Charging AGM Batteries from Car Alternator

kelly · January 2015

I would like to have the option to charge my trailer battery from my car while towing. I have come up with the following plan:

1. Connect directly to the tow vehicle battery positive with a solenoid battery isolator controlled by the ignition circuit.
2. Use heavy gauge wire to the back of the tow vehicle and install a quick-connector to a heavy gauge wire on the trailer.
3. Install a charge controller to prevent overcharging (AGM batteries prefer a lower float voltage than starter batteries).

My first question is whether it is worth wiring the negative battery terminal all the way back, or whether I should just connect to the vehicle frame near the hitch.

Question number two is whether the Trimetric solar charge controller (SC-2030) would be a good choice for this. It is PWM, so it shouldn't care that it is a car alternator not a solar panel, plus it is hooked up to a trimetric battery monitor, so it should do a particularly excellent job of preventing overcharging.

BB. · January 2015

Re: Charging AGM Batteries from Car Alternator

kelly wrote: »

1. Connect directly to the tow vehicle battery positive with a solenoid battery isolator controlled by the ignition circuit.

There are battery isolators (relays, diodes) for RV's/vehicles. Does work OK, but you will still need another charging source to fully/quickly recharge a deep cycle battery. They "like" higher voltages (~14.8 volts) vs what a car battery usually receives (~13.8 to 14.2 volts or so).

Also, automotive alternators tend to get hot when outputting lots of current, which tends to reduce their output current and charging voltage. Again, reduces ability to fully/quickly recharge a flooded cell lead acid battery bank.

2. Use heavy gauge wire to the back of the tow vehicle and install a quick-connector to a heavy gauge wire on the trailer.

This is a bit problem with 12 volt wiring... You have a pretty long path from the vehicle's alternator/battery back to the trailer's battery bank. With any "practical" size of copper wire (unless you start using welding cable), you probably will not see much more that 10 amps or so of charging current. For a smaller battery bank and lots of driving, this may be useful--Or not.

3. Install a charge controller to prevent overcharging (AGM batteries prefer a lower float voltage than starter batteries).

AGM should be fine without a charge controller when paralleled to an automotive battery/alternator. In general, automotive alternators do not output more than ~14.2 to 14.4 volts--Not usually a problem for AGM.

For a "simple" charge controller between the two battery systems:

http://www.solar-electric.com/batteries-meters-accessories/bach2/me-sbc.html
http://xantrex.com/power-products/power-accessories/auxiliary-battery-charger.aspx

If you need "serious" battery charging--You might think about putting a good AC inverter connected to the vehicle battery and send a 120 VAC cord back to the trailer and use an AC to DC battery charger in the trailer. You will have much better charging current/voltage at the battery bank--Independent of the charging voltage/wiring voltage drops in the system.

My first question is whether it is worth wiring the negative battery terminal all the way back, or whether I should just connect to the vehicle frame near the hitch.

You probably should do that... Otherwise, you may end up cooking the ground wire in the trailer light circuit.

Question number two is whether the Trimetric solar charge controller (SC-2030) would be a good choice for this. It is PWM, so it shouldn't care that it is a car alternator not a solar panel, plus it is hooked up to a trimetric battery monitor, so it should do a particularly excellent job of preventing overcharging.

In general, yes, it should work OK... But I would suggest the other two I linked to above. Includes a battery disconnect circuit--Plus if you like it, the RV solar array can recharge the battery in the two vehicle too (at the the Midnite will do this--The Xantrex may too).

If vehicle towing time is going to be a significant source of charging power--I would look at the Inverter=>AC battery charger. Much better overall charging current/charging function.

You can also look for DC to DC battery chargers... They will take "12 volts" and output a higher voltage (13.8 volts or more) to charge the battery bank. Here is one (don't know anything about the vendor or product, just a starting point for your research):

http://www.powerstream.com/DCC.htm

-Bill

kelly · January 2015

Re: Charging AGM Batteries from Car Alternator

BB,

Thanks for your recommendations. The two charge controllers that you recommend are really just replacements for the battery isolator that I was going to install, but your alternatives are definitely worth considering. I respect the theoretical efficacy of the inverter followed by a charger, but that brings me to a significant limitation in my system: I do not have an inverter with a built-in battery charger in my RV; all I have is a WF power converter. Even if I could get AC power to the trailer, the charging from that power converter would not be very effective. Honestly, I am a bit squeamish about mounting an inverter under the hood of my car. Using an inverter in the cab and then snaking the wires out windows seems a bit Mickey Mouse.

My AGMs are supposed to float at 13.2-13.4V. My concern is that I could be driving around with fully charged batteries and the alternator will maintain my AGMs at 14.2V in float - which might be for hours. I think I need a charge controller that will prevent overcharging.

BB. · January 2015

Re: Charging AGM Batteries from Car Alternator

The third one is an actual battery charger...

But, I would not worry about "floating" AGMs vs "charging" at 14.2 volts. Floating is great if you have AC power (24x7) and/or batteries that sit unused when "charging power is available" for long periods of time (i.e., RV that is mostly AC powered--Perhaps weekend/seasonal trips).

