Understanding Battery separators versus battery isolators, for charging via a car alternator.

[F350joe]

Just do this except go with 4ga and use a breaker to match the length of your cable run. Parts list in post number 7. The isolater he links to shuts off if no battery is detected so its just like stock for daily duties with no house battery plugged in. Nice thing about this system is you can move it to a bear box or under a table. You wont get a full charge into the agm but the vehicle system will give you another day or 2 to run the fridge in about 10-20 minutes of running. http://campinnforum.com/community/threads/auxiliary-battery-for-refrigerator.4190/

 

[CampStewart]

So how many amps can a discharged AGM take? How long to charge via the alternator? I am torn between using a selonoid, a dc/dc charger, or an inverter and a 110v charger while on the road camping. I have a maintainer where the battery lives 99 percent of its life in the garage. Vehicle is a Subaru Outback

 

[BobInTheAspens]

Keep it simple. The suggestions above that you should charge from your alternator with a $40 solenoid are right on. Think of it this way: When the engine is running, you want the batteries connected. When the engine isn't running, disconnected. Basically, it's that simple, and that's exactly what the solenoid will do.

See evldave , elsewhere in this forum, for a good description, with a serious diagram!

Be sure you buy good quality, perhaps Cole Hersee. Get SPST continuous duty. I suggest 200 amp rating, no sense in skimping here.

That solenoid will draw 1 - 2 amps for the coil. I wasn't confident that the source I found for 12V ignition switched voltage in the fuse box would handle that. So I put in a relay between the fuse box source and the solenoid coil. That might be overkill, but it only cost a few bucks. Plus, I found a time delay relay on Amazon, that delays the connection for 8 seconds, giving things a chance to stabilize.

About wiring: No problem putting the battery in the trunk (Assuming it is a sealed unit like an AGM). And you won't need a huge wire to feed it. I suggest #4 or #6 wire. That will tend to limit the amperage when the battery is very low, but that's probably good. It will still easily carry the 10 or 20 amps that will be your normal condition. Overheating the wire will not be a problem.

About voltage: Both your main battery and your 2nd battery need 14.4 - 14.6 volts at 70 deg. F. Colder temperatures need more. If the battery is really hot, like 120 deg., the voltage should be more like 13.9 V. Your car probably came from the factory set up for a lower voltage. That will soon kill both batteries. Get the voltage up or you will be disappointed. On most newer cars that is an easy fix.

Cred: I've had this scheme in my 4Runner for 4 years, including 3 months overland to Costa Rica, and 3 six week trips to Baja. Works great. My main battery is a Toyota brand flooded, my 2nd battery is a Optima Yellow Top. All still going strong.

And, it's really nice on a road trip to pull into camp will full batteries!

 

[Buddha.]

Get the voltage up or you will be disappointed. On most newer cars that is an easy fix.

Ok, so what's the fix?

 

[rayra]

Our GM trucks put out that much voltage but those Japanese cars don’t. I think 13.8 is the norm, and then there’s voltage drop and losses at the connectors. The point about the DC to DC charger reducing the output from 100amps to the 20/40 the charger puts out is kinda moot. The battery isn’t gonna take 100amps of charge anyway.
The cheap solenoid is still a decent option for OP, his AGM house battery might have somewhat reduced life from not getting as high a voltage for as long as it wants. If the battery is sitting in his garage most of the time though he could just leave it on a good trickle charger and it wouldn’t matter.

All true and requires that the 'expedition' start out with the AGM / House topped off. I'm thinking several camping days / nights later and a shift to a new camp. And in that scenario I think a DC-DC charger of an Amp rating well below the potential output of the vehicle alternator would wind up being a bad choice. A chokepoint for pushing a big charge into that depleted House battery while driving down the road. all depends how much load is on it while camping, and what makeup charge comes to it via a solar setup. I think an extended glamping trip will show the DC-DC idea comes up short.

/I was getting ~14.1 out of my Sub's factory Bosch 130A and now 14.6-7 out of my chicom aftermarket '220A' HO ALT. So I think I could go with an AGM / Odyssey / etc that needs the higher voltage applied. But there again my design choice is to stick with commonly / widely available (and matching) grp78 flooded batts

One of these days in confinement I'm going to spend the time for a comprehensive one-topic description of my electrical mods. There's nothing expensive, trendy or sechsy about it. But I think I've put together a highly capable setup. And I want to put it all in one place for easier reference and referral. The pieces are spread all over the place as it is. I even have trouble finding some of it sometimes.

