Battery set up question

[loren85022]

If and when the next OE has a workshop by DHW, I'll be there. Good stuff

 

[dwh]

...Again, as the battery reaches full charge, amperage drops, voltage drop goes down, and the battery will see the full 13.5-14V.

Exactly. The problem though, is that 13.5v-14v might not fully charge the battery.

Looking at the Trojan battery manual:

http://www.trojanbattery.com/Tech-Support/documents/UsersGuide_0708_English_003.pdf

On page 12 is where the charge profiles start. The first one - for a regular flooded battery - shows that 2.35v per cell is roughly 90% charged. So 2.35v x 6 cells = 14.1v.

To be fully charged, the battery voltage has to be pushed up to between 2.45v - 2.7v per cell, which works out to between 14.7v - 16.2v.

13.5v / 6 = 2.25v which looks to be somewhere down around 70-80% charged.

So yea, getting the battery up to 14v ain't bad. Holding it there for some hours will even drive the battery up to 95% eventually. (Which begs the question: Will the truck's voltage regulator hold it there? For how long?)

The Gel and AGM profiles are a bit different from the flooded, but similar to each other. Get them up to a certain voltage and hold them there long enough and they'll eventually reach 100% charge.

For the AGM that's between 14.1v - 14.7v.


So again, if the charging rig can get the battery to 14v and hold it there long enough the battery will get pretty close to full.

Still not completely full though. :ylsmoke:

The risk here, in hooking up the trailer battery with 4ga, is what happens if the main battery is very low. Even at 100Amps charge rate, the voltage drop is only 0.5V. The 7 pin connector can't handle 100A, and it won't be good for a flooded lead acid battery.

That's pretty much a non-issue because the vehicle charging system is not a constant current type charging system. It's a constant voltage system, so all it will do is supply however many amps are needed to hold the voltage. How many amps the battery draws will depend on the battery's internal resistance, and whatever the charge voltage is at the battery. If all it can absorb is 10a @ 14v, then that's all that will flow.

The battery won't be getting slammed with the max output of the alternator (unless it's an Optima or Concorde which have radically low internal resistance). If the battery is only drawing 10a than that's all that is needed to hold the bus voltage up so that's all the alternator will put out. As the battery charges, and the internal resistance drops then the amp flow will go up, but it'll be a gradual ramp up.

Bigger wire will not hurt the battery - a normal battery on a constant voltage charger will draw only as much as it can absorb.

And any battery with a low enough resistance to actually draw 100a from a constant voltage charging system will be able to handle 100a charge rate.

But yea, big wire will be too big for a 7-pin connector.

I don't think we disagree... I'm just being more cautious with the advice, and how fast I want to charge my battery, and how much complexity I want.

Yea, we're pretty much on the same page except for deliberately dropping the voltage with a smaller wire. I don't agree that it protects the battery.

My camper van has two batteries under the hood - one for engine and one for aux. They are connected with nice fat battery cables to a split-charge relay. When the engine is running they are tied into a single bank and the aux battery is allowed to absorb all it can. There is no need to limit the amperage because the battery's internal resistance does that.

If it doesn't hurt the aux battery to use a normal battery cable when it's under the hood - why would it hurt to use a normal battery cable if the battery happens to be in a trailer?


There is also an old 12.8v 10a Shumacher battery charger hard wired to the aux battery. It's a constant voltage charger. When the battery is dead and I hook up that charger, the battery will absorb 1a or less for quite a while before the amperage starts to rise. Eventually the amperage will rise and as the battery gets full, the amperage once again drops. Normal constant voltage charge behavior.

But here's the thing, that charger can put out up to 10a, but only at 12.8v constant. I've never seen that battery absorb more than 6a peak from that charger. So that's the limit. That battery simply cannot absorb more than 6a if the supply voltage is 12.8v.

It doesn't matter if I were to hook up a 100a charger using 1/0 wire - at 12.8v the battery still won't draw more than 6a. And when the battery is really low, it won't even allow 1a to flow through it at that voltage.*

Of course, with the engine running and supplying 14.6v, then more amps will flow - but still the battery sets the limit.


