Original charger as a backup to aftermarket inverter/charger

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Original charger as a backup to aftermarket inverter/charger

July 06, 2025, 05:25:39 pm

When the shop installed my Magnum Inverter/Charger back in 2013, they disconnected the original charger, installed by the factory. This left an unused charger and an unused breaker in the 120V panel. I wanted to see if I could reconnect the unit to have it as a backup if the Magnum failed. Turns out the input was disconnected, but not the output, which answered the question whether the output caps would bother any of the other chargers - evidently they don’t.

So I reconnected the input to the charger to the unused breaker, and it works fine. It produces 13.88 VDC open circuit voltage, and 13.7 or so when topping off the Lithiums which sit at rest at 13.6 or so. Cool, so now I have a ready backup charger. I’ll leave the breaker off so as not to accumulate time on the backup charger input circuitry, and to eliminate the possibility that the charger would attempt to charge the batteries when the inverter is supplying 120 VAC. I also turned both on and verified that nothing smoked.

I have a lingering question, maybe someone can educate me: I note that the A/C is powered by shore power, or by the generator, but not by the inverter. How is this mechanized? Does the transfer switch handle power to the A/C differently from the other 120 VAC circuits, such as the utility outlets, the microwave, and the refrigerator (which do receive power from the inverter)? I would consider whether I would want to exclude the refrigerator from inverter power, to avoid the issue where turning on the inverter causes the refrigerator to automatically switch from gas to AC, which consumes battery power at a high rate. I put a note near the inverter switch to remind ourselves to switch the refrigerator over to gas mode.

Re: Original charger as a backup to aftermarket inverter/charger

Reply #1 – July 08, 2025, 12:04:58 am

It sounds like we have a very similar setup. The shop that upgraded our solar and installed a “whole house” inverter (minus the air conditioner) disconnected a wire and left the existing converter-charger in the power center in place.

I don’t have an answer to your question but I’m wondering about the issue with the fridge when the inverter is turned on. I need to test to see if the fridge is excluded, but it seems to me that if the fridge is in “Auto” mode it might possibly switch to AC when the inverter is turned on but wouldn’t it also automatically revert back to gas when the inverter is turned back off?

I only leave the inverter on when actually using it for AC power.

Re: Original charger as a backup to aftermarket inverter/charger

Reply #2 – July 08, 2025, 11:03:11 am

Yes, when in auto mode, the fridge will switch back to gas when 120 VAC power is lost.

When running on inverter power, a typical absorption fridge will draw 350 to 375 watts, so that's something to keep in mind. It can drain your batteries pretty quickly. (A compressor fridge will draw something like 40 to 80 watts.)

Re: Original charger as a backup to aftermarket inverter/charger

Reply #3 – July 08, 2025, 01:11:13 pm

120 VAC power from the inverter doesn’t get supplied to the air conditioner. 120 VAC power from the inverter does get supplied to the refrigerator. The refrigerator when power source is set to “AUTO” prefers 120 VAC over propane, so it switches to 120 VAC. This is not an unreasonable strategy, except for when the 120 VAC is supplied by the inverter, which draws down the battery charge unless I manually switch the power source to “GAS”.

I asked about how the system avoids supplying 120 VAC to the air conditioner when running on the inverter, because that strategy is what I might want for the refrigerator also.

Re: Original charger as a backup to aftermarket inverter/charger

Reply #4 – July 08, 2025, 02:29:27 pm

Well, you could wire the fridge to the air conditioner circuit. I don't know how much trouble that would be.

Re: Original charger as a backup to aftermarket inverter/charger

Reply #5 – July 08, 2025, 02:38:55 pm

When using the inverter - presumably not connected to shorepower. No reason, then, not to leave fridge on 'gas only' mode. On AC the fridge is extremely inefficient, so even if using the inverter only briefly, why waste battery power for the fridge?

Steve

Re: Original charger as a backup to aftermarket inverter/charger

Reply #6 – July 08, 2025, 08:49:29 pm

Right, there is no need to have the refrigerator switching back and forth between 120 VAC and gas in response to the inverter powering on. The refrigerator always wakes up in AUTO mode, requiring someone to manually switch it over to GAS-only. That is the problem: training everyone to do that switch. On power-up, it is in the AUTO state.

Re: Original charger as a backup to aftermarket inverter/charger

Reply #7 – July 08, 2025, 08:57:46 pm

Quote from: Andy Baird - July 08, 2025, 02:29:27 pm

Well, you could wire the fridge to the air conditioner circuit. I don't know how much trouble that would be.

You touch on my question, how does the air conditioner power get selectively applied based on the source of AC: shore, generator, or inverter? Is the inverter wired directly to the output of the transfer switch, downstream of the switch, so to speak? Does the transfer switch actually have multiple switches (what in a relay would be called multi-pole), one for the air conditioning, and another for the refrigerator and microwave?

Re: Original charger as a backup to aftermarket inverter/charger

Reply #8 – July 09, 2025, 12:25:38 am

Good question Keith, and I don't know the answer. Hopefully somebody here does.

Re: Original charger as a backup to aftermarket inverter/charger

Reply #9 – July 09, 2025, 09:30:04 am

Hi Keith, the air conditioner has a separate circuit breaker just for itself, and so does the microwave. The outlets are all supplied by the GFCI circuit breaker. So just that breaker is supplied by the inverter, and passed through when the inverter is off. That 110v AC supplies the fridge also. If you want the refrigerator to not operate off of the inverter, you would need to isolate it, and have that outlet connected to the microwave for instance. Not much hazard. The metal parts of the fridge are still connected to ground.
The refrigerator would not detect the 110V AC, and would stay on gas. RonB
(in Florence Italy)

Re: Original charger as a backup to aftermarket inverter/charger

Reply #10 – July 15, 2025, 02:47:02 pm

Quote from: Andy Baird - July 08, 2025, 11:03:11 am

Yes, when in auto mode, the fridge will switch back to gas when 120 VAC power is lost.

