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Old 09-21-2020, 08:51 AM   #1
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Batteries: OK to use two charging sources?

My camper has two 12V batteries hooked together (would that be in series?). It's currently in for some repair (not battery related) and we're taking it camping the day after I get it back. The batteries were pretty depleted before I took it in so i will need to charge them quickly when it comes back from repair. Can I do so using both the onboard converter and a trickle charger at the same time? Or is that not a good idea. Also, when using a trickle charger, do I clamp the positive lead to one battery and the negative to the other battery? vs. both to one battery.
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Old 09-21-2020, 09:01 AM   #2
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Are you going to be Boondocking?

If not, and your tow vehicle can provide power to the camper when towing, I wouldn't worry about it. Just plug it in when you get home for a few hours before you leave. The inverter should charge the battery sufficiently for an emergency situation.
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Old 09-21-2020, 09:12 AM   #3
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My camper has two 12V batteries hooked together (would that be in series?). It's currently in for some repair (not battery related) and we're taking it camping the day after I get it back. The batteries were pretty depleted before I took it in so i will need to charge them quickly when it comes back from repair. Can I do so using both the onboard converter and a trickle charger at the same time? Or is that not a good idea. Also, when using a trickle charger, do I clamp the positive lead to one battery and the negative to the other battery? vs. both to one battery.
Two 12 volt batteries are in parallel not series.

You need to use one or the other chargers and not both simitanacly.
If you do each charger will try to figure out if the batteries need a high charge or lower charge.
If you put both on they will not fully charge the batteries.

My suggestion is to use the largest amperage charge you have and shut down or not use the smallest amperag charger you have.

Like the previous msg don't you have your vehicle charging your batteries while driving? This is the best way to keep them charged.
Do you have a 7 pin connector?
Look and test pin 4 should be your vehicle to trailer charge cirucit.
Sorry no pictures.
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Old 09-21-2020, 09:37 AM   #4
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Thanks! yes we will be boondocking, and hopefully using the furnace a bit. It's only a 48-hr trip so should be able to get by on fully charged batteries, but the shop previously advised us that it would take several days on either a trickle charger or the converter to bring 2 depleted 12V batteries up to par. Oh well, we'll do the best we can, but it sounds like we should not use the two chargers simultaneously.
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Old 09-21-2020, 10:00 AM   #5
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It is fine to use two chargers simultaneously. Look at the full timers with large battery banks and solar arrays. Do you think they unhook their kilowatt array when they plug in to shore power?

If your batteries were depleted when you took it to the dealer they will probably be dead when you pick it up unless you disconnected them.
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Old 09-21-2020, 10:19 AM   #6
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It is fine to use two chargers simultaneously. Look at the full timers with large battery banks and solar arrays. Do you think they unhook their kilowatt array when they plug in to shore power?

If your batteries were depleted when you took it to the dealer they will probably be dead when you pick it up unless you disconnected them.
The larger amperage will run the show and the smaller one will be a waste of time.
If they were the extact equals then yes it would be good.
You need to undertsand basic electricla theory before saying something like this.
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Old 09-21-2020, 10:22 AM   #7
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Thanks! yes we will be boondocking, and hopefully using the furnace a bit. It's only a 48-hr trip so should be able to get by on fully charged batteries, but the shop previously advised us that it would take several days on either a trickle charger or the converter to bring 2 depleted 12V batteries up to par. Oh well, we'll do the best we can, but it sounds like we should not use the two chargers simultaneously.
If you have it hooked up to your truck charging system will go into high charge mode because it will sense a battery that is lower.
That is only if you have the trailer, plug and cable was properly wired.
Like I said teminal 4 wiring is the key if you have or don't have it wired already.
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Old 09-21-2020, 10:43 AM   #8
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Thanks, we do have the 7-wire setup. But I'm not sure if it's working properly because although the trailer's turn signals and brake lights work properly, for some reason the tail and clearance lights do not, even when the TV tails are on. So I don't know if I can trust it's actually charging the battery during towing. This stuff is frustrating.
BTW we bought the trailer used from a dealer and had the TV wiring installed at another shop that does their wiring.
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Old 09-21-2020, 10:49 AM   #9
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It is fine to use two chargers simultaneously. Look at the full timers with large battery banks and solar arrays. Do you think they unhook their kilowatt array when they plug in to shore power?

If your batteries were depleted when you took it to the dealer they will probably be dead when you pick it up unless you disconnected them.
Solar power controllers are specifically designed to work in conjunction with other chargers. That's not true of a trickle charger, or even an RV converter.

The solar controller senses the voltage at the batteries. If the batteries are being bulk charged by the converter (14.4V), the solar will stop charging. Lead acid batteries have a maximum rate at which they can be charged - about .25C (where C is amp-hour capacity). If you have a single 80AH Group 24 battery, it can't/won't take more than 20 amps charge maximum, and then only for a short period before it tapers to less.

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Old 09-21-2020, 12:31 PM   #10
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If you do in fact have a trickle charger, then it will do very little is a short period to charge the battery... a trickle charger will charge a lightly discharged battery in a day or more, but anything more needed will take several days of time. A trickle charger typically charges at 1 to 3 amps, which is fine if you have the time. Larger battery chargers will charge from 5 to 10 to 15 amps which reduces charge time to overnight or a day at most.

