Question about RV battery and inverter (Scamp 5th wheel)

[John in Santa Cruz]

Originally Posted by Air Doug

One thing so far not mentioned is grounding the inverter. I am no expert, but I read a lot about it and multiple grounds are supposed to risk ground loops, which can cause electrical interference in radios.
Good info is in Grounding Made Simpler, here:
https://diysolarforum.com/resources/...e-systems.159/

In my Snoozy, I had four separate grounds to the trailer chassis:
- on the tongue where the 7-pin connector entered the frame tube;
- beneath the battery compartment where all the ground wires connect to the battery;
- beneath the converter/charger;
- at a center marker/running light on the rear bumper.

I combined them all onto a single buss near the 3000 watt inverter and ran 8 ga wire to the frame and inverter.

Another thing to pay close attention to is the wire gauge specified by the inverter manufacturer. A lot of ampacity charts don't go past 200 amps at 12 V, but that is easily exceeded with a larger inverter. My inverter required 4/0 wire.

if the inverter needs 4/0, then the negative/ground side also should be 4/0 all the way from the battery.

[computerspook]

Originally Posted by John in Santa Cruz

with newer vehicles using smart alternators, thats no longer a valid assumption. my wife's 2016 Mercedes, the system voltage can be anywhere from 12.6 to 14.4V depending on many conditions, and it can change, quite a lot, on the fly as you drive. I know newer Ford trucks do the same thing, I assume GM and Ram are much the same too. Now, the Ford trucks, if they have the full HD tow package, AND you put the truck in 'tow/haul' mode, then they maintain a higher minimum voltage

If that is what is going to the battery then her car is messing up the battery. Not that this is not already being done a lot by a lot of not so smart vehicles. Now the voltage in some places can differ quite a bit also. But if you are feeding from the battery it should be 13.8. If you stay under that too long you mess up the battery by discharging it and leaving it that way. If you go over that anything over that is being turned into heat and batteries hate heat.

The main reason that the batteries in battery backups for PCs go bad is that most of them use 14, 15, 16 volts to the battery. That cooks and warps the battery and then it no longer works. If you then leave it in the unit too long (measured in years) it can finally go dry and over heat and actually produce a bunch of very bad smoke. Have had to deal with that at several client sites. It can take more than a day to get it properly aired out.

Now one thing that is interesting with the computers on cars now is that some of them actually will turn off the alternator when under load. If they they go into reversion mode while under load the alternator will stay off. And some of them even without that will leave the alternator off long enough that if you are running an extra load your battery can become discharged. Of course that can trigger reversion mode.

[PearlBeachBob]

I put a pretty big (1500w I think, maybe bigger) Harbor Freight Inverter in my 16 Scamp and not happy with it

[computerspook]

Originally Posted by PearlBeachBob

I put a pretty big (1500w I think, maybe bigger) Harbor Freight Inverter in my 16 Scamp and not happy with it

What kinds of problems are you having with it?

[John in Santa Cruz]

I have the 2000W Renogy Pure Sine inverter, and so far its run every load I've plugged into it. I did not use a transfer switch, instead, I installed an inverter-only dual outlet, and its been used for hair drying, coffee maker, vacuum etc.

I'm pleasantly surprised at how efficient it is, its current draw with no load is miniscule, but naturally if you run a 1800 watt hair dryer on it, expect a 150 amp load on your batteries, and the DC wiring has to be up to that.

[parmm]

Originally Posted by Ottoscamp

Wouldn't the battery stay charged while traveling? I thought it was designed to recharge from the electrical harness connected to the TV? Or do you mean it will discharge despite the input from the TV?

To get a good charge you need to run a separate heavy gauge wire from you TV battery to you house batteries. The 7 wire plug running from your tow to your trailer does not have a heavy enough gauge wire. I'm running a separate 12 vdc to 12 vdc battery charger from my truck battery to my house battery with 6 gauge wire.


I also got rid of the "Scamp battery". Replaced it with two Battleborn LiFePO3 100 ah batteries. I can go several days without hooking up. I run my Phillips Dreamstation cpap and my computer on 12 vdc. I do have a small inverter to run a 20" TV. You can also use your computer as a TV. I'm connecting to my computer with my phone and streaming TV programs using Dish anywhere. Just make sure you have unlimited data!!! You can get 12 volt compressor refrigerators to replace the the frig you have. They are very efficient and dependable.

[John in Santa Cruz]

heavier gauge wire allows more amps, but the typical 12 gauge vehicle side wiring will still charge your trailer battery and/or keep it charged... what happens is, as the current goes up, the voltage drops, when the voltage is lower, there's less current demand so it charges slower... but it still charges.