When running on inverter power, a typical absorption fridge will draw 350 to 375 watts, so that's something to keep in mind. It can drain your batteries pretty quickly. (A compressor fridge will draw something like 40 to 80 watts.)

Just tested the fridge. Started out with it in auto mode with the propane on and the inverter off. Then turned the inverter on and it switched to AC, as expected. Was drawing about 330 watts initially, now consistently around 275 watts. Just wanted to test it out for myself for fun!

Re: Original charger as a backup to aftermarket inverter/charger

Reply #11 – July 15, 2025, 03:18:02 pm

My inverter is always on. But the fridge is set to gas only, unless…

On sunny drive days, I have my fridge use AC as I’ll be generating more than I use, but I do have to post a reminder on the fridge to switch it back at night

Re: Original charger as a backup to aftermarket inverter/charger

Reply #12 – July 15, 2025, 04:59:37 pm

When I had a 300AH lithium & 2 KW inverter installed, I tested my running the reefer from room temp on the inverter. Chilled nicely overnight and battery was down by 210 A. Second part of test - recharge time ran into problems. A series of atmospheric rivers brought daily rain over the next 9 days.

Re: Original charger as a backup to aftermarket inverter/charger

Reply #13 – July 15, 2025, 07:34:03 pm

Yeah, that's the problem. Lithium batteries can supply power for a long time, so it's easy to forget that it takes an even longer time to put it all back. I'm in that situation now: with 460 amp-hours of lithium storage I can run for days, but a series of cloudy and rainy days (monsoon season, I guess) are slowly lowering the average charge, day by day.

I set out my 200 watt solar suitcase this afternoon, so now I have 600 watts, but with a string of rainy days ahead, that may not be enough to bring the battery back up to a full charge. I'm not panicking, but I really prefer to keep it fairly full, and not lose charge from day to day.

Anyway, I know the situation you're talking about. A big battery bank is great, but you have to keep in mind how long it takes to recharge it.

Re: Original charger as a backup to aftermarket inverter/charger

Reply #14 – July 15, 2025, 07:37:32 pm

I think I understand the issue now. Engineers and technicians can read on. Normal people can skip!

I found a wiring diagram for another MH that uses the Magnum Inverter, which offers one possible explanation. There are two AC buses, the original one (call it Main) with the DC load center that provides the DC connections and their fuses, and one on the output of the inverter (call it Subsidiary, or Sub for short).

The air conditioner is on its own circuit on the Main bus. The Inverter and factory charger are also on the Main bus. This bus is only energized by the shore power plug and the generator, not by the inverter. This is why the A/C doesn’t try to run on the Inverter. The Magnum Inverter/Charger is smart about passing through AC power to the Sub bus: when it sees AC power on the Main bus it connects this to the Sub bus, just as if it wasn’t there. The AC outlets throughout the coach are connected to the Sub bus. The microwave and refrigerator are plugged into these AC outlets on the Sub bus.

When the Inverter is turned ON, it is enabled to invert the DC power from the battery bank to produce AC power that it feeds to the Sub bus. I said enabled because the Inverter monitors the Main bus AC voltage and silently passes through power from the Main bus if it is good enough, otherwise it does its thing and produces AC power itself for the Sub bus. This is why the refrigerator can’t tell the source of its AC power. The Inverter is acting as a UPS (Uninterruptible Power Supply). (In my coach, the shop that installed the Magnum mounted the GFCI in the cabinet under the sink, where the TEST button occasionally gets bumped by a cooking pot and shuts off the AC outlets. Once I figured out that my SO wasn’t messing with my sanity and a pot was actually guilty, I installed a guard over the TEST button.)

As it was mentioned earlier, if I wanted to make the refrigerator unable to run off of the Inverter, I could disconnect the outlet where the refrigerator plugs in from the Sub bus and run wires from the Main bus to it.

Re: Original charger as a backup to aftermarket inverter/charger

Reply #15 – July 15, 2025, 07:42:29 pm

Long dry spells of poor solar production, ironically resulting from long wet spells, call for some generator time, which Lithiums are good at making efficient use of.

Re: Original charger as a backup to aftermarket inverter/charger

Reply #16 – July 15, 2025, 08:16:29 pm

That's true, Keith, so this situation isn't a problem for most Lazy Daze owners. But with the minimal rig I'm traveling in now (19' Airstream plus small pickup), it's just not convenient to lug around a generator, although I do own one. I've been getting by OK up until recently, but I think my heavy use of the Starlink Mini (because there's no cell signal where I am) is pushing things too far. :: sigh :: I'll just have to cut back. Or fire up my steam-powered laptop. ;-)

Re: Original charger as a backup to aftermarket inverter/charger

Reply #17 – July 15, 2025, 09:37:30 pm

" I would consider whether I would want to exclude the refrigerator from inverter power, to avoid the issue where turning on the inverter causes the refrigerator to automatically switch from gas to AC, which consumes battery power at a high rate. I put a note near the inverter switch to remind ourselves to switch the refrigerator over to gas mode."

How about just unplugging the fridge? Then the fridge will always run on gas whether the inverter is on or off. You can plug it back in if you plug into regular shore power. Then get into a habit when unplugging from shore power unplug the fridge. Plugging into shore power, plug the fridge back in.

JohnF

Re: Original charger as a backup to aftermarket inverter/charger

Reply #18 – July 16, 2025, 01:45:16 am

That is a good idea!

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