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Old 11-25-2021, 04:41 PM   #1
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Why Tow Vehicle Charging Amps Low?

I have a Victron battery monitor on my Flying Cloud. I have noticed going down the road that the charging amps from the TV (Toyota Tundra) is only 7 amps even with fairly depleted AGM batteries.

What is keeping amps so low? Alternator is 100 amps. Is it voltage drop in wire from alternator to hitch? Is there some controller on TV keeping amps low? A controller in trailer?

I am thinking of installing a Victron DC-DC charger as part of a lithium upgrade but if 7 amps is the max for some reason, it seems like it would be a waste. 20-30 amps would make it worthwhile. What is holding things back?
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Old 11-25-2021, 04:54 PM   #2
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It's charging according to what the TV battery needs, not the AS.

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Old 11-25-2021, 05:13 PM   #3
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Thanks!

So, how do I make TV alternator put out amps for trailer wire and still protect TV battery?

If I install a DC-DC charger, can I expect more amps up to capacity of charger?
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Old 11-25-2021, 06:17 PM   #4
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Originally Posted by jnm30327 View Post
Thanks!

So, how do I make TV alternator put out amps for trailer wire and still protect TV battery?

If I install a DC-DC charger, can I expect more amps up to capacity of charger?
Yes, it's the most efficient way to charge while towing.

I can't answer any more questions because I don't use one, and I've never really thought of using/buying one.

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Old 11-25-2021, 06:31 PM   #5
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Why Tow Vehicle Charging Amps Low?

On the tundra you are limited to the current that can travel on the small guage constant 12v line on your 7-way. 2-5 amps is typical. It’s designed for a maintenance charge current only.

You need to rewire your 7-way with a heavier 10ga wire for constant 12v hot and common ground, or run a dedicated +/- line to your DC-DC charger.

Your alternator will be the next bottleneck, potentially. I’m not sure what your truck is specked with…
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Old 11-25-2021, 06:34 PM   #6
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I have not added the DC to DC converter yet. It is sitting on the workbench waiting. Yes, the TV alternator is responding to the state of charge of the starting battery. Yes, the long run of small wire reduces the charging. The DC - DC converter will increase the rate of charge limited by either how fast the battery will take a charge at its charge level and by how many amps the TV can provide to the DC to DC charger. That is going to depend upon the wire and fuse size in your TV. That limitation can be eliminated by running a separate circuit back to the DC to DC converter and a separate plug between the TV and the trailer. Lead acid batteries charging speed varies a lot with how charged they are. The alternator is rated for a high output but on a very limited duty cycle. That might be a factor with a high output DC - DC and deeply discharged lithium batteries.

Lithium batteries will take about whatever you can provide to them. My truck is fused 30 amp on the trailer circuit. I bought a 18 amp DC to DC. I am going to install it next spring and do the lithium conversion probably. You might want to check the fuse and the wire size for your TV.
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Old 11-26-2021, 09:51 AM   #7
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Why Tow Vehicle Charging Amps Low?

I would add that alternator output / charging behavior on modern vehicles is carefully controlled by the BCU.

In my truck I hardwired 4ga wire to the TV batteries (I have 2) and installed both a 12v bus and 500w inverter in the bed.

This allows me to pull straight off the batteries or invert to AC 120v for higher draw applications.
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Old 11-26-2021, 10:04 AM   #8
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A Victron DC to DC charger also controls when the current from your battery/alternator flows to our AGMs. It watches your charging voltage and decides when to connect and charge your battery. This keeps your AGMs from sucking your chassis battery down while parked (Assuming you have a direct connection to your battery/alternator). Victron also has algorithms that can effect use a 'Smart Alternator'. There's a chance that you have one. It could be effecting your current charge current.
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Old 11-26-2021, 10:31 AM   #9
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I missed , if you gave details of TV , but nowdays 100amps is low , most seems to be 120 - 160 amps .
And these are engineered systems --- intended to quickly recharge starting batteries / not long term high charge rate - needed for deep cycle multi battery system .

So have having someone that really knows your TV / and what is need to do what you want --- many times WHAT YOU WANT - IS NOT POSSIBLE , without modifications .

With more than one battery systems , secondary battery systems need to be isolated from starting battery - so the trailer / deepcycle batteries do not kill starting battery --- also the charging regulator senses the state of charge - another reason to isolate - charging each battery separately -- typically using a battery ISOLATOR , min. of 3 connections on isolator one comes from alternator and the other 2 to each battery system - starting & deep cycle .

This is where having someone that knows how , the stock alternator is not designed for this , too much high load , for too long , so upgrading the charging system with isolator , larger - longer charging --- HEAVIER gage cable [ not upgrading cable could start a fire - small wire get too hot carrying too much current .