Unless you are driving 8 hours per day 5+ days per week with fully charge battery bank--I would not worry about needing a float capable charger. Most people drive a couple of days for 8 hours, then "camp" for days at a time. Then drive back home (with AC power) for many more weeks before the next trip.

A few hours randomly at 14.2 volts vs 13.x volts--I think the batteries will age/cycle out before failing from over charging/venting.

Check the charging voltage of your car's battery... And the current/voltage at the trailer battery bank. If the "float" current is less than 1% (100 AH battery bank, or 1 amp) when full and tow vehicle is running--Would not even give it a second thought.

If, however, you are feeding 2.5% or more into the AGMs when they are full and driving--That would worry me if that kept up for hours/days at a time.

-Bill

scrubjaysnest · March 2015

This is why I have stayed away from AGM and Gel batteries. The way our TV is wired, I have a 40 foot run from batteries to alternator, voltages from the TV run between 14.7 to 15 volts at the battery.

KenZ71 · March 2015

Dont some vehicle mfg installed tow packages provide for trailer battery charging through the wiring harness? If you have that already in place and the tow vehicle alternator has the capacity you might be farther ahead than you think.

If not and your amount of towing is relatively short would it be easier to have extra battery mounted under the vehicle hood while driving then move back to the trailer when you arrive? Many Chevy/ GMC trucks have a mount for a 2nd battery and available relays to charge under various scenarios.

scrubjaysnest · March 2015

The 7 pin connector that come with tow packages have a charge line. Ford and Chevy use isolation relays on their newer vehicles, Our 2008 Dodge came with no isolation and a 22 awg charge line. After re-wiring the system plus adding electronic isolation is why I can over charge AGM's or Gels. Some people can get away with that but my luck doesn't work that way.

kelly · October 2015

I have been charging my AGM batteries from the car alternator without a controller. The battery is usually below 80% when I start driving, just from usage. I have not had any issues with overcharging that I have noticed. My trimetric once indicated 101% charge on the batteries when I arrived, but more commonly it only charges up into the mid 90's. So far, it appears that the system doesn't need an additional charge controller.

Mountain Don · October 2015

Isn't there a potential issue with overcharging the TV (tow vehicle) battery when using a simple direct connection between TV and trailer? IF the battery in the trailer is low but the TV battery is full, then IF the TV charge system "sees" the lower voltage of the trailer battery the TV battery might receive an overcharge. Make sense???

I've never liked the diode based battery isolators. Even the best shottky diode based isolators have a voltage drop. I've always used a continual duty relay to make the connection from TV to trailer as they are cheaper than any of the isolators that do not have a voltage drop.

kelly · October 2015

The alternator doesn't see two batteries, it just sees a voltage. In the case you describe, the TV battery voltage will be drawn down as it sends current back to the trailer. The alternator will run to maintain its setpoint voltage - the TV can't receive an overcharge since the alternator is maintaining the same voltage, regardless of what the trailer system is doing.

Mountain Don · October 2015

that makes some sense I guess....

But then is that a different scenario than running batteries parallel in an off grid application. We are told that parallel batteries are bad as differences in sp gr occur in one parallel string it can affect the rate of charge = over or undercharge in another parallel string???

kelly · October 2015

It would be bad to just leave the TV and trailer batteries connected in parallel all the time. I have a "smart battery combiner" that only connects them when the voltage is high.

PNjunction · November 2015

If you don't mind a smaller 2A charger, then Optimate makes the model TM500, which is a dc-dc smart charger, which works from a "source" battery, or of course a dc power supply if you want.

I have one for odd situations, and it is weird seeing this orange charger with two sets of battery clamps - on on each end. Works well, and has protection for the source battery so you don't absolutely kill it. The Optimate 6 is my favorite workhorse in the low amperage range (5A max), but that only works with ac.

oil pan 4 · November 2015

I have used AGM deep cycle batteries as vehicle starting batteries. The alternator does not over charge them.

mcgivor · July 2017

@haddymontange Your comment is very opinionated, but you fail to back up your comment with rationale as to why it was made in the first place, please understand I've no personal opinion in the matter , but for the purpose of discussion, it would be helpful to include your opinions as to why it a bad idea, as all may benifit from constructive criticism, however judging from the language used, that probably was not the internet.

littleharbor2 · July 2017

When your trailer battery reaches the full output voltage of the charging source the amperage being accepted by it will taper down to a minimal amount, basically an absorption charge. I don't see a problem with this. Most batteries don't get long enough of an absorption charge as it is to get 100% fully charged.

Marc Kurth · July 2017

littleharbor2 said:

When your trailer battery reaches the full output voltage of the charging source the amperage being accepted by it will taper down to a minimal amount, basically an absorption charge. I don't see a problem with this. Most batteries don't get long enough of an absorption charge as it is to get 100% fully charged.

Absolutely correct because In most RV applications, the run time is normally limited to hours instead of weeks/months.

