Wiring Q: camper to truck: simple Q from electrical ignoramus

dwh

Tail-End Charlie
Take a look at a specification sheet for a modern deep cycle battery. If you care to keep the blinders on, don't even seek out the internal resistance curves for AGM or Gel type batteries or for that matter any battery made by Concorde.

Spec sheet, eh? Sounds nifty. I might have heard of that before. Is it anything like a bath sheet? Bath sheets are cool.


The internal resistance in a modern design deep cycle lead acid battery only goes up a few percent from 12V to 10V. It is only when the cell voltage has dropped way down to 3 or 4 Volts that the internal resistance gets greater than 2X of the fully charged amount.

Cell voltage of lead-acid batteries is generally around 2v. Even a 12v battery pumped up to 14.8v is still under 2.5v per cell. I'm not sure what you're referring to when you say cell voltage of 3 or 4 volts.


Of course temperature changes this, and the warmer it is, the less the internal resistance rises at low charge voltages. I used to have internal resistance curves for Trojan deep cycle batteries, but I can't seam to find them. May have had it in print form and thus tossed. Odyssey has resistance curves for their AGM lines on the web, but not deep cycle lines. If you want to find the internal resistance numbers, look for batteries that meet military specifications. They aren't published for most consumer batteries.

BTW, there are a few errors and omissions with the SmartGauge writeup. Are you only going to be using it at 0F? CCA is when it is bloody cold out, ie Cold Cranking Amps. Lead acid batteries output higher currents when warm, like even at room temperature. Roughly 40% to 45% of capacity is lost from 70F to 0F. So their 1500CCA is more like 2500 warm cranking amps when it is 70F out.

That's a bit of a red herring. If the high battery voltage is A, and the low battery voltage is B, and the resistance of the circuit is C - then the current flow is going to be D.

Whether the high battery has the capability of producing 1500, 2500 or 25 million cranking amps doesn't matter - the amperage flow is still going to be D.

Changing the voltage(s), or changing the resistance will change the current, but not adding more cranking amps capacity.



That means the resistance of their engine cranking battery is lower than they calculated. "Dead Band" voltage only governs when charging occurs. At voltage differences as little as 0.01V current will still flow, and it will flow at the rate governed by the sum of the inline resistances time voltage difference. This is why paralleling of different types of batteries or batteries of different use ages is bad. The lower voltage/weaker battery will drain the higher voltage/stronger one.

That's one reason it's bad. Another is that an older/weaker/smaller battery is going to make the other batteries in the bank do more of the work during charge/discharge and they'll wear out sooner.

Even a parallel bank of perfectly matched batteries can have such problems simply if the cabling isn't also perfectly matched.

In your example of a voltage difference of 0.01v, there isn't going to be much of a "drain" before they end up equalized.

So sure, current can flow, and if it does it will drain the other battery, but in your example, it's only going to drain it by 0.01v.

But I think the dead band applies in the SmartGauge example, because he's not talking about a 0.01v difference. He's talking about a larger voltage differential between the high and low batteries. Thus, when they are connected - charging of the low battery occurs.


The saving grace for this is it only applies when the batteries are at rest. A small draw or charge current negates this issue. Rest voltages are surface voltages and only influence initial current rates. Current rates will rapidly change to ones based on actual charge state voltages. At rest also assumes the discharged battery has been allowed to rest. Most likely the user won't cut absolutely all loads, then wait and hour and instead will just start the engine to charge the battery back up. What happens then? The voltage difference will be much higher than the 0.6V they say. They also don't properly take into account paralleled house batteries. For internal resistance, paralleling works the same as for resistors. resistance total = 1/(1/R1+1/R2+1/R3...) So, two paralleled 0.02 Ohm batteries ends up with 0.01 Ohm resistance, and three is 0.0067 Ohms. So for their calculation substituting 2 0.02 ohm batteries into the dead battery pack means a total ohms of 0.012 for 50 Amps current. That is just for battery to battery transfer. We haven't added in what will come from the alternator after the engine starts or for the fact that the 0.3V shouldn't have been subtracted out.

Of course, that's assuming the aux bank he's referring to is a parallel bank, not a series bank.

But yes, he *should* have addressed that because he does repeatedly refer to auxiliary "bank".


EDIT: Sorry about the hijack OP.
 
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olsen_karl

Adventurer
Should I bother wiring in the running lights on the camper?

I suppose it won't take much to do so, just tap the headlight circuit under the hood. I hope the fuse is in the under-hood box -- will have to look...
 

dwh

Tail-End Charlie
I would, but I like lights. If your truck has a trailer light hookup, then you could tap that for the camper running lights.
 

olsen_karl

Adventurer
If your truck has a trailer light hookup, then you could tap that for the camper running lights.

Good idea. The truck has wiring for a 7-pin trailer hookup. I'll look under the hood first; since I'm running wiring from the solenoid to the camper battery anyway, it'd be easier and cleaner if I route the running light wire in the same bundle.
 

olsen_karl

Adventurer
Thanks Bogo - I'm planning on wiring in the running lights, just haven't gotten to it yet. This weekend will be the wiring project of connecting the camper to the truck via the solenoid dwh linked to, and I'll add wiring for the running lights at that time.
 

olsen_karl

Adventurer
Finally finished -- everything works!

I basically copied the setup in the rv.net link I posted, and used the solenoid that dwh posted a link to, plus a 30A auto-resetting breaker between the solenoid and the camper. I just used 10ga wire for the connections, and ran wire to the 7-pin plug on the camper, using a Hopkins RV jack. I decided not to gut the camper wiring and run 4ga (or heavier) as posted, as I don't need to rely on the vehicle to charge up the camper batteries -- I just wanted a trickle charge to help keep the batteries charged while driving. Anything larger than 10ga and I wouldn't have been able to use the 7-pin plug, as the large gauge terminals seem too large to fit inside the 7-pin connector.

I used "add-a-fuse" taps to tap into the underhood fuse box for an ignition-on lead for the solenoid, and also to run the running lights on the camper. The add-a-fuse tap packaging says they're good for 10A only, so I added them to 10A circuits. Shouldn't be a problem for the solenoid, but I'm not sure the running lights will continue to work -- might pop a fuse quickly there. If so, I'll just rewire that part to a relay to run the running lights.

I tested the 12V current in the camper with the truck idling, and it read out at over 14V, so it's working. Thanks again everyone.
 

dwh

Tail-End Charlie
Sweet. Electrical work is very satisfying when you get it done and hit the switch and works. Like magic! Woot!
 

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