is putting out in terms of volts at cruising RPM.

Worst case scenario that you want to design for is a heavily discharged battery (12.2v or less). To charge the camper batteries, you want about .5v more than that at the camper battery, after losses over the connecting wires. When the battery is nearly fully charged, the current over those wires is far les and so is the voltage drop. I use 2g welding cable for my alternator camper battery connection and can get 70A charging currents!

Your battery. Exide AGM you say. Find the Exide spec sheet and see what they say that battery wants for charging voltages. You will see terms like Bulk/Absorb/Float. Your 4 stage charger is hopefully setup to do those stages at close to what your particular AGM battery wants.

Morningstar solar charger. Depending on how clever this guy is (programmable Bulk/Absorb/Float?) it is likely doing the best job of charging your batteries. My Victron 75/15 is very good at this, and yes... while charging you will see voltages like the 14.4 you mention. Once the Morningstar drops down to Float stage, this will be closer to 12.8 or so.

 

[ardvark]

Welcome to world of battery charging. Thanks for providing lots of good info on your system, it makes commenting/advising a whole lot easier!

Battery voltage is not a great way to measure SOC (State of Charge) but it is a start. 12.2V in an AGM is about 50% charged. And this implies NO Load on the battery, and that it has been "just sitting" for a while. That means no load and no charging on it for a few hours. Since this rarely happens when we are looking at our batteries, you can see why the voltage measurement isn't that useful. And that is why we use battery monitors with shunts that measure all the current going in and out of our batteries to get a much better SOC reading. Victron BMV 712 or Trimetric 2030 monitors come to mind.

Next, your alternator. Different manufacturers spec different voltages for their trucks. My 2002 GMC puts out 15+ volts, many Toyota's are far lower. Given enough volts at the source, a bigger wire does make a huge difference. So, first, find out what the alternator is putting out in terms of volts at cruising RPM.

Worst case scenario that you want to design for is a heavily discharged battery (12.2v or less). To charge the camper batteries, you want about .5v more than that at the camper battery, after losses over the connecting wires. When the battery is nearly fully charged, the current over those wires is far les and so is the voltage drop. I use 2g welding cable for my alternator camper battery connection and can get 70A charging currents!

Your battery. Exide AGM you say. Find the Exide spec sheet and see what they say that battery wants for charging voltages. You will see terms like Bulk/Absorb/Float. Your 4 stage charger is hopefully setup to do those stages at close to what your particular AGM battery wants.

Morningstar solar charger. Depending on how clever this guy is (programmable Bulk/Absorb/Float?) it is likely doing the best job of charging your batteries. My Victron 75/15 is very good at this, and yes... while charging you will see voltages like the 14.4 you mention. Once the Morningstar drops down to Float stage, this will be closer to 12.8 or so.

Right on. Simple answer here is the battery is not being charged to a higher level. It doesn't get fuller than full. You can see surface charges, but turn something one without a charger source and see what you get.

 

[BillTheHiker]

Turned the refer on without any charging source and the multimeter shows 12.55 volts.

 

[Vic Harder]

Turned the refer on without any charging source and the multimeter shows 12.55 volts.

And that will slowly drop as the battery discharges. When it gets to 12.2V you might think you are drained (50% SOC), but turn off the refer and wait 30 minutes, and it will be higher than 12.2, even without any charging. So you CAN go lower than 12.2V with a load on, but you won't KNOW your actual SOC without a shunt based metering system (a good one).

Do you have any questions about my longer post, above?

 

[BillTheHiker]

Vic,

I will find out about the alternator specs and try measuring output at 2500 rpm. Not sure what would be involved in replacing the wire to the alternator but it would be beyond my skills. the guy at the auto shop said it would not be worth it, but he doesnt care if I dont have cold beer in the desert.

 

[alano]

If you are concerned about your battery charging, it is much cheaper and easier to add another solar panel than it is to change the wiring. It would also allow you to stay in place for a day or more depending on the weather.