In practice, the voltage drop on the stock, long wiring run to the trailer tends to limit charging. Also note that I am seeing many cars and trucks with alternator regulators that drop their output voltage on long drives. Even my wife's older (2011) Chevy Traverse drops down to around 13v after charging at 14.5v

I have customers who refuse to drive their RV more than 4-5 hours per day. Others may run 14+ hours. As I often say here, it is important to select the right AGM battery for the specific application - they are not all the same. Same thing holds true for specific applications......they are not all the same.

This of course needs to be reviewed for blue water boats, long haul trucks or buses with team drivers, because the engine may stay running for very, very long periods. It is common in those cases to use a three stage charge controller like the Xantrex and Balmar units.

Lifeline has a 5 year warranty on their AGM batteries used in RV and Marine applications being charged by a standard alternator. FullThrottle series AGM's have a 4 year warranty. Obviously, neither company is concerned about alternator charging.

I sell Lifeline and Fullriver AGM's for a living - around 2,500 batteries per year. When I offer an opinion, it is based on years of application experience. If my veracity is doubted, call Lifeline or Fullriver and ask the engineers if I know what I'm doing.

I can assure you that we are not seeing early and horrible deaths...........

BB. · July 2017

I have banned and deleted @haddymontange post.... It was not useful.

-Bill

jonr · July 2017

> the voltage drop on the stock, long wiring run to the trailer tends to limit charging.

Old topic, but I agree. If you want proper charging (vs just offsetting loads to prevent drain), some type of voltage step-up and independent charge control for the trailer makes sense to me. Also as others have said, consider that complete lead acid charging needs time - shorter drive times would require some other power source every week or two.

vankeys · July 2017

Here's a really informative video on the subject

westbranch · July 2017

Apart from the 14.8V as a standard for ALL AGM instead of 14.4V, overall a reasonable primer...

Lumisol · July 2017

He lost me when he said papel instead of people. Lol
Honestly there may be some information there but I couldn't understand what he was trying to say. A professional narrator would have made the video understandable for me.

softdown · March 2019

https://forum.solar-electric.com/discussion/comment/395945#Comment_395945

Paragraphs would like to be your friend.

There is one thing I have a great deal of experience with . Trailering with 7 pin connectors. The 22 gauge battery charging wire is meant for that tiny battery that operates the brakes if the trailer becomes disconnected under tow. Nice idea but those little batteries are both expensive and very short lived in my experience. 22 gauge is very, very far removed from what is likely to be a need for something much closer to 2 AWG to receive substantive 12 volt power ~10 yards away. I used 2/0, for example, aluminum to run my power hungry winch in the rear receiver.

I don't particularly love any of the solutions listed to date. Too much weather and vibration for many electronics. I might consider charging additional deep cycle batteries under the hood with quick disconnects and simply carrying them back to the trailer as needed. Or charge them from an AC power source at home or campground or... Just another idea - exactly what this thread needs.

Larman66 · March 2019

Well now that I got my story out there and it was discussed I could be a little more brief I don't want to turn to solar panels I have no desire for that I am running my house on 3 12 volt batteries and inverter and I charge the batteries through either my generator my car depending on what time it is in my neighborhood lately I've just been doing it with the car and it's I'm talking to you so but I have three different kinds I have a deep cell I have an AGM from Germany from a Porsche and I have a very good regular LED battery for my truck in parallel circuits I've had no trouble so far but I can't run my refrigerator or my dryer unless I do it off my generator and it it switches Cycles too much to wash your so I won't do it 5500 watt generator I'm looking more toward better batteries that can run my house even if I got to shut some breakers off which I do cuz I got the 10 gauge to 2:40 running from my generator to the Box just for my well I run my generator 5-10 minutes it takes me to Phyllis once maybe twice a day depending of people are here I live alone couple lights are on I got a propane hot water heater now propane stove no refrigerator no washer yet and I'm in the process of changing over to gas dryer what kind of batteries can I get and a different way to charge them cuz I know eventually the alternator is going to go that's what I'm after I have no desire to turn solar I just don't want to pay the electric company decide bills anymore because my entire house is electric until I started to convert over to propane I need help with the battery selection how many and what I could use for a charging Source other than having my power on I'm getting by just fine right now but it took me a while to get here I just wanted to get better I want my refrigerator and I want to be able to do my laundry here that's all here I go a long paragraph again LOL

mike95490 · March 2019

Welcome

Both a fridge, and a washer are pretty stout loads, and a 12vdc system is going to be "working hard" to run either one of them, if your fridge kicks in while you are doing laundry, that's going to have an impact.

I'd suggest at least considering a small 800 - 1200w PV system to keep the batteries charged and healthy between generator runs. The components are not too expensive, and will at least cut the generator usage in half (when you have sun)

We have an energy star fridge, which I know from experience, pulls over 2kw when starting it's conventional compressor - it would kick a eu2000 generator off line when it starts.

i also can speak to our front-loader washer (samsung) that it draws a very variable load when it's running, averages 200-300w, but has lots of motor reversals as the drum changes direction. I hear my 6kw inverter "groan" at each reversal. This might be a case for dual inverters, one in the 800-1200w class for your fridge and daily loads, and another +2kw just to power up for the washing machine.

Charging AGM Batteries from Car Alternator

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