 

[rayra]

And, it's really nice on a road trip to pull into camp will full batteries!

That right there is exactly what I'm talking about. I'm doubting that a glamper* going on a 7-14day trip across 4-5 locations will find that to be the case in the back half of the trip, relying on a 20A DC-DC to re-fill the House batt between trips.


* I dont mean 'glamper' as a perjorative, just referring to the modern electrical everything, fridge, gadgets, blender laptops, tablets, cellphones, DSLR, drones, electric blanket, portable A/C, rope lights, night lights, ''scene' lights etc 'Overlander'.

 

[DCH109]

That depends on charging voltage, size of battery, how low its been discharged and its condition of sulfation.

I think the problem with a discharged AGM is when it is almost completely dead. There is a procedure to get them back up and running and regular charging will not do it. My Battery charger is modern and designed for AGM battteries however when it drained down to far the charger would not charge it.
Look at this from Odyssey. https://www.odysseybattery.com/documents/ProceduretorecoverdeeplydischargedODYSSEY.pdf
I ran into this issue with the one on my FJ60, the battery was totally flat, or so I though it was. . Following the procedure it is back 100%. (I also found my battery drain in my 60).
Checking Optima it seems to also be an issue https://www.optimabatteries.com/en-us/support/charging/resuscitate-deeply-discharged-battery . So I am sure this is an issue with just about any AGM.

 

[BobInTheAspens]

So, how to boost the alternator charge voltage? This ought to be its own thread, but, for now, a quick once over.

Before you start, I strongly recommend that you install a voltmeter in your car, where you can see it as you drive. I have a Scan-Gauge, or you can get the $5 item that plugs into your cigarette lighter. Watch what your alternator is doing. Observe warm vs. cold, idle vs. cruise, discharged vs. charged, high load vs. low load, etc. Note that if your alternator voltage sags lower at low rpm or high load, boosting the desired voltage won't make any difference.

Some theory:

Voltage: With one exception, everything in your car is fine with voltage from 12 - 16 volts. (The things that need an accurate voltage, like the ECU, have their own voltage regulators). The one exception is your batteries. They must have the right voltage, or they will soon die. Either over or under charging will kill batteries. When you search the web, you will find lots of mis-information about voltages. All I can say is, If it was easy, anybody could do it!

Voltage vs. Temperature: The voltage your batteries need depends a lot on the temperature of the battery. Note that these voltages apply to lead-acid batteries, both flooded and AGM. Your alternator adjusts its output voltage according to temperature. Hopefully, the alternator output voltage should be about:

Temp. F/C Voltage
122/50 13.80
104/40 13.98
86/30 14.19
77/25 14.34
68/20 14.49
50/10 14.82
32/0 15.24
14/(-10) 15.90

So, at 70 degrees F. the charge voltage should be about 14.4V. What if your alternator is set for a lower voltage than it should be? You can trick it to raise its setpoint fairly easily. As it runs, the alternator constantly compares the voltage at the battery to what it thinks it should be. If the battery voltage is a little low, the alternator cranks itself up a little. If it is too high, it backs off a little.

The Reference Wire: To make these adjustments, the alternator needs to monitor what the battery voltage is. It uses a wire that is just for that purpose, called the Reference Wire.

************************************************************************
What we need to do is find the Reference Wire, and without telling the alternator, slightly drop the voltage that the wire is carrying. That will make the alternator think the battery voltage is a little low. It will then up its output voltage slightly to adjust. That's just what we want! So, how to slightly lower the voltage in that wire?
************************************************************************

Find the Wire: On my 4Runner, it's easy because the Reference Wire has its own fuse. On your Subaru, I don't think it does. However, on Subaru, the alternator is right on top, so your can see the wire as it comes to the alternator. It is one of the three wires in the green 3 wire connector on top. Which one? I'm not a Subaru expert. More research needed!

Drop the Voltage: A diode is normally used as a one-way valve for electricity. We will ignore that, and take advantage of the fact that a diode will drop the voltage of electricity that runs through it by either .3V or .7V, depending on which kind of diode we have. That's just what we want! So, basically we will put a diode in the Reference Wire.