Alan

 

[BillTheHiker]

If anything, I have more than enough solar. It is just a 100 watt panel on roof but the only thing it runs is a 35 watt Truckfridge. As I mentioned above, it takes only a few hours in good sun to fully charge the battery while the fridge is running. I will be travelling to coastal areas that get a lot of fog and also some rainy and cloudy days so if I do anything it would be a bigger battery. If my panel cannot make a larger battery happy then I would get another panel and maybe a portable one

 

[BillTheHiker]

After 3 hours the voltage dropped from 12.55 to 12.44. All measurements were taken while the fridge compressor was running. What is interesting is when the compressor shuts off, the voltage increases one tenth of a volt in just a few seconds, then levels off. Must be what Vic was referring to about the battery recovering a bit when no load is on it.

 

[Taku]

That's why when I get around to putting in solar and changing my battery system, I am heading to Canada to visit Vic.....

 

[klahanie]

I have found that my solar panel will charge the camper battery (1 year old Exide AGM) up to 14.4 volts, according to my multimeter. ...

I put the camper in the garage and plugged in my new "smart" battery charger, which is a 4 stage charger. The next moring the charger showed 100% charge and 12.9 volts. The multimeter also showed 12.9 volts....

So why does my Morningstar controller allow the battery to be charged more than the alternator or charger?

I'm going to say 12.9v SOC is prob fully charged - have no reason to think the home smart charger is not working properly. I don't think your AGM can be charged to hold 14.4v. Likely the "up to 14.4v" from the Morningstar is that chargers output voltage at the time of measuring. The line voltage at that time was 14.4v but the battery will only charge until it is "full", say ~12.9 resting voltage (measured after a period of no charge or discharge of say 4 hrs).

Your home smart charger might also output up to ~14.4v
For eg mine is rated at: 14.8 bulk, 14.2 absorb, 13.6 float

took the camper to a shop that specializes in auto-electric. With the refrigerator running and the truck engine running and the solar panel unplugged they measured 6 amps going to the camper battery. they said the wire from the alternator to the camper is 16 gauge...

Would be useful to know what the terminal voltage was during that test, measured at the camper battery or fridge. Likely the 6 amps was almost entirely feeding the fridge as the voltage was prob too low to charge the camper battery much. 16ga is tiny for charging in this scenario. The fridge load will drop the voltage running thru the relatively high resistance of the small ga wire (perhaps by 1.8-2v in a 40' circuit) . Larger gauge wire has less resistance and less voltage drop. Likewise for shorter vs longer distance.

and said replacing it with a heavier gage would charge the battery faster, but likely would not take it past 12.9 volts. they also said the alternator and smart charger have a voltage regulator that prevents them from charging more than what they think the battery needs.

Again, 12.9 battery resting voltage is prob close to 100%. If a larger wire gets the battery to that voltage, great. But it takes a higher voltage than 12.9 to fully charge.

Presumably the alternator is charging the truck battery satisfactorily so it should stand a chance to charge the camper battery, maybe not 100% but to a decent, usable level.

If your alternator output voltage is lower than what your battery requires, clearly it won't charge. Likewise for the charge voltage at the load end (camper battery) which is affected by voltage drop. Also the alternator may not sense the need for charging and who knows what the pcm battery charge logic is .

I can't find easily the exide spec but for eg my agms require the following for best charging: 14.2-14.4 bulk, absorb, 13.2-13.4 float

EDIT to add quotes

 

[DanoT]

Electricity and batteries are like women; we talk about them a lot but no one really understands them.

That's all I got, except that instead of replacing the solenoid and thin wire installed by Rob at Coyote RV, I just added a 6ga. wire with a big fuse and manual disconnect switch from truck battery to camper batteries.

I mostly use my camper in winter, staying at ski area parking lots with propane fridge and furnace.So no solar but I do have a Honda generator.

 

[JaSAn]

I have found that my solar panel will charge the camper battery (1 year old Exide AGM) up to 14.4 volts, according to my multimeter. If I run the refrigerator at night the battery will drop to a low 12 number, typically 12.1 - 12-3 volts. On my last day of camping it was cloudy and raining on the way home and after a three hour drive the battery was at 12.58 volts. I put the camper in the garage and plugged in my new "smart" battery charger, which is a 4 stage charger. The next moring the charger showed 100% charge and 12.9 volts. The multimeter also showed 12.9 volts. I let the charger run in trickle mode a couple days and still showed 12.9 volts.