Assume your batteries are nearly depleted at the repair shop... ( do you have a cheap meter to measure the voltage of the batteries? $5-$10 at any hardware store. A fully charged battery is 12.6 to 12.7 volts ( measured after charger is off and battery sets unhooked for 3-4 hours). A discharged battery will be less than 12 VDC. If you hit 10.1 your battery will have lost some capability permanently.

take a picture of the connections with your phone, ( see diagram below) then take the batteries out and to your home and charge 1 at a time with something other than a trickle charger. You can get a 5 amp to 10 amp charger for $60 or less at Walmart or amazon. I found a good deal on one from amazon for you see below...

You probably have little to no charging capability from your tow vehicle to your camper, unless you know differently for sure. Often a fuse needs to be added to the TV to make it work.

You will have difficulty with electric 12 VDC power even for one night if you don't prepare ahead of time.
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Old 09-21-2020, 12:33 PM   #11
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Some years of F150s require a relay and fuse installed in order to get the charge to the trailer. There would be a bag in the glove box or on the door storage with the relay and fuse.
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Old 09-21-2020, 01:09 PM   #12
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With 2 12V batteries...you have between 150 and 210 amp hours of battery to refill.
Lets say you fully emptied them by accident...instead of only to 50% which is best to avoid battery damage and insure the most life cycles. This would mean that with a modern 3 stage charger...you could put in a maximum of about 50 amps of charging current for a while before the battery chemistry would resist sufficiently to taper down that current and slow the charging more and more as you get fuller and fuller.
So...if you had a charger that could deliver that 50 amps...that would be quicker...perhaps 6-8 hours...but even at 1/2 that (25 AMPS) would still have you charged in 12 hours max.
If you only had 10 or 12amps for charging from your converter...then your mechanic would be correct.
I note that many modern chargers CANNOT charge a fully depleted (dead) battery,,, and you'll need to use and old fashioned brute force charger from a service station or auto parts place to bring your batteries up above 25% before you can put them on your own converter/charger.
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Old 09-21-2020, 01:20 PM   #13
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If it were me and the batteries were in a low state of charge I would consider disconnecting the batteries from each other, connect one battery alone to the converter and the other charge with a stand alone charger (not the trickle charger). This way each battery gets full attention from its own charger and charging time should be almost cut in half. Take pictures before disconnecting to be sure to re-connect properly.
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Old 09-21-2020, 01:38 PM   #14
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12v batteries are in parallel, not in series.
I agree that the batteries will probably be dead.

First thing you should do is plug the trailer into shore power first. Let the converter recharge the batteries.

Second thing is install a battery disconnect switch.

Third thing is to test the 7-pin connector socket on your TV. It may not be providing a charge through the umbilical cord. Most tow vehicles only provide a trickle charge at best. So if the batteries are low, you would need to drive 8-10 hours. A short trip will provide very little recharging.
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Old 09-21-2020, 01:38 PM   #15
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If it were me and the batteries were in a low state of charge I would consider disconnecting the batteries from each other, connect one battery alone to the converter and the other charge with a stand alone charger (not the trickle charger). This way each battery gets full attention from its own charger and charging time should be almost cut in half. Take pictures before disconnecting to be sure to re-connect properly.

That would be the best way to bring them both up and one charger won't conflict with the other.
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Old 09-21-2020, 01:45 PM   #16
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The larger amperage will run the show and the smaller one will be a waste of time.
If they were the extact equals then yes it would be good.
You need to undertsand basic electricla theory before saying something like this.
I understand it quite well and theoretically you can and one issue is that the electrolyte level will evaporate a lot quicker which if you don't watch it could permanently damage your battery. Now I'm not sure if a few hours will really do a lot to diminish the levels so I'd check them before you start and once your done if its only for a few hours. Technically this is feasible until you bring it to full charge, once this happens then you could have issues with the two chargers, in layman's terms fighting each other and the chargers and even the battery could be damaged. Of course threshold voltages of the chargers and its internal resistance come into play too and one could shut the other down until equalized. So like I said it can but anyone including myself we need a lot more detail to calculate if its worth it or not. So I guess in short "do you fell lucky"
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Old 09-21-2020, 01:52 PM   #17
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Two chargers will not be fighting each other. I can turn on my converter or my solar and see the current add when I put them on at the same time. When the batteries get fully charged you might get some slightly different behavior but then it hardly matters.
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Old 09-21-2020, 01:53 PM   #18
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Wow what a lot of great information, thanks everybody!
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Old 09-21-2020, 01:55 PM   #19
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Thanks, we do have the 7-wire setup. But I'm not sure if it's working properly because although the trailer's turn signals and brake lights work properly, for some reason the tail and clearance lights do not, even when the TV tails are on. So I don't know if I can trust it's actually charging the battery during towing. This stuff is frustrating.
BTW we bought the trailer used from a dealer and had the TV wiring installed at another shop that does their wiring.

Um I may be missing something but couldn't you just have the repair facility put them on a charge for you?
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Old 09-21-2020, 02:39 PM   #20
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You are right and that's what I will do, thanks! This all will come in handy for future reference, though.
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