Diode Location in Toyota, Nissan, GM: Find the fuse for the Reference Wire. Replace it with a combined fuse and diode. You can make one for a few bucks, or buy one on Ebay for $20.

Diode Location in Subaru: I think you need to find the wire on top of the alternator. Cut it and put the diode in it.

Diode Details: For a .7V increase, go to Amazon and buy Rectifier Diodes, 1 amp, 80 volts. Package of 10 for $5. Or 2 amp, or 50 volts, it's not critical. For a .3V increase buy Shottky Rectifier Diodes.

Diode Install: Diodes have to be installed in the right direction. You can avoid the issue by taking two diodes, putting them side by side, facing in the opposite directions. (One end of each diode is rounded, so just put the rounded end of one going one way, and the rounded end of the other going the other way. ) Twist the wires together, and whichever way you install it will be OK.

I'll make a new thread about all this, but now I'm off to the desert. Yes, I'm serious about distancing, but I figure that the desert west of Green River, Utah, at the end of a rough 2-track, is about the right distance from the rest of the world. As a result, I'll be off the web for a week or so. Talk some more later.....

 

[FJR Colorado]

All this makes a DC-DC charger sound like an easy no-brainer...

I'm sure the electrical police will correct me in short order

 

[moose545]

A DC-DC charger would do exactly what you want to do without the need to run heavy 1AWG or 0AWG cables.

Here's the writeup on the one I installed on my F-150 last weekend. Should work in any vehicle. Biggest issue is going to be getting a pair of 8AWG wires from the battery compartment to the trunk. If you can do that, everything else is easy.

Installing Renogy DC-DC charger in 2018 F-150

Posting this for anyone who might be interested in doing something similar. Background: The idea behind this came from my previous vehicle, a 2004 Suburban. When we camp we use an Indel-B TB 41 refrigerator and in the past we've had "issues" with it discharging the battery enough to cause...

www.expeditionportal.com

I was planning on sacrificing a set of jumper cables at 4AWG to run back to another 27F or similar 100Ah battery, but your saying 8AWG would work? If so that'd be much easier to work with, seems too small from all I've read though?

 

[Rando]

So, how to boost the alternator charge voltage? This ought to be its own thread, but, for now, a quick once over.

Before you start, I strongly recommend that you install a voltmeter in your car, where you can see it as you drive. I have a Scan-Gauge, or you can get the $5 item that plugs into your cigarette lighter. Watch what your alternator is doing. Observe warm vs. cold, idle vs. cruise, discharged vs. charged, high load vs. low load, etc. Note that if your alternator voltage sags lower at low rpm or high load, boosting the desired voltage won't make any difference.

Some theory:

Voltage: With one exception, everything in your car is fine with voltage from 12 - 16 volts. (The things that need an accurate voltage, like the ECU, have their own voltage regulators). The one exception is your batteries. They must have the right voltage, or they will soon die. Either over or under charging will kill batteries. When you search the web, you will find lots of mis-information about voltages. All I can say is, If it was easy, anybody could do it!

Voltage vs. Temperature: The voltage your batteries need depends a lot on the temperature of the battery. Note that these voltages apply to lead-acid batteries, both flooded and AGM. Your alternator adjusts its output voltage according to temperature. Hopefully, the alternator output voltage should be about:

Temp. F/C Voltage
122/50 13.80
104/40 13.98
86/30 14.19
77/25 14.34
68/20 14.49
50/10 14.82
32/0 15.24
14/(-10) 15.90

So, at 70 degrees F. the charge voltage should be about 14.4V. What if your alternator is set for a lower voltage than it should be? You can trick it to raise its setpoint fairly easily. As it runs, the alternator constantly compares the voltage at the battery to what it thinks it should be. If the battery voltage is a little low, the alternator cranks itself up a little. If it is too high, it backs off a little.

The Reference Wire: To make these adjustments, the alternator needs to monitor what the battery voltage is. It uses a wire that is just for that purpose, called the Reference Wire.

