charging a deep cycle trailer battery off the towing vehicle?

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dlb
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charging a deep cycle trailer battery off the towing vehicle?

Post by dlb » Mon Apr 15, 2019 8:55 pm

I've got another random electrical question, unrelated to yesterday's.

My wife's horse trailer has a deep cycle battery (DCB) for powering the inside lights and fan. We want to charge the DCB with towing vehicle but something weird is happening: the wire with the vehicle charge only shows 14v when it is not attached to anything and when it is the only wire connected to the DCB. As soon as the battery has any other wires (ground strap to frame, wires for interior lights and fan, etc) connected to it, they battery only shows 12.8v.

I don't understand why the wire has 14v going to it but that disappears once it's connected to the battery with any other wire. But I don't have much experience with DCB's, and my understanding of electricity is lousy. Does anyone here have an idea what might be happening?

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Re: charging a deep cycle trailer battery off the towing vehicle?

Post by teranfirbt » Tue Apr 16, 2019 6:22 am

It depends a lot on the charge state of the battery. If it is fairly drained and needs a lot of charge, it will attempt to pull a lot of current from the charge vehicle. Since trailer wiring is fairly small, it will overwhelm the wiring and the voltage will drop at the battery.
To fix this, you'll either need to limit current flow to the battery with a low resistance, high power resistor, or (much better) get a DC-DC battery charger that is designed to perform this job.
Think of voltage, current, and resistance like your garden hose. If you connect a lot of things to the same hose bibb on your house, the pressure and flow to each of the things will be much lower than if you have a single sprayer end on one hose. This is the same idea as your battery situation, you're trying to pull too much charge (water) through a small wire (hose) too quickly, so your voltage (pressure) drops. Eventually, if the wiring doesn't burn up, it will charge the battery and it will read the same voltage as the charging vehicle.

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Re: charging a deep cycle trailer battery off the towing vehicle?

Post by dlb » Tue Apr 16, 2019 9:53 am

Thanks for that, Teranfirbt. That's a good explanation. I'm going to say it back to you, please tell me if I'm understanding you correctly.

So it sounds like you're saying the alternator only puts out so much charge, and by asking it to charge a DCB in addition to the tow vehicles two batteries (it's an old f-250 diesel) we are spreading that charge around too much so that each of the three batteries is basically only getting a trickle. Is that right?

Maybe today I will check the charge at the vehicle batteries and compare that to what the DCB is getting.

For what it's worth, the wiring to the DCB is 10 gauge. I thought that would be enough. Also, the DCB is fully charged at 12.8v.

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Re: charging a deep cycle trailer battery off the towing vehicle?

Post by dlb » Tue Apr 16, 2019 3:41 pm

I just did another test and confirmed what I found before. While running, the vehicle is putting out 14.7 V to both its own batteries and the end of the wire that is supposed to connect to the DCB. But as soon as the wire is connected to the DCB, it drops down to the battery's resting charge of 12.8 V.

From what Teranfirbt explained, I thought I would find that all three batteries (the DCB and both vehicle batteries) would only be getting 12.8 V when the DCB is connected. I'm probably misunderstanding something here.

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Re: charging a deep cycle trailer battery off the towing vehicle?

Post by Petros » Tue Apr 16, 2019 4:28 pm

We has several small travel trailers that had deep cycle batteries. They are usually slow to charge, but fully charged it should be showing 13.2 volts.

either it was never fully charged, or has some minor internal breakdown that will not allow it to charge all the way. it will show less voltage than the supply voltage until it is fully charged.
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Re: charging a deep cycle trailer battery off the towing vehicle?

Post by dlb » Tue Apr 16, 2019 4:43 pm

Hmmm, I didn't realize DCB's should read 13.2 V when full. I assumed it was the same as regular car batteries where 12.8 was full. Maybe that's the problem. I'll try putting it back on the charger for a day or two and see what happens.

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Re: charging a deep cycle trailer battery off the towing vehicle?

Post by keith » Fri Apr 19, 2019 3:37 pm

Right after charging any lead acid 6 cell battery, it may show up to 13.2 volts, but a small part of that is a capacitive charge on the surface of the plates. It usually bleeds off after a short time and the battery will read its open circuit charge of 12.6 volts.

Your charge wire is pretty long and has some resistance. If it is open ended, you will read the open circuit voltage of the source, aka alternator. As soon as you touch it to the battery terminal, current will flow as the battery is at a lower potential than the alternator output (engine running). Part of that higher voltage from the alternator will be dropped by the resistance of the wire, which is higher than other wires in your truck because of length and a connector it has to go through.

If all the batteries are pretty much charged up, the alternator won't have much trouble charging all three batteries. It would be a problem if all three batteries are almost fully discharged. That could damage the alternator, specifically the diode bridge rectifier. There are some options to avoid damaging the alternator. First, if the batteries are low, avoid reving up the engine. At low RPMs, the current available for charging is lower so the voltage out will be lower. It will still charge the batteries but take much longer.

You could also install a switch on the dash to isolate the charge wire to the DCB. Keep it off until the starting batteries in the truck have recharged, then switch it on to charge the DCB while cruising to your destination. Personally, I would build a circuit with a momentary push switch that sends current to a relay coil to flip the relay, once flipped, the relay would send current to the battery and to its own coil to keep it on. Then when you shut down the truck, the relay would flip off and disconnect the DCB until the truck is started and the button is pushed again.

Now about your question of why the voltage changes when you connect a load. First you indicated the ground wire was disconnected. Not sure how you measured the voltage under this condition but if the battery is not grounded, then your voltage reading may not be accurate due to a condition known as a floating ground. Not going into that here. But once you hook up the load to the battery, you may have some loads that are either always on or have a parasitic draw. Would need full schematics of the trailer and all appliances plugged into them to offer any further advice on this.

Edit: one advantage to an isolation switch or relay circuit is that it would keep the trailer from discharging the truck batteries when parked.

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Re: charging a deep cycle trailer battery off the towing vehicle?

Post by dlb » Fri Apr 19, 2019 4:58 pm

Today we tried this with a regular vehicle battery and it worked just fine so either there's something wrong with the DCB, or for some reason a DCB won't work in this application. I'm leaning toward the former.

Hmmm, I installed something a long time ago to prevent the trailer battery from drawing on the vehicle batteries but what you're saying has me thinking about a better system, Keith. I'll get this sorted out and then look into that next.

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Re: charging a deep cycle trailer battery off the towing vehicle?

Post by teranfirbt » Fri Apr 19, 2019 7:56 pm

Probably worth taking it to an auto part store and having them run a battery test on it.

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