Not everyone would do this , but having separate gauges for independently monitoring each battery , do not want to end up in the middle of no-where and a dead starting battery ;(
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Old 11-26-2021, 10:41 AM   #10
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Not many amps can go 25 ‘ in an 18 ga wire
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Old 11-26-2021, 11:28 AM   #11
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Not many amps can go 25 ‘ in an 18 ga wire
I would be surprised if it is only an 18ga....but I don't know tundras very well. My truck has a 10Ga wire and 30 Amp fuse. An 18 amp victron DC-DC converter is perfect for my application. I believe a 30 amp model would pop the fuse when AS batts are low.
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Old 11-26-2021, 04:04 PM   #12
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Quote:
Originally Posted by dznf0g View Post
I would be surprised if it is only an 18ga....but I don't know tundras very well. My truck has a 10Ga wire and 30 Amp fuse. An 18 amp victron DC-DC converter is perfect for my application. I believe a 30 amp model would pop the fuse when AS batts are low.
The 10ga wire and 30 amp fuse is the standard, unfortunately not all manufacturers follow the standard so everyone wanting to add a DC-DC converter should check their system. Personally I believe if your TV complies to the standard the 18 amp DC-DC converter is the biggest safe choice before you need to start modifying things.

Actually the OP getting 7 amps on a stock system is doing great.
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Old 11-26-2021, 04:46 PM   #13
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Originally Posted by Wazbro View Post
The 10ga wire and 30 amp fuse is the standard, unfortunately not all manufacturers follow the standard so everyone wanting to add a DC-DC converter should check their system. Personally I believe if your TV complies to the standard the 18 amp DC-DC converter is the biggest safe choice before you need to start modifying things.

Actually the OP getting 7 amps on a stock system is doing great.
I regularly saw 11 amps and once saw 14 on my Silverado....bone stock.
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Old 11-26-2021, 05:32 PM   #14
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Quote:
Originally Posted by dznf0g View Post
I would be surprised if it is only an 18ga....but I don't know tundras very well. My truck has a 10Ga wire and 30 Amp fuse. An 18 amp victron DC-DC converter is perfect for my application. I believe a 30 amp model would pop the fuse when AS batts are low.
Small wire..small charge…batteries used up..drove 4.5 hours and the batteries were still down..I looked it up for my Ram, all say 2-4 amps…
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Old 11-26-2021, 05:40 PM   #15
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I found on a Toyota site…lucky to get 5 amps:
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Old 11-26-2021, 07:17 PM   #16
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There are several useful threads regarding limited charging from the tow vehicle and how to improve the situation. I decided to go Full Monty and route welding cable from the engine compartment to the bumper, use a high amperage connector at the hitch, then more welding cable, and finally a DC-DC converter in the trailer. Works great. Not everyone will want to spend the $400+ I spent but it works and I have no regrets. You can read about it here:

https://www.airforums.com/forums/f37...le-202849.html
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Old 11-26-2021, 07:44 PM   #17
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I do know from personal experience the condition of those copper contacts within the trailer female plug can deteriorate due to weather and exposure. I used to own a Hi-Lo travel trailer which had a 12 volt refrigerator. As time progressed I found times at which I would arrive at my destination with the battery discharged to the point that I couldn't power they hydraulic system that raised the trailer's top to its operational height. Apparently I wasn't getting enough power to offset the refrigerator operation on battery as I was traveling.

After spending time looking for the reason, I looked into the plug on the trailer and saw a green coating on the flat copper contacts which indicated the copper was deteriorating due to exposure to weather. I bought a can of spray electrical contact cleaner and sprayed those contacts in the plug and also into the bargeman connector on the tow vehicle. That took all that crud of those copper surfaces and I never had problems again.

So from that point on I made it a normal practice to spray those connectors prior to going on a trip. I looked in the garage and over 35 years since I got this stuff, the can still sprays and the mixture inside is still viable from what I can see. I noticed the ingredients on the can and I think it's not sold any more. My neighbor looked at the can and said he didn't think it was very ozone friendly.

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Old 11-26-2021, 08:09 PM   #18
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Get one of these.

https://www.amazon.com/IPA-Tools-Inn...a-570352745245
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Old 11-27-2021, 07:39 AM   #19
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OP here. With further effort, I have found out that the Tundra alternator on my 2018 Crew Max with tow package is actually 170 Amps. The fuse for the charging line to the trailer is 40 amps. I have not been able to determine the wire gauge of the wire to the trailer harness. It is tempting to add the DC-DC 30 amp Victron charger and see what happens, but I suppose the wisest move is to go ahead and run the 6 AWG wire.

The other warning I have gotten is to be careful not to use the alternator for a task it was not designed to perform. For example, several hours at 50 amps. I do not know how to get to the bottom of that issue.

Getting detailed specifications on the Tundra is not easy. There is next to nothing in the owner's manual. Everything online seems to be second hand.
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Old 11-27-2021, 07:48 AM   #20
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OP here. With further effort, I have found out that the Tundra alternator on my 2018 Crew Max with tow package is actually 170 Amps. The fuse for the charging line to the trailer is 40 amps. I have not been able to determine the wire gauge of the wire to the trailer harness. It is tempting to add the DC-DC 30 amp Victron charger and see what happens, but I suppose the wisest move is to go ahead and run the 6 AWG wire.

The other warning I have gotten in this thread is to be careful not to use the alternator for a task it was not designed to perform. For example, several hours at 50 amps. I do not know how to get to the bottom of that issue.

Getting detailed specifications on the Tundra is not easy. There is next to nothing in the owner's manual. Everything online seems to be second hand.
If it truly has a 40 amp fuse (and thus assume the wire is at LEAST 10 gauge...maybe 8ga), I don't see any need for an additional wire for a 30 amp DC-DC. Even when it is running full tilt, you would still have some 25% margin on that circuit. I would investigate further on the wire gauge and verify 40 amp fuse and hook it up and go.
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