I took the camper to a shop that specializes in auto-electric. With the refrigerator running and the truck engine running and the solar panel unplugged they measured 6 amps going to the camper battery. they said the wire from the alternator to the camper is 16 gauge and said replacing it with a heavier gage would charge the battery faster, but likely would not take it past 12.9 volts. they also said the alternator and smart charger have a voltage regulator that prevents them from charging more than what they think the battery needs.

So why does my Morningstar controller allow the battery to be charged more than the alternator or charger?

Does all this sound reasonable to you folks?

A 12 volt lead acid battery is fully charged at 12.7 volts nominal (2.12 volts per cell). If you are reading 14+ volts from the solar controller while charging you are reading the voltage of the controller. Ditto with the smart charger. Voltage is electrical pressure, it needs to be higher at the charger than the battery to push electrons into the battery. The battery specs will tell you what the the bulk, absorb, and float charger voltages need to be for efficient charging and long battery life, and the Morningstar should be programmable.

If you are reading voltages higher than ~12.7 volts when the battery is not being charged, you are reading surface charge. You need to let a lead acid battery rest for at least 4 hours (preferably 12 hours or more) for the surface charge to migrate into the lead plates (the thicker the plates the longer this takes). Voltage reading is a very slow, lagging indicator of battery charge.

 

[Old Crow]

Below is a link to a Battery Charging and Storage Specifications sheet from late 2015 for Exide batteries.

I believe the bottom row applies, i.e., the Flat Plate Exide Edge. I say this under the assumption that when Bill says he has an Exide AGM battery, he probably has the Exide Edge model AGM24DP I've seen others mention.

Exide Battery Charging and Storage Specifications

Unfortunately, the chart may raise more questions than it answers. The SOC voltages, for instance, challenge our (well, MY) notions of what percentage discharge a specific open-circuit voltage indicates.

Perhaps I've just not been aware how much variation there can be between battery manufacturers. When I compare to this Fullriver 85ah Group 24 battery, I'm struck by how different the SOC and charging-voltage numbers (and the operating temperatures) are.

.

 

[BillTheHiker]

Klahanie: my Morningstar does hot have a digital display, just three different color lights for battery status. All my posted voltage readings are from my multimeter directly on the battery terminals.

 

[klahanie]

^ understood.

to your question, "So why does my Morningstar controller allow the battery to be charged more than the alternator or charger?"

with respect to the charger, I not sure that it does because using the charger overnight resulted in a 12.9v reading (I'm guessing soc close to 100%). Meaning both controller and charger can charge the camp batt to "full". As they should, that's what they are designed to do, with sufficient wiring and time.

with respect to the alternator, because the load end voltage and current, measured at the camper battery is inferior. Also because the controller is "smart" and the alternator likely isn't, in the same way.


While driving that 16ga wire can run a fridge, lights etc. but not charge your camper agm much.

 

[klahanie]

Not sure what would be involved in replacing the wire to the alternator but it would be beyond my skills

fwiw, for charging, I connected truck battery terminal(s) to camper battery terminal(s), didn't touch the alternator.

I can't say for sure you'll be satisfied with a larger charge wire but I will say your truck alternator is also designed to charge and I expect the oem charge wire to the truck battery is much heavier than 16ga. Plus the distance is much shorter. Size and distance are key factors.

If your camper is hooked up to the truck towing harness and back to the caper battery that's added distance.

As you mentioned,second camper battery might be the way go for you. Fully charge at home, hope for sunnier weather. Only kicker is having the space and the dreaded "both batteries should be the same type, age, production run, etc". Bow wow.

or ... could you learn to love warm beer ??

I mean if you had too ...

 

[Vic Harder]

After 3 hours the voltage dropped from 12.55 to 12.44. All measurements were taken while the fridge compressor was running. What is interesting is when the compressor shuts off, the voltage increases one tenth of a volt in just a few seconds, then levels off. Must be what Vic was referring to about the battery recovering a bit when no load is on it.

yup

 

[PaulT]

Yeah, but Calgary is in such a nice neighborhood.

Paul