Charging From The Tow Vehicle - Good Idea or Not?

[Alphacarina]

More than you ever wanted to know about "storage batteries"....

http://www.batteryfaq.org/

Another good one here

http://www.windsun.com/Batteries/Battery_FAQ.htm

Don

[Joanne]

I just took delivery of a new Little Guy 4-Wide Platform TD. I ordered it with a deep cycle battery and wiring for 12 volts in the TD. It is also set up to charge from the tow vehicle while driving.

I am new to all this and so I read everything I could in sdtripper2's index related to batteries. That was a BIG help. To keep the battery in good condition, I ordered the Battery Tender Plus.

One thing confused me though. In one of the posts, it was stated that having the battery hooked up to the tow vehicle is not a good idea. As I remember it, the writer said that if I start out on a trip with a fully charged battery (as mine will be having been tended by the Battery Tender Plus), that I will actually lose charge as I drive and that I will arrive with only 70% charge. That would not be good at all.

1. Does that seem correct?
2. If so, should I disconnect the charging wire (or put a switch on it to keep it off when not needed)?
3. Also, if so, is there something I can do to use the tow vehicle's power to keep the TD's battery topped off correctly.

Finally, can someone enlighten me on why this is or is not true?

Thanks, everyone, for making this forum such a great resource.

Hi Chuc,

I solved this issue by using a solenoid and a switch. The solenoid is only activated when the ignition is on AND the switch is on. This lets me control when the trailer battery is connected to the tow vehicle.

By tapping off of the ignition switch I can't accidentally leave the two batteries connected together when the engine isn't running. No run-down starting battery!

Here's a diagram.



Joanne

[Alphacarina]

Joanne

Excellent diagram - That's an economical alternative to the much more expensive ACR I mentioned . . . . though not quite as foolproof since it can still accidently be left off if you forget about it . . . . or left on with the engine not running, again if you forget about it

I hope what you have labeled as a 'solenoid' is really a 'relay' because soleniods were designed for intermittant duty, such as energizing a starter for a few seconds at a time and not being engaged for hours on end. You don't need anything capable of handling 200 amps or more like a soleniod can - A cheap 20 amp relay would do you just fine

If it is really a solenoid, check it (carefully) with your hand after it's been turned on for half an hour or so - It will likely be very HOT! and that may not be a good thing, longterm

Don

[brian_bp]

I have essentially the same thing as Joanne. Yes, I can forget to turn it on... I only put the switch in so that the relay is not on the 99% of time during which the trailer is not attached. I turn it on at the beginning of a trip, and off at the end (which might be a week later).

The relay is a continuous-duty unit sold for this purpose by all of the RV shops. Really, it is a solenoid-actuated contactor, as are all of the devices used for this purpose. A "solenoid" is the electromagnet coil, but has fallen into use to mean a "starter solenoid", which is in turn a solenoid-actuated contactor to feed an engine's starter motor; a "relay" is a device which relays information, so if the output power is used to charge a battery (not just to trigger another device or display information) it shouldn't be called a relay... but now it means all sorts of electrically controlled switches. The point? The name isn't important, but the ability to be on continuously while carrying a few tens of amps of DC current is. A regular starter solenoid might not be a good idea, as Don explained.

[Joanne]

Joanne

Excellent diagram - That's an economical alternative to the much more expensive ACR I mentioned . . . . though not quite as foolproof since it can still accidently be left off if you forget about it . . . . or left on with the engine not running, again if you forget about it

I hope what you have labeled as a 'solenoid' is really a 'relay' because soleniods were designed for intermittant duty, such as energizing a starter for a few seconds at a time and not being engaged for hours on end. You don't need anything capable of handling 200 amps or more like a soleniod can - A cheap 20 amp relay would do you just fine

If it is really a solenoid, check it (carefully) with your hand after it's been turned on for half an hour or so - It will likely be very HOT! and that may not be a good thing, longterm

Don

Hey Don,

You are right about the need for a continuous duty relay! That is what I installed.

Although I run the risk of forgetting to turn the switch on, I like having the choice of when of when I charge or don't charge.

I don't understand how the relay will stay on with the engine off though, because the ignition key is in series with the switch. Removing the ignition key OR turning off the switch will deactivate the relay.

Joanne

[brian_bp]

... though not quite as foolproof since it can still accidently be left off if you forget about it . . . . or left on with the engine not running, again if you forget about it...

...I don't understand how the relay will stay on with the engine off though, because the ignition key is in series with the switch. Removing the ignition key OR turning off the switch will deactivate the relay.

Joanne, I agree. The risk of being left "on" with the engine not running is just that the next time you do run the engine, the relay will turn on, even if you don't need it to. That's me, getting back from vacation and not turning the switch off until days or weeks later: no harm done, just no benefit to the switch when it isn't used!

I think that the ACR setup is the slickest solution... I just went simpler.