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Old 11-08-2007, 05:07 PM   #1
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Required voltage to charge batteries

My '07 Tundra has 13.8 v. at idle and 13.9 v. at 2,000 rpm (a common rpm while driving) at the trailer plug. My understanding is that 14.4 was the usual output voltage in the past, but some auto & truck companies have reduced it to make the truck/car batteries last longer.

Will 13.8 or 13.9 v. charge the trailer batteries (assuming they are not completely drawn down)?

Will the fact there was no load on the battery output circuit—at least at the plug (or I couldn't test it there)—change the voltage?

Thanks for any information.

Gene
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Old 11-08-2007, 05:33 PM   #2
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yes that is hi enough to provide a charge.

Many battery chargers drop down to 13.6 for a fully charge battery.

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Old 11-08-2007, 05:55 PM   #3
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Gene,

Proper 3-stage charging requires 14.2-14.4 VDC for both the bulk and acceptance charging and 13.2-13.3 for the float charge. A lower initial charge will NOT make your batteries last longer......to the contrary.....they will hasten their death from insufficient charging. In fact, if you only have a single phase charger, it should be putting out 14.2-14.4 VDC and should be left on charge until the internal voltage of the battery reaches 13.2-13.3VDC when it should be manually dinconnected.

Most 3-stage chargers on the market today use the voltages above. I have no idea why the OEM auto guys would use a lower voltage, unless they are gell cells which are extremely voltage sensitive and use lower charging voltages. Both liquid lead acid and AGM's use a higher voltage with AGM's being the highest.

It might also be possible that your alternator charging system will not put out the full 14.2-14.4 VDC until a certain RPM is reached and will no show this at idle.
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Old 11-08-2007, 07:44 PM   #4
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I think the Tundra battery is liquid lead acid as they are the cheapest. Toyota used to use sealed lead acid, but no more. The Tundra put out 14.1 when started, running at about 1,200 rpm, but when idle went down to 700 or so, voltage dropped to 13.8 and only went up .1 at 2,000. I expect the higher voltage may have something to do with topping off the Toyota battery. I think it might be more accurate to check voltage when the Airstream is hitched, umbilical plugged in, and get the voltage somewhere else. Not sure where yet: there must be an input on the converter—seems like the logical place.

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Old 11-08-2007, 08:09 PM   #5
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Quote:
Originally Posted by CrawfordGene
My '07 Tundra has 13.8 v. at idle and 13.9 v. at 2,000 rpm (a common rpm while driving) at the trailer plug.
voltage at the trailer plug is somewhat like hp at the wheels...

many issues contribute to the reduction down line...

have you measured output from the alternator at the source?

what is the amp/hour rating for the alternator?

regardless,

charging the trailer batteries via the 7 pin is a slow slow process.

it takes many hours of driving (days really) to replenish significantly depleted trailer batteries via the 7 pin...

and just as long to fully top off batteries not depleted.

there are several threads here that work through the issues, like alt output, other draws, wire size and so on...

but i seem to recall a dislike for too much calculating?

cheers
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Old 11-08-2007, 09:05 PM   #6
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13.8 to 14.2. My truck runs at 14.1 down to 13.9 depending on the load. I have a large alternator.
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Old 11-08-2007, 10:40 PM   #7
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the voltage from the alternator will vary depending on the state of charge of the tv battery and the temperature. actually "charging" discharged trailer battery takes ages unless the tv battery is similarly discharged.
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Old 11-16-2007, 11:30 AM   #8
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Good answers. I would add if I may that the higher voltage IS required at some point to overcome the internal resistance of the battery and complete the charge. Voltage is the force needed to overcome the resistance.
Like mentioned, there will be a drop in voltage through the charge line. Also like mentioned, temperature does play a part.
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Old 11-17-2007, 08:24 AM   #9
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It seems that most car and truck alternators charge at only 5 amps, and often at lower than 14.4 volts. Lead acid batteries last longer when charged at a rate of 25% of capacity (25 amps for a 100 amp battery) and at 14.4 volts. Is there such a thing as a regulator for the TV that will provide this higher capacity and still charge the TV battery at the lower rate?

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Old 11-20-2007, 09:05 PM   #10
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More facts or possibly factoids. After driving about 200 miles I stopped at a campground and while the truck engine was idling, checked the voltage at the converter. The trailer courtesy lights and inside lights were on, so there was a load, though small, on both truck and trailer electrical systems. I checked the voltage at the converter (battery + and - terminals) and it was 12.5. The solar control panel showed trailer batteries were fully charged.

As for Terry's question about a "regulator". Doesn't the isolator perform that function?

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Old 11-20-2007, 10:04 PM   #11
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Gene,

I think the isolator just keeps the truck battery from being discharged by the trailer's batteries. I was wondering about this scenario: dry camping for a couple of days, then maybe 3-5 hours on the road to the next campsite.
If the alternator is only charging at say 5 amps, it will never get the batteries back up to full charge. And since the regulators in most cars and trucks are really designed for keeping the starting battery topped off, does the regulator sense this and decrease the charge rate to the trailer's batteries? Constant volt regulators will vary the charging amperage depending on the charge state of the battery, but what if the truck battery is full--does the regulator reduce voltage to the trailer? Just wondering.

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Old 11-21-2007, 01:58 PM   #12
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the experts will chime in but here's my input.

if the batteries are hooked together you get an "average" charge rate. if the batteries are equal, you get an even charge. with one weak battery, the weak battery gets a weak charge and the strong battery gets overcharged.
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Old 11-24-2007, 07:55 PM   #13
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To make sure the truck battery doesn't get discharged when boondocking, disconnect the umbilical. I think the isolator does that, but why take a chance based on "I'm not sure how this thing works?"

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Old 11-24-2007, 09:33 PM   #14
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Hi, your low voltage and low amperage charge is determined by battery condition and load. Run your battery down, and turn on your A/C and headlights right after starting your engine; Now with a fast idle, check out the volts and amps. Now you might be reading 14.8 volts and 30 to 60 or more amps depending on the load and state of discharge of your battery. If your tow vehicle is in a normal state of charge, then only a nominal amount current will be going to your trailer. [slow charge as many have already stated]
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