I did install a Victron Orion Tr 12/12-18 as part of my battery upgrade and I did not upgrade the truck wiring. At the Orion in the back of my trailer, I will see as low as 10 volts from the truck, at 30+ amps so the Orion can output 13-14 volts at 18 amps into my batteries. I'm not sure it was worth installing, but then I didn't want my LFP batteries supplying power to the truck either.

[jwcolby123]

Originally Posted by PearlBeachBob

I put a pretty big (1500w I think, maybe bigger) Harbor Freight Inverter in my 16 Scamp and not happy with it

You don't mention what kind of battery you have. Lead Acid batteries are less than ideal for running high wattage inverters because of the Peukert effect. The voltage immediately droops pretty bad under high loads and the inverter struggles. The lower the voltage, the higher the current the inverter tries to draw. The higher the current the more the droop (the lower the voltage).

You can see where we are going there.

It is important to remember that an inverter draws current more or less directly depending on the AC load. So a high wattage inverter will do fine when you just plug in the laptop because it isn't drawing much current. Now turn on a microwave and look out.

For low current apps such as led lights and the occasional water pump draw or even a low wattage inverter (for the laptop etc) a lead acid is fine. For high current draw a LiFePO4 is a better match.

A 1500 watt inverter may draw well over 100 amps. Watts = amps X volts so 1500 / 120 (roughly) = 12.5 AC amps.

However the inverter now has to convert DC into AC so divide the AC voltage by the battery voltage. Under high load a lead acid battery will droop badly so best case... (120v ac / 12v dc) = 10 amps DC for every 1 amp AC for lead acid. A LiFePO4 also does have a Peukert effect but it is lower, so unloaded it might be 13.4v, under high load it might be 13v. Divide that 120 volts AC by 13 and you have 9.23 amps DC per amp AC.

Now multiply the required amps AC by the amps DC. 12.5 amps AC X 10 amps DC = 125 amps DC for the lead acid, vs 12.5 amps AC X 9.23 amps DC = 115 amps DC for the LiFePO4.

These numbers matter! A LiFePO4 battery has a battery management system (BMS) which has a max amp rating. Typically a 100 amp hour LiFePO4 will handle 5=10% more than the rated but still, it needs to be kept in mind.

And then there is the matter of wiring. These currents can quickly overheat a small gauge wire.

The point here is that whatever battery you use, whether lead acid or LiFePO4, you need to be aware how much current you will actually draw out of the inverter (and for how long), as well as how the battery will be affected by drawing that much current. Or for that matter, whether the battery can even supply that much current.

[John in Santa Cruz]

Yeah, peak DC current I've measured from my 2000 W pure sine inverter is about 180 amps. Definitely need 2 LFP batts in parallel for that.

[David Tilston]

Originally Posted by John in Santa Cruz

absorption fridges in FG trailer sizes generally draw 300-700 watts when on 120VAC, maybe half the time, or more if its particularly hot out.

I know that I'm splitting hairs, but:
An RM211, (Trillium 1300) has a 95 watt heater
An RM36, (Trillium 4500 up to 1978) has a 120 watt heater.
An RM360, (Trillium 4500 1979 on) has a 125 watt heater.

Somewhat less than 300 watts.

You may also want to upgrade the wires to your inverter. If your drawing 180 amps you are into 3/0 territory.

You could get away with 2/0 if your using 90°C rated wire.

[Perryb67]

OTOH, there are thousands of campers out there that have been running microwaves and toasters for years on two 6v deep cycles batteries, many running both at once. The draw only lasts a couple of minutes and the batteries recover.

So, if you camp like us, and only want to charge electronics, run a toaster, CPAP, or microwave leaded batteries are just fine. If you own a mobile home camper and need to run everything under the sun at once you’d better have 400 or more useable ah’s.

Enjoy,

Perry

[John in Santa Cruz]

Originally Posted by David Tilston

I know that I'm splitting hairs, but:
An RM211, (Trillium 1300) has a 95 watt heater
An RM36, (Trillium 4500 up to 1978) has a 120 watt heater.
An RM360, (Trillium 4500 1979 on) has a 125 watt heater.

Somewhat less than 300 watts.

You may also want to upgrade the wires to your inverter. If your drawing 180 amps you are into 3/0 territory.

You could get away with 2/0 if your using 90°C rated wire.

those are all tiny fridges,
the one that was in my Escape, RMD8555, 6.7 cubic feet, was rated 250W on AC or 170W on DC. I guess I was looking at numbers for the big double door fridges found in big stickie trailers