With a constant voltage charger, there is no seed to set any limit other than the maximum voltage - the battery will limit the amps by its own internal resistance, so why deliberately choke it off even further? Better to let it have all it wants.




*(Also, at 12.8v that charger never does get my aux battery completely full. That only happens when I drive the truck on the highway all day. I really should buy a proper converter/charger...but it woks, so why fix it? Like I said, I beat up my aux battery; I run it way below 50%, every now and then I run it pretty much dead, it usually only gets partially recharged most of the time. That's why I buy a cheap battery. It is a real deep cycle battery though, not a rv/marine so it can take the abuse for somewhat longer before I have to scrap it.)

 

[dwh]

My truck came from the factory with a 7pin and already has the power from the battery to the back of the truck in place.

Does it have some sort of isolator? If not then draining the trailer battery will also drain the engine battery unless you remember to unplug the trailer.

Any suggestions on parts? I do not need state of the art equipment here just solid working stuff. The solar part is really just an add on. I don't know if it is even worth the hassle. I can just run an extension cord to the trailer when not in use to a battery tender. I was just thinking a solar panel would be cool to help give a maintenance juice to the battery while camping?

If there is no isolator, you'll probably want one. Diode type (with the heat sink that you see in the auto parts store) aren't good because they drop the voltage. Split-charge relay type is the good stuff.

As for solar...gotta leave that to someone else for now. I'm heading out and likely won't be online much, if at all, for the next week or so.

 

[Martyn]

Most vehicles do not have isolators to stop the trailer drawing down the vehicle battery. You could put a diode in, we prefer to keep it very simple and tell people to get into the habit of unplugging the trailer from the vehicle if it's going to be sitting for any length of time.

 

[Paladin]

Exactly.

<snip>

Well, I've been schooled.

The problem though, is that 13.5v-14v might not fully charge the battery.

Wonder if that's why my Odyssey battery died so early. Seems I'm only getting 13.5V-13.9V out of the alternator.

But yea, big wire will be too big for a 7-pin connector.

But there's still this we agree on.

I would think that it's possible to run 10ga to the 7 pin as normal. Then you could have an auxilliary Anderson connector with 4ga power and ground that you could connect when you want to fast charge. Both systems can run in parallel safely. And then if you're ever towing somebody else's trailer, your 7 pin setup will work just fine without hooking up the big system.

 

[McZippie]

1. Isn't the 10 gauge wire on 7 pin connector primarily there to charge the small battery used for the emergency disconnect trailer brake???

2. The 'real' problem with trying to engine alternator charge a large auxiliary trailer battery with small gauge wire...
The alternator is sending full amps to the thick gauge wired starting battery. While this charging cycle is happening, reduced amps are going to the small wire gauge trailer battery.
The Starting battery reaches a full charge and the alternator reduces amp output before the trailer battery is charged.
As posted above the trailer battery is set-up for an early death.

 

[RubiconGeoff]

Charging a battery through small-gauge wire, passing through a diode, or through poor connections will still charge the battery, but may prevent it from being properly "peaked." This will shorten the battery's life as it never actually reaches full charge, and sulphates are allowed to build up.

Proper battery care would require plugging it into a battery maintainer once the trailer is parked back at home. If you're really worried about fully "peaking" the batteries (as opposed to simply charging them) on an extended expo trip, you could always use a small solar panel to hook up directly to the trailer's batteries once at camp. This will bump up the voltage and properly "peak" the batteries to 100% after receiving a 98% charge from the tow vehicle's alternator through the 7-pin or Anderson connector.

 

[NuggetHoarder]

10 gauge is fine for the battery supply, in fact, it's preferred over a big fat wire. Unfortunately this is another example of ExpoSexual creeping in.

ExpoSexual - that's a great word for it. I've noticed this in a lot of different areas, whether it's vehicle mods or camping gear. Around here, if there is a $500 solution to a $5 problem, the $500 solution wins.

I can see how people get caught up in it. We work all day, and time is short, and we don't have time to build something or use primitive methods, so we reach for the easy solution even if it's complete overkill and terribly overpriced. Lucky for me, I'm pretty tight with my money so I only indulge in exposexual style solutions as a last resort.

Thanks for the new, and funny, dictionary entry!