************************************************************************
What we need to do is find the Reference Wire, and without telling the alternator, slightly drop the voltage that the wire is carrying. That will make the alternator think the battery voltage is a little low. It will then up its output voltage slightly to adjust. That's just what we want! So, how to slightly lower the voltage in that wire?
************************************************************************

Find the Wire: On my 4Runner, it's easy because the Reference Wire has its own fuse. On your Subaru, I don't think it does. However, on Subaru, the alternator is right on top, so your can see the wire as it comes to the alternator. It is one of the three wires in the green 3 wire connector on top. Which one? I'm not a Subaru expert. More research needed!

Drop the Voltage: A diode is normally used as a one-way valve for electricity. We will ignore that, and take advantage of the fact that a diode will drop the voltage of electricity that runs through it by either .3V or .7V, depending on which kind of diode we have. That's just what we want! So, basically we will put a diode in the Reference Wire.

Diode Location in Toyota, Nissan, GM: Find the fuse for the Reference Wire. Replace it with a combined fuse and diode. You can make one for a few bucks, or buy one on Ebay for $20.

Diode Location in Subaru: I think you need to find the wire on top of the alternator. Cut it and put the diode in it.

Diode Details: For a .7V increase, go to Amazon and buy Rectifier Diodes, 1 amp, 80 volts. Package of 10 for $5. Or 2 amp, or 50 volts, it's not critical. For a .3V increase buy Shottky Rectifier Diodes.

Diode Install: Diodes have to be installed in the right direction. You can avoid the issue by taking two diodes, putting them side by side, facing in the opposite directions. (One end of each diode is rounded, so just put the rounded end of one going one way, and the rounded end of the other going the other way. ) Twist the wires together, and whichever way you install it will be OK.

I'll make a new thread about all this, but now I'm off to the desert. Yes, I'm serious about distancing, but I figure that the desert west of Green River, Utah, at the end of a rough 2-track, is about the right distance from the rest of the world. As a result, I'll be off the web for a week or so. Talk some more later.....

This does work for older vehicles with an externally controlled regulator. However this will not work for newer ECU controlled alternators (for instance most 2016+ Toyotas) where the regulation happens within the alternator itself and the communication with the alternator is digital.

You also may be trading one shortcoming for another. Adding a 0.7V drop diode to the sense circuit will move all the set points up by 0.7V. So while the absorption charge voltage may now have moved from 13.9 to 14.6V, the float voltage also moved from 13.5 to 14.2. On long road trips, your starting battery may not enjoy sitting at 14.x for hours at a time.

 

[Rando]

I was planning on sacrificing a set of jumper cables at 4AWG to run back to another 27F or similar 100Ah battery, but your saying 8AWG would work? If so that'd be much easier to work with, seems too small from all I've read though?

With a 20A DC-DC charger, 12AWG would be fine but 10AWG would give you some margin. 8AWG is borderline overkill. It is the wire ampacity that matters.

 

[moose545]

With a 20A DC-DC charger, 12AWG would be fine but 10AWG would give you some margin. 8AWG is borderline overkill. It is the wire ampacity that matters.

I just plan to run the aux for my fridge and accessories and use the aux as a backup starting battery should the primary ever have a problem, is that possible on 8AWG? If so then that's excellent, since it's much easier to work with and run. My AUX battery will be 100Ah, and using the Redarc BCDC1225D DC to DC charger and solar controller.

 

[rayra]

I was planning on sacrificing a set of jumper cables at 4AWG to run back to another 27F or similar 100Ah battery, but your saying 8AWG would work? If so that'd be much easier to work with, seems too small from all I've read though?

Big difference between wire gauge sufficient for passing max charging amps from your alternator rating, vs the juice needing to be passed for an starter (which on most vehicles is 1/0 or 4awg. I would not be relying on 8awg to power a starter. Guess it depends on the draw of your starter.

btw, I can vouch for these inexpensive 20' 4awg jumper cables, they are a true fine-strand copper wire set

https://www.amazon.com/gp/product/B00F9RNIPI/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

even with the bunch of cheap accessories in the kit, even at full kit price for just the wire it comes to <75cents/foot.

 

[Buddha.]

I’m genuinely curious about my Silverado.
If alternators output lower voltage the warmer they are, and batteries don’t like to sit at 14+ volts, then why does my voltage get the highest after 45 min of driving?
Ive got a voltage readout on my cigarette lighter plug and I see my highest voltage 14.8-15.3 after I get to work(45min drive) and let the truck idle for a few min. Longish 2 hr drives do the same while still driving. Initial voltage is something like 14.3.